Drift of aquatic invertebrates above and below an area of agricultural nitrate input in a small Michigan river.

Abstract

This study, inspired by an earlier discovery of a nitrate ""pulse"" in the study stream, sampled drifting aquatic invertebrates above and below an area of agricultural nutrient input (run off from cattle fields) to determine whether those inputs had an effect on the amount of invertebrates drifting. I hypothesized that greater invertebrate densities would be found in areas of higher nutrient concentration due to increase in primary production. (A concurrent study looked at the affects of the increased nitrate on the periphyton). Invertebrates were sampled with drift nets and ribbon substrates at two sites on the West Branch of the Maple River in Emmet County, Michigan during one week in August, 1991. Nitrate data were collected. T tests (alpha=0.05) were used to determine if there was a statistical difference between the sample of invertebrates and also the water chemistry at the two sites. While there was a significant difference found in the water chemistry (P<0.02) at the two sites there was not a consistent significant difference found in the invertebrate populations (Simuliidae, Ephemeroptera and a group of organisms (called Group C) should a difference (P<0.05) while Chironomidae (P<0.5), Culicidae (P<0.1), and two other groups of invertebrates (called group A and B) did not (P<0.1 and P<0.5, respectively).