Biotic responses to low-concentration urban road runoff

Abstract

Abstract A major effect of urbanization on streams is the input of stormwater (SW) runoff from impervious surfaces. This water may contain excess nutrients, heavy metals, and other organic contaminants. These inputs are potential stressors or stimulants for algae, invertebrates, and fish within the aquatic community. We conducted a controlled experiment in mesocosms (∼1300-L tanks) to evaluate the effect of SW quality on different trophic levels. SW runoff was collected before it entered Little Black Creek, which is a tributary draining an urbanized subcatchment in western Michigan. The captured runoff was used to create the following treatments: 0% SW (control, n  =  4), 50% SW (n  =  4), or 100% SW (n  =  4). Nested within each mesocosm were exclosure treatments containing different combinations of algae, snails (Physa sp.), and fish (pumpkinseed: Lepomis gibbosus). SW did not have an overall effect on algal biomass or metabolic activity in the mesocosms. Algal community composition in the 0% SW treatment was significantly different from the 100% treatment at the end of the experiment. Mortality and growth rates of the fish and snails were not significantly affected by SW treatment. The absence of a strong effect of SW on algal biomass and metabolism or fish and snail growth in the mesocosms might have been the result of relatively low concentrations of contaminants in the SW. Changes in algal community composition in the mesocosms suggest that community composition is a more sensitive measure of water quality than either algal biomass or mortality and growth rates of fish and snails. SW quality is extremely variable, even among different storm events at the same location, so extrapolating results from one storm or one system to another should be done with caution. Our findings show that SW quality does not necessarily have negative impacts on stream biota.