Effect of Black Fly Ingestion and Assimilation on Seston Transport in a Quebec Lake Outlet

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Abstract
Ingestion and assimilation rates of simullid (Diptera) larvae feeding on natural seston were estimated in an artificial stream and used to predict the impact of simuliids on seston transport in a 40-m section of the outlet stream of Petit Lac Fraser in southern Quebec. This prediction was then compared with changes in the seston flux above and below the population. Finally, we selectively killed simuliids with a larvicide (Bacillus thuringiensis serovar. israelensis) to observe changes in seston transport after removal of simuliids. The stream population of simuliid larvae ingested between 32 and 55% of the seston flux and assimilated between 17 and 25% of the dry mass of ingested seston. The change in assimilation of seston in the stream section before and after the larvicide treatment was similar to that predicted from assimilation rate of larvae and standing stock. Larvae ingested 0.8–1.4% of the seston per linear meter of stream, about 10–1000 times more than what has been reported for other populations of stream filter-feeders. The high values observed can be explained by high specific ingestion rates, high standing stock, and low discharge. Our results suggest that simuliid populations can be food limited and may help to explain the downstream decrease in abundance and growth rates of filter-feeders in lake outlets.