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Home , Hydrobiology, Nutrient depletion, Particle (ecology), River ecosystem, Stream metabolism, Urbanization

EFFECTS OF ON

Lei Zheng, PhD and Michael J. Paul, PhD

Tetra Tech, Inc.

Abstract

This paper describes the effects of nutrient enrichment on the structure and function of stream ecosystems. It starts with the currently well documented direct effects of nutrient enrichment on algal and the resulting impacts on stream chemistry. The paper continues with an explanation of the less well documented indirect ecological effects of nutrient enrichment on stream structure and function, including effects of excess growth on physical , and alterations to aquatic structure from the microbial assemblage to and mammals. The paper also dicusses effects on the level including changes to , respiration, , carbon and other geochemical cycles. The paper ends by discussing the significance of these direct and indirect effects of nutrient enrichment on designated uses - especially recreational, aquatic life, and drinking .

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1. Introduction

1.1 Stream processes

Streams are all flowing natural , regardless of size. To understand the processes that influence the pattern and character of and reduce natural variation of different streams, several stream classification systems (including ecoregional, fluvial geomorphological, and stream order classification) have been adopted by and national programs. Ecoregional classification is based on , , , dominant uses, and natural vegetation (Omernik 1987). Fluvial geomorphological classification explains stream and slope processes through the application of physical principles. Rosgen (1994) classified stream channels in the into seven major stream types based on morphological characteristics, including entrenchment, gradient, width/depth ratio, and in various land forms. These morphological characteristics affect stream ecosystem processes and community structure and functions. Stream order classification (Strahler 1964) is also widely applied for organizing drainage networks in the United States. These stream classification systems describe and material , which in turn influence physical, chemical, and biological processes.

Another classification scheme is to classify streams based on nutrient conditions (EPA 2001a). EPA divides the country into 14 level III nutrient (Omernik 2000) with common characteristics to better assess background nutrient concentrations in different geographic regions. This classification reflects spatial and geographic variations that influence nutrient concentrations in streams (Rohm et al. 2002, Wickham 2005) and natural background nutrient concentrations should be established for each region (Smith et al. 2003). Dodds (1998, 2006) proposed classifying streams into trophic state classes similar to those developed for and reservoirs (EPA 2001b).

One of the most important processes in streams is nutrient cycling. Stream channels receive nutrients from upstream, terrestrial runoff, ground water, and the atmosphere. The proportion of each source is variable depending on stream geology, elevation, and regional setting. Different landforms (forest vs. agricultural catchments) and spatial and temporal variables also significantly affect nutrient concentrations and loadings into streams (Arheimer and Liden 2000). Internal nutrient cycling also provides nutrients to streams (Mulholland 1996). Stream biota use nutrients and convert them into biomass; thus, nutrients are important to ecosystem structure and function.

Two major nutrients, (N) and (P), occur in streams in various forms as ions or dissolved in solution. Aquatic plants convert dissolved inorganic forms of nitrogen (nitrate, nitrite, and ammonium) and phosphorus (orthophosphate) into organic or particulate forms for use in higher trophic production. The right balance of nitrogen and phosphorus is essential for maintaining natural biological communities and ecosystem functions in aquatic systems. In freshwater systems, phosphorus and nitrogen

3 are limiting nutrients, that is, the levels of these nutrients limit the biological productivity of such systems.

1.2 Limiting nutrients in streams

Stream primary producers, i.e., and macrophytes, absorb natural energy from sunlight to fix carbon and convert inorganic forms of N and P into organic forms through , storing the energy produced in their cells. In most streams, either N or P concentrations or both limit this process. Different algae have been reported to require different N and P concentrations for growth. One study found that require less P (0.3-0.6 µg/L P, Bothwell 1988) to saturate growth than filamentous green algae (25-50 µg/L P, Bothwell 1989). Nitrogen limitation has been reported when ambient N concentration was 55 µg/L in a desert stream in Arizona (Grimm and Fish 1986) and when it was less than 100 µg/L in an Ozark stream (Lohman et al. 1991). Rier and Stevenson (2006) found that algal growth was 90% of maximum rates or higher in nutrient concentrations of 16 µg/L P and 86 µg/L N. The Redfield ratio (molar ratio of 106:16:1 for C:N:P) has been proposed as a community-wide optimum nutrient ratio (Redfield 1958, Borchardt 1996). High ambient or cellular N:P ratios (N:P >20:1) indicate P is limiting growth; low N:P ratios suggest that N is limiting (N:P<10:1). However, levels of nutrient concentrations and ratios for nutrient limitation are also regulated by other abiotic and biotic factors.

Regional differences may determine limiting nutrients for plant growth. Phosphorus used to be and is still considered the sole limiting nutrient in aquatic systems by a number of authors (Huchinson 1957, Correll 1998, Khan and Ansari 2005). With increasing experimental manipulation of nutrient limitation, especially bioassays using nutrient diffusing substrates and artificial streams, N limitation and N and P co-limitation are quite commonly discovered (Grimm and Fisher 1986, Peterson and Grimm 1992). Borchardt (1996) reviewed studies in and concluded that roughly the northern half of the United States is P limited while the Southwest and Missouri Ozarks are N limited. The Pacific Northwest may be limited by both N and P. A meta-analysis of 237 nutrient enrichment studies in temperate streams revealed that 16.5% indicated an N response, 18.1% indicated a P response, 23.2% required N and P be added together for a response, 5% had N or P inhibition, and 43% had no response to N or P (Francoeur 2001). These proportions have been confirmed by a similar literature review (Tank and Dodds 2003).

4 1.3 Eutrophication problems

Eutrophication means “good food”. In freshwater systems, eutrophication is a process whereby waterbodies receive excess inorganic nutrients, especially N and P, which stimulate excessive growth of plants and algae. Eutrophication can happen naturally in the normal succession of some freshwater ecosystems. However, when the nutrient enrichment is due to the activities of humans, sometimes referred to as “cultural eutrophication”, the rate of this natural process is greatly intensified. Eutrophication was recognized as a problem in North American lakes and reservoirs in the mid- 20th century (Rohde 1969). Although has long been recognized as a major problem in streams and (USEPA 2000), the concept of eutrophication has been less commonly used with respect to nutrient enrichment problems in streams (Dodds 1998, 2006).

Nutrient enrichment of streams in the United States is widespread (Carpenter 1998, Correll 1998, Smith et al. 1999, 2006). EPA assessed approximately 840,000 and stream miles nationwide and reported that 10% of assessed rivers and streams had nutrient enrichment problems, which contributed to 30% of reported water-quality problems in the impaired rivers and streams (~ 291,000 miles). (USEPA 2002). Nitrate concentration has more than doubled in the Mississippi River since 1965 and concentrations in many major rivers in the Northeast have increased by from 3- to 10-fold since the early 1900s (see reviewed by Vitousek 1997). Smith et al. (1987) found that at 381 riverine sites in the continental United States, the mean total phosphorus concentration was 130 mg/m3, which is almost double the threshold value for eutrophication (75 mg/m3) proposed by Dodds (1998) for streams.

