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J. Phycol. 34, 741–749 (1998)

MINIREVIEW

USE OF IN LARGE RIVER MANAGEMENT1

John D. Wehr2 Louis Calder Center-Biological Station, Fordham University, P.O. Box K, Armonk, New York 10504 and Jean-Pierre Descy Faculte´s Universitaires Notre-Dame de la Paix, Unit of Freshwater , 61 Rue de Bruxelles, B-5000 Namur, Belgium

Key index words: freshwater; large rivers; phytoplankton; some authors define ‘‘large river’’ as a fluvial system river impoundments large enough to intimidate researchers (Hynes Abbreviations: POC, particulate organic carbon; RCC, riv- 1989) or too large to study without a boat and spe- er continuum concept; SRP, soluble reactive phosphate cialized equipment (Stalnaker et al. 1989). Streams and rivers are classified by geomorphologists accord- Historically, rivers have served as sources of drink- ing to fluvial and watershed properties, which rec- ing water, fisheries resources, transportation routes, ognize a continuum of physical features (channel irrigation supplies, and waste removal systems. Hu- width, average depth, discharge, catchment area) as man civilization has had many major effects on riv- one progresses downstream (Morisawa 1968). When ers, dating back more than 5000 years when Egyp- viewed from this perspective, the distinction be- tians built dams on the Nile to supply water for tween large and smaller rivers may be considered crops and consumption. Today, manage- arbitrary (Stalnaker et al. 1989, Townsend 1996). ment of large rivers requires a balance between hu- Ecological theory developed for river , man needs and ecological integrity, although until such as the RCC (Vannote et al. 1980) and Nutrient quite recently, ecological principles have played a Spiraling model (Newbold et al. 1981), employ lon- minor role in river management (Edwards 1995). gitudinally integrated processes as factors affecting Planktonic are an important part of these supply and function. The rele- issues because they play a central role in the func- vance and usefulness of the RCC for large rivers has tioning of large rivers. Algal communities are major been questioned (Sedell et al. 1989, Thorp and De- producers of organic carbon in larger rivers, are a long 1994, Townsend 1996), but the definition of food source for planktonic consumers, and may rep- ‘‘large river’’ that has emerged from these studies is resent the primary source in many low-gra- dient rivers (Thorp and Delong 1994, Ko¨hler 1995, that of rivers greater than sixth order (based on Reynolds and Descy 1996). Phytoplankton are re- Strahler 1957). Other workers examining the im- sponsive to excessive supplies of inorganic nutrients portance of phytoplankton production to carbon and may pose problems in long stretches of rivers supply and system function have suggested that sim- with cultural , but may also enhance ply ‘‘low-gradient’’ or ‘‘lowland’’ rivers may be more water quality for in rivers affected by agri- useful concepts because their metabolism is fre- cultural or industrial uses. Algal communities of riv- quently dominated by algal or other in situ autotro- er systems consist not only of suspended algae, but phic organisms, despite the fact that many are light- also a diverse benthic assemblage of macrophytic limited due to depth and turbidity (Baker and Baker forms, smaller epilithic , epiphytes, and sed- 1979, Descy and Gosselain 1994, Ko¨hler 1995, Reyn- iment-dwelling forms (Reynolds 1996). This review olds and Descy 1996). This latter perspective has focuses on the use of planktonic algae in river man- merit, because downstream reaches of smaller rivers agement because of their central importance in larg- possess many features common to large rivers er rivers and because of the growing need for eco- (floodplain influences, high turbidity, lateral carbon system-level studies on river . inputs, lower gradient). Other authors maintain that WHAT ARE LARGE RIVERS? large rivers should be viewed as systems distinct from upland sections, with ecological features and Ecologists know much more about small tributar- ies than larger rivers, in large part because they are management problems that deserve particular atten- simply easier to study. Undoubtedly, this is why tion (Sedell et al. 1989, Admiraal et al. 1994). In- deed, large rivers are widely recognized and even defined as manipulated or regulated ecosystems 1 Received 5 November 1997. Accepted 8 June 1998. 2 Author for reprint requests; (Ward and Stanford 1983, De´camps 1996). There- e-mail: [email protected]. fore, our review focuses on management concerns 741 742 JOHN D. WEHR AND JEAN-PIERRE DESCY

