ORGANIC-MATTER PROCESSING BY LARVAL BLACK FLIES IN A TEMPERATE WOODLAND STREAM

We studied the role of a dense aggregation of filter-feeding black fly larvae in organic matter processing in a north temperate woodland str eam. Black fly guts contained algae, individual bacterial cells, bacte rial clusters, and detritus, but bacterial clusters and especially det ritus were a much larger proportion of gut material. Particle size dis tribution and size variance within the largest size fraction of fine p articulate organic matter (FPOM) studied (> 100 mu m) decreased downst ream of black flies, due to the high proportion of uniform-sized fecal pellets in this size category. Ash free dry mass (AFDM) of FPOM was 2 8% higher downstream of the aggregation than upstream, even though bla ck flies were actively feeding. As a control for factors other than bl ack flies that might alter FPOM mass, we killed the black fly larvae w ith a highly specific microbial larvicide (Bacillus thuringiensis var. israelensis or Bti), then remeasured FPOM mass at the same stations. Following Bti treatment, FPOM mass was only about 5.6% higher downstre am of the aggregation, an increase that was likely because FPOM floccu lates from dissolved organic matter (DOM) due to turbulence. These dat a suggested that black flies were feeding from another pool of organic matter in addition to suspended FPOM, which we hypothesized to be DOM . Laboratory experiments using filtered (0.2 mu m) stream water demons trated that black flies ingested C-14-labeled DOM. Our results indicat e that the quantitative effects of black flies on FPOM in streams are linked to DOM ingestion. We suggest that a more complete examination o f these processes is needed for a comprehensive understanding of organ ic matter processing in streams.