1.4 Sources of nutrient enrichment: point and nonpoint sources

Nutrient concentrations in streams and rivers have been strongly correlated with human land use and gradients. Both N and P enrichment are linked to agricultural and urban land uses in the watershed. Fluxes of total N in temperate-zone rivers surrounding the North Atlantic are highly correlated with net anthropogenic input of N to their watersheds (Howarth et al. 1996). Total N and nitrate fluxes and concentrations in rivers are also correlated with human density (Cole et al. 1993, Howarth et al. 1996). Nitrogen fertilization is the main source of N in streams and rivers (Goolsby and Battaglin 2001). Similarly, urbanization generally leads to higher phosphorus concentrations in urban catchments (see review by Paul and Meyer 2001). Increasing imperviousness, increased runoff from urbanized surfaces, and increased municipal and industrial discharges all result in increased loadings of nutrients to urban streams. This makes urbanization second only to as the major cause of stream impairment, even though the total area covered by urban land in the United States is minor compared to agricultural area (Paul and Meyer 2001).

Nutrient enrichment of aquatic systems from anthropogenic sources includes point and nonpoint sources (Table 1, adapted from Carpenter et al.1998). Both have degraded aquatic systems. Point sources of nutrients include wastewater (both municipal

5 and industrial) and storm sewer . In contrast to point sources of nutrients that are relatively easy to monitor and regulate, nonpoint sources such as livestock, crop fertilizers, and exhibit more spatial and temporal variability. Following strong regulation of point source inputs in response to the , nutrients from nonpoint sources are now the major source of in the United States (Carpenter et al. 1998).

Table 1. Characteristics of point and nonpoint sources of chemical inputs to receiving waters recognized by statutes of the United States (from Carpenter et al. 1998). • Point Sources -- Wastewater effluent (municipal and industrial) -- Runoff and from waste disposal sites -- Runoff and from animal feedlots -- Runoff from mines, oil fields, unsewered industrial sites -- Storm sewer outfalls from with a population >100,000 -- Overflows of combined storm and sanitary sewers -- Runoff from construction sites >2 ha • Nonpoint Sources -- Runoff from agriculture (including return flow from irrigated agriculture) -- Runoff from pasture and range -- Urban runoff from unsewered and sewered areas with a population <100,000 -- Septic tank leachate and runoff from failed septic systems -- Runoff from construction sites -- Runoff from abandoned mines -- Atmospheric over a water surface -- Activities on land that generate contaminants, such as logging, conversion, construction, and development of land or waterways

Seasonal variability of nutrient loading into streams is dramatic in both agricultural and woodland streams impacted by fertilization. Fertilization of timberlands with nitrogen to increase production is a common practice for the timber industry in the United States. Typically, concentrations of nitrate-N in streams increase rapidly during rainstorms after fertilization in the fall. Peak concentrations have ranged from less than 100 µg/L to greater than 10,000 µg/L (exceeds EPA drinking water level) during high runoff (Anderson 2002). Ambient NO3-N concentrations in forest streams are typically elevated as much as from 2- to 10-fold for the entire winter and following a fall fertilization (Bisson et al. 1992). Stream uptake reduces NO3-N concentrations to background levels by summer in most cases (Mulholland 1992, Mulholland and Rosemond 1992). Then a secondary NO3-N peak is observed in the subsequent fall period, which indicates that applied nitrogen fertilizer remains available for leaching to streams beyond the spring and summer growing seasons. An extreme example of long-term availability is reported at Fernow Experimental Forest (in the Appalachians), where NO3-N remained elevated relative to control streams 10 years after fertilization with ammonium nitrate at 336 Kg N/ha (Edwards et al. 1991).

6 1.5 Consequences of eutrophication

Allan (2004) has summarized the various impacts of land use on streams and rivers. Eutrophication, as one of the main causes of stream impairment in the United States, imposes severe threats to ecosystem structure and function. The direct impact of nutrient enrichment is to increase autotrophic production and change species assemblages including proliferation of filamentous algae. Nutrient enrichment also accelerates breakdown rates by and fungi. The dramatic changes at lower trophic levels may also lead to “trophic cascading”. As nutrient concentrations increase and destabilize the primary producer assemblage and water chemistry, macroinvertebrates and fish may shift from sensitive species to more tolerant, often non-. Changes in the may also cause changes in ecosystem function and further alter stream physical habitat and water chemistry, e.g., decreasing dissolved .

1.6 The objective of this document

The trophic structure of stream ecosystems can be divided into producers and consumers. Primary producers often refers to algae, , ferns, and higher plants. They convert inorganic nutrients into organic forms and are the main energy source of streams. Primary producers are consumed by primary consumers () or after they die, the organic matter is added to the detrital cycle and either decomposed by (bacteria and fungi) or ingested by . Secondary consumers (predators) rely on energy sources from primary consumers. Nutrient enrichment of streams directly changes the nutrient supplies for primary producers, which then affects consumers and nutrient cycling processes, leading to changes in ecosystem structure and function (i.e., eutrophication).

The objective of this document is to provide an understanding of the effects of nutrient enrichment on the structure and function of stream ecosystems. Several authors have documented the relationship between stream enrichment and algal biomass (e.g., New Interstate Water Pollution Control Commission 2001, Virginia Research Center 2006, Dodds 2006). And EPA’s nutrient criteria technical guidance manual for streams also provides background on nutrient impacts. However, the ecological effects of enrichment on stream structure and function are less well discussed. To develop ecologically sound nutrient criteria to protect streams, it is critical to understand how ecological processes that affect stream nutrient dynamics and ecosystem function will change with elevating nutrient pollution.

Although it is important to understand nutrient cycling processes, including nutrient exchange between terrestrial, aquatic, and atmospheric systems (Mulholland 1992, Valett et al. 1994, Boulton et al.1998, McMahon et al. 1994, Arheimer and Liden 2000), they are beyond the scope of discussion for this document. Readers interested in nutrient cycling should see some of the many reviews on this topic (e.g., Stream Solute Workshop 1990).

7 2. Effect on physical habitat and water chemistry

2.1 Physical habitat modification

Nutrient enrichment can substantially change stream habitat. The direct effect of nutrient enrichment in streams is excessive accumulation of filamentous benthic algae during the peak summer growing season, altering flow environment; physical benthic habitat used by stream and vertebrate (Welch et al. 1989, Chessman et al. 1992). Filamentous green algae, such as Cladophora, Ulothrix, and Rhizoclonium, favor nutrient enriched environments. Under suitable enrichment, they can be several meters long in high-velocity areas. When they are aging, they often detach and float near the surface (Power 1990). Vaucheria and Chara can trap to form knolls more than a meter in length in the sandy streams of the north central United States. Chain forming diatoms, such as Terpsinoe, can also form long filaments in flowing water. Cladophora and other filamentous algae are often habitat for small invertebrates, such as chironomids, amphipods, and many meiofauna (see review by Dodds and Gudder 1992).