chemical, and biological processes, which reach a higher degree of complexity downstream. There are also longitudinal differences in the time scales of chemical and biological processes. Such complexity renders it difficult to design policies and assess the results of management actions. Recently, studies have taken into account riverine, watershed, and land use variables, as well as differences in spatial scale in devising management models for river water and fish quality (Hunsaker and Levine 1995, Roth et al. 1995, Smitz et al. 1997). FIG. 1. Schematic representation of alterations and disturbances Use of phytoplankton in management has had a that affect ecosystem integrity in large rivers. A management fairly long history. Concern most often centers on scheme may alter the number and position of sampling stations adverse effects, such as taste and odor problems, fil- in an attempt to monitor specific effects, but due to the direc- ter clogging at water treatment , and effects tional flow of rivers, each location will represent a combination (and perhaps interaction of) factors from further upstream. on fisheries (Palmer 1962). Fewer studies have doc- umented the beneficial uses of algae in manage- ment practice. How algae are applied depends on of lowland rivers greater than sixth order and the the problem, as well as on river size and flow regime. role that phytoplankton may play in these systems. For example, in the highly regulated Buffalo River (South Africa), high nutrient loads were reduced by MANAGEMENT ISSUES sedimenting algal populations along impounded Many studies document the effects of human dis- stretches, resulting in the ability to release relatively turbances on communities in large and small rivers, low-nutrient water below the dams (O’Keefe et al. but few of these findings have been adequately com- 1990). Studies using algal structure or municated to water management agencies (Petts et accumulation of toxic metals have been employed al. 1995). This condition exists despite the fact that in large-scale analyses of water quality in many most large rivers of the world are now or will soon streams and rivers (del Giorgio et al. 1991, Whitton be substantially altered by human activities (Sparks et al. 1991, Lowe and Pan 1996). An important com- 1995). A few of the important disturbances include plication to their use in management is that the channelization, navigation or hydroelectric dams, composition, dynamics, and production of algal removal of adjacent , toxic pollutants, and communities in large rivers is less well understood cultural eutrophication (Johnson et al. 1995, Petts than in either lakes or small streams. et al. 1995). Management of these large-river distur- bances is complicated by the fundamental feature RIVER PHYTOPLANKTON ECOLOGY: A SHORT REVIEW common to all flowing waters: activities or distur- Various aspects of potamoplankton, the commu- bances at one location affect processes and organ- nity of suspended algae in flowing water, have been isms downstream (Fig. 1). Due to the longitudinal reviewed by Reynolds and Descy (1996). These au- of rivers, management problems that arise thors highlight several key points specific to the are frequently more difficult to manage than in composition and ecology of planktonic algal assem- lakes. For example, excessive nutrients may enter blages in large rivers. Ironically, many of the most the system from several points along the main chan- important features emphasized were understood nel, and the forms of nutrients also depend on var- long ago (Kofoid 1908, Krieger 1927, Reinhard ious watershed processes (e.g. hydrogeological pro- 1931, Butcher 1932, Welch 1952). Among questions cesses as nutrients transfer through soils and left unresolved, the origin of phytoplankton assem- groundwater), which are not well understood. blages in running waters is most often raised. Even Downstream transport of dissolved and particulate the earliest studies perceived that algal inocula from substances often requires that management schemes various sources contributed to the main channel, consider large river stretches (Ͼ102–103 km) or en- but potamoplankton is also ‘‘native’’ to rivers, as tire basins, as well as adjacent drainage areas. demonstrated more precisely by Reynolds (1995) Human disturbances affect not only riverine or- and Reynolds and Glaister (1993). The actual ganisms and processes, but also the links between a sources of inocula (backwaters connected to the river and its watershed, by disrupting lateral linkages main channel or dead zones within the channel) are with floodplains and groundwater (Townsend still uncertain and likely vary among river systems. 