The trophic basis of production of the macroinvertebrate community of a southeastern U.S.A. blackwater stream

Abstract

The trophic basis of production of the macroinvertebrate communities at three sites on a second‐order, low gradient blackwater stream in southeastern U.S.A. was determined. The sampling sites were located above, within and below a low‐flow swamp system. From 47–64% of macroinvertebrate production was supported by FPOM at the three sites, with dependence on FPOM being greatest at the swamp site. Algae (filamentous species and diatoms) supported 15–31% of production, indicating that algae can be of considerable importance even in fully canopied headwater streams. The production of some collector‐gatherers including Stenonema modestum (55%), Hexagenia munda (58%) and Baetis spp. (78%), was supported predominantly by algae. Algae also supported 61–79% of Hydropsychidae production and 68% of Simuliidae production. Animal material supported 16–26% of macroinvertebrate production at the three sites. CPOM was of minor direct importance to the macroinvertebrate community of this headwater stream, supporting only 1–3% of macroinvertebrate production. Shredders ingested only 1–3 g m−2 y−1 of CPOM, or about 1% of the annual direct leaf fall to this stream. Assuming a 10% assimilation efficiency for CPOM, shredders produced <3 g m−2 y−1 of FPOM through CPOM processing, this being approximately 2 orders of magnitude less than reported for high gradient headwater streams. These results indicate that low‐order coastal plain streams vary somewhat from the River Continuum Concept in that they exhibit little utilization of and dependence on CPOM as a direct energy source. Only the smallest first‐order streams and especially the extensive floodplains may be the functional headwaters of these stream systems.