Jeq50433 1110..1117

A better understanding of the controls on algae and dissolved O2 in agricultural streams of Illinois is needed to aid in development of nutrient standards. We investigated the relationships between dissolved nutrients, algal abundance, and dissolved O2 in five streams in east-central Illinois from March through November 2004. The streams drained watersheds from 25 to 777 km that were dominated by row crop agriculture. Three sites had open canopies and twowere bordered by a narrow forest of deciduous trees. Algal abundance was measured as chlorophyll-a (chl-a) concentration in the water column (sestonic) and on the streambed (periphytic). Mean NO3–N concentrations ranged from 5.5 to 8.8 mg N L and did not relate to algal abundance. Sestonic chl-a values ranged from nearly zero to .15 mg m with no differences between open and shaded streams and only a weak correlation with dissolved reactive P (mean concentrations were 44– 479 mg L). The results suggest that sestonic chl-a is a poor criterion for assessing nutrient-related problems in these streams. Greatest periphytic chl-a occurred during low flow from August through October, but periphyton occurred consistently in only two of the five streams. The abundance of filamentous algae explained 64% of the variation in diel O2 saturation, but was not correlated with nutrients. Currently it appears that hydrology and light, rather than nutrients, control algal abundance in these streams, and in the agricultural landscape of east-central Illinois, it may not be possible to reduce nutrient concentrations sufficiently to limit filamentous algal blooms. ADEQUATE dissolved O2 is vital for the survival of aquatic organisms and is therefore an important variable in the assessment and monitoring of water quality. Short periods of anoxia can be fatal to aquatic organisms, and prolonged exposure to low O2 concentrations can increase susceptibility to other environmental stressors (Horne and Goldman, 1994). Although O2 concentrations in streams can vary naturally over diel and seasonal time scales, large fluctuations in O2 concentrations often indicate excessive productivity resulting from nutrient enrichment (Walling andWebb, 1992). As algal biomass increases, respiration during nighttime can deplete O2 concentrations to values that kill susceptible organisms and result in generally impaired biotic integrity (Portielje and Lijklema, 1995; Miltner and Rankin, 1998). In eutrophic streams and rivers, dissolved O2 can range from supersaturated during daylight to nearly anoxic at night. Less productive, and presumably less impaired, streams are generally characterized by dissolved O2 concentrations near saturation, with some moderate diurnal fluctuation caused by temperature and metabolism (Walling andWebb, 1992). In freshwater systems, increased inputs of P are of particular concern because it commonly is the limiting nutrient for productivity in freshwater ecosystems. Phosphorus loading to streams can increase the biomass of periphyton, macroalgae, and sestonic algae, as measured by chl-a (Welch et al., 1989; Van Nieuwenhuyse and Jones, 1996; Dodds et al., 1998); however, identifying strong relationships between nutrient enrichment, chl-a concentrations, and biotic integrity in streams has been difficult because of confounding environmental factors such as shading, turbidity, scouring of biomass during floods, substrate characteristics, and herbivory (Miltner and Rankin, 1998; Dodds and Welch, 2000). Therefore, establishing defensible nutrient criteria for streams, as mandated by the USEPA for all states and tribes, requires an understanding of how environmental factors can influence the relationship between nutrients, chl-a, and dissolved O2. Nutrient enrichment and eutrophication are linked to a variety of human activities that can decrease water quality, such as agriculture, sewage effluent discharge, and urbanization (Biggs, 2000; Dodds and Welch, 2000). Major inputs of N and P to surface waters in the USA are from nonpoint sources, such as agricultural and urban activities (Carpenter et al., 1998). In the midwestern USA, N and P concentrations in streams tend to be high due to the dominance of fertilized agriculture and extensive artificial drainage (e.g., David and Gentry, 2000). The purpose of this study was to examine the relationships between nutrient concentrations, algal biomass, and dissolved O2 patterns in streams draining agricultural watersheds in east-central Illinois. Algae in these streams occur as sestonic cells in the water column, as periphyton attached to hard surfaces, and as mats of the filamentous algae, Cladophora. Our first objective was to determine how these different groups varied in abundance through time and if that variation could be explained by P availability. Our second objective was to examine the role of physical factors, such as light and discharge, in controlling algal biomass in streams of this region. Our final objective was to determine the extent to which patterns in dissolved O2 were related to the abundance of sestonic, periphytic, and filamentous mats of algae. MATERIALS AND METHODS