1996). Perhaps this is one reason why computer It is tempting to speculate that most planktonic al- models that have attempted to predict phytoplank- gae in rivers are meroplanktonic (planktonic with a ton production or species composition in large riv- benthic stage) and recolonize littoral areas or ers have met with only modest success (Billen et al. the river bed for survival and perennation. The ben- 1994, Ruse and Love 1997). River ecosystems are thos clearly serves as the origin of suspended algae actually networks of interactions among physical, in smaller streams (Swanson and Bachmann 1976). PHYTOPLANKTON AND LARGE RIVER MANAGEMENT 743

In such cases, the a flux is positively re- (several rivers: Jones 1984, Moss et al. 1984). The lated to discharge. However in large rivers, phyto- largest values are comparable to those reported for plankton varies inversely with discharge highly eutrophic . It is in the higher range (Schmidt 1994); this distinction is an important where potamoplankton may cause harmful effects characteristic that separates small from large rivers. for water use and even decreases in oxygen concen- Composition and selection. The most successful algal trations. However, as algal is often groups in large rivers are and green algae, the major supplier of oxygen to slowly flowing rivers, the latter tending to dominate in summer condi- phytoplankton loss may also result in water quality tions, although this pattern is far from consistent. problems (Descy 1992). Cryptophytes and may also at times be That potamoplankton biomass exhibits variations abundant, whereas other groups, chrysophytes, di- along the course of a river has been known for many noflagellates, and euglenophytes are usually less de- decades (Welch 1952). Simulation models show this veloped, despite records of blooms of particular taxa pattern develops in four phases, progressing down- in specific conditions (e.g. Ohio River: Wehr and stream (no plankton in the headwaters, increase, Thorp 1997, St. Lawrence River: Hudon et al. 1996). maximum, decline), and results from interactions Plankton surveys provide lists of up to several hun- among river morphology, hydrology, light availabil- dred taxa, but the list of common and truly plank- ity, and algal growth rate (Descy and Gosselain 1994, tonic taxa is much less (Reynolds and Descy 1996), Garnier et al. 1995). Growth rates in eutrophic riv- and (unlike benthic macroalgal species), few if any ers strongly depend on the balance between algal planktonic taxa are specific to rivers (Round 1981). photosynthesis and respiration, which is positive in The principal selective factors affecting river phy- shallow, transparent river channels and may become toplankton were clearly summarized by Reynolds negative in deep and turbid reaches. This deficit in (1988, 1995): phytoplankton growth is particularly true in river es- tuaries, such as the tidal Hudson River, New York ● Nutrient limitation is unlikely in rivers, despite (Cole et al. 1992). Phytoplankton in lower reaches the influence of algal growth on nutrient (Si, P) are likely imported from upstream (less turbid) levels. Nutrients in rivers are often in consider- stretches that support positive net algal production. able excess of algal requirements, such that mod- These longitudinal dynamics are driven by down- els based on nutrient resource ratios in lakes (Til- stream transport of plankton into reaches where man et al. 1982) are not applicable in most rivers. growth fluctuates in response to physical factors; ● Phytoplankton and production are therefore, measures against eutrophication must controlled by discharge. This is related to resi- consider the entire river system. dence