Feeding behavior of black fly larvae and retention of fine particulate organic matter in a high-gradient blackwater stream

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Abstract
We studied the feeding behavior of black fly larvae and aspects of fine particulate organic matter (FPOM) retention in four riffles in a high-gradient blackwater stream in northeastern Minnesota, U.S.A., using fluorescent particulate dyes as tracers of FPOM. Radiant Red and Radiant Deep Green dye particles were added sequentially to the stream above the upper riffle during two sequential 10-min pulses separated by a 5-min interval with no dye addition. Gut analyses showed that green particles were ingested progressively more at downstream riffles well after the dye pulse had passed, but red particles showed the opposite pattern. Samples of Cladophora sp. collected at the same intervals showed that green dye particles were preferentially retained over red ones by filamentous algae, and thus would have been more available than red particles to larvae functioning as collector-gatherers. Our work strongly suggests that collector-gatherer behavior, in addition to collector-filterer behavior, is important for obtaining very fine particles by larval black flies. Our work also shows that although fluorescent dye particles are useful tracers of very fine particulate organic matter for feeding studies, the two similar-sized but different-colored particles used in this study behaved differently in terms of retention on Cladophora sp. By analogy, other similar-sized FPOM, which would be more heterogeneous chemically and physically than the dye particles, might also be expected to exhibit diverse behavior, affecting its availability to consumers.

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