Periphyton and macrophytes can also alter water velocity in streams. Dodds and Biggs (2002) found that algae attenuate velocity more than macrophytes, and of the different types of algae, dense aggregations of diatoms (primarily Cymbella) attenuate velocity more than filamentous green algae or red algae. Benthic aquatic plants can cause an exponential decline in velocity with depth. In urban channelized streams, Cladophora and Ulothrix attach to concrete , reducing flow rate and capacity. Aquatic plants clogging stream and drainage ditches causes water backup, flooding, and water loss through evaporation from floating or emergent plant surfaces.

Nutrient enrichment can also lead to excessive growth. Excessive planktonic algal growth in slow moving water can reduce light penetration and consequently limit the growth of submerged aquatic plants, decreasing available habitat and shelter for fish and their food organisms (Sand-Jensen et al. 2000). Lack of aquatic plants can also cause and instability of the stream .

2.2 Water chemistry

2.2.1 Dissolved Oxygen (DO)

Nutrient enrichment leads to excessive growth of primary producers as well as heterotrophic bacteria and fungi, which increases the metabolic activities of stream water and may lead to a depletion of dissolved oxygen (Mallin et al. 2006). During the day, photosynthesis by primary producers provides a large amount of oxygen to the water. At night, photosynthesis stops and elevated respiration by algae and bacteria continues to consume dissolved oxygen, which can deplete DO. Furthermore, as primary producers die, they are decomposed by bacteria that consume oxygen. Large of decomposers consume more dissolved oxygen, which increases the severity of DO depletion.

8 DO depletion has been reported in many streams through diel studies but is more frequently studied in lowland, slow moving streams (House and Denison 1997, Kaenel et al. 2000, Sabster et al. 2000, Wilcock and Nagels 2001). Daily oxygen fluctuations in enriched streams at low flow were reported to range from a high of approximately 25 mg/L at noon to a low of approximately 3 mg/L at night (Wong and Clark 1976). Decomposition and microbial activities are likely enhanced during summer when rising temperatures lead to lower DO saturation. Sometimes streams become anoxic (House and Denison 1997). Excessive amounts of nutrients in streams and rivers may also negatively impact the dissolved oxygen levels of downstream receiving waters. For example, a zone of (< 2 mg/L) in the Gulf of Mexico has been linked to high nutrient inputs from the Mississippi River (USEPA 2000, Mallin et al. 2006).

Long-term nutrient enrichment (of streams) leads to long term declines in average DO concentrations (Vagnetti et al. 2003, Parr and Mason 2003, 2004). One long-term study (Parr and Mason 2004) found a significant decline in DO during1955-1998 in the River Brett, England. That stream has shifted from an autotrophic to a heterotrophic system.

The composition of the community can affect the DO response. Wilcock and Nagels (2001) found that streams dominated by submerged macrophytes exhibited the greatest amplitude swings in DO and pH, and led to DO levels of ~ 86-128% saturation. Parr and Mason (2004) estimated that macrophytes accounted for 45% of community respiration at the study site while accounted for 36%, and suggested that removal of plants would restore DO. Kaenel et al. (2000) suggested that an increase in the oxygen concentration after plant cutting would only be transient in unshaded, nutrient-rich streams.

2.2.2 pH

Diel changes in pH often occur in regions with low acid neutralizing capacity and are related to excessive algal growth and stream . The “normal” pH range in streams is from 6 to 9 with 7 being neutral, less than 7 indicating acidic conditions, and greater than 7 indicating basic conditions. During photosynthesis, carbon dioxide (CO2) and water are converted by sunlight into oxygen and carbohydrate. Hydroxyl ions (OH-) are produced, raising the pH. In addition, plants use a large amount of dissolved CO2 for photosynthesis, resulting in lower levels of carbonic acid (H2CO3) in the water column. Thus, photosynthesis increases water column pH. At night, increased respiration from biota increases the release of CO2 into the water, increasing the production of carbonic acid and hydroxyl ions, which, in turn, increases the acidity.

Extremely high or low pH values in streams are harmful to aquatic organisms. For example, high pH levels can be toxic to fish and other organisms. High pH levels damage fish gills, eyes, and skin, and affect fish reproduction. High pH levels also increase the toxicity of some substances, such as ammonia. Low pH levels can make heavy metals in stream sediment more bioavailable (e.g., Al). pH levels also influence the availability of some nutrients, further exacerbating enrichment problems.

9 2.2.3 Other chemicals

Toxic effects of chemicals released from certain have been reported in lakes; very few studies have found in streams. Pfiesteria, a toxic substance produced by dinoflagellates that cause fish kills, has also been reported in coastal rivers associated with nutrient enrichment (Burkholder 1999). A relatively new golden alga, Prymnesium parvum, has been reported to be toxic in Texas. The prymnesin affects -breathing organisms including fish, tadpoles, and (Rhodes and Hubbs 1992) and has been responsible for an estimated 2.5 million dead fish and millions of dead clams in the Pecos, the Colorado, and Brazos river basins in Texas.

Other chemicals can taint drinking water supplies and recreational waters. 2- methylisoborneol and geosmin are two chemicals produced by cyanobacteria that can cause taste and odor problems in drinking water. Livestock that drink water contaminated with cyanobacteria have died (Dodds and Welch 2000). Humans who drink or swim in water that contains high concentrations of from cyanobacteria may experience gastroenteritis, skin irritation, allergic responses, or liver damage (CDC 2004). In 1991, one of the largest recorded riverine blooms of toxic cyanobacteria occurred in the Murray-Darling River Basin in , resulting in a state of emergency being declared to protect water supplies drawn from the river (Oliver et al. 1999).

3. Direct biological responses of streams to eutrophication: primary producers

Although it is generally recognized that nutrient enrichment stimulates algal growth in nutrient limited streams, increased algal productivity may or may not result in accumulation of algal biomass. A number of factors, i.e., light availability, grazer intensity, and physical disturbance, can affect algal biomass accumulation in streams. The fate of biomass produced in aquatic ecosystems follows four different paths (Lamberti 1996): (1) accumulation as a standing crop of algae, (2) respiration to CO2 (decomposition), (3) consumption by herbivores (grazing), and (4) exported to suspended matter. The specific fate of biomass will depend heavily on grazing regimes (Lamberti et al. 1987, 1993). In heavily grazed systems, is either consumed or exported, while reduced grazing pressure allows accumulation of algal biomass and its decomposition.