time, channel depth, and dilution rate and The view that potamoplankton development in affects water transparency and sedimentation. large rivers takes place mostly in the main channel ● Maintaining stable or persistent phytoplankton and is heavily dependent on physical characteristics communities is impossible in large (deep and tur- is applicable not just to regulated or eutrophic riv- bid) rivers if these systems exist as fully mixed ers. The same processes may govern phytoplankton reaches. Therefore, river mixing must be incom- growth in rivers with extensive floodplains, as shown plete (Margalef 1960), which is provided by het- by Lewis (1988) for the Orinoco River. On an an- erogeneity in channel morphology. nual basis, plankton gross production was 9.5 g The river environment selects for fast-growing C·mϪ2; the floodplain contributed only 0.25 g species able to cope with wide variations in light con- C·mϪ2. At the upper end of the production range, ditions, which depend on incident light, water trans- the River Loire, France (Lair and Sargos 1993, Lair parency, and depth (not unlike turbid, well-mixed and Reyes-Marchant 1997), provides an example of shallow lakes; Reynolds et al. 1994). There are very a river with massive potamoplankton growth, but few studies demonstrating nutrient limitation in limited flow regulation, where exchanges (nutrients, larger rivers (Moss and Balls 1989) or dependence organisms) with the littoral zone substantially en- of algal growth upon P availability (Reynolds and hance plankton development. Descy [1996], but see Basu and Pick [1996] for a Despite many studies to the contrary, nutrient lim- contrary view). Such issues are important when deal- itation should not be excluded as a factor in the ing with the effects of eutrophication in rivers. Anal- longitudinal development of river phytoplankton. ysis of long-term data shows that increases in algal Data from rivers in southeastern Ontario and west- biomass that have occurred in recent decades in ern Quebec reveal a positive relationship between many European and North American rivers may total P and chlorophyll a (Basu and Pick 1996). We have resulted from increased nutrient inputs cou- suggest, however, that a clear demonstration of nu- pled with hydrological changes and river regulation. trient limitation in river systems is generally still lack- Biomass, longitudinal variation, and driving variables. ing. Such a relation is likely true in systems with few A wide range of phytoplankton biomass has been nutrient sources, long residence times, and high wa- reported for rivers, from Ͻ1 ␮g (Orinoco River, Bra- ter clarity (or shallow depth). Few large rivers world- zil: Lewis 1988) up to ഠ400 ␮g chlorophyll a·LϪ1 wide combine these conditions, and many if not 744 JOHN D. WEHR AND JEAN-PIERRE DESCY most are already significantly affected by human in- fluences. Role of loss processes in river phytoplankton dynamics. Phytoplankton development in some rivers may be favored because of lower pressure and re- duced sedimentation rates (Cole et al. 1991, 1992). There are some data indicating that biomass in lotic environments is lower than in lakes (Pace et al. 1992, Thorp et al. 1994) and that phy- toplankton–zooplankton interactions are weak in most river systems (Ko¨hler 1995, Basu and Pick 1996). However, declines in potamoplankton num- bers or biomass do occur in otherwise favorable con- ditions for algal growth (de Ruyter van Steveninck et al. 1992, Gosselain et al. 1994). These declines may result from poor light availability combined with zooplankton grazing and sedimentation losses. However, few field studies have quantified the im- pact of zooplankton grazing on phytoplankton in large rivers (Thorp et al. 1994, Gosselain et al. 1998). The primary constraint in rivers is residence time, which affects both phytoplankton and their grazers (Pace et al. 1992, Thorp et al. 1994). Rivers usually select for small-bodied zooplankton with an ability to grow rapidly enough to compensate for limited residence times (Viroux 1997), but which may have a low capacity to regulate phytoplankton biomass. Several authors have reported decreases in phytoplankton numbers brought about by benthic filter feeders, especially zebra mussels (Dreissena poly- FIG. 2. Seasonal changes in maximum and 8- year average phy- morpha: Effler