RUI: Organic Matter Processing by Filter Feeders in Stream Ecosystems

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Hershey Fine particulate organic matter (FPOM) dominates macroconsumer food webs in most stream ecosystems. However, dissolved organic matter (DOM), thought to be available only to microconsumers, is often the largest pool of organic matter present. Dense aggregations of black flies, which often occur at lake outlets an on bedrock cascades, appear to filter a very large portion of the FPOM entering the aggregation, but mass of this pool does not decrease as it passes across the aggregations; it increases. Preliminary data suggest that the FPOM entering these aggregations is augmented by DOM, via black fly feeding and egestion. The objectives of the proposed work are: 1) to determine the importance of DOM and FPOM feeding to FPOM and DOM pools in streams; and 2) to evaluate the role in black fly feeding. To meet these objectives, a series of upstream and downstream comparisons, stable isotope and dye tracer experiments, filter-feeder removal experiments, and laboratory feeding experiments will be used to measure: 1) FPOM passing through a filter-feeder aggregation, including mass, size distribution of particles, and chlorophyll concentration; 2) transformation of DOM to FPOM; and 3) availability of different size fractions (molecular weights) of the DOM pool to black flies. The same processes that occur in these dense aggregations could also occur in more diverse stream communities; however, effects would be harder to detect. Thus, although these experiments focus on special cases, they may have broad applicability to stream organic matter dynamics.