10 3.1. Responses of algal biomass to nutrient enrichment

A number of authors have documented the positive relationship between benthic algal biomass and nutrient concentrations (see reviews by ENSR 2001, Virginia WRRC 2006, Dodds 2002, 2006). These studies include both field manipulations of nutrient levels to assess benthic algal growth (Bothwell 1989, Walton et al. 1995, Stevenson et al. 2006, Rier and Stevenson 2006) and large scale surveys that investigate relationships between nutrient enrichment and periphyton biomass across different streams (Dodds et al .1997, Biggs and Close 1989, Welch et al. 1992, Biggs 2000, Lohman et al. 1992, Chetelat et al. 1999). These studies established that total N and total P in the water column are significantly related to benthic algal biomass. Empirical models derived from these studies allow one to predict the mean summer chlorophyll a level in a stream based on nutrient concentrations (e.g., Biggs 2000).

The positive relationship between algal biomass and nutrient concentrations is also observed in phytoplankton (Jones et al. 1984, van Nieuwenhuyse and Jones 1996, Basu and Pick 1995, Lohman and Jones 1999). Phytoplankton in streams receive less attention because they are low in and frequency. However, in many slow moving embayments, algal concentrations can be more than 40 times that in the of the river (Reynolds and Descy 1996). Van Nieuwenhuyse and Jones (1996) used data from 292 temperate streams and found that summer mean sestonic (suspended) chlorophyll concentration showed a strong curvilinear relationship with summer mean total phosphorus concentration (TP), although much of that sestonic chlorophyll was likely due to dislodged benthic algae technically called tychoplankton.

A fair number of nutrient enrichment studies in streams find no relationship between algal biomass and nutrient concentrations. Many hypotheses have been proposed to explain these results. One possible hypothesis is that nutrients exceeded the maximum concentration to saturate algal growth (e.g., Munn et al. 1989). However, under most circumstances, a number of other factors, such as hydrologic regime (e.g., Welch et al. 1988, Biggs 1989, 1995), light availability (e.g., Lowe et al. 1986), grazers (e.g., Rosemond 1998) and interactions of these factors (Hill et al. 1995) could have contributed to the lack of correlations between benthic algal biomass and nutrient concentrations. Resources and disturbances are the main factors (Figure 1, Biggs 1996) that influence algal accrual in a stream.

11 Biomass Accrual Biomass Loss High Biomass Erect, stalked Resources and/or filamentous Disturbance taxa Substratum Instability Nutrients Velocity Light Suspended solids Temperature Grazing

Invertebrates Low growing, tightly adhering Fish taxa

Low Biomass Figure 1. Summary of the disturbance- supply-grazer concept for the control of benthic algal development in streams. The relative balance of “biomass accrual” and “biomass loss” is depicted by the width of the triangles that make up the central rectangle. The physiognomy of the community likely to dominate each end of the gradient is also shown. After Biggs 1996.

Hydrological events such as scours significantly reduce the effects of nutrients on periphyton biomass (Welch et al. 1988, Ghosh and Gaur 1994, Bourassa and Cataneo 1998). Biggs (2000) developed a comprehensive model linking hydrologic regime and nutrients to algal accrual. This model was developed using data from rivers and streams across a wide range of land use practices and hydrologic patterns. Although the relationship between nutrients and algal biomass was relatively weak (R2=0.30), including the time of accrual (time since the last scouring ) increased R2 values to approximately 70%, indicating that eutrophication effects are stronger under stable flow conditions.

Grazers significantly reduce the amount of algal biomass in streams (Steinman 1996). Rosemond et al. (1993) manipulated both nutrient and abundances in an experimental mesocosm and found that grazers could moderate or eliminate the observed increase in periphyton biomass with increasing nutrient levels. Other studies also found that increased densities of grazers recruited from nearby areas (Lowe et al. 1986, Welch et al. 1988, Rosemond 1994, Wellnitz et al. 1996, Bourassa and Cataneo 1998, Hillebrand 2002, Roll et al. 2005) could eliminate increases of algal biomass to nutrient additions.

Light is a key factor for algal growth and consequent algal biomass (Hill 1996). Algal biomass can be 4 or 5 times higher in stream segments with open canopies than at sites with more closed canopy cover (Lowe et al. 1986). Light effects increase with trophic state and algal biomass, indicating enhanced importance of light limitation and self-

12 shading at high nutrient supply (Hillebrand 2005). Light and nutrient limitations also interact, so increased light availability or nutrients alone sometimes do not guarantee an elevated biomass. In a nitrogen limited system, increased light level did not increase accumulation of algal biomass without nitrogen additions (Taulbee et al. 2005). Similarly, several studies have found that nutrient addition alone does not stimulate algal biomass accrual unless light availability is also increased (Bernhardt and Likens 2004, Mallory and Richardson 2005). These results demonstrate that for essential resources such as light and nutrients, the magnitude of the response to enrichment by one resource depends on the relative availability of another.

Interactions of light, nutrients, and grazers may confound the relationship between nutrients and algal biomass in streams. In a meta-analysis of experimental manipulation studies, Hillebrand (2005) found that increased light (light enhancement) increases grazer effect size, whereas grazer presence reduces light effects, which indicates that high light favors algal growth but increased algal biomass is easily offset by increased grazer densities. Similarly, Hill et al. (1995) found light-enhanced primary productivity was reflected by increased grazer densities instead of increased algal biomass.

3.2. Responses of algal species composition to nutrient enrichment

Algal species composition changes with elevated nutrient concentrations (Stevenson 1996, Pan et al. 1996, Stevenson and Smol 2001). Because of their small scale, periphytic algae composition receives less public attention, while problematic macroalgae (e.g., Cladophora) and cyanobacteria receive more. Under most circumstances, a dominated algal community represents healthy, non-enriched stream , while a predominance of filamentous algae may indicate problems with nutrient enrichment. Since algae are often the problem associated with enrichment, a change of taxonomic composition in a stream can show whether nuisance algae are present and can indicate long or short-term changes in point and nonpoint source pollution (Lowe and Pan 1996) that cannot be detected by a one-time sampling of water chemistry. Thus, algal species composition could be considered an important indicator of nutrient pollution.

Diatom species composition is increasingly used as an indicator of environmental conditions, especially nutrient enrichment in streams (Stevenson 1999). European scientists initially used diatoms as indicators of organic pollution and developed tolerance values (e.g., Lange-Bertalot 1979) that were adapted in the United States (Lowe 1974, Bahls 1992, Stevenson and Bahls 1999, KDOW 2002). Recent studies have used a variety of algal attributes to develop biological indices (Hill et al. 2000, 2003, Fore and Grafe 2002, Wang et al. 2005). Diatom indicators are most sensitive to nutrient enrichment, and several authors (van et al. 1994, Kelly and Whitton 1995, Kelly et al. 1998, Winter and Duthie 2000) have reported this sensitivity. A variety of studies have also developed diatom autoecological preferences for nutrients using weighted average regression techniques like those used in paleoecology (Pan and Stevenson 1996, Potapova et al. 2004, Potapova and Charles 2005).