et al. 1996, Roditi et al. 1996, Strayer toplankton densities in the Hungarian section of the River Dan- et al. 1996) and other invaders, such as Corbicula sp. ube during 1957–1965 and 1979–1987. The 10-fold increase in (nonwinter) algal densities occurred during a period of similar and Corophium curvispinum (Bachmann et al. 1995). nutrient levels, but marked changes in the hydrological condi- Voracious feeding rates coupled with very large pop- tions brought on by upstream reservoir construction (redrawn ulations have dramatic effects on phytoplankton, wa- after Kiss 1994). ter quality, and ecosystem processes in large rivers. These recent and dramatic developments in many large rivers (e.g. Mississippi, Ohio, St. Lawrence, growth is commonly thought of as resulting princi- Hudson) call for specific management actions. pally from increased nutrient loading, it is far from Almost no studies have shown that sedimentation clear that this is true in most rivers. For example, in in rivers is an important loss process for phytoplank- the case of River Danube (Hungary), there are ex- ton. To date, sedimentation losses have been mostly cellent algological records spanning Ͼ40 years (Sze- calculated, taking into account water velocity, depth, mes 1967, Kiss 1994). A 10-fold increase in phyto- and specific settling velocity of algal species (Carling plankton densities occurred during the 1970s (Fig. 1992). However, it is likely that sedimentation is a 2) without a noticeable increase in the nutrient sup- selective factor that may suppress numbers ply since the end of the 1950s. Rather, changes in and perhaps prevent their in shallow riv- suspended matter transport, brought about by res- er stretches, especially in regions of very low flow ervoirs in the German and Austrian sections, im- (Reynolds 1995). proved water transparency and light availability for algal growth. It is also likely that increased residence CASE STUDIES AND OTHER MANAGEMENT ISSUES time from the exploitation of the upper Danube fa- Eutrophication in large rivers. Most large European vored the development of euplanktonic species in rivers have a long record of eutrophication attrib- the river. uted to human influences (Friedrich and Viehweg A similar phytoplankton increase was observed in 1984, Descy 1987), which raises concern about the the River Meuse, Belgium, by water supply compa- sustainability of domestic uses of river water. This is nies since the early 1970s (Descy 1992). Strong in- especially true when river water is abstracted and creases in mean annual chlorophyll a occurred with intensively used, such as with the River Meuse (Bel- no clear trend in P concentrations within the Bel- gium), a source of drinking water for about six mil- gian sector (SRP: 50–100 ␮g·LϪ1). Despite increases Ϫ lion people (Descy 1992). Although increased algal in N concentrations in the Meuse (especially NO3 ), PHYTOPLANKTON AND LARGE RIVER MANAGEMENT 745 as in other European rivers (Van Dijck 1996), levels Յ10 ␮g P·LϪ1 (Van Donk and Kilham 1990, Lam- in the 1970s were already in the mg·LϪ1 range, well pert and Sommer 1997). This is Ͻ10% of current P exceeding algal demand. Phytoplankton blooms in concentrations in many rivers. Furthermore, taking the Meuse may have built up in the French sector into account intracellular storage and downstream (Le´glize and Salleron 1988) as a result of water displacement of water masses, careful treatment treatment plants lacking P-stripping and other ter- strategies must be designed to induce P limitation tiary treatment capabilities. The Meuse received in- in large rivers. To develop and test such strategies, creased P in river reaches that already had optimal reliable simulation models must quantify nutrient hydrological features for phytoplankton growth. As inputs, river network features, and biological re- a consequence, algal biomass represented an aver- sponses. age of 58% of the total POC transported in the up- Potamoplankton occupies a key place in the oxy- per Belgian section of the Meuse; similar values have gen budget of a river at least during the growing been reported for other large rivers (Descy and Gos- season. On one hand, algal photosynthesis is a ma- selain 1994). jor source of oxygen production in most large and/ Similar long-term, high nutrient levels (1960 to or lowland rivers. On the other