MACROINVERTEBRATE COMMUNITY STRUCTURE AND SEDIMENT BIOASSAY RESULTS FROM NEARSHORE AREAS OF NORTH-AMERICAN GREAT-LAKES

Benthic invertebrate communities of 50 nearshore reference sites in th e North American Great Lakes were evaluated by replicate (n = 5), quan titative sampling. Also, sediments collected at the 50 sites were used in eight, replicated (n = 3), lethal and sublethal bioassays in the l aboratory. We quantified the magnitude of variation and the proportion of variation among sires, as opposed to among replicates within sites , for densities of major community members and all bioassay endpoints. Tetra and bioassay endpoints with a large amount of variation, primar ily among sites, best described the magnitude and nature of variation among unpolluted reference sites. Sponges (Porifera) and worms (Oligoc haeta) were the most descriptive benthic taxa, with relatively high am ounts of variation, mostly (>80%) among sites. Growth of Hexagenia lim bata and tubificid growth and reproduction best described variation in bioassay endpoints among the reference sites, with a considerable amo unt of variation, mostly (>60%) among sites. In general, bioassay endp oints showed less variation than taxon abundances. A Mantel's test sho wed a strong (r = 0.20; p<0.004) relationship between community struct ure, as reflected in the density of the fifteen major benthic taxa, an d sediment toxicity, as I reflected in the eight bioassays. Semi-stron g hybrid multi-dimensional scaling of the community and bioassay matri ces showed three correlated sets of sites: (i) depauperate sites with poor Hexagenia and tubificid performance in bioassays, (ii) high spong e sites with good Hexagenia and tubificid performance in bioassays; an d (iii) high worm sites with poor Hexagenia and moderate tubificid per formance in bioassays. This study has illustrated both the magnitude a nd nature of variation in benthic communities and sediment toxicity am ong reference sites in the North American Great Lakes, as well as the covariation of community and bioassay measures of ecosystem structure and function.