3.3 Responses of macrophytes to nutrient enrichment

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The definition of macrophytes varies. According to Wetzel (1975), macrophytes represent a taxonomically diverse group of aquatic plants including vascular plants, , ferns, and macroalgae. However, people generally exclude macroalgae from this group. Most macrophytes (i.e., emergent, floating-leaved, and submerged groups) are rooted in the sediments so they can persist in current flow and absorb nutrients from sediments. Rooted plants live in a more predictable light climate but may experience severe self-shading and generally grow more slowly than algae. The nutrient requirements of rooted macrophytes are lower than those of microalgae because of their low growth rates, high internal C:N:P ratios, and nutrient conserving mechanisms. Nutrient limitation is less important because the plants exploit rich nutrient pools in the sediment (Sandjensen and Borum 1991). One group of macrophytes (e.g., duckweeds), however, are unattached and rely entirely on nutrients from the water column. They are easily affected by current and wind, and are most frequently found in backwaters.

Nutrient effects on macrophytes are poorly studied (Chambers et al. 1999). Several factors have constrained this research. First, light is the most crucial factor regulating species composition and distribution of macrophytes in streams (Chambers and Kalff 1985). Light availability is easily affected by water clarity, riparian canopy cover, and water depth. between phytoplankton and periphyton for nutrients and light may also reduce the direct effects of nutrient enrichment (Sandjensen and Borum 1991). Second, macrophytes mostly use nutrients from sediments. Any nutrient loadings in stream water have to be incorporated into the sediments before being available to the plants. However, nutrient supply can affect plant attributes.

Most studies of nutrient enrichment on plants focus on wastewater effluent. Nutrient enrichment in streams and rivers leads to increasing plant biomass (Chambers and Prepas 1994, Gucker et al. 2006), declines in plant richness (Thiebaut and Muller 1998, San- Jensen et al. 2000), and increases in plant tissue phosphorus (Thiebaut and Muller 2003). Rattray et al. (1991) found that the size and tissue phosphorus content of plants grown on eutrophic sediments was approximately twice that of those grown on oligotrophic sediments, indicating that nutrient supply strongly affects plant stoichiometry. Removal of aquatic plant biomass with its accumulated phosphorus has been proposed as a eutrophication control strategy (Thiebaut and Muller 2003). Reduction of nutrient (particularly N) input from municipal wastewater sources led to macrophyte biomass declines in the Bow River (Alberta) (Sosiak 2002). Other authors have (e.g., Schneider and Melzer 2003) developed plant based trophic indices (Trophic Index of Macrophytes) based on the concentrations of soluble reactive phosphorus in both the water column and the sediment pores of streams in .

14 4. Indirect biological responses of streams to eutrophication: microbial cycling

In heterogeneous stream reaches, microbial biomass is patchily distributed and is controlled by a number of factors, including light, substrate carbon availability, temperature, nutrients, and current velocity. The main components of stream microbial biomass are bacteria and fungi. Fungi dominate large substrates such as leaves and wood, while bacteria dominate fine organic substrate such as sand (Findley et al. 2002). The organisms are important components of stream food webs and play a key role in carbon cycling.

4.1 Bacteria

Similar to algae, bacteria are also limited by nutrients in aquatic systems, especially in planktonic forms (Cole 1982). Bacteria can outcompete algae for nutrients because of their higher surface area to volume ratio (Fuhs et al. 1972, Currie and Kalff 1984a). Bacterioplankton have substantially higher phosphorus requirements, higher phosphorus contents, and higher net consumption of phosphorus than phytoplankton (Wetzel 2001). Therefore, bacterioplankton can outcompete algae for phosphorus under a wide range of phosphorus supply rates.

Stream periphytic are mainly composed of algae and bacteria. Bacteria can either inhibit algae by outcompeting it when nutrients are limited (Currie and Kalf 1984b, Grover 2000, Biddanda 2001), or they may interact positively with algae by using its photosynthetic products and decomposing dead plant and algal biomass and nutrients. A number of studies (Geesey et al. 1978, Hudson et al. 1992, Hepinstall and Fuller 1994, Rier and Stevenson 2001, Carr et al. 2005) have demonstrated positive interactions between algae and bacteria in periphyton, though caution should be used when interpreting these correlations due to the complexity of the environment (Findlay et al. 1993, Bott 1996). Some (Rier and Stevenson 2002) speculate the positive correlation between bacteria and algae in biofilms to be a result of the co-dependence between the two for space.

Nutrient enrichment tends to increase both algal and bacteria biomass (Carr et al. 2005). Sobczak (1996) found that interactions between bacteria and algae are weakened in the presence of a labile source of allochthonous DOC, under extreme light limitation, or under extremely oligotrophic conditions where algae are severely nutrient limited. Similarly, Rier & Stevenson (2001) found only a positive statistical relationship between algae and bacteria in streams when streams with periphyton chlorophyll a values greater than 5 µg cm−2 were included in the analysis. Nutrient addition generally increases the coupling of algae and bacteria biomass (Rier and Stevenson, 2001, 2002, Carr et al. 2005).

Changes in bacteria taxonomic composition and abundance in response to nutrient enrichment are less studied (see Findley et al. 2003, Olapade and Leff 2005). Changes in taxonomic abundance are highly dependent on seasons, level of DOM, and interactions between nutrients and DOM. However, certain bacteria are sensitive to nutrient

15 enrichment and excessive growth on sensitive has been proposed as an indicator of nutrient enrichment (Lemly 1998, 2000).

4.2 Fungi

Similar to bacteria, fungi also play an important role in detrital decomposition in streams. Fungal communities in many streams are also limited by nutrients (Grattan and Suberkropp 2001, Tank and Dodds 2003). This limitation can be released by nutrient additions that lead to significantly higher fungal biomass. Experimental enrichment increased aquatic hyphomycete conidia in the water of a treated stream by 4 to 7 times more than the controlled streams (Gulis and Suberkropp 2004). Other effects observed in the same study included increased number of fungal species detected on each sampling date, and changes in patterns and relative abundances of individual species. Nutrient pollution may also change aquatic hyphomycete diversity and sporulation (Pascoal et al. 2005b).

Bacteria and fungi also compete with each other for nutrients (Gulis and Suberkropp 2003b). Gulis and Suberkropp (2003b) found that fungi inhibit bacterial growth and reduce bacterial biomass by 2-fold at low nutrient concentrations, suggesting that nutrient availability may modify microbial interactions. Fungi seem to be a superior competitor than bacteria on leaves. Fungal biomass can be one or two order of magnitudes higher than bacterial biomass in polluted streams (Gulis and Suberkropp 2003a, Pascoal et al. 2005a).

