FEEDING-BEHAVIOR OF BLACK FLY LARVAE AND RETENTION OF FINE PARTICULATE ORGANIC-MATTER IN A HIGH-GRADIENT BLACKWATER STREAM

We studied the feeding behavior of black fly larvae and aspects of fin e particulate organic matter (FPOM) retention in four riffles in a hig h-gradient blackwater stream in northeastern Minnesota, U.S.A., using fluorescent particulate dyes as tracers of FPOM. Radiant Red and Radia nt Deep Green dye particles were added sequentially to the stream abov e the upper riffle during two sequential 10-min pulses separated by a 5-min interval with no dye addition. Gut analyses showed that green pa rticles were ingested progressively more at downstream riffles well af ter the dye pulse had passed, but red particles showed the opposite pa ttern. Samples of Cladophora sp. collected at the same intervals showe d 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 stro ngly suggests that collector-gatherer behavior, in addition to collect or-filterer behavior, is important for obtaining very fine particles b y larval black flies. Our work also shows that although fluorescent dy e particles are useful tracers of very fin:: particulate organic matte r for feeding studies, the two similar-sized but different-colored par ticles used in this study behaved differently in terms of retention on Cladophora sp. By analogy, other similar-sized FPOM, which would be m ore heterogeneous chemically and physically than the dye particles, mi ght also be expected to exhibit diverse behavior, affecting its availa bility to consumers.