hand, organic mat- present) and flow regulation have also prevailed in ter production in eutrophic rivers in excess of grazer the Ohio River (Seilheimer 1963, Nall 1965, Wehr requirements is principally degraded by heterotro- and Thorp 1997). Under these conditions, it is un- phic following phytoplankton mortality, clear whether hydraulic or chemical factors play the and can lead to severe oxygen depletion. These del- major role in favoring high biomass. The upper Mis- eterious effects may be seen as a result of river-based sissippi also supports large populations of algal spe- power plants, as demonstrated in model simulations cies regarded as typical of eutrophic waters (Micro- of the James River , Virginia (Smith and Jen- cystis aeruginosa, Aphanizomenon flos- aquae, Aulacosei- sen 1974). These studies show how increased eutro- ra granulata, A. italica), and although nutrient levels phication may interfere with industrial uses of river were generally high, there was no evidence that water, affecting algal mortality and physiological standing crop correlated with N or P concentration changes from thermal shock. Further effects include (Lange and Rada 1993, Huff 1986). Authors also chlorine treatment and entrainment, the effects of contend that discharge and temperature were the which are often difficult to sort out within the com- most likely controlling factors of algal biomass plex situations in the field. Finally, eutrophication through navigation dams and channelization in the of large rivers discharging into the sea have also had Mississippi. Such conditions are common in most serious effects on water quality of coastal regions, large rivers of the world and illustrate that even through increases in nutrient loading and changes when long-term nutrient data are available, it is not in nutrient ratios (Rabalais et al. 1996, Billen and always straightforward to make conclusions about Garnier 1997, Lorenz et al. 1997). the primary causes of eutrophication. Effects of exotic invaders. Recent literature has fo- Some eutrophic rivers carry such heavy burdens cused on phytoplankton declines resulting from bi- of N and P that identification of the critical limiting ological invasions in large rivers, as well as lakes. nutrient for regulation by a management agency Such decreases in phytoplankton biomass have been may not be possible. This is because standard nutri- documented from several European and North ent addition bioassays (e.g. Cain and Trainor 1973, American rivers (de Ruyter van Steveninck et al. Gerhart and Likens 1975) will not produce the nec- 1992, Descy 1993, Effler and Siegfried 1994, Gosse- essary growth responses when supplies already ex- lain et al. 1994, Garnier et al. 1995, Ko¨hler 1995), ceed demand. An alternative method has been de- although not all have received a definitive explana- vised for the periodically hypereutrophic Neuse Riv- tion. Zooplankton grazing may play a role in some er (North Carolina), in which ‘‘dilution bioassays’’ cases (Gosselain et al. 1998), although short-lived apply stepwise dilutions of N, P, Fe, and trace metals zooplankton that develop in rivers are generally un- (Paerl and Bowles 1987). Such tests identify the nu- able to control algal biomass over longer periods of trient(s) most responsible for triggering massive al- time (i.e. several weeks). The most studied filter gal growth and help set target nutrient levels that feeders affecting phytoplankton declines is the ze- management agencies may recommend for river sys- bra mussel Dreissena polymorpha, which invaded tems. North American rivers in the early 1990s (Effler et Solutions to reduce nuisance blooms in these very al. 1996, Roditi et al. 1996, Strayer et al. 1996, Cope eutrophic rivers may lie in hydraulic control (i.e. et al. 1997, Gist et al. 1997). In the Hudson River, natural flow conditions) whenever possible but must large populations built up between 1992 and 1994 also involve P reduction. At least a 90% reduction and reduced phytoplankton abundance by a factor in P is required in many large rivers because most of 5 to 10 compared to preinvasion levels (Caraco of the algal species that thrive in large rivers such et al. 1997). Moreover, unlike small planktonic her- as in the Danube, Meuse, and Ohio (e.g. small spe- bivores, zebra mussels apparently feed effectively on cies of