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Effects of Eutrophication on Stream Ecosystems

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Effects of Eutrophication on Stream Ecosystems EFFECTS OF EUTROPHICATION ON STREAM ECOSYSTEMS 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 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. 2 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 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., 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). 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 pollution problem in North American lakes and reservoirs in the mid- 20th 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
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