5. Indirect biological responses of streams to eutrophication: herbivores

Nutrient enrichment accelerates autotrophic production and algal biomass in streams, and consequently changes ecosystem structure at other trophic levels. Long-term (16 years) stream fertilization (P addition) in an arctic stream ecosystem resulted in a dramatic change in the community structure with positive response to fertilization at all trophic levels (e.g., increases in epilithic algal stocks, densities, and fish growth). Both top-down and bottom-up control of different trophic levels have been observed in stream ecosystems (Shurin et al. 2002) and has led to development of “” models for streams (Power 1992).

5.1 Invertebrates

A large number of observations and experimental manipulations have shown that invertebrates are food limited in streams (Hill 1992, Rosemond et al. 1993, Biggs and Lowe 1994, Lamerti 1996, Biggs et al. 2000). These studies examined the effect of algal biomass on growth of invertebrates and found that abundance could be strongly stimulated by increasing algal availability. Nutrient enrichment, in many cases, did not increase the total biomass of algae in streams; rather the energy was converted into increased invertebrate density through herbivory (Hill et al. 1995).

16 Changes in macroinvertebrate composition with nutrient enrichment are more complicated than changes of abundance. , a group of invertebrates that are considered sensitive to environmental , show highest relative abundance when algal biomass is at intermediate levels (Miltner and Rankin 1998). The abundance of scrapers, a functional group that is closely related to grazers, is highest when nutrient levels are elevated, indicating positive effects from increased algal availability (Miltner and Rankin 1998). Similarly, scrapers (e.g., Ancylus fluviatilis) and detritivores (e.g., Oligochaeta, Lumbriculidae) have shown significant increases in density or biomass on certain substrata with enrichment even while total macroinvertebrate density or biomass did not (Sabater et al. 2005). Benthic invertebrate composition has also been shown to shift from chironomid-amphipod to an oligochaete-gastropod dominated assemblage in response to decreases in DIN and changes in benthic algal abundance and sediment organic carbon concentrations (Chambers et al. 2006).

Enrichment may also alter benthic habitat for macroinvertebrates. In addition to food sources for invertebrates, benthic algae, especially macroalgae, are important habitat for macroinvertebrates (Dudley et al. 1986). Some algal species or growth forms are grazer- resistant (e.g., Oedogonium spp.) and are good habitat for many invertebrates (Steinman et al. 1992). For example, long-term additions of P in an arctic stream resulted in an increase in moss density. The increase in physical habitat associated with this shift was responsible for changes in the macroinvertebrate community (Lee and Hershey 2000).

5.2 Fish

Fish may benefit from increases in food availability when nutrient additions increase primary and secondary production. Enrichment of oligotrophic streams and rivers may result in increased algal biomass, increased benthic invertebrates, and . Long-term enrichment studies in Vancouver Island, British Columbia (e.g., Slaney and Ward 1993, Slaney and Ashley 1997) reported increased fish size up to 2 times. Similar studies (Deegan and Peterson 1992, Slavik 2004) also observed increased fish growth rates in long-term fertilization studies in rivers. However, it is difficult to assess how much of the primary production from nutrient enrichment flows through to fish, and the effect of nutrient addition on fish is largely unpredictable. van Dam et al. (2002) estimate that periphyton ingestion by many herbivorous and omnivorous fish ranges from 0.24 to 112 mg (³dmg fish)-1 d-1 in a fishpond.

One of the consequences of nutrient enrichment may be loss of sensitive fish taxa and increases in tolerant taxa. The strong correlation between fish metrics and nutrient pollution (Miltner and Rankin 1998) indicates that nutrient enrichment has contributed to changes in the structure of fish assemblages. While nutrient enrichment could potentially benefit fish production in the short term, the ecological consequence of nutrient addition could have severe impacts on stream ecosystems (Stockner et al. 2000). The obvious impact at high nutrient loads is the reduction in DO, which would exclude many sensitive taxa. In addition, excess algal growth would eliminate important feeding and respiration habitat, further reducing survivorship. While it is evident that some nutrient subsidy

17 benefits the growth of select species, the overall impact is negative, especially at stressful nutrient levels.

5.3 Food web structure

Carpenter et al. (1985) adapted the “cascade of effects” concept developed from top- down marine food webs to lakes and described it as “cascading trophic interactions”. They posited that each is controlled by both predators (top-down control) and resources (bottom-up control). Changes at one trophic level would alter material cycling and other trophic levels in the food web (trophic cascading). Several authors have discussed trophic cascading in streams (e.g., Lamberti 1996, Biggs 2000).

Long-term fertilization studies have demonstrated the cascading effect of nutrient enrichment at several trophic levels. Huntsman (1948) first recognized that fertilizers stimulate downstream algal growth, and lead to increased insect and fish densities. Since then, more quantitative studies (Peterson et al. 1993, Slaney and Ashley 1998) have shown that nutrient additions increase algal biomass at least at the beginning of the enrichment. Later, top-down forces take effect to control primary consumers and consequently algal biomass. Generally, grazing has demonstrated a larger effect than resource limitation in influencing algal biomass and composition (Steiman 1996, Lamberti 1996, Flecker et al. 2002). However, interactions among different trophic levels could be regulated by many factors, which sometimes lead to unexpected responses to nutrient additions at higher trophic levels (Deegan et al. 1997). In either case, plant- herbivore interactions are considered central to food web structure and in aquatic ecosystems (Lamberti 1996).

While nutrient additions affect higher trophic levels, predators also play an important role in influencing nutrient demand and nutrient supply. Flecker et al. (2002) examined nutrient limitation in the presence and absence of fishes and found that the response to nitrogen enrichment is significantly greater on substrates accessible to natural fish assemblages compared to substrates where grazing fishes are excluded. Many experiments (e.g., Biggs 2000) demonstrate simultaneous and interactive effects of top- down and bottom-up factors in limiting primary producers in streams.

6. Responses of ecosystem function to nutrient enrichment

6.1 Primary production and respiration

Production- Three empirical models have been developed to simulate algal nutrient uptake kinetics: the Michaelis-Menten model, the Monod model, and the Droop model (Borchardt 1996). All three models are similar to each other in form. The Michaelis- Menten model emphasizes nutrient uptake kinetics (Dugdale 1967). The Monod model examines algal specific growth rate along external nutrient concentration (Monod 1950), while the Droop model takes into account intracellular concentration of the limiting nutrient (Droop 1968). These models postulate that algae accrual follows a logistic growth rate and reaches maximum saturation at some concentration.

