IDENTIFICATION OF PREDOMINANT ENVIRONMENTAL-FACTORS STRUCTURING STREAM MACROINVERTEBRATE COMMUNITIES WITHIN A LARGE AGRICULTURAL CATCHMENT

1. Patterns of macroinvertebrate community composition were examined in streams within a 40000-kM2 catchment in central Michigan, U.S.A., to identify the major environmental gradients influencing community variation. Agriculture and associated clay and sandy soils predominated in much of the region. 2. Eighty macroinvertebrate taxa were collected from stream surveys conducted during May and August 1990. Community composition varied primarily by the proportions of Plecoptera and Ephemeroptera. Benthic communities from the heaviest agricultural zones were most different from those at other sites. 3. Chemical composition among the sites varied most in relation to nutrients (NH3, NO3, PO4). Other parameters were relatively similar. Physical characteristics of the sites were scored in six habitat categories: (i) substrate characteristics, (ii) instream cover, (iii) channel morphology, (iv) riparian zone and stream-bank conditions, (v) riffle/run quality, (vi) pool quality. Most physical habitat scores were lowest in the intense agriculture zones. 4. The relative importance of physical and chemical variables in explaining variation in macroinvertebrate communities was quantified using canonical correspondence analysis. Substrate characteristics were most important in both surveys. Significant correlations (P < 0.05 and P < 0. 10) were observed between substrate quality and total numbers of Ephemeropteran, Plecopteran, and Trichopteran taxa. These relationships reflected correlations from sites in the clay soil-type region (P < 0.01 and P < 0. 10) which contrasted with non-significant results from the less impacted, sandy soil-type region. 5. Effective stream restoration efforts in this region will require the alteration of local land-use activities that influence the physical habitat. Further development of empirical relationships between catchment activities and substrate and channel characteristics within natural geomorphic regions is essential for the evaluation of restoration projects.