Algal growth response in two illinois rivers receiving sewage effluent

Phosphorus (P) primarily enters streams in Illinois as effluent released from sewage treatment plants and runoff from agricultural fields. As a result, water quality can be affected and large amounts of algal growth are possible. We determined the growth of periphytic algae (as chla) relative to differing amounts of P (factor of 10) released in sewage effluent in two rivers. The Salt Fork Vermilion River and the Copper Slough branch of the Kaskaskia River both have a sewage treatment plant near their sources. Periphytic algal growth was assayed in each river with unglazed ceramic tiles (five week period) at 10 sites, each 10 km apart downstream from where the treatment plant was located. Field measurements included canopy cover, turbidity, water depth (to the tile surface), and water temperature. The concentrations of sestonic algae (as chla), total P, dissolved reactive P, nitrate-N, dissolved organic carbon, and Si were determined in water samples. Total P concentrations were different between the two rivers, ranging from 1.9 mg L⁻¹ just below the Salt Fork Vermilion River plant to 0.67 mg L⁻¹ 90 km downstream; corresponding values were 0.19 and 0.16 mg L⁻¹ for the Kaskaskia River. Phosphorus concentrations were not related to sestonic or tile periphytic chla in either river. Canopy cover, turbidity, and unstable sediments apparently regulated algal growth by limiting the penetration of light. Therefore, P was not the primary regulator of algal growth, and removing sewage effluent P from these rivers is unlikely to alter algal growth.