EFFECTS OF EUTROPHICATION ON 

STREAM ECOSYSTEMS 

Lei Zheng, PhD and Michael J. Paul, PhD 

Tetra Tech, Inc. 
2

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 biomass 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 habitat, and alterations to aquatic life community structure from the microbial 

assemblage to fish and mammals. The paper also dicusses effects on the ecosystem level 

including changes to productivity, respiration, decomposition, 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 water. 
3

1. Introduction 

1.1 Stream processes 

Streams are all flowing natural waters, regardless of size. To understand the processes 

that influence the pattern and character of streams and reduce natural variation of 

different streams, several stream classification systems (including ecoregional, fluvial 

geomorphological, and stream order classification) have been adopted by state and 

national programs. Ecoregional classification is based on geology, soils, geomorphology, 

dominant land 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 United States into seven 

major stream types based on morphological characteristics, including entrenchment, 

gradient, width/depth ratio, and sinuosity 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 hydrology 

and material transport, 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 ecoregions (Omernik 2000) 

with common land use 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 lakes 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, nitrogen (N) and phosphorus (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 
4

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., algae and macrophytes, absorb natural energy from 

sunlight to fix carbon and convert inorganic forms of N and P into organic forms through 

photosynthesis, 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 diatoms 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 North America 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). 
5

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 pollution problem in North American lakes and reservoirs in the mid20th century (Rohde 1969). Although nutrient pollution has long been recognized as a 

major problem in streams and rivers (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 river 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 disturbance 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 Ocean 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 population 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 agriculture 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 effluent (both municipal 
6

and industrial) and storm sewer discharge. In contrast to point sources of nutrients that 

are relatively easy to monitor and regulate, nonpoint sources such as livestock, crop 

fertilizers, and urban runoff exhibit more spatial and temporal variability. Following 

strong regulation of point source inputs in response to the Clean Water Act, nutrients 

from nonpoint sources are now the major source of water pollution 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 leachate from waste disposal sites 

-- Runoff and infiltration from animal feedlots 

-- Runoff from mines, oil fields, unsewered industrial sites 

-- Storm sewer outfalls from cities 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 deposition over a water surface 

-- Activities on land that generate contaminants, such as logging, wetland 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 spring 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).
7

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 litter 

breakdown rates by bacteria 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-native species. Changes in the food 

web may also cause changes in ecosystem function and further alter stream physical 

habitat and water chemistry, e.g., decreasing dissolved oxygen. 

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, moss, 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 (herbivores) or after they die, the organic 

matter is added to the detrital cycle and either decomposed by decomposers (bacteria and 

fungi) or ingested by detritivores. 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 

England Interstate Water Pollution Control Commission 2001, Virginia Water Resources 

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). 
8

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 invertebrates and vertebrate organisms (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 sediments 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 current 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 irrigation canals, 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 phytoplankton 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 erosion and instability of the stream channel. 

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 populations of 

decomposers consume more dissolved oxygen, which increases the severity of DO 

depletion. 

 
9

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 hypoxia (< 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 aquatic plant 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 sediment 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 metabolism. 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 water column 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. 
10

2.2.3 Other chemicals 

Toxic effects of chemicals released from certain cyanobacteria have been reported in 

lakes; very few studies have found cyanotoxins 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 toxin prymnesin affects 

gill-breathing organisms including fish, tadpoles, and clams (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 toxins 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 Australia, 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 periphyton 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, primary production is either consumed or 

exported, while reduced grazing pressure allows accumulation of algal biomass and its 

decomposition. 
11

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 abundance and frequency. However, in many slow moving 

embayments, algal concentrations can be more than 40 times that in the main stem 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. 
12

Figure 1. Summary of the disturbance-resource 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 New Zealand 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 flood) 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 herbivore 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 selfHigh Biomass 

Low Biomass 

Biomass Accrual Biomass Loss 

Grazing 

Disturbance 

Fish 

Substratum Instability 

Velocity 

Suspended solids 

Invertebrates 

Temperature 

Resources 

Nutrients 

Light 

Low growing, 

tightly adhering 

taxa 

Erect, stalked 

and/or filamentous 

taxa 
13

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 diatom 

dominated algal community represents healthy, non-enriched stream water quality, 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 Dam 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 
14

The definition of macrophytes varies. According to Wetzel (1975), macrophytes 

represent a taxonomically diverse group of aquatic plants including vascular plants, 

mosses, 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. Competition 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, SanJensen 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 Germany. 
15

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 biofilms 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 recycling 

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 
16

enrichment and excessive growth on sensitive insects 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 dominance 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, insect 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 “trophic cascade” 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 invertebrate 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). 
17

Changes in macroinvertebrate composition with nutrient enrichment are more 

complicated than changes of abundance. Mayflies, a group of invertebrates that are 

considered sensitive to environmental pollutants, 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 grazerresistant (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 fishes. 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 
18

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 topdown marine food webs to lakes and described it as “cascading trophic interactions”. 

They posited that each trophic level 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, plantherbivore interactions are considered central to food web structure and energy flow 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 topdown 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 MichaelisMenten 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. 
19

The Monod equation has been applied in a number of studies to investigate the nutrientalgal 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 
20

(photosynthetic and heterotrophic activity) (Mallin et al. 2004). Nitrogen additions 

increase chlorophyll a production and significantly stimulate BOD. Combined organicinorganic 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 salamander 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). 
21

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 particle 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 dissolved organic carbon sources. Thus, nutrient enrichment will 

enhance carbon cycling, especially in slow moving streams. 
22

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 nutrientdiffusing 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 Lake 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 consumer 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. 
23

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 water treatment 

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. 
24

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. 
25

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