Freshwater Habitats of Algae

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2 FRESHWATER HABITATS OF ALGAE

John D. Wehr Robert G. Sheath Louis Calder Center — Biological Station Office of Provost and Vice President for and Department of Biological Sciences Academic Affairs Fordham University, California State University, San Marcos Armonk, New York 10504 San Marcos, California 92096

I. What is Fresh Water? IV. Wetlands II. Lentic Environments A. Functional Importance of Algae A. Major Lakes of North America in Wetlands B. Lake Basins B. Algal Diversity in Freshwater C. Lake Community Structure and Wetlands Productivity C. Algal Communities of Bogs D. Ponds, Temporary Pools, and Bogs V. Thermal and Acidic Environments E. Phytoplankton of Lakes and Ponds A. Thermal Springs F. Benthic Algal Assemblages of Lakes B. Acid Environments III. Lotic Environments VI. Unusual Environments A. Major Rivers of North America A. Saline Lakes and Streams B. Geomorphology of Rivers B. Snow and Ice C. The River Continuum and C. Other Unusual Habitats Other Models Literature Cited D. Benthic Algal Communities of Rivers E. Phytoplankton Communities of Rivers

I. WHAT IS FRESH WATER? freshwater algae fall into a large, but ecologically meaningful collection of environments: all habitats that The study of freshwater algae is really the study of are at least slightly wet, other than oceans and estuaries. organisms from many diverse habitats, some of which One reason for such a broad scope is that inland saline are not entirely “fresh.” Although the oceans are clear- lakes, snow and ice, damp soils, and wetlands are ly saline ( 35 g salts L–1) and most lakes are relatively studied by phycologists and ecologists who also exam- dilute (world average < 0.1 g L–1; Wetzel, 1983a), there ine more traditional freshwater environments. Some is enormous variation in the chemical composition of genera with terrestrial species, such as Vaucheria, the nonmarine habitats that algae occupy. Conditions Nostoc, Chlorella, and Prasiola, also have species in lakes and rivers vary not only in salinity, but also found principally in streams or lakes (Smith, 1950; in size, depth, transparency, nutrient conditions, pH, Whitton, 1975). In North America, the variety of fresh- pollution, and many other important factors. Aquatic water habitats colonized by algae is very rich, and ecologists also use the term “inland” waters to encom- offers an enormous and fascinating range of environ- pass a greater range of aquatic ecosystems. Even this ments for their study. term may be unsatisfactory, because algae occupy The distinction between marine and freshwater many other habitats, such as snow, soils, cave walls, habitats is revealed in the variety of algae that occur and symbiotic associations (Round, 1981). in these environments. There are no exclusively fresh- Organisms grouped together in this volume as water divisions of algae, but certain groups exhibit

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greater abundance and diversity within fresh waters, (Hutchinson, 1957, 1967, 1975; Frey, 1963; Wetzel, especially Cyanobacteria, Chlorophyta, and Charophyta 1983a; Cole, 1994; Abel et al., 2000). This section (Smith, 1950). Within the green algae, conjugating summarizes some features of lentic environments as greens and desmids (Zygnematales, Chap. 9) comprise they pertain to the ecology and distribution of fresh- a very rich collection of species that almost exclusively water algae. occupy fresh water. Other groups, such as the diatoms and chrysophytes, are well represented in both spheres. A. Major Lakes of North America Other groups, particularly the Phaeophyta, Pyrrophyta, and Rhodophyta, exhibit greater diversity in marine Worldwide, the single largest volume of freshwater waters (Smith, 1950; Bourrelly, 1985). Most freshwater — nearly 20% of the world’s total — is located in Lake algae are best described as cosmopolitan, although Baikal, Siberia (23,000 km3), but the North American there are reports of endemic chrysophytes, green algae, Great Lakes (Fig. 1A) collectively represent the largest rhodophytes, and diatoms (Tyler, 1996; Kociolek et al. total volume of nonsaline water on Earth, approxi- 1998), and at least some species of cyanobacteria mately 24,600 km3 (Wetzel, 1983a). North America is (Hoffmann, 1996). Many algal taxa have particular home to many spectacular large and deep freshwater environmental tolerances or requirements, and are ecosystems, nearly half of all the world’s lakes greater than logically restricted, but still geographically widespread. 500 km2 (Hutchinson, 1957). Two of the most impres- The euglenophyte Colacium is almost exclusively epi- sive lakes are subarctic: Great Slave Lake (28,200 km2; zooic on aquatic invertebrates, but is widely distributed 614 m deep; deepest in North America) and Great Bear throughout North America (Smith, 1950; Chap. 10). Lake (30,200 km2; > 300 m deep) in the Northwest The chrysophyte Hydrurus foetidus is an exclusive Territories (Hutchinson, 1957). Crater Lake in Oregon inhabitant of cold mountain streams, but is distributed (Fig. 1B) is much smaller (64 km2 in area), but is the worldwide (Smith, 1950; Whitton, 1975, Chap. 12). deepest lake in the United States (608 m) and seventh Even specialized taxa such as Basicladia chelonum deepest in the world (Edmondson, 1963). The largest (Chlorophyceae), which is restricted mainly to the lakes on the continent are located in northern and shells of turtles, has been collected from many habitats temperate regions, although Great Salt Lake (Utah) is a throughout eastern North America (Smith, 1950; massive remnant lake (> 6000 km2) that has a mean Prescott, 1962; Colt et al., 1995). The actual distribu- depth ( 9 m) and very high salinity (130–280 g L–1) tion of apparently disjunct freshwater species must that fluctuate with available moisture, and occupies a therefore be viewed with some caution until detailed portion of the Pleistocene Lake Bonneville, which surveys have been conducted (see, for example, Linne had an area > 51,000 km2 and a depth of 320 m von Berg and Kowallik, 1996; Müller et al., 1998). (Hutchinson, 1957). Inland waters represent only about 0.02% of all water in the biosphere, and nearly 90% of this total is B. Lake Basins contained within only about 250 of the world’s largest lakes (Wetzel, 1983a). Nonetheless, it is fresh water that Sizes and shapes of lake basins (their morphome- is most important for human consumption and is most try) have profound effects on the physics, chemistry, threatened by human activities. Algal ecologists play and biology of lake ecosystems, and influence the com- an important role in the understanding of aquatic position of algal communities and their productivity. ecosystems, their productivity, and water quality issues Lake basins differ in morphometry as a result of the (Round, 1981; Brock, 1985a; Hoffmann, 1998; Dow forces that created them, many of which were cata- and Swoboda, 2000; Oliver and Ganf, 2000, strophic events from the past, principally glacial, Chaps. 23 and 24). This chapter examines the habitats seismic, and volcanic activity. Hutchinson (1957) of freshwater algae and how differences in these distinguished 76 different lake types based on their systems affect algal communities. origins; these were classified into a simpler scheme by Wetzel (1983a) that is summarized in Table I. Glacial activity is the most important agent in II. LENTIC ENVIRONMENTS North America. It created millions of small and large basins from the arctic south to the southern extent of Lentic environments include standing waters from the Wisconsin ice sheet. In this period (15,000–5000 the smallest ponds (a few square meters) to enormous years BP), many basins became closed by morainal bodies of water (e.g., Laurentian Great Lakes: deposits, including the Laurentian Great Lakes. Some 245,000 km2). Their formation, geography, limnology, morainal lakes occur at the ends of long valleys after and conservation have been covered in several texts glaciers have receded, including the Finger Lakes of 741550.02.qxd 16/9/2002 2:25 PM Page 13

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FIGURE 1 Examples of different types of lakes in North America: A, Laurentian Great Lakes; B, Crater Lake, Oregon, a deep caldera lake; C, New York Finger Lakes; D, kettle lakes in Becher’s Prairie, central British Columbia; E, pothole lakes in Qu’appelle Valley, Saskatchewan; F, Louise Lake, a cirque in Mt. Rainier National Park, Washington. Photos A and C courtesy of U.S. Geological Survey EROS Data Center, reproduced with permission; photo B by R. G. Sheath; photo D by R. J. Cannings; photo E by P. R. Leavitt, reproduced with permission; photo F by J. D. Wehr.

New York (Fig. 1C), which are elongate, radially basins within narrow valleys may form deep fjord lakes arranged basins that range from small ponds to large (Fig. 2A), or a chain of smaller lakes known as pater- lakes, such as Seneca (175 km2 area, 188 m depth; nosters. Several forces, including glacial scour and lava Hutchinson, 1957; Berg, 1963). However, most flow, combined to form the small (9.9 km2) but deep glacially formed lakes are small kettles scattered across (259 m) fjordlike Garibaldi Lake (Northcote and the continent (Fig. 1D and E). Glacial scouring in Larkin, 1963; Fig. 2B). Ice-formed (thermokarst) lakes, mountainous terrain may form deep amphitheater-like which result from freezing and thawing action in ice cirques (Fig. 1F), which are common from Alaska and soil, are common in the Arctic. All are shallow but through the western mountain ranges south to tropical vary from large elliptical basins (up 70 km2 area) to locations in Costa Rica (Haberyan et al., 1995). Glacial small (10–50 m diameter), “polygon” ponds (Fig. 2C), 741550.02.qxd 16/9/2002 2:25 PM Page 14

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TABLE I Major Lake Basin Types, Grouped According to the Principal Forces That Shaped Them

Type of forces Basins Principal force North American examples

Catastrophic 1. Glacial Glacial scouring Great Slave Lake, NWT Moraine deposits Finger Lakes, NY; Moraine Lake, AK Kettles Linsley Pond, MA; Cedar Bog Lake, MN Cryogenic Many polygon lakes, AK 2. Tectonic Graben Lake Tahoe, CA-NV Uplift Lake Okeechobee, FL Landslide Mountain Lake, VA 3. Volcanic Caldera Crater Lake, OR Maar Zuni Salt Lake, Mexico 4. Meteor Meteor New Quebec Lake, PQ Noncatastrophic 5. Solution Doline Deep Lake, FL; limestone areas of KY-IN-TN Salt collapse Montezuma Well, AZ; Bottomless Lakes, NM 6. Rivers Oxbows Lake Providence, LA Plunge Fayetteville Green Lake, NY 7. Coastal Shoreline Along Laurentian Great Lakes; Cape Cod area 8. Wind Deflation Moses Lake, WA; Sandhills region, NB 9. Organic Beaver Many locations in northern regions Human Lake Mead, AZ-NV; Cherokee Reservoir, TN

FIGURE 2 Other glacial and ice-formed lakes: A, Okanagan Lake (BC), a fjord lake; B, Garibaldi Lake, formed by glacial scour and lava damming; C, arctic polygon ponds. Photos A courtesy of NASA, reproduced with permission; photos B and C by R. J. Cannings, reproduced with permission.

which are estimated at more than a million in number Tahoe, a symmetrical, deep (505 m), and steep-sided (Livingstone, 1963; Sheath, 1986). lake (Fig. 3A). Tahoe (double fault lines) and Lakes Tectonic basins are formed by movements of the Baikal and Tanganyika (single faults) include the Earth’s crust. Among these, grabens form when fault deepest lakes in the world, although less spectacular lines create often enormous depressions, such as Lake examples also occur. Lago de Peten (Guatemala) is 741550.02.qxd 16/9/2002 2:25 PM Page 15

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FIGURE 3 Tectonic lakes: A, Lake Tahoe, a graben, viewed from the northeast; B, Spirit Lake, Washington, a landslide lake near Mt. St. Helens; C, Lake Okeechobee, Florida (larger lake on lower right), and surrounding lakes. Photos A and B by J. D. Wehr; photo C reproduced with permission of the South Florida Water Management District.

the largest (567 km2) and deepest (> 32 m) lake on the Lake (WY) and Surprise (AK) Lake, have hydrothermal

Yucatan Peninsula (Covich, 1976), and the largest lake vents that influence temperature, pH, and O2 condi- in Mexico, Lago Chapala (1109 km2), is also a tectonic tions, and may contribute trace metals (Pierce, 1987; trench (Serruya and Pollingher, 1983). Landslide lakes Larson, 1989; Cameron and Larson, 1993). Other form when water flows through an existing depression volcanic lakes formed from violent explosions of cinder and is blocked by rock or other material, as in as cones (maars) and are often nearly circular in outline, Spirit Lake, near Mount St. Helens (WA; Fig. 3B) and such as Big Soda Lake in Nevada, Lago Chamico in Mountain Lake (VA; Parker et al., 1975). In the El Salvador (Cole, 1963), and Laguna Hule in Costa Pliocene, shallow marine areas were raised above sea Rica (Umana-Villalobos, 1993). Volcanic lakes also level and existing depressions filled with freshwater, as occur in Mexico, Guatemala, Nicaragua, and El with Lake Okeechobee (Fig. 3C), a large (1840 km2), Salvador (but not all are deep), and some were formed shallow (4 m) subtropical lake in Florida. as recently as 500 year ago (Hutchinson, 1957; Cole, Volcanic lakes are among the deepest and most 1963). steep-sided lakes on the continent. The exceptionally In limestone regions, solution or sinkhole lakes clear water in Crater Lake (OR), a collapsed caldera form from the dissolution of bedrock by surface and

(Fig. 1B), has Secchi depths between 20 and 30 m, with underground waters charged with CO2 (Cole, 1994). 1% surface light down to 100 m (Larson et al., 1996). Sinks may be circular, elliptical, or irregular in outline, Lake Atitlán in Guatemala is an alpine tropical caldera and occur throughout Kentucky, Indiana, Tennessee, (8.2 km2; 1550 m elevation) that reaches a depth of Florida, Mexico, and Guatemala (Fig. 4C). Florida is 341 m, making its ratio of depth to surface area four especially rich in sinkholes, with several hundred lakes times greater than better known Crater Lake. Lake and ponds ranging from less than 1 ha to several Nicaragua (Fig. 4A; 7700 km2 area; depth 60 m) was square kilometers (Fig. 3C). In north Florida, some formed by volcanic lava damming an existing valley lakes are relatively dilute and colored with organic (Cole, 1963). Some volcanic lakes, such as Yellowstone matter from pine litter, whereas others are clear, 741550.02.qxd 16/9/2002 2:25 PM Page 16

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FIGURE 4 Volcanic and solution lakes: A, Lakes Managua (near) and Nicaragua, two large calderas; B, Volcan Maderas (Nicaragua), a small caldera located on an island in Lake Nicaragua; C, unnamed, deep limestone sink in the Yucatan Peninsula; D, Montezuma Well (AZ), a collapsed travertine limestone sink. Photo A courtesy of NASA, reproduced with permission; photo B by A. Merola; photo C by L. P. Burney; photo D by D. W. Blinn, reproduced with permission.

hardwater systems (Shannon and Brezonik, 1972). the beaver (Castor canadensis), which dams smaller Spring-fed sinkholes may become isolated or thermally rivers to form lakes and ponds (Hutchinson, 1957; constant, such as Montezuma Well (AZ; Fig. 4D), Fig. 5C). Today, reservoirs are a more important group creating an environment with high algal productivity of lentic ecosystems, the size and number of which (600gCm–2 y–1; Boucher et al., 1984) and unusual are increasing worldwide (Fig. 5D). The physical and communities. Montezuma is a collapsed travertine chemical properties of reservoirs differ from natural system with several endemic invertebrates, but no fish, lakes with respect to dendritic or eccentric morpho- rotifers, or cladocerans (Cole, 1994). metry (deepest near the dam), shorter flushing period, Lakes may form through wind action, whereby irregular water level, greater dissolved and suspended deposited sand blocks existing valleys (e.g., Moses solids, and less stable littoral zone (Wetzel, 1990). Lake, WA) or forms depressions in dunes, as in Because reservoirs serve hydroelectric, flood control, or Nebraska and Texas (Cole, 1963; Edmondson, 1963). drinking water uses, they occur in many biomes. River-formed lakes, including oxbows, occur across A few lakes may have been formed by meteor North America where rivers traverse level terrain, impact, such as New Quebec Lake, a nearly perfectly enabling siltation of meandering valleys (Fig. 5A). circular basin (3.4 km diameter) in a region of irregu- Other small basins form in the plunge pools of water- lar, glacially formed lakes in northern Quebec (Cole, falls (Fig. 5B). Fayetteville Green Lake is a relatively 1994). Carolina Bays, which are not truly bays, are a deep (59 m; 0.3 km2 area) plunge-pool lake in central series of roughly 150,000 small, shallow, elliptical New York that apparently has never fully mixed; it was basins, with a distinctive NW–SE orientation, and con- formed during the Pleistocene when melting glaciers centrated along the Atlantic coast from New Jersey formed a vast waterfall (Berg, 1963; Brunskill and south to Florida. It is their directional orientation that Ludlam, 1969). has caused some to speculate that their origin may be For many centuries the principal biological agent from meteor showers, whereas others have suggested responsible for creating lakes in North America was wind action or artesian springs (Hutchinson, 1967; 741550.02.qxd 16/9/2002 2:25 PM Page 17

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FIGURE 5 Lakes formed by river action and other agents: A, an oxbow lake in Texas; B, a plunge pool below Waimea Falls, Hawaii; C, a beaver-dam lake (southern British Columbia); D, reservoir (eastern Colorado). Photo A by J. Cotner; photo B by D. Burney, reproduced with permission; photos C and D by J. D. Wehr.

