Chironomid grazing on benthic microalgae in a Louisiana salt marsh

Field experiments were conducted to examine spatial and temporal variation in chironomid (predominantly Tanypus clavatus) abundance, and their trophic relationship with benthic microalgae. High performance liquid chromatography (HPLC) analysis of chironomid gut pigments indicated that diatoms comprised the bulk of the microalgae ingested by chironomids. C-14-feeding studies were used to obtain quantitative estimates of chironomid, copepod, ostracod and nematode grazing on benthic microalgae. Daily consumption of standing microalgal biomass by chironomids ranged from 0.12% (January) to 125% (May), but was highly variable. There were no significant diel, temporal (over the scale of months), or spatial patterns in individual chironomid grazing rates. There was significant temporal variation in the proportion of microalgal biomass consumed by the total meiofaunal assemblage, and highest grazing impacts occurred in May coincident with high abundances of chironomids, harpacticoid copepods, and ostracods. The grazing impact of chironomids was comparable to or greater than that of other known grazers of microalgae (copepods, ostracods). Functional-response experiments performed in the laboratory revealed that chironomid ingestion rates increased with increasing food availability over short (1 to 2 h) time scales. Field data did not indicate a functional response to food availability over longer (mo) time scales, possibly because of other environmental influences. Gut residence time (determined using fluorescently labeled beads) changed with variable feeding rates, which were in turn a function of variable food availability. Chironomid larvae have the ability to consume significant fractions of the microphytobenthos in absolute terms, and relative to other meiofauna, indicating that they are an important component of the salt marsh food web.