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Influence of organic matter on invertebrate colonization of sand substrata in a northern Michigan stream

Abstract Sand is a common substratum in many streams, especially in lacustrine geologies, but has been less studied as a habitat for invertebrates than other substrata such as gravel and cobble. We hypothesized that benthic organic matter (BOM) content would influence the abundance and community structure of macroinvertebrates in sand habitats. Levels of coarse BOM (no = 0, low = 1%, high = 5% organic matter, as dry mass) were manipulated in 45 colonization chambers (volume = 539 cm3) implanted in a sand-dominated reach of Shane Creek in the Ottawa National Forest, Michigan. Chironomidae, Tipulidae, and Trichoptera were common colonizers of chambers. At the end of the 32-d experiment, invertebrate abundance (F2,12 = 7.5, p = 0.015) and biomass (F2,12 = 8.7, p = 0.010) were significantly higher in chambers with low BOM than with no BOM, but the high-BOM treatment did not differ from the no-BOM treatment. Throughout the experiment, functional feeding groups (FFG) were dominated numerically by gathering collectors in all treatments, but predators increased over time in the no-BOM and high-BOM treatments. At the beginning of the experiment, FFG biomass was dominated by shredders, and predator biomass increased in the no-BOM and high-BOM treatments over time. In general, taxon richness was higher in the low-BOM treatment than in the high-BOM treatment (F2,108 = 4.1, p = 0.025). Overall, the low-BOM treatment supported more invertebrates than the no-BOM and high-BOM treatments, perhaps because of fewer predators. Results suggest that local patterns of BOM accumulation may affect macroinvertebrate abundance and distribution in sand habitats in a nonlinear manner.