Effects of silt clay sediment in a phosphorus-limited algal-grazer food chain

Background/Question/Methods Silt clay sediment is a major pollutant of streams and rivers in the United States, causing problems that include turbidity, shading, and declines in water quality. Clay sediments have varying phosphorus (P) sorption properties, and as a result of urban development and intensive agriculture practices, an increase in erosion rates and impervious surfaces allow rising levels of sediment to be washed into streams. Often a limiting nutrient in streams, P is a valuable nutrient for algal and grazer growth. To determine if silt clay sediments have a positive or negative effect on algal growth, and if an indirect effect on grazer growth occurs, a 6-week experiment was conducted. Microcosms were used to mimic stream environments and were filled with modified Woods Hole MBL algal growth medium, with frosted glass slides used as substrate. Three sediment treatments (silt clay sediment from the banks of two urban streams in Baltimore County, Maryland and a control) and two grazer treatments (with and without grazers) were used. Algal growth was determined by measuring chlorophyll a concentrations and phosphorus limitation of algae was measured using an alkaline phosphatase activity (APA) assay. Grazer growth was measured using a length/ash-free dry mass regression. Results/Conclusions There was no significant difference in algal biomass among sediment treatments but there was a significant decrease in algal biomass in grazer treatments. Treatments with grazers were also significantly more P-limited. One explanation for these results is that the high P requirement for grazer growth did not leave sufficient P for algae to offset consumption by grazers. Results also showed that grazer growth significantly decreased in treatments containing sediment, most likely as a result of a decline in food quality.