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TECHNICAL REPORTS

Surface Water Quality

Relationships among Nutrients, Chlorophyll-a, and Dissolved Oxygen in Agricultural Streams in Illinois

^a Department of Natural Resources and Environmental Sciences, University of Illinois at Urbana-Champaign, 1102 S. Goodwin Ave, Urbana, IL 61801
^b School of Public and Environmental Affairs, Indiana University, 1315 E. Tenth St., Bloomington, IN 47405

^* Corresponding author (troyer{at}indiana.edu)

Received for publication November 17, 2005. 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 km² that were dominated by row crop agriculture. Three sites had open canopies and two were 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^–1 and did not relate to algal abundance. Sestonic chl-a values ranged from nearly zero to >15 mg m^–3 with no differences between open and shaded streams and only a weak correlation with dissolved reactive P (mean concentrations were 44–479 µg L^–1). 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.

Abbreviations: chl-a, chlorophyll-a • DRP, dissolved reactive phosphorus

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