Relative Importance of Substrate Particle Size and Detritus to Stream Benthic Macroinvertebrate Microdistribution

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Abstract
Manipulative field experiments were conducted in Carnation Creek, British Columbia, to determine whether particle-size composition of the stony substrate influenced macroinvertebrate microdistribution if substrate detritus was standardized. A standardized quantity of alder (Alnus rubra) detritus was added to five substrate mixtures ranging from homogeneous gravel to a heterogeneous gravel, pebble, and cobble mixture, and the substrates imbedded in a riffle to allow macroinvertebrate colonization. Densities and biomasses of most macroinvertebrate taxa (16 of 19) were not significantly different among the wide range of substrate types containing the standardized alder detritus, even though surface area, intra-substrate current velocity, and interstitial space varied significantly between treatments. However, in the treatment that did not have the standardized detritus, the biomasses and densities of nine taxa, and the totals of all taxa combined, were significantly lower than in an identical substrate mixture that contained the detritus. We concluded that the differential colonization of substrates demonstrated for many macroinvertebrate taxa in previous studies was more likely related to differences in organic sedimentation. Although detritus is a major determinant of detritivore microdistribution, substrate composition may be an important factor to other trophic groups such as filter-feeders.