Cole, 1994). Many are now filled, but those with ing plants (mosses, liverworts) also occur (Hutchinson, aquatic habitats are shallow and have extensive macro- 1975). Vertical zones also develop in temperate phyte beds and low algal production (Schalles and regions. In early spring, most temperate lakes are well Shure, 1989). mixed, with similar temperatures and chemical conditions from top to bottom. As temperatures increase, upper mixed waters become thermally isolated from C. Lake Community Structure and Productivity deeper and colder waters, a process which is termed stratification. The upper epilimnion continues to 1. Lake Zones and Thermal Patterns become warmer, receives greater irradiance, and is well Regions within lakes exhibit physical and chemical mixed and oxygenated; deeper waters remain cool (ca. differences that affect algal communities. The open 4°C in deep lakes) and dissolved gases are consumed water region of lakes is termed the pelagic (or limnetic) by microbial activity. At an intermediate depth (the zone, whereas close to shore is the littoral zone, where metalimnion), temperature declines, often sharply, with the greatest exchange between nutrient-rich sediments reduced heat penetration and reduced mixing and the water occurs (Fig. 6). The littoral zone is colo- (= thermocline if ≥∆1° m–1). Density gradients formed nized by submersed (e.g., Ceratophyllum, Potamogeton, by this thermal barrier may be sufficient to support and Vallisneria) and emergent (e.g., Scirpus and Typha) dense algal populations (Pick et al., 1984); here light is flowering plants, although some macroalgae (e.g., adequate for photosynthesis coupled with a greater Chara, Nitella, and Batrachospermum) and nonflower- supply of nutrients. Algal production may create metal- 741550.02.qxd 16/9/2002 2:25 PM Page 18

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FIGURE 6 Diagram that represents the zones and algal habitats within typical oligotrophic and eutrophic lakes (EZ = euphotic zone).

imnetic oxygen maxima in clear oligotrophic lakes summer or late spring, occur mainly in alpine and (Wetzel, 1983a; Parker et al., 1991). The hypolimnion arctic areas (temperatures ≤ 4°C). Oligomictic lakes is a deeper, cooler region with greater nutrient supply, have rare mixing periods (less than once per year), but reduced (approaching zero) oxygen levels; light where temperature strata (summer epilimnion > 4°C) may be too low for photosynthetic algal growth except may remain for some years; this pattern is most com- in very clear lakes. mon in deep tropical lakes (Wetzel, 1983a). Polymictic Patterns of thermal stratification and mixing differ lakes are shallow systems with frequent or continuous with altitude and across biomes. A dimictic pattern, in mixing, and occur in tropical and equatorial areas such which lakes stratify in the summer, mix in the autumn, as Lake Managua (Xolotlán), Nicaragua (Erikson et stratify in the winter after ice cover, and mix in the al., 1997). Amictic lakes, uncommon in North America spring after ice-out, is most common in temperate (some in Greenland; common in the Antarctic), are climates. Warm monomictic lakes (stratification and perennially ice covered and do not turn over. A special one mixing period; summer epilimnion > 4°C) occur in class of lakes in which upper waters (mixolimnion) warmer climates or in large basins without ice cover mix, but deeper waters (monimolimnion) never circu- (Wetzel, 1983a). Examples include the Great Lakes, late, are termed meromictic (Wetzel, 1983a). These larger Finger Lakes, Lake Tahoe, lakes in warm or lakes have a very stable chemical and temperature coastal climates, and subtropical, high altitude lakes. density gradient, known as a chemocline, that results

Cold monomictic lakes, with a single turnover in in anoxic conditions, H2S, and purple sulfur bacteria in 741550.02.qxd 16/9/2002 2:25 PM Page 19

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the water column. A strong depth to surface area ratio Temporary ponds are located in low-lying areas is usually necessary to maintain meromixis; Fayetteville that fill with snow melt or spring runoff, but dry up in Green Lake in New York and Hot Lake in Washington the summer (Wetzel, 1983a); resident algal populations are examples (Hutchinson, 1957). may be quite substantial during the growing season and rely on resistant resting stages (e.g., dinoflagellate 2. Lake Productivity cysts [Chap. 20], cyanobacterial akinetes [Chap. 4], Limnologists distinguish lakes according to a gra- and zygospores of Zygnematales [Chap. 9]) capable dient of primary production (14C uptake, algal growth) of surviving extended adverse conditions (Reynolds, or biomass, from oligotrophic (annual average < 50– 1984a, b). Arctic tundra ponds typically freeze solid for 300 mg C m–2 d–1; < 0.05–1 µg chlorophyll-a L–1) to many months, resulting in a growing season of only eutrophic (> 1000 mg C m–2 d–1; 15–100 µg chloro- 60–100 days (Wetzel, 1983a; Sheath, 1986). Algal phyll-a L–1), and these levels are influenced, at least in communities often consist of small flagellates (particu- part, by the properties of lake basins (Wetzel, 1983a; larly Cryptophyceae and Chrysophyceae; see Chaps. 21 Likens, 1985). Oligotrophic lakes are poor in nutrients, and 12–14) that bloom during the brief summer of usually deep and steep-sided, and have high trans- long daylight, although many species apparently do not parency, a narrow littoral zone, abundant dissolved form resting stages and survive freezing conditions in oxygen with depth, and larger relative hypolimnion their vegetative condition (Sheath, 1986). volume. Lake Tahoe is an ultra-oligotrophic lake that has exceptional water clarity, although average Secchi E. Phytoplankton of Lakes and Ponds depths have declined from about 30 to 23 m and primary productivity levels have more than doubled Assemblages of planktonic algae vary greatly (40–100 mg C m–2 d–1) since the late 1950s, following among lake basins and biogeographic regions. They increased nutrient loading from regional development include members of all algal divisions except the (Goldman, 1988). Eutrophic lakes are nutrient-rich, Rhodophyta and Phaeophyta. Their ecology has been often shallower with a broad littoral zone, and have the subject of many reviews, including Hutchinson depleted summer hypolimnetic oxygen and reduced (1967), Kalff and Knoechel (1978), Round (1981), transparency. Eutrophication of lakes often result Reynolds (1984a), Munawar and Talling (1986), when nutrients are added as sewage, detergents, or Sandgren (1988), Munawar and Munawar (1996, fertilizers (Wetzel, 1983a). Lake Mendota, Wisconsin, 2000), Stoermer and Smol (1999), and Whitton and is an example of a larger lake (39 km2) that has an Potts (2000). average depth of only 12 m and receives substantial input from agricultural and urban sources. The lake 1. Phytoplankton Diversity, Composition, and has experienced algal blooms and high nutrient levels Seasonal Succession for more than a century, and water transparency has Every collection of freshwater algae is characterized been consistently low for more than 80 years (Brock, by a fascinating and perplexing diversity of species, 1985a). many of which are potential competitors for common resources. Up to several hundred algal species may D. Ponds, Temporary Pools, and Bogs comprise the phytoplankton community of a typical north-temperate lake (Kalff and Knoechel, 1978). This Smaller lentic environments, often called ponds, paradox of many potentially competing phytoplankton may seem very similar to lakes except for their size, species coexisting in a relatively uniform habitat was but they have distinct properties. Ponds are shallow proposed by Hutchinson (1961) to be possible because enough either to support rooted aquatic vegetation of the numerous niches within a lake, as well as varia- across the entire basin or to fail to stratify during the tion within the environment over time. Many factors summer. The term “pond,” however, is not a precise contribute to phytoplankton diversity and production, concept, despite its frequent usage. In New England, including temporal and spatial variations in nutrient the word is often applied to fairly substantial lakes, supply, grazing, temperature, and parasitism (Turpin such as Linsley Pond (9.4 ha, max depth 14.8 m; and Harrison, 1979; Crumpton and Wetzel, 1982; Brooks and Deevey, 1963) and Long Pond (40 ha, Sommer, 1984; Bergquist and Carpenter, 1986). The 22 m; Canavan and Siver, 1995), both of which stratify biomass of phytoplankton is thought to be driven in summer. Small limnetic systems can be divided into mainly by nutrient supply and herbivory in all lakes, those that contain water year round, often called per- but their temporal dynamics differ in eutrophic and manent, and temporary waters that become dry each oligotrophic systems (Walters et al., 1987; Carpenter year and are termed vernal ponds (Wetzel, 1983a). et al., 1993). 741550.02.qxd 16/9/2002 2:25 PM Page 20

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Ecosystem-level studies of lake food webs in some P-limited conditions (Suttle et al., 1987, 1988; Wehr, cases have ignored phytoplankton species composition, 1989) and are grazed mainly by micro-zooplankton and instead treat this component as a black box. This (ciliates, flagellates), rather than cladocerans or approach is incomplete because the functional pro- copepods (Pernthaler et al., 1996; Hadas et al., 1998), perties of algal assemblages vary strongly with species making them important links between microbial and composition. Taxonomic information is important for classical food webs (Christoffersen et al., 1990; ecological studies because many of the features used to Sommaruga, 1995). classify algae, such as photosynthetic pigments, storage Size affects sinking rate and thus the ability of cells products, motility, reproduction, cell ultrastructure, to remain in the euphotic zone. Smaller cells tend to and even DNA sequence information, have functional be spherical or ellipsoid and thus sink more slowly, importance. For example, among freshwater phyto- whereas larger forms have more elongate or complex plankton, only cyanobacteria and some cryptomonads shapes to reduce sinking. The dinoflagellate Ceratium possess the red accessory pigment phycoerythrin, hirundinella is a large planktonic alga (up to 400 µm) which has an absorption maximum (540–560 nm) that common in mesotrophic and eutrophic lakes with broadens the photosynthetic capacity of cells and may stable stratification (Chap. 20); the alga regulates its facilitate growth at greater depths (Goodwin, 1974, position in the water column by active migration and Chap. 21). Similarly, only certain species of cyano- perhaps by changes in the shape and size of hornlike

bacteria are able to fix N2, mainly those that possess projections (Heaney and Furnass, 1980; Pollingher, heterocysts (e.g. Anabaena, Aphanizomenon, and 1987; Heaney et al., 1988). The silica walls of diatoms Nostoc), although some non-heterocystous forms with result in heavier cellular densities, making them thick sheaths or that form dense aggregations susceptible to sinking. Diatoms are estimated to have (Gloeocapsa, Oscillatoria, Microcoleus, and 3–16 times faster sinking rates than nonsiliceous algae Plectonema; see Chap. 4) also have this ability (Bothe, of equivalent sizes (Sommer, 1988). Larger colonial 1982; Paerl et al., 1989). Algal flagellates are a poly- diatoms, such as Asterionella formosa, Fragilaria croto- phyletic collection of different protists, but they form nensis, and Tabellaria fenestrata (see Chap. 16), have an important ecological group because of their tenden- more elongate or elaborate morphologies, which may cy toward mixotrophy (Porter, 1988). This fact is not reduce sinking rates or cause some cells to rotate coincidental, because the flagellar apparatus is directly (Smayda, 1970; Barber and Haworth, 1981). Plank- involved in the capture of bacterial prey (Andersen and tonic cyanobacteria and desmids produce extracellular Wetherbee, 1992). mucilage, which may aid in buoyancy (Round, 1981). Diversity in size is also an important property of Some cyanobacteria, such as Anabaena flos-aquae and phytoplankton communities. One scheme (Sieburth et Microcystis aeruginosa, also maintian their position in al., 1978) categorizes sizes into groups that differ over the water column using gas vacuoles (Chaps. 3 and 4). orders of magnitude: picoplankton (> 0.2–2 µm), nano- Observed changes in the population size may be the plankton (> 2–20 µm), microplankton (> 20–200 µm), result of differences in vertical migration and sinking to and mesoplankton (> 200–2000 µm) include most algal lower strata, rather than actual changes in numbers. cells and colonies in freshwater. Expressed in terms of Regular seasonal changes (seasonal succession) in volume, the sizes of freshwater phytoplankton span phytoplankton populations over many years have been at least 8 orders of magnitude (Reynolds, 1984b). observed widely and reported in long-term limnological The bacteria-sized picoplankton have attracted recent records. Few species have been documented as thor- interest because they have been found to dominate (at oughly as Asterionella formosa, a diatom found in least numerically) many phytoplankton communities in mesotrophic, temperate lakes (Round, 1981; Reynolds, lakes and marine systems (Stockner et al., 2000). They 1984a, Chap. 16). In Lake Windermere, populations occur in great numbers (105–106 mL–1), possess rapid increase after turnover and peak in the spring when growth rates, and are highly productive; most often light levels and temperatures are increasing and the Si:P reported are cyanobacteria (e.g., Cyanobium, Cyano- ratio is near maximum (Fig. 7). Populations decline thece, Synechococcus, and Synechocystis; see Chap. 3) over the summer and have a second (usually smaller) and some green algae (e.g., Nannochloris). Their peak in the autumn (Lund, 1964; Reynolds, 1984a; importance (percentage of biomass or primary produc- Neale et al., 1991). Asterionella was the spring domi- tion) seems to be greatest in oligotrophic and least in nant in Lake Erie from at least 1931 until about 1950, eutrophic lakes (Burns and Stockner, 1991; Hawley when nutrient enrichment selected for diatom genera and Whitton, 1991; Wehr, 1991), although there are such as Stephanodiscus, Aulacoseira (Melosira; see exceptions (Wehr, 1990; Weisse, 1993). Autotrophic Chap. 15), and Fragilaria, and filamentous cyanobacte- picoplankton have a strong competitive ability in ria such as Anabaena and Aphanizomenon (Davis, 741550.02.qxd 16/9/2002 2:26 PM Page 21

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FIGURE 7 Seasonal periodicity in the abundances (cells per liter) of Asterionella formosa (solid line), Fragilaria crotonoensis (dashed line), and Tabellaria flocculosa (dotted line), and concentrations of dissolved silica (upper, black; milligrams per liter) in Windermere, English Lake District 1945–1960. Reproduced with permission from A. Horne and C. R. Goldman, Limnology, 2nd ed. Copyright © 1994, McGraw–Hill, New York.

1964). As eutrophication receded, there was a 70–98% ods: times in the lake cycle, such as turnover, that lead reduction in numbers of Stephanodiscus spp. and to sharp changes in chemical or physical conditions. Aphanizomenon flos-aquae, and the reappearance of Lakes with less predictable conditions or more shock Asterionella (Makarewicz, 1993). Further details are periods exhibit frequent changes in species composition given in Munawar and Munawar (1996). and shorter growth peaks (Fig. 8A), but when condi- Population dynamics generally follow predictable tions are more stable, populations may persist over changes in temperature, sunlight, nutrients, and other longer time periods (Fig. 8B). Furthermore, coexistence factors. However, algal population dynamics exhibit (temporal overlap) among species will result if a greater more abrupt changes than these gradual trends predict, variety of habitats are available, perhaps through suggesting that other factors may drive seasonal succes- stratification, basin complexity, or multiple inflows sion. Round (1971) described these as “shock” peri- (Fig. 8C). These temporal patterns repeat only if nutrient 741550.02.qxd 16/9/2002 2:26 PM Page 22

22 John D. Wehr and Robert G. Sheath

FIGURE 9 Predicted (lines) and observed (points) outcomes of competition between Asterionella formosa (stars = dominant) and Cyclotella (Stephanocyclus) meneghiniana (diamonds = dominant) under varying levels of Si and P, indicating that coexistence (solid cir- cles) is possible in an intermediate range of Si:P ratios (From Tilman, D.; Resource Competition and Community Structure. Copyright © 1982 by Princeton University Press. Reprinted by permission of Princeton University Press.

resources, such as C:P and N:P (Kilham and Kilham, 1978; Rhee and Gotham, 1980; Sommer and Kilham, 1985; Olsen et al., 1989). In situ manipulations of phosphorus and light have demonstrated clear differences among species: some respond positively to P FIGURE 8 Diagrammatic representation of seasonal growth curves addition alone (e.g., Synedra radians), whereas others, of freshwater phytoplankton species in lakes with different ecological conditions and habitat complexity: A, variable ecological conditions such as chrysophytes (e.g., Dinobryon sertularia and or frequent shock periods, with short peaks and low overlap; B, Synura uvella), increase under greater light (de more stable conditions or longer stratification periods, with greater Noyelles et al., 1980; Wehr, 1993). Species composition temporal overlap; C, complex lake basins with more habitats and and size structure each can be influenced by nutrient peaks of different duration, overlap, and frequency. Redrawn and supply. With higher N:P supply ratios, assemblages in adapted from Round (1972). an oligotrophic lake were dominated by pico-cyanobacteria (Synechococcus sp.; see Chap. 3), but at lower supply ratios, larger diatoms (Nitzschia and Synedra) or other conditions are stable and other disturbances dominated (Suttle et al., 1987). Whole-lake N and P are kept to a minimum. For example, in Lake Michigan additions to oligotrophic Kennedy Lake (BC), used to Tabellaria sp. and Asterionella formosa exhibited enhance production of sockeye salmon, also affected regular peaks in the spring phytoplankton community competitive interactions among phytoplankton for several decades, but Asterionella numbers have (Stockner and Shortreed, 1988). Loadings at N:P ratios declined in more recent years, while Stephanodiscus between 10 and 25 increased algal biomass with a

and filamentous cyanobacteria have increased follow- summer community dominated by N2-fixing Anabaena ing increases in P supply (Makarewicz and Baybutt, circinalis; increasing the N:P ratio to 35 retained the 1981). higher biomass, but shifted the community dominance Laboratory experiments with Stephanocyclus to small-celled Synechococcus spp. (Cyclotella) meneghiniana and Asterionella formosa, Manipulations of fish densities in a small which have different Si and P requirements and con- Wisconsin lake varied predation pressure on zooplank- sumption rates, have indentified the levels of these ton, which in turn created different levels of grazing nutrients at which the two species may coexist (Fig. 9, pressure on phytoplankton; nested nutrient-permeable Tilman, 1977, 1982). Studies have quantified resource chambers separated effects of recycled nutrients from competition among other species and with other relaxed herbivory (Vanni and Layne, 1997). Phyto- 741550.02.qxd 16/9/2002 2:26 PM Page 23