Stephanodiscus, Cyclotella, and Chlorella-like most phytoplankton sizes, thereby controlling a green algae) have half-saturation growth constants large fraction of riverine algal biomass. Similar ef- 746 JOHN D. WEHR AND JEAN-PIERRE DESCY fects have been described in the Seneca River, New ification is also usually necessary for the develop- York (Effler et al. 1996), where large densities of ment of Anabaena circinalis blooms. It is interesting mussels also contribute to low oxygen concentra- to note that the model used for simulating the Au- tions. Although perhaps less dramatic, some Euro- lacoseira granulata–Anabaena sp. alternation in the pean rivers have suffered from recent invasions by Murray River comprised a detailed physical submo- Dreissena and other exotic filter feeders (Bachmann del and a simple algal submodel based on algal et al. 1995). What measures should be taken to re- growth (as a function of light) and a buoyancy co- duce the spread of these organisms is not clear, and efficient for the two taxa. Sufficient discharge seems there is a debate about whether they may be limited the best measure to suppress cyanobacterial blooms, by availability of substratum or food (Strayer et al. perhaps along with artificial destratification (Web- 1996). It is reasonable to assume, however, that pol- ster et al. 1996). lution control measures to reduce algal proliferation While the risk of potentially harmful cyanobacter- (e.g. reductions in P inputs) may have at least two ia blooms in large temperate rivers is small, some beneficial effects on these rivers: (1) decreases in systems with high residence times or connected to the intensity and frequency of nuisance blooms and eutrophic ponds or lakes may still be prone to (2) a smaller food supply for zebra mussels. Some bloom development (e.g. River Bure, Moss et al. predictions may be possible through simulation 1984). Nonetheless, it is important to remain con- models, provided that reliable data on filter feeder cerned with the possible expansion of cyanobacter- densities, , and in situ filtering ial species (Coute´ et al. 1997, Padisa´k 1997) and to rates are available. But long-term algal monitoring be able to assess the risk of harmful blooms in fluvial coupled with reductions in nutrient loading is still systems. necessary. Cyanobacteria in rivers. A large contribution of co- ARE POTAMOPLANKTON USEFUL INDICATORS OF THE lonial cyanobacteria to potamoplankton has been ECOLOGICAL STATUS OF RIVERS? most often reported from downstream reaches of In lakes, phytoplankton biomass has been used warm-temperate and tropical rivers, such as the Rio for decades to assess trophic status and to identify Salado, Argentina (O’Farrell 1993), Nagdonk River, artificial eutrophication induced by human activities South Korea (Ha et al. 1998), Neuse River, North (Wetzel 1983, Harper 1992). Moreover, detailed Carolina (Paerl and Bowles 1987), and Darling Riv- studies based on long-term records of phytoplank- er, Australia (Ho¨tzel and Croome 1994). However, ton numbers and taxonomic composition have cyanobacteria blooms comprising potentially toxic shown that, in general, phytoplankton are very sen- strains have been reported periodically in cold tem- sitive indicators of various environmental changes perate rivers. These include the Ohio River over at (Maberly et al. 1994). So far, no such biomass–nu- least three decades (Seilheimer 1963, Nall 1965, Pe- trient indicator system has been successfully applied tersen and Stevenson 1989, Wehr and Thorp 1997) in large or lowland rivers, mainly because longitu- and the upper Mississippi since the 1930s (e.g. Rein- dinal dynamics prevent the reliable use of simple hard 1931, Baker and Baker 1979, Huff 1986). trophic scales based on chlorophyll a. Potamoplank- Clearly, cyanobacterial blooms in large rivers are not ton dynamics respond primarily to physical factors a recent phenomenon: Microcystis blooms have been and may fluctuate considerably in time and space. reported since 1930 in the Potomac River, Maryland Perhaps this is why phytoplankton abundance and (Krogmann et al. 1986). However, slow growth rates biomass data from large rivers frequently do not cor- of many of these organisms likely do not allow them relate strongly with nutrient chemistry, such as in to develop large populations in large rivers unless