PRODUCTION DYNAMICS OF RIVERINE CHIRONOMIDS - EXTREMELY HIGH BIOMASS TURNOVER RATES OF PRIMARY CONSUMERS

A critical step in understanding food webs and trophic dynamics of com munities is quantification of the role of the primary consumers, and a major aspect of such quantification is determination of their product ion. Annual production and biomass turnover were estimated for the lar val chironomid (midge) assemblage found on the submerged woody (snag) habitat of a Coastal Plain blackwater river. Temperature-specific grow th rate equations, generated from field growth studies, were applied t o the biomass values from quantitative field samples to obtain mean da ily production on a monthly basis throughout the year. The most produc tive genera were the filtering collectors, Rheoranytarsus (31.1 g dry mass.m(-2).yr(-1) for snag surface area) and the gathering collectors, Polypedilum (11.3 g.m(-2).yr(-1)) and Rheocricotopus (9.8 g.m(-2).yr( -1)). Total production was 65.4 g.m(-2).yr(-1) based on summing values of individual taxa, and 69.9 g.m(-2).yr(-1) when applying a family-le vel equation to biomass of all taxa. When converted to production per square meter of river channel bottom, values were about one-third of t he snag surface area estimates (20.1 g.m(-2).yr(-1)). Total chironomid production was somewhat higher in a second sampling year (81.9 g.m(-2 ).yr(-1) for snag surfaces and 26.8 g.m(-2).yr(-1) for channel bottom) . Production was relatively high throughout both years but tended to b e lowest in winter with peaks in either summer or fall. These producti on estimates are among the highest ever reported for chironomids in fr eshwater systems, primarily owing to extremely high biomass turnover. Annual production/biomass (P/B) values were 158 for Rheocricotopus, 25 8 for Polypedilum, 196 for Rheotanytarsus, and 202 for total Chironomi dae. For Polypedilum, such high annual P/B represents a biomass turnov er rate of almost once per day. The P/B values are among the highest e stimated for aquatic metazoans (including zooplankton) and are similar to or exceed turnover rates of microbes in many aquatic ecosystems. S uch high biomass turnover may be more widespread among freshwater bent hic invertebrates than is commonly supposed and may have profound impl ications for trophic dynamics.