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plankton biomass and the abundance of many algal cyanobacteria from 100 to 10% or less (Edmondson taxa (e.g., Peridinium inconspicuum, Chrysochromulina and Lehman, 1981). sp., and Staurastrum dejectum) increased with greater Eutrophic lakes with low N:P ratios favor blooms

fish biomass, although a similar effect was seen in of N2-fixing cyanobacteria; hence, nutrient cycling grazer-free diffusion chambers for some species, which within these lakes is closely coupled these organisms suggested that the positive effect of fish may have been (Schindler, 1977, 1985). Algal species have different mediated through recycled nutrients, rather than lower micronutrient requirements, such as Si, Mg, Ca, Fe, grazing pressure. Mo, and Se. Diatom dominance in phytoplankton Some algae occupy both planktonic and benthic assemblages is dependent on recycled Si following habitats at different periods of the year or even daily. spring turnover; lakes with lower Si levels may lack a This strategy may involve overwintering as inactive springtime diatom pulse altogether. The haptophyte stages (e.g., algal cysts) or active recruitment of benthic Chrysochromulina breviturrita develops large popula- forms into the pelagic zone. In many systems, flagel- tions in dilute lakes undergoing the early stages of lates such as Ceratium, Cryptomonas, and Euglena, acidification (Nicholls et al., 1982, Chap. 13). Its + exhibit diurnal vertical migrations toward the surface requirement for Se and NH4 , and its inability to use – by day and into deeper strata at night (Palmer and NO3 favor its success in oligotrophic lakes within the Round, 1965; Heaney and Talling, 1980; Hansson et pH range 5.5–6.5 (Wehr and Brown, 1985; Wehr et al., al., 1994), responding to patterns in light availability, 1987). Nutrient requirements may have important temperature cues, mixing conditions, or nutrient supply. interactions, for example, Mo, which is an essential In spring and early summer, the cyanobacterium micronutrient for some cyanobacteria as a co-factor

Gloeotrichia echinulata colonizes shallow sediments or for N2 fixation; its assimilation may be inhibited by 2– submersed plants in eutrophic lakes, but in late sum- elevated SO4 , which is common in saline lakes mer, gas-vacuolate colonies migrate into the pelagic (Howarth and Cole, 1985; Marino et al., 1990). zone, representing as much as 40% of the planktonic Light supply within the water column is a critical assemblage (Barbiero and Welch, 1992). In subtropical factor that also affects phytoplankton production and Lake Apopka, Florida, benthic and settled planktonic species composition in lakes. Although the abundance diatoms (and their resting stages) are regularly resus- of many species is greatest in the epilimnion where pended into the water column by wind action, making irradiance is greatest, other species, including several a major contribution to the chlorophyll budget of the algal flagellates are adapted to deeper waters (Lund lake (Schelske et al., 1995). In these shallow lakes, and Reynolds, 1982). Under relatively warm and buoyant cyanobacteria contribute to recycling phos- calm conditions in Blelham Tarn, more than 90% of phorus (internal loading) from sediments back into the Uroglena sp. colonies and Eudorina elegans aggregated water column (Salonen et al., 1984; Pettersson et al., in the upper 2 m, but in October, Trachelomonas 1993; Moss et al. 1997). occupied a narrow band near the thermocline (6–8 m depth). In eutrophic or highly turbid waters, some 2. Factors That Regulate Phytoplankton Production algae remain buoyant using gas vesicles (e.g., in Lakes Anabaena, Aphanizomenon, Coelosphaerium, and A large body of data clearly indicates that phyto- Microcystis; see Chaps. 3 and 4) or flagella (e.g., plankton production and biomass in most lentic Ceratium, Chlamydomonas, and Euglena). Species also systems is controlled by P supply (Schindler, 1978; exhibit different photosynthesis–irradiance and temper- Wetzel, 1983a; Hecky and Kilham, 1988). Although ature optima. Some species, such as Asterionella for- other factors (e.g., grazing, light availability, tempera- mosa, have especially high photosynthetic efficiency at ture) are clearly involved in regulating phytoplankton low light (Reynolds, 1984a), whereas others, including production, only nutrients are amenable to regulation species of Oscillatoria, may vary their chlorophyll-a (Schindler, 1978). This concept was demonstrated levels and employ accessory pigments that saturate at in Lake Washington, which received secondary sewage lower and spectrally altered light levels (Mur and (a source of P) from the city of Seattle. Blooms of Bejsdorf, 1978). Flagellates such as Dinobryon, planktonic cyanobacteria, especially Planktothrix Poterioochromonas, Cryptomonas, and Ceratium may (Oscillatoria) rubescens, were common in past decades ingest bacteria under low light conditions or under ice; (Edmondson, 1977). Today eutrophication in Lake these mixotrophic species switch between photosyn- Washington has been largely reversed through monitor- thetic and bacterivorous metabolism (Bird and Kalff, ing and sewage diversion, which reduced P inputs to 1987; Porter, 1988; Berninger et al., 1992; Caron et al., near zero, summertime chlorophyll-a from about 1993). This strategy may also serve to supplement 45 to 5 µgL–1 and the percentage of phytoplankton as inorganic nutrients as well as organic-C needs. 741550.02.qxd 16/9/2002 2:26 PM Page 24

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F. Benthic Algal Assemblages of Lakes such as Cryptomonas, Euglena, and Eudorina, and chains of cells such as Tabellaria (Hutchinson, 1975; Benthic algae—those attached to or closely asso- Graham and Vinebrooke, 1998). ciated with various substrata or bottom surfaces— The range of sizes in freshwater benthic algae occupy an enormous variety of microhabitats, exceeds that of planktonic forms. The smallest include including stones, macrophytes, sediments, sand grains, unicells like Nannochloris or Synechococcus (0.8–2 µm and logs, as well as a variety of artificial substrata. All diameter) to actual macrophytes, such as Chara, divisions of algae have benthic representatives, although Cladophora, and Hydrodictyon, which range from a many more freshwater species in Chrysophyta few centimeters to more than a meter in length. In (Chaps. 12–14), Xanthophyta (Chap. 11), Cryptophyta terms of length, this range is more than 6 orders of (Chap. 21), and Pyrrophyta (Chap. 20) are planktonic, magnitude, and in biovolume, perhaps as great as a whereas all species of freshwater Phaeophyta and factor of 1010. The tremendous variety of microhabi- Rhodophyta are benthic (but rare in lakes). Perhaps the tats, morphologies, sizes, and architectures found in most widely used term for benthic algae is periphyton, benthic algal associations has led to the suggestion that a word that has obscure etymology and debatable these organisms may represent a more diverse commu- usage (Sládeckova, 1962; Round, 1981; Wetzel, 1983a; nity and greater trophic complexity than phytoplank- Aloi, 1990; Stevenson, 1996a). The term may have ton (Havens et al., 1996; Stevenson, 1996a). In shallow been coined in the 1920s by Russian limnologists lakes, production of epiphytic algae often equals or (Sládecková, 1962) to refer to a collection of organisms exceeds that of phytoplankton per unit area (Wetzel, (bacteria, fungi, protozoa) and detritus (Wetzel, 1983c). 1983a). Much of the literature on freshwater benthic The word is analogous to the German aufwuchs, which algae has been reviewed in several important works, means “to grow upon,” but some authors (e.g., Round, including Hutchinson (1975), Round (1981), Wetzel 1981) argue against using it because it is often used (1983b), and Stevenson et al. (1996). incorrectly to describe only the algal community and is imprecise with regard to habitat. Its use probably will 1. Epiphytic Communities not be abandoned, however. We recommend the use Epiphytic algae colonize submersed and emergent of the most precise and descriptive terminology for plants. These are the most widely studied group of particular algal communities (e.g., epilithic diatom) benthic algae in lakes, perhaps because of their obvious that includes the nature of the substratum. Otherwise accumulation in the littoral zone. Larger forms, the term “benthic” is perhaps most suitable for general such as Cladophora, Chara, Hydrodictyon, and uses or when the substratum is not defined. Oedogonium, serve as additional substrata for micro- Modes of algal attachment are diverse. Some are algae. Epiphytic algae are important in macrophyte firmly attached or encrusting, such as Chamaesiphon, communities, because greater densities may cover and Coleochaete, and Cocconeis, making them resistant to shade their hosts (Losee and Wetzel, 1983). Evidence wave scour or other disturbances, but susceptible to includes negative relationships between epiphyte and competition from canopy-forming morphologies, such macrophyte biomass (Sand-Jensen and Søndergaard, as Stigeoclonium or Ulothrix (Hoagland and Peterson, 1981; Cattaneo et al., 1998) and more rapid host 1990; Maltais and Vincent, 1997; Graham and senescence with greater epiphyte cover (Neely, 1994). Vinebrooke, 1998). Diatoms, such as Frustulia, Navicula, There are often differences in species composition Nitzschia, Pinnularia, and Stauroneis, maintain contact and biomass of epiphytic algae among different macro- with various surfaces (and glide among these micro- phyte host species. Prowse (1959) recognized that habitats) by means of a slit in the wall that is termed a densities of three common epiphytes, Gomphonema raphe (Chaps. 16–19). Other diatoms (e.g., Cymbella gracile, Eunotia pectinalis, and Oedogonium sp., and Gomphonema) attach by means of gelatinous pads differed among three macrophyte species in one small or stalks. Some filamentous cyanobacteria, such as pond. Many subsequent studies have reported differ- Oscillatoria, Hapalosiphon, Lyngbya, and Micro- ences in epiphyte biomass or species composition on coleus, exhibit motility by gliding, although the mecha- different plant hosts (Gough and Woelkerling, 1976; nisms are not clear (Castenholz, 1982). Filamentous Eminson and Moss, 1980; Lodge, 1986; Blindow, green algae, such as Cladophora, Oedogonium (see 1987; Douglas and Smol, 1995; Hawes and Schwarz, Chap. 8), Spirogyra, and Zygnema (Chap. 9) produce 1996), although not in all cases (Siver, 1977). In one holdfast-like rhizoids that enable them to remain shallow lake, epiphyte biomass on submersed macro- attached in turbulent conditions. Other benthic forms phytes (Myriophyllum spicatum, Ceratophyllum are loosely associated with plants or sediments, and demersum, and Najas marina) was 10–40 times greater include filamentous species like Mougeotia, flagellates than on floating-leaved plants (Trapa natans), but 741550.02.qxd 16/9/2002 2:26 PM Page 25

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species diversity was less (Cattaneo et al., 1998). In the Synedra ulna and Fragilaria vauchariae (Blinn et al., Great Lakes, Cladophora glomerata is a host to many 1989). Grazers of epiphytic algae may have indirect microalgal epiphytes, but the red alga Chroodactylon effects on host plants by reducing shade and enhancing ornatum (as C. ramosum) is attached only to this plant growth (Lodge et al., 1994). species (Sheath and Morrison, 1982). The three-dimen- Because of the difficulties of sampling epiphytic sional architecture of epiphyte assemblages also varies algae, artificial substrata, such as glass slides, plastic with the type of substratum. Colonization by epiphytic flagging, styrofoam floats, plexiglas plates, and plastic algae has been compared to terrestrial plant succession, aquarium plants, are employed. Advantages include which comprises temporal changes in vertical structure reduced variability, known surface area, standardized and diversity, an increase in the dominance of larger conditions, and no nutritional or chlorophyll artifacts organisms, and possible facilitative effects of earlier from the host. An implicit assumption in their use is colonizers (Hoagland et al., 1982). that the community sampled is representative of the The reasons for differences in epiphytic communi- “true” epiphyte community on aquatic plants, but ties among host plant species can be attributed to studies suggest this is rarely true (Tippet, 1970; features of the macrophyte, such as leaf orientation, Robinson, 1983; Aloi, 1990; Cattaneo and Amireault, texture, or chemical properties. One survey revealed a 1992). Glass microscope slides were among the first correspondence between epiphytic communities and materials used (Sládecková, 1962), but differences in species of submersed macrophytes in less productive biomass, seasonal patterns, and community structure lakes, but little pattern was observed in eutrophic lakes (different species proportions ) suggest this approach where nutrient macrophyte interactions might be less may provide unreliable estimates (Tippet, 1970). (Eminson and Moss, 1980). However, plants may Evidence suggests that biomass of most epiphytic algae inhabit different zones within lakes that indirectly offer is overestimated when some types of artificial substrata different ecological conditions for algal colonization. are used, although green algae and cyanobacteria may Nonetheless, direct evidence shows that living macro- be undersampled (Aloi, 1990; Cattaneo and Amireault, phytes translocate and release small quantities of P 1992). Synthetic materials are much simpler in surface (about 3.5 µgPg–1 macrophyte shoot), which can be texture and chemistry than natural substrata, and this taken up by algal epiphytes, and that algal species is likely to affect the grazing, production, and commu- differ in their ability to sequester released P (Moeller nity structure of epiphytes. Although artificial substrata et al., 1988). A Synedra–Fragilaria complex obtained should not be assumed to mimic natural habitats fully, more than 50% of released P, but erect forms such as they can be useful in comparative analyses or replicated Mougeotia and Lyngbya, and stalked Gomphonema studies on the effects of disturbances on benthic algal obtained most of their P from the surrounding water. communities (Robinson, 1983; Aloi, 1990). Alkaline phosphatase activity of epiphytic algae on artificial (plastic) plants was shown to be greater than 2. Epilithic Communities on natural plants under similar conditions (Burkholder Epilithic algae colonize stones, boulders, and and Wetzel, 1990). bedrock in lakes, and may dominate wave-swept lit- Epiphytic communities are important and complex toral zones and oligotrophic lakes that have minimal components of lake food webs. In mesotrophic Lake macrophyte cover (Loeb et al., 1983). Species composi- Mann (WI), herbivorous snails consume and regulate tion differs more strongly from phytoplankton than do benthic algal biomass, but pumpkinseed sunfish also epiphytic communities, but do exhibit pronounced exert predatory control on snails (Brönmark et al., seasonality in response to changes in nutrients, tem- 1992). Algal-feeding snails, benthic insects, and other perature, and other factors (Hutchinson, 1975; Lowe, invertebrates also have a qualitative impact on epiphytic 1996). communities, because many consumers graze more Epilithic communities in turbulent littoral habitats effectively on erect or filamentous forms, thereby shift- are distinct from epiphytic communities within the same ing the community toward more compact or adherent lake and comprise species known mainly from streams, forms like Cocconeis placentula and Coleochaete spp. such as Chamaesiphon spp., Gongrosira incrustans, (Kesler, 1981; Lodge, 1986; Marks and Lowe, 1993). Hildenbrandia rivularis, Tolypothrix distorta, or In eutrophic lakes, snails similarly avoid larger colonies Heribaudiella fluviatilis (Kann, 1941, 1978; Auer et al., of epiphytic Gloeotrichia (Cattaneo, 1983; Brönmark 1983). Vertical zonation is often observed. Many et al., 1992). In contrast, the limpet Ferrissia fragilis epilithic species are restricted to the upper littoral zone, grazes mainly understory species, such as Epithemia whereas others occur in deeper waters where wave spp., Cocconeis placentula, and Achnanthidium action is less severe (Hoagland and Peterson, 1990; minutissimum, and avoids upright forms such as Lowe, 1996). Bangia atropurpurea and Ulothrix zonata 741550.02.qxd 16/9/2002 2:26 PM Page 26