the River Severn, U.K. (Ruse and Love 1997), or there are relatively high residence times. According- may even correlate negatively, as in the Ohio River ly, in the Rhine and Meuse (Germany and Bel- (Wehr and Thorp 1997). gium), bloom-forming cyanobacteria build large Physical factors, along with nutrients and biotic populations in lower reaches of these rivers, but only interactions (grazing), also influence community for short periods during summer (Ibelings et al. composition, but we are a long way from being able 1998). Interestingly, the 1995 survey in the Rhine to sort out which are the most important determi- indicated that cyanobacteria developed large popu- nants at the species level. Even at the level of major lations in summer in the Untersee, but quickly dis- taxonomic groups, only basic trends can be identi- appeared in the river downstream. A significant por- fied (Reynolds 1994). Thus, our present limited un- tion of the bloom (Planktothrix agardhii, plus small derstanding of processes that determine potamo- Chroococcales) was later observed near Lobith, plankton composition currently prevents manage- ഠ800 km downstream of Untersee. ment agencies from developing predictive tools for The conditions under which cyanobacterial assessing ecological quality based on phytoplankton blooms develop in the Murray River (Australia) have community structure in large rivers. Attempts to use been studied in detail by Bormans et al. (1997). Al- planktonic algae as indicators of river water quality though slow flow (current speed Ͻ0.05 m·sϪ1) has using a community structure approach similar to been put forward as a key factor, temperature strat- methods employed for benthic diatoms and ma- PHYTOPLANKTON AND LARGE RIVER MANAGEMENT 747 croinvertebrates (Plafkin et al. 1989, Lowe and Pan activity of the phytoplankton in the upper Mississippi River. 1996) have so far been unsuccessful (Williams 1964, Freshwater Biol. 9:191–8. Basu, B. K. & Pick, F. R. 1996. Factors regulating phytoplankton 1972, del Giorgio et al. 1991). Studies have shown and zooplankton biomass in temperate rivers. Limnol. Ocean- that changes in phytoplankton composition reflect ogr. 41:1572–7. not only variations in water quality, but also changes Billen, G. & Garnier, J. 1997. The Phison River plume: coastal in physical variables and biotic interactions. Varia- eutrophication in response to changes in land use and water management in the watershed. Aquat. Microbiol. Ecol. 13:3– tions in water chemistry may alter relative propor- 17. tions of a few dominant taxa but often have little Billen, G., Garnier, J. & Hanset, P. 1994. Modelling phytoplank- effect on the overall assemblage. ton development in whole drainage networks: the RIVER- The ability to apply phytoplankton-based assess- STRAHLER model applied to the Seine river system. Hydro- biologia 289:119–37. ments of the ecological status of large and lowland Bormans, M., Maier, H., Burch, M. & Baker, P. 1997. Tempera- rivers lies in a better understanding of the growth ture stratification in the lower River Murray, Australia: impli- rates and requirements of suspended algal species cation for cyanobacterial bloom development. Mar. Freshwater in rivers. As some studies have shown (e.g. Kiss Res. 48:647–54. Butcher, R. W. 1932. Studies on the ecology of rivers. II. The 1994), major changes in a watershed can be detect- microflora of rivers with special reference to the algae on the ed by biomass changes and by alteration of the river bed. Ann. Bot. 46:813–61. growth pattern of certain potamoplankton species. Cain, J. R. & Trainor, F. R. 1973. A bioassay compromise. Phycol- Therefore, a system may be devised that evaluates ogia 12:227–32. ecological change by contrasting the present com- Caraco, N. F., Cole, J. J., Raymond, P. A., Strayer, D. L., Pace, M. L., Findlay, S. E. G. & Fischer, D. T. 1997. in- munity with reference data from unaltered stretches vasion in a large, turbid river: phytoplankton response to in- or time periods. Moreover, as good historical data creased grazing. Ecology 78:588–602. are rarely available, the reference situation may be Carling, P. A. 1992. In-stream hydraulics and sediment transport. simulated through a comprehensive river phyto- In Calow, P. & Petts, G. E. [Eds.] The Rivers Handbook, vol. 1. 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