Stream macroinvertebrate occurrence along gradients in organic pollution and eutrophication

1. We analysed a large number of concurrent samples of macroinvertebrate communities and chemical indicators of eutrophication and organic pollution [total-P, total-N, NH₄-N, biological oxygen demand (BOD₅)] from 594 Danish stream sites. Samples were taken over an 11-year time span as part of the Danish monitoring programme on the aquatic environment. Macroinvertebrate communities were sampled in spring using a standardised kick-sampling procedure whereas chemical variables were sampled six to 24 times per year per site. Habitat variables were assessed once when macroinvertebrates were sampled. 2. The plecopteran Leuctra showed a significant negative exponential relationship (r² = 0.90) with BOD₅ and occurred at only 16% of the sites with BOD₅ above 1.6 mg L⁻¹. Sharp declines with increasing BOD₅ levels were found for the trichopteran families Sericostomatidae and Glossosomatidae although they appeared to be slightly less sensitive than Leuctra. Other plecopterans such as Isoperla showed a similar type of response curve to Leuctra (negative exponential) but occurred at sites with relatively high concentrations of BOD₅ up to 3-4 mg L⁻¹. In contrast, the response curve of the isopod Asellus aquaticus followed a saturation function reaching a plateau above 3-4 mg L⁻¹ BOD₅ and the dipteran Chironomus showed an exponential increase in occurrence with increasing BOD₅ concentration. 3. Macroinvertebrate occurrence appeared to be related primarily to concentrations of BOD₅, NH₄-N and total-P whereas there were almost no relationships to total-N. Occurrence of a number of taxa showed a stronger relationship to habitat conditions (width and substrate) than chemical variables. 4. Important macroinvertebrate taxa are reduced at concentrations of BOD₅ that are normally perceived as indicating unimpacted stream site conditions. Our results confirmed sensitivity/tolerance patterns used by existing bioassessment systems only to some degree.