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occur on rocks in the upper splash zone of the Pennsylvania (Fairchild et al., 1989). The epilithon of Laurentian Great Lakes, just above a mat of meso-oligotrophic Flathead Lake (Marks and Lowe, Phormidium sp., whereas a Cladophora glomerata 1993) was limited principally by N and P, but individ- zone is found in deeper water (Sheath and Cole, 1980). ual species differed in their response to nutrient and In Lake Traunsee (Austria), Kann (1959) documented shading manipulations. In sublittoral Lake Tahoe, that zonation patterns can differ among regions in a epilithic populations of Calothrix, Tolypothrix, and lake according to differences in slope. Epilithic algae Nostoc were strongly N-limited and exhibited N-fixa- form distinct communities in different regions of olig- tion activity, in contrast to resident phytoplankton otrophic Lac à l’Eau Claire (subarctic Quebec), that are (Reuter et al., 1986). Recent increases in atmospheric influenced by ice scour and wave action (Maltais and N deposition may change the nutrient economy toward Vincent, 1997). A Gloeocapsa-dominated community P limitation (Jassby et al., 1995). colonized shallow areas, while a filamentous community Grazing by benthic invertebrates is also important dominated by Ulothrix zonata occurred in open, south- to epilithic algae. Snails (Planorbis contortus) and facing shores. In a study of 35 arctic ponds, freezing limpets (Ancylus fluviatilis) that inhabit the stony and other habitat factors were found to be of greater littoral zone of a small calcareous lake consumed sub- importance to benthic communities than were chemical stantial quantities of algae and detritus, and each pref- variables (Douglas and Smol, 1995). Light or other erentially grazed certain algal species (Calow, 1973a, factors may interact with the effects of wave action. b). Selectivity and more intense grazing activity by An epilithic population of U. zonata exhibited a greater limpets exerted greater effects on algal community photosynthesis irradiance optimum (1200 µmol pho- structure than did snails. Light availability and grazing tons m–2 s–1) than C. glomerata (300 µmol photons m–2 s–1) pressure are factors that logically would be expected to isolated from the same region of Lake Huron (Auer be more important than nutrients for epilithic algae et al., 1983). Temperature tolerances and nutrient in eutrophic lakes, but grazing had minor impacts on requirements may further interact with irradiance opti- biomass and seasonal patterns of epilithic algae in ma, and affect local and regional distributions Crosmere, a eutrophic lake in the English Midlands, (Graham et al., 1985). Bangia, recent invader to the although caddisfly larvae may have contributed to spa- Great Lakes, has displaced Ulothrix zonata in many tial patchiness of Cladophora (Harrison and Hildrew, locations, perhaps because of its ability to produce 1998). Studies on epilithic food webs that consisted more durable holdfasts (Lin and Blum, 1977) or an of crayfish, invertebrates, and macrophytes in one ability to resist epiphyte cover by sloughing its cell wall Swedish lake found little top-down control by crayfish (Lowe et al., 1982). Interestingly, Kann (1959) similarly on epilithic algae (Nyström et al., 1996). The general observed Bangia occupying rocks in the splash zones of importance of benthic algae in lake food webs is not the Traunsee, but co-occurring with another filamen- well established, in part because of difficulties in their tous green alga, Mougeotia. In the calmer epilithic quantification. However, a study of arctic, temperate, community of Montezuma Well (AZ), only 8 of the 83 and tropical lakes using stable isotopes suggests benthic diatom taxa identified were restricted to this that previous efforts may have underestimated the habitat (Czarnecki, 1979). The upper littoral zone can importance of algae in benthic food webs (Hecky be a harsh habitat, where algal cells experience abra- and Hesslein, 1995). Prior studies were based on net sive turbulence or desiccation during an annual cycle. production of phytoplankton, benthic algae, and Controls on epilithic production vary among macrophytes, instead of ease of grazing, edibility, or different lake types. The epilithon of softwater, oligo- nutritional quality; all of these qualities were predicted trophic Lakes in the Experimental lakes Area (ELA; to be greatest in benthic algae. The importance of ON) was dominated by diatoms and filamentous green epilithic algae in some lakes may be increasing, due to algae; production levels tended to be low but quite the expansion of filamentous algae in many acidifying variable (Stockner and Armstrong, 1971; Schindler et lakes (Stokes, 1986; Turner et al., 1995). In one neutral al., 1973). Comparing nutrient-amended, pH-manipu- lake, tadpoles suppressed the growth of filamentous lated, and reference lakes in the ELA, Turner et al. algae on tiles, and favored communities of adherent (1994) concluded that epilithon production is unrelated and encrusting species (Coleochaete scutata, Achnani- to N or P supply (despite positive effects on phyto- dium minutissimum); grazers had no such effects on plankton), but is limited by dissolved inorganic carbon epilithic communities in acidified lakes (Graham and (DIC). Experiments using nutrient-diffusing substrata Vinebrooke, 1998). Transplants of epilithic algae (on determined that DIC and P supply were the most natural quartz tiles) across lakes of varying acidity, important influences on biomass and species com- coupled with grazer exclosures confirmed that pH position in another oligotrophic, softwater lake in was the key factor that regulated algal communities, 741550.02.qxd 16/9/2002 2:26 PM Page 27

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but grazer control was important in neutral lakes more than nine months. In acidified Woods Lake (NY), (Vinebrooke, 1996). several epipelic species were abundant from May through October, such as Navicula tenuicephala 3. Epipelic and Epipsammic Communities at 1 m and Hapalosiphon pumulis at 7 m (Roberts and Algal communities that colonize sediments (epipel- Boylen, 1988). Following liming, several acidophilic ic) and sand (epipsammic) are among the least studied species were replaced by other taxa, but diatom species benthic associations. This knowledge gap is surprising, still dominated the epipelon (Roberts and Boylen, considering the large area of many lakes not covered by 1989). Round (1972) suggested that epipelic habitats stones or aquatic plants. It is difficult to separate provide greater habitat diversity than the water column, epipelic from epipsammic habitats because these enabling more species to coexist. substrata are often mixed and, due to wave action, are especially unstable (Hickman, 1978; Kingston et al., 4. Benthic Macroalgae 1983). Methods used to quantify living algal cells from Several species of macroalgae, those that form lake sediments require considerable care and may still macroscopic or plantlike morphologies, can be impor- result in fairly high relative error (30% or greater), tant in benthic lake communities. Some investigators which may not be consistent among taxa (Eaton and (e.g., Hutchinson, 1975) restrict this category to taxa Moss, 1966). that are attached by means of rhizoids, which include Epipelic and epipsammic communities occur in all only charophytes (Chara, Lamprothamnion, Nitella, lake types, but their relative importance is greatest in Tolypella, and Nitellopsis). From an ecological perspec- small, shallow systems. Diatoms are the most common tive it makes sense to include other algae, such as algal group in most systems (numbers and biomass), Cladophora, Enteromorpha, and Nostoc, which also although cyanobacteria, cryptomonads, desmids, may function as structuring elements within the benthic euglenoids, and colonial and filamentous green algae zone and as hosts for epiphytic microalgae. Astounding often are observed (Gruendling, 1971; Round, 1972; colonies of Nostoc greater than 30 cm in diameter Hickman, 1978; Roberts and Boylen, 1988). Epipelic (“mare’s eggs”) have been observed (Dodds et al., algae may live on or within the first few millimeters of 1995), and tubes of Enteromorpha may exceed 1 m in sediment and so must be able to tolerate conditions of length (Wehr, unpublished). Chara may form large very low light or oxygen, making motility a distinct underwater meadows in the littoral zone of calcareous, advantage. Nonmotile epipelic and epipsammic forms nutrient-poor lakes and frequently is encrusted with

may be capable of heterotrophic growth, which allows marl (CaCO3). them to tolerate dark conditions and utilize greater In clear, oligotrophic lakes, charophytes colonize levels of dissolved nutrients. Despite these constraints, lake bottoms down to depths of 30 m or more. In epipelic and epipsammic communities are often very nonturbid systems, wave action is suggested to be the diverse. In a study of arctic ponds, Moore (1974) iden- primary limiting factor, rather than light (Schwarz tified 357 algal taxa from sediments, of which 226 and Hawes, 1997). Various species of Chara and were benthic diatoms. Very high standing crops were Nitella differ in their depth distributions, presumably also measured, some exceeding 108 cells cm–2, which as a function of individual light requirements (Wood, perhaps was influenced by long photoperiods during 1950). The lower depth boundary for Nitella meadows arctic summers. A diverse assemblage of 255 taxa was in more productive lakes is influenced by total irradi- observed along a depth gradient in Lake Michigan, ance and the supply of red light; in deep, clear lakes, where benthic forms predominated in shallow and mid- their distribution is limited by the availability of blue depth communities (6–15 m) and settled, living plank- light (Stross et al., 1995). Reduced charophyte abun- tonic forms were more common in deep (23–27 m), dance in lakes with eutrophication has been attributed low light conditions (Stevenson and Stoermer, 1981). to excessive or even toxic levels of P (Hutchinson, Patterns of seasonal succession seem to differ from 1975; Phillips et al., 1978). However, the macrophyte those observed for lake phytoplankton. Over a three- community of eutrophic Lake Luknajno (Poland) is year period, Round (1972) observed that populations dominated (90% of total dry mass) by seven charophyte of epipelic algae in two shallow ponds had distinct and species (codominants: C. aculeolata and C. tomentosa), reproducible periods of abundance peaks, but their where the mean biomass is greater than 1 kg dry mass duration was longer (months rather than weeks) and m–2 (Krolikowska, 1997). Nitella hookeri was found varied markedly among species. Stauroneis anceps and to grow best at very high P concentrations, about Oscillatoria sp. persisted for several winter months, 20 mg L–1 (Starling et al., 1974). Reduced charophyte whereas other species like Navicula hungarica and abundance in eutrophic lakes may be the result of light N. cryptocephala maintained sizeable populations for limitation, given that some species do grow at greater 741550.02.qxd 16/9/2002 2:26 PM Page 28

28 John D. Wehr and Robert G. Sheath

depths but only in clear lakes (Blindow, 1992; given in the treatises by Leopold et al., (1964) and Steinman et al., 1997). Recovery of a previously Morisawa (1968), and in syntheses by Hynes (1970) eutrophic lake resulted in a 15-fold increase in the ben- and Beaumont (1975). Several important reviews thic cover of charophytes in just a two-year period discuss ecological feature of rivers, in particular, those (Meijer and Hosper, 1997). by Hynes (1970), Whitton (1975, 1984), Lock and The epiphytic algae that colonize charophytes can Williams (1981), Allan (1995), and Petts and Callow be dense and may differ in composition among specific (1996). charophyte taxa. In one deep oligotrophic lake, epiphytes on charophytes differed strongly with depth; A. Major Rivers of North America filamentous green algae and stalked diatoms predominated in shallow water (≤ 5 m), while adnate forms, The Mississippi–Missouri River system is longest such as Cocconeis placentula, predominated at greater in the world (6970 km), has the third largest drainage depths (Hawes and Schwarz, 1996). Littoral epiphyte area (3270 103 km2), and is the sixth largest in terms densities in one eutrophic Swedish lake were greater of discharge (18,390 m3 s–1; Hynes, 1970; Milliman on C. tomentosa, C. globularis, and Nitellopsis obtusa and Meade, 1983). At least 10 other rivers in North than on Potamogeton pectinatus (Blindow, 1987). America exceed 1000 km in length. Most of them have Nitellopsis harbored the highest total densities, but been altered substantially by navigation or hydroelectric differences in epiphyte species composition were most dams, channelization, wetland removal, and pollutants pronounced between the two Chara species, probably a (Sparks, 1995; Wehr and Descy, 1998). The Missouri result of differences in marl encrustation. Benthic inver- River (3770 km) has six major impoundments over tebrates and zebra mussel larvae also colonize species 1230 km (33%) of its length; another 1200 km (32%) of Chara more readily than other macrophyte hosts have been channelized. Only 35% (1330 km) of all (Lewandowski and Ozimek, 1997; Van Den Berg et al., river sections remain free-flowing, although discharge 1997). still is influenced by reservoir conditions upriver (Hesse et al., 1989). No major impoundments are located on the Ohio or Mississippi Rivers, but navigation dams III. LOTIC ENVIRONMENTS and channelization have been built throughout their lengths to facilitate ship traffic. Running water ecosystems, from headwater streams to the largest rivers, are termed lotic environ- B. Geomorphology of Rivers ments and differ from standing waters in several important respects. Lotic systems are turbulent and Streams and rivers are part of a network of con- generally well mixed; hence stratification is uncommon nected, increasing tributaries that have hydrological except for brief periods in slowly flowing, lowland features that vary in predictable ways. Many features, rivers (Hynes, 1970). Waters with greater current such as discharge, substratum size, stream width, and velocity tend to have abundant dissolved oxygen; even depth, affect the species composition and productivity large rivers with substantial current speed, such as the of lotic algae and their consumers. For example, sizes Ohio, are generally well mixed (Thorp et al., 1994). In of substrata available for colonization vary with differ- shallow rivers or littoral regions of large rivers, organ- ences in current velocity (Table II). Physical features of isms experience less temperature fluctuation than rivers may be described in a system of stream orders, organisms in lakes of comparable depths. Many organ- which assign increasing numbers to streams when two isms adopt a benthic habit and are attached to a variety tributaries of equal order join. The most widely adopted of substrata, the sizes of which are a function of system (Strahler, 1957) defines a headwater stream or current velocity and discharge. Rapid rivers are typified spring with no (permanent) tributaries as first order by large stones and boulders, whereas the bottom and the junction of two such streams a second order material in more slowly flowing systems consists of (Fig. 10). A second-order stream increases only when it sand and silt. is joined by another second-order stream and so on. Rivers are intimately connected with the surround- Larger order streams are wider and longer segments, ing watershed, which is responsible for regulating drain larger areas, and have a more gradual slope than water and chemical balances within the aquatic envi- smaller streams. The network of these stream segments ronment (Likens et al., 1977). Rivers are open systems forms a treelike structure that is used in hydrological that transport materials and energy from one part of models to predict average discharge, behavior of flood the watershed to downstream areas. Descriptions of the events, and quantity of suspended matter (Beaumont, physical and geological attributes of river systems are 1975). A simpler scheme divides rivers into three zones 741550.02.qxd 16/9/2002 2:26 PM Page 29

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TABLE II Relationship between Minimum Current Velocity Although many aspects of drainage basins follow and Mean Size of Stone Substrata That Can Be Moved along an ordered structure within an individual watershed a Streambed (based on Hynes, 1970; Reid and Wood 1976) (catchment), physical conditions and biological communities in streams of equal order at two locations Current Particle velocity size Stream bed may be quite different. Current velocity, stream width, (m s-1) (cm) characteristics Habitat and substrata in a second- or third-order mountain stream in New England differ from a similar order 3.0 180 Bedrock Torrential cool-desert stream in the western Great Basin 2.0 80 Boulders Rapids (Minshall, 1978; Minshall et al., 1983; Benke et al., 1.0 20 Large stones Riffles 0.8 10 Stones and gravel Riffles 1988). Landscape factors such as climate, terrestrial 0.5 5 Gravel & coarse sand Run vegetation, and external nutrient supplies, may exert a 0.2 1 Sand Run substantial effect on the biological properties of rivers. 0.1 0.2 Silt Pool For example, rivers that flow through a limestone region will possess greater concentrations of certain ions (Ca, Mg) than would be found in streams flowing through a region composed of granite or basalt. (Schumm, 1977). Zone 1 (erosional) includes head- River basins also differ in the amount of interac- water and small order streams that function as the tion between river channels and their watershed, which source of water and sediments for downstream reaches. is mainly influenced by geological features of the region In Zone 2 (conveyance), water and sediments are trans- and amount of interface between groundwater and sur- ported along the mainstem with no net gain or loss of face waters (Dahm et al., 1998). Some rivers are geo- materials. Zone 3 (deposition) includes lower reaches logically constricted, such as the Hudson River and that receive sediments from upriver, including river large sections of the Ohio and Columbia. Floodplain deltas and estuaries. Some studies suggest that benthic rivers have substantial watershed interaction, such as algal communities differ similarly along these zones the lower Mississippi. The floodplain includes portions (Rott and Pfister, 1988). of the watershed (tributaries, adjacent wetlands, flood-

FIGURE 10 Structure of tributaries in a watershed, indicating numbering of stream orders. 741550.02.qxd 16/9/2002 2:26 PM Page 30

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plain lakes, riparian zones) that are seasonally in- seven stream-widths apart (Hynes, 1970). These undated during periods of high flow. Meandering rivers regions have greater turbulence and concentrations of and those with more islands have greater littoral dissolved gases, which may provide a physiologically and floodplain interaction, and more complex and richer habitat for benthic algae. Deeper pools form varied current regimes than rivers with less sinuous downstream of riffles, where organisms experience courses (Fig. 11A and B). This increased habitat com- reduced current velocity, possibly depleted dissolved plexity may offer refuge to larger, more slowly growing gases, and perhaps light limitation. Bottom materials algae. Small “islands” also form in lowland rivers from consist of smaller particles, primarily sand and silt, and large stands of submersed angiosperms (Butcher, 1933; current velocity is reduced during average flow periods. Holmes and Whitton, 1977). However, during flooding, pools lack stable substrata, Within a given reach, there are alternating regions which makes them susceptible to scouring and erosion. of erosion and deposition. Bottom materials are eroded from river margins and deposited downstream in point C. The River Continuum and Other Models bars. Differences in current velocity and substratum also create regular, alternating patterns of riffles and For many decades ecologists lacked broad concep- pools (Fig. 11C and D). Riffles are shallow sections tual models for describing and testing patterns in the with larger substrata and greater current velocity, and structure and function of river communities. Early con- are spaced at fairly regular intervals, about five to cepts, some of which were developed by algal biolo-

FIGURE 11 Examples of river features: A, B, meanders in lowland rivers; C, riffle (arrows) and pool sequences in a larger (Firehole River, WY) and smaller (Little Beaverkill, NY) streams. All photos by J. D. Wehr. 741550.02.qxd 16/9/2002 2:26 PM Page 31