18

The Monod equation has been applied in a number of studies to investigate the nutrient- algal production relationship (Rier and Stevenson 2006). The equation is as follows: µ=µmax *S/(Kµ+S), where µ is the specific growth rate, µmax is the maximum specific growth rate, Kµ is the half-saturation constant for growth for the limiting nutrient, and S is the external nutrient concentration (µmole/L).

Experimental approaches have confirmed that periphytic algal growth increases along nutrient gradients (Horner et al. 1983, Bothwell 1989, Walton et al. 1995, Rier and Stevenson 2006, Stevenson et al. 2006). However, while algal biomass accumulation is a reflection of algal production, maximum algal production does not lead to maximum areal biomass. Bothwell (1989) examined both cellular growth rates and maximum areal biomass, and demonstrated that the amount of P needed to saturate cellular growth rates during early stages of colonization was two orders of magnitude lower than concentrations needed to produce maximum areal biomass. Rier and Stevenson (2006) manipulated nutrient concentrations to examine the effect on the growth rate of periphyton in artificial streams, and found that saturating concentrations for algal growth rates were 3 to 5 times lower than concentrations needed to produce maximum biomass. These growth patterns can be fit in to modified Monod models, demonstrating the exponential growth of algal biomass along nutrient gradients.

Correlation analysis between nutrient concentrations and algal biomass and algal productivity in streams also indicate that nutrient enrichment enhances algal production (Dodds et al. 2002, see review by Dodds 2006). Dodds (2006) plotted algal production against biomass using Bott et al.’s (1985) compiled data from the literature and found a positive correlation between algal primary production and algal biomass in streams, though only 24% of the total variance was explained. Field observations are consistent with experimental manipulations of nutrient enrichment and algal production. In addition to algal production, macrophyte production is also stimulated by nutrient enrichment (Gucher and Brauns 2006).

Respiration- It is generally accepted that higher production leads to higher respiration. Dodds (2006) demonstrated that production and respiration by algae in streams are strongly correlated with each other with a correlation coefficient close to 0.90. Similarly, nutrient enrichment increases bacterial and fungal production, which in turn increases aerobic respiration from these decomposers. Thus, the total respiration in enriched streams is higher than production. High respiration:production ratios lead to high biological oxygen demand (BOD) in downstream water (in nutrient enriched streams). Thus, nutrient enrichment will generally lead to elevated gross primary production and whole-stream community respiration, and decreased stream DO concentrations (Chambers and Prepas 1994, Gucker and Brauns 2006).

A good example is blackwater streams in the eastern United States. Studies of nutrient loading on phytoplankton, bacterioplankton, and respiration in blackwater streams indicate that changes in nutrient loading stimulate two different biological pathways

19 (photosynthetic and heterotrophic activity) (Mallin et al. 2004). Nitrogen additions increase chlorophyll a production and significantly stimulate BOD. Combined organic- inorganic phosphorus additions significantly stimulate bacterial abundance, ATP, and BOD on most occasions. In blackwater streams, nitrogen inputs stimulate phytoplankton growth, which in turn dies and decomposes in deeper, higher order streams, becoming a source of BOD and lowering DO. Phosphorus inputs directly stimulate bacterial growth, increase BOD, and lower stream DO concentrations. In some circumstances, hypoxia is expected (Mallin et al. 2006).

6.2 Secondary production and predators

Generally, secondary producers (consumers) in streams are food limited, and their production and biomass are expected to increase in streams with higher primary production due to N and P enrichment. Elwood et al. (1981) found that the initial response to stream nutrient enrichment in a Tennessee woodland stream was increased algal standing crop, which was then consumed by a large increase in grazer abundances. Continuous enrichment of P-limited streams on Vancouver Island, British Columbia led to substantial increases in secondary producers (Perrin et al. 1987, Slaney and Ward 1993). Benthic invertebrate biomass increased by from 2 to 7-fold and fish size by from 1.4 to 2-fold (Slaney and Ward 1993). Macroinvertebrates also ingest bacterial production (Fuller et al. 2004). Benthic invertebrate biomass increased 4.5-fold and fish (cutthroat trout) increased 6.3-fold in a stream enriched by carbon that elevated bacterial biomass (Warren et al. 1964). In another study, enrichment did not cause a general increase in macroinvertebrate density or biomass, but altered assemblage composition in the enriched reach (e.g., Sabater et al. 2005).

Secondary production clearly responds to enrichment and the response may be more clear in oligotrophic than eutrophic streams. Fish, especially herbivores, can obviously benefit from increasing biomass of primary producers. Carnivorous or omnivorous vertebrates also prosper from the increased biomass of primary consumers (e.g., Deegan and Peterson 1992). A recent study (Johnson and Wallace 2005, Johnson et al. 2006) on the growth of larvae, top predators in southern Appalachian streams, indicated that nutrient enrichment stimulates salamander larval production by increasing detrital quality and quantity. Although some enrichment may benefit fish production, a transition region in enrichment from beneficial to detrimental effects has not been defined to the extent that it has for lakes and reservoirs (Welch 1992). It probably exists for different physical types of streams and rivers. Two recent stream studies have provided independent estimates of target nutrient concentrations that should be maintained in order to ensure acceptable water quality needed for fish growth if fish growth is the primary concern (Stockner et al. 2001, Compton et al. 2005).

20 6.3 Decomposition rates

Microbial decomposition rates are affected by a number of environmental factors, i.e., current velocity, feeding by detritivores, sedimentation, and nutrient enrichment. Microbial decomposition rates are strongly affected by fungal and bacterial productivity and their relative contributions to total microbial biomass (Hieber and Gessner 2002). Findlay et al. (2002) found that bacterial abundance was higher on fine particles, while fungal biomass was significantly greater on larger size classes, i.e., on leaves and wood. The relative contribution of reach-scale fungal biomass ranged from 10 to 90% of microbial biomass in this study depending on the quantity of leaves and small wood in the streams. In streams with abundant leaves, fungi dominate the total microbial biomass (98.4 to 99.8%) and cumulative production (97.3 and 96.5%) (Gulis and Suberkropp 2003a). Nutrient enrichment generally increases bacterial and fungal productivity and biomass, but does not change the roles of each assemblage. Gulis and Suberkropp (2003a) found that the fungal yield coefficient exceeded that of bacteria by a factor of 36 and 27 in low- and high-nutrient treatments, respectively.

Nutrient enrichment significantly increases decomposition rate, microbial respiration, fungal and bacterial biomass, and the sporulation rate of aquatic hyphomycetes associated with decomposing leaf material (Gulis and Suberkropp 2003c). Comparisons between one reference site and two downstream polluted sites with high nutrient concentrations in the Ave River, Portugal indicated that bacterial production was greater at the two polluted sites than at the reference sites, while highest fungal biomass and production corresponded to the fastest leaf breakdown. Other studies also support the contribution of fungi and bacteria to leaf litter breakdown and the enrichment of microbial activity by nutrients (Suberkropp 1995, Weyers and Suberkropp 1996, Royer and Minshall 2001, Gulis and Suberkropp 2003a, b, c).

