Production Dynamics of Riverine Chironomids: Extremely High Biomass Turnover Rates of Primary Consumers

A critical step in understanding food webs and trophic dynamics of communities is quantification of the role of the primary consumers, and a major aspect of such quantification is determination of their production. Annual production and biomass turnover were estimated for the larval chironomid (midge) assemblage found on the submerged woody (snag) habitat of a Coastal Plain blackwater river. Temperature-specific growth rate equations, generated from field growth studies, were applied to the biomass values from quantitative field samples to obtain mean daily production on a monthly basis throughout the year. The most productive genera were the filtering collectors, Rheotanytarsus (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-level equation to biomass of all taxa. When converted to production per square meter of river channel bottom, values were about one-third of the 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 be lowest in winter with peaks in either summer or fall. These production estimates are among the highest ever reported for chironomids in freshwater systems, primarily owing to extremely high biomass turnover. Annual production/biomass (P/B) values were 158 for Rheocricotopus, 258 for Polypedilum, 196 for Rheotanytarsus, and 202 for total Chironomidae. For Polypedilum, such high annual P/B represents a biomass turnover rate of almost once per day. The P/B values are among the highest estimated for aquatic metazoans (including zooplankton) and are similar to or exceed turnover rates of microbes in many aquatic ecosystems. Such high biomass turnover may be more widespread among freshwater benthic invertebrates than is commonly supposed and may have profound implications for trophic dynamics.