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gists, borrowed ideas from lake systems, such as the linked environmental (e.g., width, depth, flow) and concept of oligotrophic–eutrophic gradients and climax resource (nutrients, light) gradients. Biological com- communities (Blum, 1956; Hynes, 1970). These ideas munities respond to longitudinal changes in geomor- do not adapt easily to lotic ecosystems, and a recent phology, water chemistry, and energy sources in several synthesis of nutrient and chlorophyll data from more ways. In addition, organisms, by their own activities, than 200 temperate streams suggest that nutrient–algal influence conditions and communities downstream biomass relationships are weaker than in lakes, perhaps (Fig. 12). This is evidenced by changes in the types of because of the effects of nonalgal turbidity in running invertebrate consumers that are localized in rivers of waters (Dodds et al., 1998). Some ecologists recognize different sizes. The model was based on data largely different zones along the length of a river that possess from smaller, temperate forest streams, and broadened different physical conditions and habitats for riverine into a theory for rivers as a whole. Many other ideas organisms (Hawkes, 1975). A river’s features, however, and studies concerning nutrient spiraling, benthic do not fall into discrete zones, but rather vary continu- invertebrates, fish production, and the influence of ously along a river’s course. The combination of hydro- dams have emerged from the general framework laid logical principles with changes in biological and out in the RCC (Newbold et al., 1981; Welcomme et chemical processes along river gradients led to the al., 1989; Minshall et al., 1983; Ward and Stanford, development of the river continuum concept (RCC; 1983; Thorp and Delong, 1994). Fig. 12; Vannote et al., 1980. The role of algae and other primary producers was The RCC characterizes lotic ecosystems as a also considered in the RCC. The metabolism of network of streams with a continuum of longitudinally first- to third-order streams was viewed to be largely dependent on external or allochthonous sources of terrestrial carbon. Hence, consumers in smaller streams were mainly shredders and collectors of coarse particulate matter. Algae were viewed as a minor component of food webs in headwater communities because of light limitation due to heavy riparian shading and subsidies of terrestrial organic matter. The overall effect is a net heterotrophic community, in which system-level respiration (R) exceeds in situ primary production (P; P : R < 1). Long-term studies confirm that the mass of allochthonous litter (leaves, wood) forms the dominant organic carbon source for small streams (Benke et al., 1988; Findlay et al., 1997), although food web studies suggest that benthic algae still may be an important or even dominant food source for some benthic animals, perhaps because of greater assimilation efficiency with algae than detritus (Fuller et al., 1986; Mayer and Likens, 1987). Algal production is predicted to increase in mid-sized (fourth to sixth order) rivers, in response to greater sunlight and a reduction in subsidies of allochthonous organic matter, resulting in P : R ≥ 1. In such systems, consumers are likely to be dominated by grazers of (mainly benthic) algal material and collectors of finer organic matter transported downstream from upper reaches. The model suggests that algal (plus macrophyte) primary production still may not come to dominate river metabolism in mid-sized rivers if the reach receives substantial supplies of allochthonous organic matter from smaller tributaries, which would FIGURE 12 Diagrammatic representation of the river continuum increase system-level respiration and reduce light pene- concept, predicting changes in P/R ratios, consumer groups, and tration. In larger rivers (greater than sixth order), the sources of organic matter along the length of a river. Reprinted from R. L. Vannote et al., Canadian Journal of Fisheries and Aquatic RCC predicts that even though the river basin is open Science 55:668–681, 1980, with permission from NRC Research to full sun, higher levels of fine particulate matter from Press. upstream, greater depth, and resuspended sediments 741550.02.qxd 16/9/2002 2:26 PM Page 32

32 John D. Wehr and Robert G. Sheath

limit algal production. Although phytoplankton their flood-pulse model, Junk et al. (1989) suggested usually dominate the algal community in large rivers, that large river metabolism may be driven more by these systems are predicted to be too turbid and too batch processes—the pulses of resources from flood deep to support high levels of algal production. This events—than by the continuous processes emphasized forecast appears to be the case in the Hudson River, in the RCC. This process is undoubtedly true for rivers where depth and tidally driven mixing create a light- such as the Amazon and the lower Mississippi. limited environment for phytoplankton (Cole et al., However, not all large rivers occur in extensive flood- 1992), but studies on other large rivers have found that plains. River basins also flow through constricted substantial phytoplankton populations can develop channels that have minimal floodplain influence. Thorp (e.g., Descy and Gosselain, 1994; Lair and Reyes- and Delong (1994) proposed a river productivity Marchant, 1997; Wehr and Descy, 1998). model that suggests that local autochthonous produc- Ideas from the RCC have stimulated a great tion may have been underestimated in large, constrict- increase in research on and discussion of lotic ecosys- ed-channel rivers, which have firm substrata and less tems, but several exceptions have been raised (Wetzel, turbid water. In such systems, for example, a 350 km 1975; Lock and Williams, 1981; Benke et al., 1988). stretch of the Ohio River, substantial phytoplankton Rivers in different biomes have different climate, lithol- biomass and production have been observed, and dur- ogy, and amounts of riparian vegetation, which may ing some seasons may be the principal carbon source shift the relative importance of autochthonous versus for planktonic consumers, such as small-bodied clado- allochthonous production for river metabolism cerans and rotifers (Thorp et al., 1994; Wehr and (Minshall, 1978; Minshall et al., 1983; Wetzel and Thorp, 1997). Also, many, if not most large rivers Ward, 1992). Not all rivers (small or mid-sized) in worldwide have been altered substantially due to North America are shaded heavily (e.g., high altitudes, industrialization, which has had profound influence on deserts, agricultural areas) nor do they receive large ecological conditions. Flow regulation and nutrient subsidies of terrestrial organic matter from their water- inputs favor greater phytoplankton productivity in shed. In more open rivers, algal production may be large rivers, such that P : R ratios may exceed 1.0, at substantial. A metanalysis of studies on 30 streams least during the spring and the summer (Admiraal et from five biomes showed that gross primary produc- al., 1994; Descy and Gosselain, 1994; Wehr and Descy, tion (GPP) levels varied by more than 4 orders of 1998). magnitude; the greatest rates were in desert areas (Lamberti and Steinman, 1997). Variations in GPP D. Benthic Algal Communities of Rivers were related to variables such as watershed area, inorganic P, temperature, discharge, and canopy cover, Most studies of river algae concern benthic species. but not to stream order or latitude, further suggesting The necessity to remain in a stable position while water that variation among rivers may be greater than pre- flows downstream is an important selective force for all dicted by the RCC. River systems are also very patchy, benthic organisms in lotic environments. Earlier from differences in sunlight within a local reach to reviews pointed out the paucity of studies on lotic algae watershed-level differences in sources of carbon or relative to those on lakes (Blum, 1956; Hynes, 1970; inorganic nutrients (Pringle et al., 1988). Subsequent Whitton, 1975), yet the pace of work on benthic algae syntheses have emphasized that lotic ecosystems also in streams and rivers has increased substantially. Much can be regulated by biotic interactions, such as preda- of what has been learned in the interim has been tor–prey dynamics, competition, and food-chain summarized in several reviews (Lock et al., 1984; length, rather than solely controlled by physical (e.g., Reynolds, 1996; Biggs, 1996; Steinman, 1996), which hydraulic) factors (Power et al., 1988). In turn, physi- reveal an evolution from mainly descriptive studies to cal factors, such as the frequency of floods, have signif- structural and functional analyses of benthic algae and icant effects on food web interactions at several levels, their importance in lotic food webs. including algal biomass (Wootton et al., 1996). Other syntheses have focused on large rivers 1. Benthic Algal Diversity, Composition, (greather than sixth order). Sedell et al. (1989) and and Biogeography Junk et al. (1989) pointed out that many large rivers In streams, diatoms often comprise the dominant receive materials, energy, and organisms from the adja- algal group in terms of species number and biomass cent floodplain, often in greater quantity than may (Blum, 1956; Douglas, 1958; Round, 1981; Kawecka, have been transported from upriver. The influence of 1981). Their diversity in species and growth form algal production was regarded as minimal because (upright frustules, short- and long-stalked, rosettes, large rivers were generally turbid and light-limited. In tube-dwelling, filamentous, mucilaginous matrix, and 741550.02.qxd 16/9/2002 2:26 PM Page 33

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prostrate cells) enables them to colonize a variety of Fragilaria, and Melosira, and stalked forms, such as microhabitats. In more slowly flowing or less flood- Didymosphenia geminata and Cymbella spp., may be prone systems, filamentous species like Melosira vari- recognized by their gross appearance, but are identified ans and upright, stalked forms such as Gomphoneis only using microscopy (Holmes and Whitton, 1981; herculeana may predominate, along with filamentous Steinman and Sheath, 1984; Sheath and Cole, 1992). nondiatom species (Stevenson, 1996a; Biggs, 1996). In Some benthic stream algae have limited distribu- very rapid water, firmly attached diatoms, such as tions. Prasiola mexicana, a seaweed-like green alga Cocconeis placentula, Achnanthidium minutissimum, (Fig. 13B), thus far has been recorded only in streams and Hannaea arcus, may occur with encrusting non- located in the arctic tundra and western coniferous bio- diatoms, such as Hildenbrandia rivularis, Gongrosira mes, whereas the encrusting brown alga Heribaudiella spp., and Chamaesiphon spp., and corticated forms fluviatilis has been confirmed in North America from like Lemanea spp. (Fritsch, 1929; Whitton, 1975; streams in western Canada and the United States only Kawecka, 1980; Kann, 1978). (Sheath and Cole, 1992; Wehr and Stein, 1985). Some One of the fascinations of studying benthic algae in taxa have been regarded to be limited in geography or rivers is that many species (although not the majority) habitat, only to be found later in other are macroscopic and recognizable in the field (Holmes locations. Thorea violacea is a large red alga that was and Whitton, 1977; Kann, 1978; Entwisle, 1989; thought to be restricted to streams in warmer biomes Sheath and Cole, 1992). These include cyanobacteria, or in temperate areas only during the summer (Smith, and green and red algae, as well as chrysophytes, xan- 1950; Sheath and Hambrook, 1990, Chap. 5), but it thophytes, and brown algae (Whitton, 1975; Sheath was discovered growing profusely in the upper Hudson and Cole, 1992). Morphologies are diverse and include River in cool (15°C), rapidly flowing water (Pueschel et encrusting, turflike, filamentous, cartilaginous, al., 1995). Few long-term studies of benthic stream mucilaginous, tubular, and bladelike thalli (Fig. 13). In macroalgae exist that may help to explain the dispersal some instances, diatoms, including species of Eunotia, patterns of these organisms. In one of the few cases of

FIGURE 13 Examples of different forms of macroalgae from rivers: A, Zygnema sp., simple, flexible filaments (arrow = direction of flow); B, Prasiola mexicana, a flat, seaweed-like thallus; C, Lemanea fluviatilis, corticated tubes; D, Nostoc verrucosum, mucilaginous colonies. Photos A and D by J. D. Wehr; photos B and C by R. G. Sheath. 741550.02.qxd 16/9/2002 2:26 PM Page 34

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a long-term (more than 40 years) database, Holmes composition, diversity, architecture) with functional and Whitton (1977, 1981) were able to characterize studies (production, food web dynamics, nutrient species as previously overlooked, currently increasing, cycling). or recently extirpated from the River Tees (UK). No In all river systems, there are proximate variables such studies are known from North America. (those we measure) that affect organisms, such as light Ecological endemism in some species of river algae availability or nutrient supply, and larger scale factors may be caused by the spread of marine taxa into estu- such as climate, watershed features, and land use prac- arine environments, followed by adaptation to lower tices, that drive proximate variables (Stevenson, 1997). salinity. Such may be the case with genera such as Biggs (1996) and Biggs et al. (1998) proposed a con- Audouinella, Hildenbrandia, Prasiola, and Entero- ceptual, disturbance–resource supply–grazer model that morpha: genera that each has marine and freshwater categorizes controls on benthic algal production in species and are found in rivers but only rarely in lakes streams in terms of two processes: factors that affect (Flint, 1955; Whitton, 1975; Sheath and Cole, 1980; (1) biomass accrual and (2) biomass loss. The model Sheath et al., 1985; Hamilton and Edlund, 1994). recognizes how ecosystem-level changes, such as flood- However, other species appear to be human-accelerated plain modifications, can affect proximate controls, invaders from other freshwater systems. Populations of such as current velocity. An energetic balance sheet is the red alga Bangia, which invaded North American constructed for each side of the ledger that can be used freshwaters in the 1960s through the St. Lawrence to predict to algal production and species composition River, are not closely related to any marine populations (Fig. 14). In this scheme, biomass accrual increases as a (based on rbc-L, RuBisCo spacer, and 18s rDNA function of resource supply, whereas biomass losses are sequences) and show strong affinities with freshwater a function of disturbance and grazing. In rivers with European populations (Müller et al., 1998). These data infrequent, low-intensity floods (= disturbance) and suggest that some vector, such as a ship’s ballast water, modest grazing intensity, the model predicts that bio- enabled invasion, rather than gradual spread and mass accrual dominates to a level dictated by resources. adaptation from a marine environment. Under low resource supplies, growth continues at Biogeographic data suggest that green algae are the lower rates, favoring an adnate and turflike community most common group of stream macroalgae across all dominated by filamentous cyanobacteria (e.g., Schizo- biomes in North America. Based on 1000 stream seg- thrix, Phormidium, and Tolypothrix; Chap. 4), red ments studied, the lowest species diversity was found algae (e.g., Audouinella; Chapt. 5), and many benthic in streams in arctic tundra and the greatest in boreal diatoms (e.g., Epithemia and Navicula; Chaps. 17–19). forests (Sheath and Cole, 1992). There was no increase Under similar hydraulic and grazing conditions but in macroalgal species diversity from the arctic to the greater nutrient supply, a greater biomass of filamen- tropics (contrary to marine species), which may be the tous taxa, such as Cladophora, Ulothrix, or Melosira, result of periodic flooding common in streams across is expected. For each combination of plus and minus all regions. Arctic streams tend to have more species of factors, specific predictions can be made about the macroalgal cyanobacteria, whereas tropical streams most important variables that drive algal production have more species of Rhodophyta (Sheath and Cole, and community composition. 1992; Sheath et al., 1996; Sheath and Müller, 1997). Empirical evidence supports these predictions. In the Colorado River, where nutrients and sunlight are 2. Factors That Regulate Benthic Algae in Rivers generally nonlimiting, release of water from Glen Several reviews (Blum, 1956; Hynes, 1970; Canyon Dam (greater disturbance) decreased the Whitton, 1975; Biggs, 1996; Stevenson, 1997) discuss biomass and relative importance of Cladophora glom- many ecological factors, including current velocity, erata, and reduced total benthic primary productivity substratum, geology, nutrient conditions, grazers, (Blinn et al., 1998). The physiognomy of epiphytic temperature, pollutants, and light availability, and their algae changed from upright assemblage (Diatoma vul- effects on benthic algae in rivers. Factors often interact gare, Rhoicosphenia curvata) to more closely adherent to affect algal growth and survival, and multivariate forms (Achnanthes spp. and Cocconeis pediculus; analyses have been useful in determining the key envi- Hardwick et al., 1992). Herbivorous invertebrates have ronmental factors that affect species composition (e.g., been widely shown to reduce the density and biomass Hufford and Collins, 1976; Wehr, 1981; Lowe and of benthic algae, but losses due to grazer activity come Pan, 1996). The following discussion provides a brief from scouring as well as consumption (Allan, 1995). overview of current perspectives on the influence of Furthermore, ingestion and assimilation of benthic habitat variables on benthic stream algae and, in par- algae vary widely (30–70%), depending on the species ticular, efforts to integrate structural aspects (species of both the alga and the consumer (Lamberti et al., 741550.02.qxd 16/9/2002 2:26 PM Page 35

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chemical composition of algal assemblages, including changes in protein, total lipids, and fatty acid composition (Steinman et al., 1988). In general, cyanobacteria have different complements of fatty acids, including greater oleic, linoleic, and linolenic acids than diatoms (McIntire et al., 1969). Reductions in light availability to Kingsley Creek (NY) caused significant reductions in epilithic algal biomass and densities of a herbivorous mayfly (Baetis tricaudatus), but did not affect densities of a filter-feeding blackfly larva (Simulium spp.), which mainly consumed detritus (Fuller et al., 1986). However, most field studies on grazing are conducted under summer base-flow conditions when disturbance is low and resource factors are less important (Feminella and Hawkins, 1995). Current velocity is of great importance to benthic algae. Species that colonize areas of rapid current velocity are firmly attached to substrata using rhizoidal FIGURE 14 Conceptual model of ultimate (landscape) and proxi- or holdfast-like structures (Israelson, 1949; Whitton, mate (stream) variables that control benthic algal communities, their consumers, and interactions in streams (solid lines = strong effects; 1975). Greater current velocity provides a continuous dashed lines = weaker effects; double arrows indicate feedback inter- replenishment of nutrients from upstream and a steeper actions. Reproduced with permission from Biggs, B. J., in Stevenson, diffusion gradient near the cell surface (Whitford, R. J. et al., Eds., Algal Ecology: Freshwater Benthic Ecosystems. 1960; Horner et al., 1990). Species restricted to riffles Copyright © 1996 by Academic Press. with strong current velocity (e.g., Hydrurus foetidus and Lemanea fluviatilis) may have a higher metabolic demand for nutrients, which must be met by a greater 1989; Pandian and Marian, 1986). Resultant biomass physical supply. Studies suggest that there is no simple and compositional changes, in turn, have effects on positive relationship between current velocity and algal benthic food webs, because many invertebrates find an metabolism or growth rate; greater current speed may adherent community less easily grazed (Colletti et al., also decrease algal biomass through scouring 1987). The Riviere de L’Achigan (Quebec) is an un- (Borchardt et al., 1994; Stevenson, 1996b). Few correl- shaded stony stream interspersed with a chain of small ative studies are able to attribute differences in algal lakes that alter flow conditions immediately down- communities to current velocity effects because many stream (lower disturbance; Cattaneo, 1996). Biomass variables change along a river’s course. A more clear- and species composition of benthic algae on gravel var- cut study of parallel streams draining the same reser- ied inversely with distance from lake outlets, yet this voir found that the regulated stream had greater diatom impoundment effect was unimportant for algae that cover and large populations of Prasiola fluviatilis, colonize boulders. Only boulders supported communi- while the stream that lacked flood control had popula- ties of large filamentous and plumose forms such as tions of Hydrurus foetidus and Ulothrix zonata, and Draparnaldia (Chlorophyceae), Stigonema (Cyano- greater species number, but a lower biomass of diatoms bacteria), and Batrachospermum (Rhodophyta). An (Kawecka, 1990). experimental study in Big Sulphur Creek (CA) demon- Current velocity affects the growth form of indi- strated the interactive effects of shading (resource vidual algae, such as in Cladophora, which develops supply) and invertebrate grazing on epilithic algal compact tufts with narrow branching in greater current communities (Feminella et al., 1989). Algal biomass in velocity and develops plumose forms with widely low-grazer conditions declined by 75% with greater branched filaments in calmer flow (Whitton, 1975; (15–95%) canopy cover, but was unaffected by light Dodds and Gudder, 1992, Chap. 8). The architecture availability at normal grazer densities. The food web— of algal communities affects their response to current resource supply interaction in Big Sulphur Creek is speed. A mucilaginous, stalked diatom community complicated by the fact that densities of trichopteran (e.g., Gomphoneis herculeana and Navicula avenacea) grazers (e.g., Gumaga nigricula) declined significantly exhibited increased production with greater near-bed in shaded conditions, while other species were unaffect- current velocity, whereas a community consisting of long ed. Abundances of algal grazers are often driven by filamentous algae (Oedogonium sp. and Phormidium food supply, but differences in irradiance also affect the sp.) experienced decreaseed biomass accrual (Biggs 741550.02.qxd 16/9/2002 2:26 PM Page 36