In addition to microbial biomass in streams, decomposition rates can also be regulated by other factors, especially the presence of detritivores. Sponseller and Benfield (2001) found that shredder presence and abundance was critical to leaf breakdown rate in Appalachian headwater streams. Increased sedimentation from agricultural input in some streams may limit the distribution of shredders and thus influence leaf breakdown in these streams. Similarly, Pascoal et al. (2005a) attributed low leaf breakdown rates to low shredder densities in a polluted river.

6.4 Other functional responses

Carbon cycling-Carbon is not considered limiting for periphyton growth in streams because CO2 can be dissolved in water and provides sufficient inorganic carbon for use in algal production. However, bacteria are limited by carbon in many streams with relatively low DOC inputs. Increases in primary production due to nutrient enrichment increase DOC and the coupling of periphyton and bacteria since bacteria will rely more heavily on organic carbon sources produced by algae and other plants. Microbial decomposition further increases sources. Thus, nutrient enrichment will enhance carbon cycling, especially in slow moving streams.

21 Nitrogen fixation- Nitrogen fixation by algae and bacteria may be affected by nutrient additions and changes in N:P ratios. Nitrogen fixation by cyanobacteria and other microbes is favored by slow moving or standing water, high temperatures, low DIN and high DON (Dodds 1995). Nitrogen fixation can provide a substantial amount of N to a stream depleted of nitrogen (Grimm and Petrone 1997). Although not much information is available about nutrient enrichment on N fixation in streams, studies in lakes indicate that decreases in the N:P ratio will generally lead to increases in N fixation rates by cyanobacteria (Smith 1990, Hendzel et al. 1994). Grimm and Petrone (1997) found that biomass specific N2 fixation was positively correlated with temperature and light, and negatively correlated with dissolved inorganic nitrogen.

Alkaline phosphatase activity –Aquatic primary producers, especially algae, can excrete an enzyme called alkaline phosphatase in response to P deficiency in the water. Various studies (e.g., Peck et al. 2006) have used the concentration of alkaline phosphatase in the water column and in periphyton as an indicator of P limitation. It is considered the most precise indicator of P limitation over epilithic N:P ratios and algal growth on nutrient- diffusing substrates (Bowman et al. 2005). It is speculated that phosphatase levels of approximately 0.003 mmol/mg chl per hour indicate moderate P limitation, whereas phosphatase levels above 0.005 mmol/mg chl per hour indicate severe P limitation (Steinman and Mulholland 1996).

Silica limitation- Silica limitation is also reported in some streams. Silica is an important component of diatom frustules. A study in eutrophic Okeechobee, Florida indicated that along with N limitation, silica also limited microalgal assemblages dominated by diatoms (Zimba 1998). In streams dominated by diatoms, silica concentrations tended to decrease downstream, implying that diatoms absorb silica from the water (Wall et al. 1998). In some cases, nutrient enrichment generally increases the production rate of green filamentous algae while diatom production decreases. It is possible that diatoms are limited by silica concentrations in these streams. The extent of this limitation is poorly understood in streams.

6.5. Nutrient enrichment impacts on designated uses

Nutrient enrichment may directly lead to excessive increases in algae and other primary producers. The indirect effects of enrichment are to increase stream production, change food web structures, and consequently alter ecosystem function. The deleterious impacts of these structural and functional changes may impair aquatic life uses, limit drinking water resources, and degrade recreational and aesthetic uses of waters. Various impacts of nutrient enrichment in streams have been summarized in Table 2 below.

22

Table 2. Effect of nutrient enrichment on designated uses of streams. Excessive nutrient levels allow excessive increases in algae and other primary producers, which, in turn, prevent streams from meeting their designated uses. The adverse effects of either high nutrient levels or the nuisance growth of primary producers include, for example: • Impairment of aquatic life uses -- Diel fluctuation of oxygen concentrations and pH values in streams may negatively impact aquatic life (fish and invertebrates). -- Ammonia toxicity (e.g., at a high level > 1 mg/L) may be derived from high nitrogen concentrations. -- Overgrowth of many algae can lead to algal blooms, several toxins from which have been found to kill fish and other aquatic life. -- Indirect effect of excess growth on physical habitat availability. -- Enrichment may lead to loss of diversity and native taxa, changes in biological community structures (algae, aquatic plant, invertebrate, and fish), and eventually loss of ecosystem function. • Negative impact on water resources -- High nitrate concentration (>10 mg/L) is toxic in drinking water. -- 2-methylisoborneol and geosmin are two chemicals produced by cyanobacteria that can cause taste and odor problems in drinking water. -- Diatoms and filamentous algae can clog intake screens and filters in plants. -- Decay of algae may lead to taste and odor problems of drinking water. -- Potentially carcinogenic disinfection by-products (trihalomethanes, THMs) may form during treatment of drinking water from eutrophic waters. • Degradation of aesthetic and recreational uses -- Many algae can form large clogs of mats either floating (Lyngbya, Oscillatoria, Hydrodictyon, Spirogyra) or attached (e.g., Cladophora and Ulothrix) to substrata, which are unappealing to swimming, fishing, and boating. -- Large odorous algal masses die off and decay after the growing season, and their presence inhibits recreation and human health. -- Cyanobacteria, such as Oscillatoria or Lyngbya wollei, can produce chemical compounds that irritate swimmers. -- Fish grown in waters with algal-derived chemicals can suffer impacts on flavor. -- Skin rashes, nasal irritation, or other health effects may result from skin contact with algal toxins. -- Slippery streams are a threat to swimmers. -- Cladophora abundance may slow water flow in canals and inhibit navigability.

23 7. Control of eutrophication: best management practices

Nutrient enrichment poses serious threats to stream ecosystems. Managing nutrient loading into streams will reduce not only the magnitude of maximum algal biomass, but also the frequency and duration of benthic algal problems in streams (Biggs 2000). To better protect and restore streams, control of both point and nonpoint sources of nutrient loadings into streams is essential. Currently, many states have proposed plans to develop nutrient criteria to control nutrient loading into streams. Control of point sources, such as treated wastewater, can be improved with new technology (Arheimer et al. 2004). Still a persistent problem for implementation of criteria will be control of nonpoint sources. It will require innovative technologies and better understanding of stream ecosystems to decrease nutrient loadings from nonpoint sources into streams. Best management practices should be implemented including riparian buffer and wetland protection, and smart use of fertilizers in agricultural and silviculture. New technologies are contributing to some improvement in nutrient pollution from nonpoint sources (Arheimer et al. 2004). More cost-effective practices should be developed to better fulfill this goal.

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