Stream macroinvertebrate occurrence along gradients in organic pollution and eutrophication.

SUMMARY 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 (r 2 = 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 ,N H 4–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.