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et al., 1998). The morphology of Nostoc parmelioides (a diversity were similar. In Mack Creek (OR), greater filamentous colonial cyanobacterium) in stony streams biovolume (but not chlorophyll-a) and diversity of is altered from spherical to ear-shaped colonies by the benthic algae were observed colonizing pieces of wood presence of an endosymbiotic midge larva (Ward et al., than clay tiles (Sabater et al., 1998). Certain taxa, such 1985), and only ear-shaped colonies exhibit greater as Cymbella minuta, Hannaea arcus, and Zygnema

photosynthesis and N2-fixation rates with greater cur- sp., were more abundant on wood, while some closely rent velocity (Dodds, 1989). Such studies that aim to adherent forms, such as A. minutissima and A. lanceo- link metabolic patterns with differences in community lata, were more abundant on clay tiles. The physical composition or form are needed to improve our under- structure of the stream bed also influences benthic standing of the factors that regulate lotic communities. communities. Experimental manipulations of rocks Several studies of benthic stream algae also have and bricks with different density and surface texture considered the influence of substratum. Several decades resulted in significant changes in the diversity and the ago, Douglas (1958) recognized that different sizes abundance of benthic invertebrates and epilithic algae of stones supported different densities and species of in the Steavenson River, Australia (Downes et al., epilithic algae, which was likely the result of differences 1998). Greater densities of Audouinella hermannii in their susceptibility to flood disturbance. A compari- were observed on substrata without large crevices, son of epilithic communities on different substratum but total biomass was greatest on surfaces that were sizes and similar in nutrient conditions found that most roughened, independent of the presence of crevices. of the variation in algal biomass was explained by total-P and seston levels, but size of the substratum E. Phytoplankton Communities of Rivers also exerted a significant effect (Cattaneo et al., 1997). Stones that are disturbed or scoured during floods may Although benthic algae typically dominate rocky have algal crusts or propagules that still remain on streams and smaller rivers, phytoplankton become their surfaces (Power and Stewart, 1987). The degree important in larger rivers and lowland streams to which algae can recolonize disturbed substrata in a (Rosemarin, 1975; Reynolds and Descy, 1996). A long stream is a function of (1) their resilience, through history of studies on river phytoplankton dates back to immigration and greater growth rates, or (2) their at least the 1890s, when Zacharias (1898) coined the resistance, as influenced by the species’ morphology term “potamoplankton,” to refer to the suspended and community physiognomy (Peterson, 1996). Benthic organisms in flowing waters. In North America, phyto- algal biomass and species composition are also influ- plankton have been studied since the early years of enced by substratum–current interactions. Diatom limnological research in rivers including the Illinois immigration onto bare substrata may increase with (Kofoid, 1903, 1908), Mississippi (Reinhard, 1931), either reduced current speed or greater surface com- Ohio (Eddy, 1934), San Joaquin (Allen, 1921), and plexity (Stevenson, 1983). Substrata conditioned with a Sacramento (Greenberg, 1964). Much of the early simulated mucilage (agar coating) were colonized twice research focused on whether a true phytoplankton as rapidly as clean surfaces, but responses were species- community (populations that survive and reproduce specific; some increased (Navicula gregaria and within rivers) actually existed, as opposed to dislodged Synedra ulna), while others declined (Achnanthidium benthic forms or plankton washed in from lakes within minutissima and Diatoma vulgare) or were unaffected the watershed. Indeed, plankton in most rivers consists (Diatoma tenue and Gomphonema olivaceum). A of all three components in varying proportions matrix of organic matter and bacteria probably facili- (Reynolds, 1988), but in a single river sample, it is tates colonization by most benthic algae in rivers difficult to distinguish these sources, although certain (Karlström, 1978; Korte and Blinn, 1983; Sheldon algal taxa may be considered typical of each. and Wellnitz, 1998). A comparison of benthic diatom Many benthic algae become suspended. These assemblages that colonize natural rocks, sterilized meroplanktonic forms can be washed out from sedi- rocks, and clay tiles in Fleming Creek (MI) found ments, plants, or other substrata. A metanalysis of 67 greater total densities and species diversity on natural studies suggests that about 50% of suspended algal rocks (Tuchman and Stevenson, 1980). Otherwise iden- taxa in rivers are either benthic or meroplanktonic tical natural substrata constructed from three rock types (Rojo et al., 1994). Among diatoms, most raphe-bear- (basalt, sandstone, and limestone) that occur in Oak ing species are likely nonplanktonic, but it is difficult to Creek (AZ) were compared for their effects on diatom distinguish true potamoplankton from those that origi- colonization of riffles (Blinn et al., 1980). Densities nated from lakes (Reynolds, 1988). One distinction on sandstone substrata were 60–80% greater than on may be found in species’ responses to flow regime and basalt and limestone, but species composition and other physical factors, as has been attempted for phyto- 741550.02.qxd 16/9/2002 2:26 PM Page 37

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plankton species in the Ohio River (Peterson and of P, but then increased further downstream as a result Stevenson, 1989; Wehr and Thorp, 1997). The abun- of impoundments and flow regulation. A rather differ- dances of most species were negatively related to dis- ent longitudinal pattern is seen in the St. Lawrence charge, but tributary rivers were found to have little River, in which a gradient of increased P and reduced effect on species composition. However, during low current velocity is counterbalanced by greater sus- flow (summer to early autumn), there were significantly pended matter, causing a net decrease in river plankton greater amounts of colonial cyanobacteria (e.g., densities downstream (Hudon et al., 1996). The con- Aphanocapsa saxicola) and certain diatoms (e.g., trary and complex influences of discharge and algal Stephanocyclus [= Cyclotella] meneghiniana) in the growth rates have been discussed in detail by Reynolds river downstream of these tributaries, suggesting that (1988, 1995). Despite these limits on algal growth, some populations may have originated from outside large accumulations of phytoplankton frequently devel- the main river. op in summer and other low-flow periods. With greater The fact that chlorophyll-a concentrations in large nutrient supplies, surface blooms of cyanobacteria rivers usually vary inversely with discharge (Schmidt, (Microcystis, Anabaena and Aphanizomenon) may 1994) strongly suggests that potamoplankton is not occur, although their prevalence appears to be greater derived primarily from benthic habitats. This pattern in warmer climates or in temperate zones under low- was observed for individual species in the River flow conditions (Baker and Baker, 1979; Krogmann Thames, although smaller cells were less affected or et al., 1986; Paerl and Bowles, 1987). Unlike many even increased with greater discharge (Ruse and Love, lakes, nutrient limitation is uncommon in larger rivers 1997). In general, smaller forms appear to be more (Reynolds, 1988; Reynolds and Descy, 1996; Wehr and successful members of the potamoplankton, perhaps Descy, 1998). Therefore, the principal factor that regu- due to greater growth rates and surface area to volume lates phytoplankton production in rivers is discharge, ratios (Reynolds, 1988; Rae and Vincent, 1998). which regulates dilution rates, turbidity, and mixing Diatoms are clearly the most diverse and abundant of cells within the water column (Reynolds and Descy, group, with Cyclotella and smaller species of 1996). Stephanodiscus especially common in larger rivers Algal productivity in larger and lowland rivers worldwide (Chap. 15). Algal flagellates rarely achieve can be substantial despite frequent turbidity and con- large numbers in river plankton, except some crypto- tinuous mixing of algal cells within the water column monads, chrysophytes, and members of the Volvocales (Descy et al., 1987, 1994; Reynolds and Descy, 1996). (see Chap. 6). In some instances, poor preservation A delicate balance exists between phytoplankton pro- techniques may cause underreporting (see Chaps. 12 duction and respiratory losses during periods of higher and 21). turbidity (Descy et al., 1994; Reynolds and Descy, Two major limitations to survival and growth of 1996). In freshwater tidal sections of the Hudson River, river phytoplankton are the continuous removal of turbidity is further complicated by tidally driven mix- organisms by downstream flow (so-called washout) ing, resulting in a net heterotrophic balance for most of and mixing within the water column, which places cells the year (Cole et al., 1992). Following invasion of the in variable and often aphotic light fields. Hence, most Hudson by zebra mussels (Dreissena polymorpha), studies conclude that riverine phytoplankton produc- phytoplankton biomass declined from a summertime tion is controlled by discharge (Baker and Baker, 1979; mean of about 30 to about 5 µg chlorophyll-a L–1 and Soballe and Kimmel, 1987; Cole et al., 1992; Reynolds species composition shifted from colonial cyanobacte- and Descy, 1996). Assuming no other limiting ria to diatoms (Caraco et al., 1997; Smith et al., 1998). resources, rivers must be sufficiently long and/or the In the River Spree, a positive autotrophic balance is flow rate sufficiently low for net positive algal growth established during the spring (mainly diatoms), but in rates. This principle was demonstrated clearly in early the summer, cyanobacteria dominate (Köhler, 1995). studies on the Sacramento River in which peaks of How large river systems maintain large phytoplankton abundance in potamoplankton became progressively populations throughout the year is still something of a more pronounced further downstream, a region that mystery, but the main channel may receive subsidies provided reduced current velocity and more time for of algae and nutrients from tributaries, wetlands, or populations to develop (Greenberg, 1964). A similar backwaters (Owens and Crumpton, 1995; Reynolds, increase was seen along the Rhine (Germany– 1996). Species composition may provide a clue: slower Netherlands; de Ruyter van Steveninck et al., 1992). growth rates of larger colonial species (e.g., Apha- In the lowland River Spree (Germany; Köhler, 1993, nizomenon and Planktothrix) may have higher respira- 1995), phytoplankton biomass declined in mid-river tory costs (P : R < 1) for maintaining populations than in response to increased turbidity and Fe precipitation smaller centric diatoms (e.g., Cyclotella spp. and 741550.02.qxd 16/9/2002 2:26 PM Page 38

38 John D. Wehr and Robert G. Sheath

Stephanodiscus hantzschii; see Chap. 15), but may be 1996; Dodds et al., 1998). Even more difficult to stable if they are ineffectively grazed by small-bodied predict are changes in species composition of phyto- zooplankton (Gosselain et al., 1998). Only zebra plankton communities in rivers. This information is mussels, which utilize a wider particle size range, may important for water management agencies, because cer- crop these larger forms, as in the Hudson River. In the tain algae affect taste and odor conditions of water that Meuse, Rhine, Danube, and upper Mississippi Rivers, may ultimately be used for domestic consumption. higher levels of nutrients coupled with less turbid con- Despite the inclusion of many physical and chemical ditions enable high levels of phytoplankton production variables, only about 20% of the total variation in to be sustained for several months of the year (Baker phytoplankton species composition for the River and Baker, 1979; Descy et al., 1987; Lange and Rada, Thames could be explained using a canonical corre- 1993; Admiraal et al., 1994; Kiss, 1994). With greater spondence model (Ruse and Hutchings, 1996). One primary production, potamoplankton influence the model, which incorporates several physical factors biogeochemical properties of large rivers, including and river order, found that hydrological conditions

dissolved O2 (Köhler, 1995; Reynolds and Descy, exert an overriding effect on potamoplankton develop- 1996), dissolved Si (Admiraal et al., 1993), and dis- ment, but biological controls (mainly grazers) are solved organic matter (Wehr et al., 1997). important during low-flow conditions (Billen et al., Phytoplankton are an important food source for 1994). Reynolds (1988, 1995) has suggested that more zooplankton in rivers, even if grazers do not regulate fundamental work is still needed to acquire the neces- algal biomass or production as effectively as in lakes. sary data to build meaningful predictive models for Grazing pressure is less important because the zooplankton communities in rivers. plankton community is usually dominated by small- bodied cladocerans and rotifers (Winner, 1975, Köhler, 1995). Rivers select for small-bodied zooplankton IV. WETLANDS because of their ability to grow rapidly enough to com- pensate for downstream losses (Viroux, 1997). Biomass Wetlands regulate nutrient fluxes between ter- and density of zooplankton in larger rivers also may be restrial and aquatic systems, serve as nurseries for less than in lakes (Pace et al., 1992; Thorp et al., many of the world’s fisheries, and are among the most 1994). Although discharge and turbidity typically drive productive and threatened ecosystems worldwide phytoplankton production in rivers on an annual basis, (Whittaker, 1975; Mitsch and Gosselink, 1993). zooplankton grazing may still be an important loss fac- Freshwater wetlands occur from arctic to tropical bio- tor during summer low-flow periods (Gosselain et al., mes across North America. Many are situated in the 1994). Grazing activity in the River Meuse (mainly upper littoral zone of lakes and rivers, and also include Bosmina and rotifers) appears to exert strong control marshes, peat bogs, fens, wet alpine meadows, cypress over summertime phytoplankton numbers and to cause swamps, and forested lowlands. Unifying features of a shift in phytoplankton size structure toward larger wetland include more or less continuously saturated celled forms (Gosselain et al., 1998). soils, shallow water depth (≤ 2 m), fluctuating water Models designed to predict phytoplankton produc- levels, an accumulation of plant detritus and organic tion in large rivers primarily have been devised for matter, and vegetation adapted to wet conditions specific conditions, such as the influence of tempera- (Mitsch and Gosselink, 1993; Goldsborough and ture and irradiance on phytoplankton production in Robinson, 1996). one reach of the Great Whale River, Quebec (Rae and Vincent, 1998). Light and temperature explained A. Functional Importance of Algae in Wetlands between 74 and 98% of the variation in photosynthetic activity in this subarctic river. A larger model for the Most wetland algae are benthic, loosely associated Rhine, which included the effects of irradiance, light with emergent plants (metaphyton), attached to plants, attenuation, flow, nutrients, phytoplankton biomass, or colonized in sediments; most suspended forms have and grazing, was successful in predicting the fate of been dislodged from various surfaces. A five-year study algal production, although some parameters (e.g., of epipelic, epiphytic, metaphytic, and planktonic zooplankton grazing) were based on data from lakes primary production in Delta Marsh, Manitoba, deter- (Admiraal et al., 1993). Efforts to develop nutrient– mined that metaphyton contributed roughly 70% of algal biomass models have been less effective than in the total algal productivity, compared with only 6% lakes, owing to weaker relationships between N or P for phytoplankton (Robinson et al., 1997). Values for and chlorophyll-a, and the complicating effects of algal productivity (400–1100gCm–2 y–1) over the year discharge and turbidity (Van Niewenhuyse and Jones, are comparable to or exceed that of the emergent 741550.02.qxd 16/9/2002 2:26 PM Page 39

