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 stream. Black fly guts contained algae, individual bacterial cells, bacterial clusters, and detritus, but bacterial clusters and especially detritus were a much larger proportion of gut material. Particle size distribution and size variance within the largest size fraction of fine particulate organic matter (FPOM) studied (> 100 mu m) decreased downstream 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 28% higher downstream of the aggregation than upstream, even though black flies were actively feeding. As a control for factors other than black flies that might alter FPOM mass, we killed the black fly larvae with 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 downstream of the aggregation, an increase that was likely because FPOM flocculates from dissolved organic matter (DOM) due to turbulence. These data 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 demonstrated that black flies ingested C-14-labeled DOM. Our results indicate that the quantitative effects of black flies on FPOM in streams are linked to DOM ingestion. We suggest that a more complete examination of these processes is needed for a comprehensive understanding of organic matter processing in streams.