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macrophytes present (aboveground: 100–1700gCm–2 water level, macrophyte plant composition, and degree y–1). Among several constructed wetlands in Illinois, of mixing with other water bodies are also important benthic algae are estimated to contribute between (Goldsborough and Robinson, 1996). In the Everglades, 1 and 65% of the total system primary production benthic algae colonize many macrophyte and sediment (Cronk and Mitsch, 1994). However, not all wetland surfaces; species composition and production vary algal communities are highly productive, presumably with species of macrophyte, water level, nutrient a result of nutrient limitation (Murkin et al., 1991; inputs, and degree of CaCO3 incrustation (Browder Goldsborough and Robinson, 1996). et al., 1994). Filamentous cyanobacteria, including Algal production is important for many inverte- Scytonema, Schizothrix, Oscillatoria, and Microcoleus, brate consumers that preferentially consume algal are often abundant. Filamentous green algae (Spiro- material over either live or detrital macrophyte tissues gyra, Bulbochaete, and Oedogonium), desmids, and (Campeau et al., 1994; Goldsborough and Robinson, diatoms (Cymbella, Gomphonema, and Mastogloia) 1996). Attached algal material may be especially are common in less calcareous conditions. Algae re- important in winter when emergent macrophytes are present between 30 and 50% of primary producer dead or senescent (Meulemans and Hienis, 1983). In biomass in these systems, and their activity is apparent 13 a wetland along western Lake Superior, δ C data in the large diurnal changes in dissolved O2 and CO2. suggest that entrained algae are an important primary Because of their close connection with water chemistry, food source for the grazing food web in addition to benthic algae help regulate water quality in wetlands, macrophyte detritus (Keough et al., 1998). Given the especially P loading from agricultural and urban runoff greater proportion of structural tissues in emergent (McCormick and Stevenson, 1998). plants and the greater turnover rates among algal cells, In nutrient-rich wetlands, the algal flora is typified the importance of algae in wetland food webs is often by filamentous green algae such as Stigeoclonium, substantial. Nutrient additions designed to enhance Oedogonium, and Cladophora, cyanobacteria such as algal biomass in wetland enclosures also resulted in Lyngbya, Oscillatoria, and Nostoc, and many diatoms, greater densities of cladocerans and copepods in including species of Amphora, Epithemia, Navicula, nearshore water, as well as benthic invertebrates such Nitzschia, and Surirella (Chaps. 17–19). Filamentous as snails and chironomids (Gabor et al., 1994). Using cyanobacteria Rivularia, Calothrix, Microcoleus, and pigment tracers, Bianchi et al. (1993) estimated that Gloeotrichia (Chap. 4), and meadows of Chara benthic diatoms comprise a major food source for (Chap. 8) typically dominate systems that have greater invertebrates in Hudson River wetlands and, combined Ca2+ levels. In more oligotrophic systems, filamentous with lower C:N ratios, may be a better resource for members of the Zygnematales (Chap. 9), including benthic consumers than detritus. Algal biomass may, Mougeotia, Zygnema, and Spirogyra, commonly occur, however, be spatially variable. In wetlands of Lake along with epiphytic diatoms such as Tabellaria, Gooimeer (Netherlands), stable isotope data indicate Eunotia, and Fragilaria (see Chaps. 17 and 18), sedi- that within Phragmites beds, macrophyte detritus is ment-dwelling species such as Frustulia and Pinnularia the major carbon source for benthic invertebrates, but (Chap. 17), and many desmids (Hooper-Reid and algal material dominates littoral food webs outside the Robinson, 1978; Livingstone and Whitton, 1984; reed bed (Boschker et al., 1995). The littoral zone of lakes Goldsborough and Robinson, 1996; Pan and Stevenson, is an important region of nutrient exchange between 1996, Chap. 9). Similar to patterns in softwater lakes, nearshore and pelagic zones, and attached algal com- wetlands too may be impacted by acidic precipitation, munities are important components of this exchange leading to increases in Mougeotia, Zygnema, and, in (Mickle and Wetzel, 1978; Aziz and Whitton, 1988; severely impacted systems, Zygogonium (Stokes, 1986; Moeller et al., 1988). During decomposition of wetland Turner et al., 1995). Invertebrate grazers mediate plants, attached algae may enhance breakdown (Neely, species composition. Experimental removal of clado- 1994), although other data suggest that dissolved cerans and copepods from marsh enclosures resulted organic carbon (DOC) released during this process may in a shift from a simple community dominated by inhibit algal growth (Cooksey and Cooksey, 1978). Stigeoclonium to a more diverse and structurally complex assemblage of diatoms, filamentous green algae, B. Algal Diversity in Freshwater Wetlands and cyanobacteria (Hann, 1991). An important reason for the success of certain The communities of algae in freshwater wetlands algal species in wetland habitats is their ability to are nearly as diverse as those found in lakes. As in tolerate variations in water level and desiccation. One other systems, nutrient conditions, climate, and geo- model predicted specific wetland algal communities logy influence species composition, but in wetlands, that depend on varying water level: dry, sheltered, or 741550.02.qxd 16/9/2002 2:26 PM Page 40

40 John D. Wehr and Robert G. Sheath

lakelike (Goldsborough and Robinson, 1996). Water Algal communities of bogs are typically species- levels may fluctuate several times in a few months or poor, although diversity, especially that of the desmids, persist for several years. Algae that occupy a variable may be much greater in systems connected to other moisture regime must have adaptations to tolerate lakes or streams (Woelkerling, 1976; Hooper, 1981; extremes of conditions. Some epipelic desmids Mataloni and Tell, 1996). A tangle of filamentous (Closterium and Micrasterias species) are capable of green algae (Zygnematales and Ulotrichales) and surviving extended periods of drying and darkness desmids (see Chap. 9) are common, but contrary to (Brook and Williamson, 1988). Other filamentous common wisdom, although desmids are numerous and forms (e.g., Oscillatoria, Lyngbya, and Oedogonium) diverse, they are rarely important in terms of algal may form thick mats during the open (flooded) state biomass (Yung et al., 1986). Further details on their that protect algal cells during a later dry phase. distribution and ecology are given in Chapter 9. At least three major algal habitats are found in bogs: (1) C. Algal Communities of Bogs pools and open water, (2) habitats associated with Sphagnum, and (3) epiphytic habitats of Nuphar or Bogs are a special class of algal habitats, and other macrophytes. Data for 31 eastern bog systems include wetlands, streams, and ponds where water demonstrated that algal species richness increases (espe- retention and chemistry are usually influenced by cially desmids) with proximity to the Atlantic coast, Sphagnum. Most bogs have lower pH (4.0–5.5), low and is less in systems with greater color and lower Ca2+, are poor in nutrients, and have high levels of pH (Yung et al., 1986). The flagellate Gonyostomum dissolved organic matter, which casts a yellow or (Raphidophyceae; Chap. 11, Sect. III) and the red alga brown stain to the water (Gorham et al., 1985; Cole, Batrachospermum turfosum (as B. keratophytum; 1994). These dystrophic systems, along with lakes high Chap. 5) are two unusual species that are characteristic in humic materials, accumulate poorly decomposed of boggy systems (Prescott, 1962; Bourelly, 1985; Yung organic matter as peat. Ombrotrophic bogs are hydro- et al., 1986; Sheath et al., 1994). logically isolated and depend on precipitation as their Diatoms are generally less diverse than desmids, water source. Bogs are scattered throughout North but certain species are frequently observed, including America, especially in cool or cold regions that have an Anomoneis brachysira, Frustulia rhomboides var. sax- excess of moisture most of the year (Wetzel, 1983a). onica, Eunotia elegans, E. exigua, Navicula subtilissi- They are common across the subarctic, New England– ma, Pinnularia viridis, and several types of Stauroneis Maritime region, northern Great Lakes, Pacific spp. (Kingston, 1982; Cochrane-Stafira and Andersen, Northwest, and scattered alpine areas (Brooks and 1984; Mataloni and Tell, 1996). Diatom species Deevey, 1963; Northcote and Larkin, 1963; Yung et composition may change with successional stage of the al., 1986). Bogs are not restricted to cool climates, surrounding vascular plant community, and apparently however. Many of the Carolina Bays are dystrophic are responsive to many of the same variables, such as ponds or bogs and exhibit low algal production Ca, pH, and specific conductance (Cochrane-Stafira (Schalles and Shure, 1989). Bogs also occur in semi- and Andersen, 1984). Although it has been argued that desert, desert, and tropical areas, such as in east-central cyanobacteria may be unable to tolerate lower pH, Texas, Arizona, and Costa Rica (Cole, 1963). Some especially values less than 4.0 (Brock, 1973), there are bogs are islands of acidic waters and soils surrounded several species that are common in these waters, by an alkaline “sea” (Glime et al., 1982). including species of Aphanocapsa, Chroococcus, In bog lakes, mats of vegetation (bryophytes, Dacytlococcopsis, Hapalosiphon, Microchaete, Nostoc, angiosperms, algae) may float out over the littoral zone and Stigonema. Because many of these species are hete- and grow toward the center for many years as the rocystous, their N-cycling role in these nutrient-poor bottom of the lake fills in with peat (Whittaker, 1975). environments deserves attention. Some species of Bogs are a stage in the long-term succession of some dinoflagellates, chrysophytes and synurophytes also lake basins that are in the gradual process of filling in. may be found, although their abundance is usually low. A pioneering limnological study of Cedar Bog Lake (MN; Lindeman, 1941a, b, 1942) documented the V. THERMAL AND ACIDIC ENVIRONMENTS vegetational history and aquatic food webs, and was one of the first estimates of freshwater algal productivity. A. Thermal Springs Lindeman’s study helped launch the trophic–dynamic concept in ecology and served as the basis for sub- Thermal springs and streams (hot springs) are sequent energetic studies of freshwater food webs and extreme environments in geologically active regions development of the ecosystem concept (Cole, 1994). where temperatures are influenced by geothermal 741550.02.qxd 16/9/2002 2:26 PM Page 41

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sources and can range from 35 to 110°C. In North sumers are associated with hot springs, because few America, thermal springs are common from Alaska metazoa tolerate temperatures greater than about south to Costa Rica, and in scattered locations in 50°C. Invertebrates include ostracods, water mites, and Arkansas, Florida, Georgia, Virginia, and north to rotifers, but little is known of their dynamics or food Massachusetts. Among the best known for aquatic webs. Adult beetles and flies are successful in some organisms is the spectacular thermal area of springs, systems. Brine flies (Paracoenia and Epihydra) lay eggs geysers, and fumaroles in Yellowstone National Park. in microbial mats found in springs in Yellowstone Park Some thermal springs are less obvious, exerting within the 30–40°C range. Both adult and larval stages their chemical and thermal effects on larger lakes and consume algal and bacterial material, which may in rivers, as in Yellowstone Lake (and adjacent lakes), and turn enhance primary productivity (Brock, 1967; Brock several streams and lakes in eastern Costa Rica (Pringle et al., 1969). et al., 1993; Theriot et al., 1997). Some thermal springs are highly acidic, which fur- The geology, chemistry, and organisms of hot ther limits their species diversity. A notable eukaryote, springs have been reviewed by Castenholz and the red alga Cyanidium caldarium, is often the sole Wickstrom (1975), Brock (1986), and Ward and photosynthetic organism in very acid (pH 2–4) hot Castenholz (2000). Temperatures can be fairly constant springs up to about 55°C. This somewhat enigmatic near the source, but can range from about 110°C (with organism was variously classified as a cyanobacterium, a high concentrations of salts) to just above ambient, green alga, cryptomonad, and an evolutionary link depending on the temperature and volume of thermal between red and green algae (Seckbach, 1991); today water, distance from the source, and volumes of non- it is placed in the division Rhodophyta based on pig- thermal surface water entering a system. Temperature ments, chloroplast structure, and molecular features is not the only extreme condition for thermal organisms; (Steinmüller et al., 1983; Pueschel, 1990). In acid hot most springs have elevated concentrations (50–150 mg springs, other enigmatic rhodophytes including –1 2+ 2+ + – 2– L ) of inorganic ions (Ca , Mg , Na , HCO3 , SO4 , Cyanidioschyzon merolae and Galdiera sulphuraria – Cl , Si, and H2S) and elevated pH (8–10). These condi- (DeLuca and Moretti, 1983), also have been reported, tions select for highly adapted organisms, especially also have. It seems to be unlikely that either high tem- chemoautotrophic and heterotrophic bacteria in very perature or low pH is solely responsible for this pecu- hot (> 70°C to 94°C) conditions (Brock, 1985b). liar flora, because alkaline hot springs and nonthermal Among photosynthetic organisms, cyanobacteria are acid springs have very different algal communities. most common, with an upper limit between 70 and 73°C; eukaryotic algae are restricted to a maximum of B. Acid Environments about 55°C. Along a thermal stream, distinct zones of bright colors and morphologies that correspond to Most highly acid springs and streams are non- different species along the thermal gradient can be thermal and support a characteristic algal flora that is observed. unlike those in other aquatic environments. Although A diversity of cyanobacteria dominate thermal bogs may exhibit relatively low pH (4.0–5.0), highly waters. They include masses of coccoid species of acidic environments typically are regarded as systems Aphanocapsa, Chroococcus, Cyanobacterium, and with H+ concentrations at least an order of magnitude Synechococcus, and filamentous species of Mastigo- greater, that is, pH values ≤ 3.0, and they usually cladus, Oscillatoria, and Phormidium (Ward and receive acidic inputs from either geological or anthropo- Castenholz, 2000, Chaps. 3 and 4). In cooler waters genic sources (Hargreaves et al., 1975). Nearly all have further from the source (35–50°C), diatoms (Achnanthes elevated concentrations of metals, including Al, Fe, and Pinnularia) and green algae (Spirogyra and Mn, Pb, Co, Cu, and Zn, which may be near satura- Mougeotia) proliferate (Stockner, 1967; Castenholz tion levels even for very low pH, resulting in the forma- and Wickstrom, 1975). Cyanobacterial assemblages tion of metal salt precipitates along stream margins may form mats several centimeters thick and have or on algal colonies. Acid springs with very high Fe

extremely high rates of primary production (> 10 g C concentrations also may have very low dissolved O2, –2 –1 m d ; Castenholz and Wickstrom, 1975). In one hot due to Fe(OH)2 and FeO(OH)2 precipitates (Van spring in Costa Rica (62°C, pH 7.0), a species of Everdingen, 1970). Laguna de Alegría, a crater lake in Oscillatoria dominated, while nearby streams with less El Salvador, is influenced by sulfur-rich fumaroles and extreme temperatures (35–36°C; pH 7.8–8.0) had a exhibits pH values as low as 2.0 (Cole, 1963). greater diversity of algal species, including cyano- The earliest detailed studies of algae in highly bacteria (Oscillatoria, Phormidium and Lyngbya) and acidic systems in North America were conducted in diatoms (Pinnularia; Pringle et al., 1993). Unusual con- acid mine drainages in Indiana, Kentucky, Ohio, 741550.02.qxd 16/9/2002 2:26 PM Page 42

42 John D. Wehr and Robert G. Sheath

Pennsylvania, and West Virginia (Lackey, 1938; the western United States and Canada, anion and Bennett, 1969; Warner, 1971). These studies, as well as cation chemistries vary significantly with latitude: lakes 2– those in the United Kingdom (Hargreaves et al., 1975), north of 47° latitude are dominated by SO4 and all reveal low species diversity and a remarkable simi- either Na+ or Mg2+, whereas lakes in more southern 2– – larity in composition. Euglena mutabilis is the most locations are dominated by CO3 or Cl in conjunction widespread and often most abundant species, occurring with Na+ ions, reflecting climatic as well as geological in systems as acidic as pH 1.5. E. mutabilis is also characteristics of each region (Blinn, 1993). common in naturally acidic streams, such as the Rio Saline lakes of athalassic (nonmarine) origin differ Agrio (pH 2.3) in Costa Rica (Pringle et al., 1993), and from oceanic systems in several important ways. Total in acidic ponds in the Smoking Hills region of the dissolved salts vary considerably more (0.5–600 g L–1) Northwest Territories (Sheath et al., 1982). The latter than in oceans (35–40 g L–1), both among systems and site also supports populations of Chlamydomonas over time. Most athalassic lakes are shallow, which acidophila. E. mutabilis is not apparent in natural makes seasonal and longer term (climatic, anthropo- acid springs in Kootenay Paint Pots (BC) although a genic) changes in salinity important for algal survival, few diatoms and green algae are present (Wehr and and results in low biotic diversity (Cole, 1994). A few Whitton, 1983). Other common elements in many are large and relatively deep, such as Pyramid Lake highly acidic environments include Klebsormidium (532 km2; 102 m deep) and Big Soda Lake (1.5 km2; (previously Hormidium) rivulare, Eunotia tenella, 64 m deep) in Nevada (Hutchinson, 1957), Mono Lake Pinnularia microstauron, P. braunii, and Gloeochrysis (150 km2; 40–50 m deep) in California (National turfosa. Acid sites in West Virginia contain many of the Academy of Sciences 1987), and Soap Lake (3.6 km2; same species found in the United Kingdom. (Bennett, 27 m deep) in Washington (Castenholz, 1960). Because 1969), although an apparent absence of G. turfosa of differences in concentrations of salts with depth, from North American sites may be the result of the many of the deeper saline lakes, such as Soap and alga being overlooked. No studies report cyanobacteria Mono Lakes, are meromictic. Water level in Mono in these highly acidic environments, in agreement Lake varies substantially as a function of water diver- with Brock’s (1973) recommendation for a lower pH sion for human usage. In Redberry Lake, Saskatchewan, limit of less than 4.0. Isolates of E. mutabilis, mean depth has decreased by about 37%, while salinity Chlamydomonas acidophila, Klebsormidium rivulare, has increased by roughly 41% since the 1940s, due to Gloeochrysis turfosa, and Stichococcus bacillaris from changes in land use (Evans et al., 1996). one acid stream (pH 2.6–3.1) were able to tolerate and Saline lakes are concentrated in arid environments, grow at pH levels less than the lowest measured in especially in the U.S. Southwest, Mexico, and interior their collecting site (Hargreaves and Whitton, 1976a). regions of California, Oregon, Washington, northern In addition, an acid strain of Klebsormidium rivulare prairies, and British Columbia. Some of the most tolerated greater Zn and Cu concentrations in the pH extensive surveys of the chemistry and biology of saline range 3.0–4.0, than at pH ≥ 6.0, suggesting a H+–metal lakes were conducted in Saskatchewan (Rawson and interaction (Hargreaves and Whitton, 1976b). Moore, 1944; Hammer et al., 1983). Few inland saline lakes are found in eastern North America, although Onondaga Lake (12 km2, 20.5 m depth) is a saline lake VI. UNUSUAL ENVIRONMENTS in the New York Finger Lakes region. Levels of [Na+ + + –1 –1 A. Saline Lakes and Streams K ] and Cl exceeded 500 and 1400 mg L , respec- tively, in part from salt springs, plus pollution from an Saline lakes and streams make up a large and adjacent soda ash facility (Berg, 1963; Sze and heterogeneous collection of water bodies that have Kingsbury, 1972). Controls on salt waste have resulted elevated total dissolved salts (> 500 mg L–1; Williams, in reduced total salinity (450 mg Cl– L–1), although levels 1996). Many are closed basins or desert playas that are still greater than pre-industrial times (ca. 230 mg gain salinity over the year as they lose water (Hammer, Cl– L–1; Effler and Owens, 1996; Rowell, 1996). 1986; Evans and Prepas, 1996). The term “saline” Algal communities of saline lakes differ among does not mean simply greater concentrations of NaCl. systems and their diversity varies inversely with salinity The ion content of inland saline lakes is influenced (Blinn, 1993; Cole, 1994). Because many saline lakes by Na+, K+, Ca2+, and Mg2+, and the major anions are are shallow and subject to wind-driven mixing, it is – 2– – 2– typically Cl , SO4 , HCO3 , and CO3 (Wetzel, 1983a; often difficult to distinguish between benthic and Hammer, 1986; Cole, 1994). Systems are usually well planktonic forms. Several studies have evaluated the – 2– buffered (high in HCO3 and/or CO3 ) and neutral to influences of salinity and ion composition on algal alkaline in pH (7.5–10.0). Among 47 saline lakes in communities (Castenholz, 1960; Hammer et al., 1983; 741550.02.qxd 16/9/2002 2:26 PM Page 43

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Blinn, 1993; Fritz et al., 1993; Evans and Prepas, 1996; portion of total production (Sze and Kingsbury, 1972; Wilson et al., 1996). In mildly saline (total salts Robarts et al., 1992). Many lakes in the Canadian 500–2000 mg L–1) lakes, the algal flora is fairly rich prairies have lower algal biomass and primary produc- and composed of a variety of diatoms (e.g., species tivity than would be predicted using standard nutrient of Amphora, Campylodiscus, Cyclotella, Epithemia, models (Campbell and Prepas, 1986), apparently Fragilaria, Navicula, Nitzschia, and Rhopalodia), green because of greater densities of macrozooplankton that algae (Crucigenia, Pediastrum, Oocystis, and Sphaero- flourish in saline waters and perhaps due to fewer cystis), and cyanobacteria (e.g., species of Anabaena, zooplanktivorous fish (Evans et al., 1996). In Great Aphanizomenon, Chroococcus, Lyngbya, Merismo- Salt Lake, weather-induced decreases in salinity pedia, Microcystis, and Oscillatoria), especially if N (250–50 g L–1) resulted in an increase in grazers from and P are high. In more strongly saline conditions one species (Artemia) to four, where rotifers and cope- (2–20 g L–1), many species are eliminated, but the com- pods were dominant (Wurtsbaugh and Berry, 1990). munity still includes taxa found in nonsaline waters, Paleoecological studies have considered whether like Cladophora glomerata, Botrycoccus braunii, these systems were saline in the past (Fritz et al., 1993; Cocconeis placentula, Mastogloia spp., Nitzschia palea, Rowell, 1996). A study of the diatoms from 219 Plagioselmis (as Rhodomonas) minuta, and several lakes in British Columbia indicated that some species cyanobacteria (Aphanizomenon flos-aquae, Anabaena are limited by salinities as dilute as 0.02 g L–1 (e.g., spp., Microcystis aeruginosa, and Oscillatoria spp.). Achnanthes pusilla), whereas others tolerate more than Species typical of higher salinities also co-occur within 500gL–1 (Amphora coffeiformis and Nitzschia frustu- this range, probably because concentrations can vary lum; Wilson et al., 1996). Models from these data have by an order of magnitude or more over a year in many been used to infer temporal and spatial differences in lakes. Under hypersaline conditions (20–600 g L–1), precipitation, which in turn influences salinity in these diversity is very low, and includes some species that are closed basins. Long-term changes in fossil algal pig- restricted to higher salt levels, such as the diatoms ments suggest that as prairie lakes became more saline, Amphora coffeiformis, Anomoneis sphaerophora, phytoplankton communities have shifted from a Navicula subinflatoides, Nitzschia communis, and N. diatom–chrysophyte–dinoflagellate community to one frustulum, cyanobacteria Nodularia spumigena and dominated by greens, cyanobacteria, and diatoms Aphanothece halophytica, and the filamentous green (Vinebrooke et al., 1998). Due to possible effects of alga Ctenocladus circinatus (Blinn, 1971; Herbst and global warming, semiarid regions of North America may Bradley, 1989; Wurtsbaugh and Berry, 1990; Kociolek experience an increase in the number of saline waters and Herbst, 1992; Reuter et al., 1993). A few taxa in the next few decades (Evans and Prepas, 1996). that have marine distributions, such as Enteromorpha intestinalis, diatoms Dunaliella salina, D. viridus, B. Snow and Ice Chaetoceros muelleri, and Thallasiosira pseudonana, and the coccolithophorid Pleurochrysis carterae, have People who have hiked in alpine regions are been observed (Sze and Kingsbury, 1972; Stephens and familiar with red snow, especially where snowfields Gillespie, 1976; National Academy of Sciences, 1987; accumulate for most of the year. Aristotle also appar- Johansen et al., 1988). Experiments in which salinity ently observed red snow many centuries ago (Kol, was varied within Mono Lake mesocosms determined 1968). This phenomenon is most often caused by the that diatom dominance, algal diversity, chlorophyll-a, green alga Chloromonas (previously recorded as and photosynthesis in benthic communities all declined Chlamydomonas) nivalis (see Chap. 6). The red color is at higher salinities, while Ctenocladus circinatus and an the result of an accumulation of secondary carotenoids, Oscillatoria spp. dominated, although the diatom mainly astaxanthin, in resting cells (Round, 1981; Nitzschia monoensis increased (Herbst and Blinn, 1998). Bidigare et al., 1993). Not all snow communities are Primary production by algae is often high (300– colored red: orange, brown, and green patches are also 1000gCm–2 y–1), along with very high chlorophyll-a seen, depending on the species present, dominant levels (50 to > 500 mg L–1), because many saline pigments, exposure to sunlight, and perhaps pH (Stein lakes are located in sunny locations and have surplus P and Amundsen, 1967; Kol, 1968; Hoham and Blinn, levels (total P: 50 to > 1000 mg L–1; Hammer, 1981; 1979). Cells aggregate near the surface, and as succes- National Academy of Sciences, 1987; Robarts et al., sive snowfalls accumulate, layers or bands of pigmented 1992). Much of this production comes from epipelic algal communities can be seen in vertical cuts through and epilithic assemblages (e.g., Ctenocladus) rather a snow bank (Hoham and Mullet, 1977). Although than phytoplankton (Wetzel, 1964). Surface blooms of motile stages can be found in snow, most cells occur cyanobacteria may at times represent a significant pro- either as thick-walled resting zygotes or as asexual 741550.02.qxd 16/9/2002 2:26 PM Page 44

44 John D. Wehr and Robert G. Sheath

hypnospores (Stein and Amundsen, 1967). Cryophilic requirements (N: 1–20 mg L–1, P: 0.1–2 mg L–1). The flora include other algal flagellates, such as Chloro- most common taxon appears to be Stigeoclonium monas nivalis, C. brevispina, Carteria nivale (= plano- tenue; lesser abundances of Chlorella spp., Nitzschia zygotes of Chloromonas sp.?), Scotiella cryophila, and palea, Oedogonium sp., Oscillatoria spp., Pleurocapsa Chromulina chionophila, and nonflagellated green minor, Pseudanabaena catanata, Scenedesmus quadri- algae, such as Raphidonema nivale and Stichococcus cauda, and Tribonema spp. also are observed (Palmer, spp. (Stein and Amundsen, 1967; Kol, 1968; Hoham, 1962; Sládecková et al., 1983; Davis et al., 1990a). 1975; Hoham and Blinn, 1979). The life cycles of Communities exhibit seasonal changes in composition: many snow algae are incompletely known; thus, some Tribonema dominates in the spring, and Oscillatoria, cells previously identified as new taxa have been shown Scenedesmus, and Stigeoclonium reach their maxima in to be zygotes of other known species (Hoham and warmer months. These shifts are largely driven by Blinn, 1979; R.W. Hoham, personal communication). changes in light and temperature, not nutrient supply Snow algae exhibit measurable but low photo- (Davis et al., 1990a). Substantial biomass and rapid synthesis rates, many of which reach a maximum near growth rates have made algae candidates for nutrient 10°C, although some peak near freezing and decline removal plans (Sládecková et al., 1983; Davis et al., at higher temperatures (Mosser et al., 1977; Round, 1990b). Treatment ponds have been devised in which 1981). A strain of Chloromonas pichinchae from > 95% of P may be removed by algal assemblages snowfields in Washington state grew best at 1°C and alone (Hoffmann, 1998). pH 6.0, whereas the optimum for an isolate of Algae colonize caves in many limestone or Raphidonema nivale from the same location was dolomite regions where fractures or underground 4–5°C, with a pH optimum ≥ 7.0, despite lower envi- streams form underground pockets or large caverns. ronmental pH (ca. 5.0) and temperature (near 0°C; Some cave-dwelling algae, including species of Hoham, 1975). Strains of Chloromonas isolated from Gloeocapsa (Cyanobacteria), survive very low light snowfields in the Adirondack Mountains (exposed to levels by having densely packed thylakoids in their cells decades of acid deposition) have significantly greater and very slow growth rates (Pentecost and Whitton, (1.5- to 2.2-fold greater) growth yields at pH 4.0 than 2000). By some observers caves may be considered to be Chloromonas isolated from the (less acidic) White dark, unproductive environments, but substantial algal Mountains of Arizona (Hoham and Mohn, 1985). floras develop where surface are open to the sunlight Algal cells that colonize snow surfaces at high altitude (fissures, mouths) or in show caverns where artificial experience extreme levels of solar (including UV) radia- lighting has been added. Cyanobacteria (e.g., Aphano- tion. Carotenoid pigments provide photoprotection by capsa, Calothrix, Chroococcus, Gloeocapsa, Hapalo- reducing total radiation and filtering certain shorter siphon, and Schizothrix) may be especially abundant wavelengths from photosynthetic pigments (Bidigare near light sources (Claus, 1962; Round, 1981). Species et al., 1993). Experiments with snow algae from Tioga diversity is typically low: Timpanogos Cave (UT) and Pass in the Sierra Nevada found that UV radiation Seneca Cavern (OH) each support an algal flora of inhibited photosynthesis in green snow by 85%, but fewer than 30 species (St. Clair and Rushforth, 1976; only 25% in red snow (Thomas and Duval, 1995). Dayner and Johansen,1991). Species include diatoms Snow algal communities also are influenced by the (Navicula tantula and N. contenta), unicellular green amount of wind-blown soil (plus factor: nutrient algae (Chlorella miniata), and the xanthophyte

source), exposure (plus or minus factor: direction and Pleurochloris commutata. CaCO3 from cave walls may quantity of sunlight), snow albedo (usually minus become incorporated into algal colonies and thalli, factor: reflective property), and water content (plus fac- which is why artificial lighting and the resultant algal tor) of the snow (Stein and Amundsen, 1967; Hoham colonization has been cited as a prime cause of cave and Blinn, 1979; Thomas and Duval, 1995). These wall destruction in show caves (Gurnee, 1994). communities have cryophilic food webs with protozoan The calcareous cave flora are similar to wet lime- consumers, and bacterial and fungal decomposers that stone seepages above ground, and exhibit an abundance are active at very low temperatures (Felip et al., 1995; of calcified cyanobacteria and diatoms (Pentecost, Thomas and Duval, 1995). 1982; Pentecost and Whitton, 2000). Algae from pools adjacent to caves may be an important carbon source C. Other Unusual Habitats for food webs inside caves (Pohlman et al., 1997). Noncalcareous, temporary rock pools that form in Algae form thick mats on tank walls, outflow weathered bedrock along lake and river margins also weirs, pipes, and other substrata in sewage treatment become algal habitats. These small systems (a few liters plants, where concentrations of nutrients greatly exceed or less) experience great variations in water level and 741550.02.qxd 16/9/2002 2:26 PM Page 45

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nutrients, and extremes in solar radiation. The most be mistaken for moss (Canter-Lund and Lund, 1995). common inhabitant is the algal flagellate Haemato- Epiphytic green algae have been used as biological indi- coccus lacustris, which accumulates red secondary cators of air quality (Hanninen et al., 1993). A number carotenoids much like snow algae. This adaptation of genera that colonize plant leaves, walls, stones, or serves a similar photoprotective role and is not influ- soils are also common in fresh waters, including many enced by nutrient levels (Yong and Lee, 1991; Lee and cyanobacteria (Nostoc, Oscillatoria, Lyngbya, and Soh, 1991). Humans have created a widespread habitat Plectonema), green algae (Chlamydomonas, Chlorella, for H. lacustris, namely birdbaths (Canter-Lund and Chlorococcum, and Klebsormidium), and diatoms Lund, 1995); hence, many people report red water in (Achnanthes, Hantzschia, and Navicula; Segal, 1969; their birdbaths after a year or so of use. Cox and Hightower, 1972; King and Ward, 1977; Endophytic algae live in cavities of higher plants Hunt et al., 1979). Diatoms from these habitats are or colonize plant tissues intracellularly. Algae colonize often desiccation-resistant and many are regarded as microhabitats created by bromeliad cups, and colonized obligate aerial taxa (Johansen, 1999). Other genera,

by algae, and differ in pH and dissolved O2 levels among such as Prasiola and Zygogonium, have very distinct different plants (Laessle, 1961). Perhaps the most eco- ecological requirements. Prasiola colonizes N-rich (e.g., logically important endophytic algae in freshwaters are guano) rocks and walls in aerial (often shaded) and species of cyanobacteria that colonize various aquatic even urban environments (Jackson, 1997; Rindi et al., plants and bryophytes. The best known is Anabaena 1999), whereas aquatic species colonize cool, nutrient- azollae, a symbiont within the water fern Azolla, which poor streams (Sheath and Cole, 1992; Kawecka, 1990; is used as a nitrogen source or “green fertilizer” for rice Hamilton and Edlund, 1994). A few terrestrial species crops worldwide (Bothe, 1982). Roots of some cycads colonize the interstitial spaces within crystalline rocks (Cyas, Encephalartos, and Macrozamia) are colonized in arid or semiarid regions, where water supply may be by several cyanobacteria, especially Anabaena, Calothrix, limited (Bell, 1993; Johansen, 1993). Further details on and Nostoc (Huang and Grobbelaar, 1989); when sec- the algal flora of soils and terrestrial habitats can tioned, cyanobacteria appear as a healthy blue–green be found in reviews by Fritsch (1922), Starks et al. color. Green algae (e.g., Chlorella, Chlamydomonas) (1981), and Johansen (1993). Some algae are parasitic and some xanthophytes (see Frost et al., 1997, within plant tissues (leaves and twigs), such as Chaps. 6 and 7) are symbionts in a variety of aquatic Cephaleuros, which is the cause of red rust disease in organisms, including freshwater sponges (e.g., higher plants (Thompson and Wujek, 1997). Perhaps Spongilla lacustris and Corvomeyenia everetti), cnidari- one of the most unusual of all algal habits is that of the ans (e.g., Hydra spp.), ciliates (e.g., Euplotes and red alga Rufusia, which lives within the hairs of two- Ophrydium), and other organisms (Slobodkin, 1964; and three-toed sloths, but apparently does not colonize Frost and Williamson, 1980; Berninger et al., 1986; nearby vegetation (Chap. 5). Sand-Jensen et al., 1997). In the spring, green, baseball- sized (up to 20 cm) green masses can be observed in ponds and lake outflows that are actually mucilaginous ACKNOWLEDGMENTS masses of amphibian eggs (Amblystoma and Rana) colonized by the green alga Chlamydomonas (syn. Thanks are due Dr. R. Jan Stevenson (Michigan Oöphila) amblystomatis. This association appears to State University) and Dr. Robert G. Wetzel (University

provide N for the alga and added O2 for developing of Alabama) for helpful reviews and advice on relevant amphibian eggs (Goff and Stein, 1978; Bachmann et literature. Dr. Ronald W. Hoham (Colgate University) al., 1986; Pinder and Friet, 1994). Much more intimate provided advice on species of snow algae. We also symbioses are found among algae and cyanobacteria thank D. W. Blinn, D. Burney, R. J. Cannnings, S. J. with fungi, as lichens. The treatise by Ahmadjian Cannings, J. Cotner, P. R. Leavitt, A. Merola, NASA, (1993) can be consulted for further information. and the South Florida Water District for permission to Many algae colonize terrestrial habitats such as reproduce photos. soils, trees, and other surfaces, and serve important ecological functions in soil and moisture retention, seed germination, nutrient dynamics, and succession of LITERATURE CITED terrestrial vegetation (Carson and Brown, 1978; Bell, 1993; Vazquez et al., 1998). Many of the more Abel, R. A., Olson, D. M., Dinerstein, E., Hurley, P. Eds. 2000. Freshwater ecoregions of North America: A conservation common terrestrial species, appear as a green “felt” on assessment. World Wildlife Fund, Washington, DC. stone walls, tree bark, and wooden fences such as Admiraal, W., Mylius, S. D., de Ruijter van Steveninck, E. D., Apatococcus and Trentepohlia (Chlorophyta), that may Tubbing, D. M. J. 1993. A model of phytoplankton production 741550.02.qxd 16/9/2002 2:26 PM Page 46

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in the lower River Rhine verified by observed changes in silicate Bianchi, T. S., Findlay, S., Dawson, R. 1993. Organic matter sources concentration. Journal of Plankton Research 15:659–682. in the water column and sediments of the Hudson River estu- Admiraal, W., Breebaart, L., Tubbing, G. M. J., Van Zanten, B., de ary: The use of plant pigments as tracers. Estuarine, Coastal Ruijter van Steveninck, E. D., Bijerk, R. 1994. Seasonal varia- and Shelf Science 36:359–376. tion in composition and production of planktonic communities Bidigare, R. R., Ondrisek, M. E., Kennicutt, M. C., Iturriaga, R., in the lower River Rhine. Freshwater Biology 32:519–531. Harvey, H. R., Hoham, R. W., Macko, S. A. 1993. Evidence for Ahmadjian, V. 1993. The lichen symbiosis. J Wiley, New York. a photoprotective function for secondary carotenoids of snow Allan, J. D., Ed. 1995. Stream ecology: Structure and function of algae. Journal of Phycology 29:427–434. running waters. Chapman & Hall, New York. Biggs, B. 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