Metabolism of Antarctic Micronektonic Crustacea Across a Summer Ice Edge Bloom: Respiration, Composition, and Enzymatic Activity
Joseph Donnelly, Helena Kawall, Stephen P. Geiger, and Joseph J. Torres
The Antarctic marginal ice zone is an important oceanic front separating the pack-ice and open water environment. During summer, the retreating pack-ice creates a meltwater lens in the euphotic zone allowing primary producers and microheterotrophs to flourish in a discrete bloom just seaward of the retreating ice edge that lasts about 60 d. The purpose of the present study was to see if the ice edge bloom had a discernible effect on the metabolism and physiological condition of the Antarctic micronektonic Crustacea similar to that observed in zooplankton species. We also wished to assess the imporantance of the summer season on the species’ life cycle. Three major data sets were collected on twenty two species of crustaceans in the following taxonomic groupings: euphausiids, amphipods, decapods, mysids, isopods, and ostracods. The first data set described the metabolic rates of individuals in areas of the ice edge with widely different levels of chlorophyll biomass in order to investigate the effect of the ice edge bloom on metabolism. The second described the summer metabolic rates on micronektonic species to compare them with data from other seasons. The third data set detailed the activities of intermediate metabolic enzymes (CS and MDH), RNA:DNA ratio, protein, and water content of micronektonic crustaceans to examine the efficacy of biochemical indices as predictive tools for metabolism. Results suggested that the mobility of the micronektonic species homogenized any effects of the bloom on metabolism. Individuals captured in very different productivity regimes showed no significant differences. Seasonal increases in metabolism from winter to summer were observed in the euphausiids Euphausia superba, E. triacantha, and Thysanoessa macrura; the amphipod Vibilia stebbingi; and the decapod Pasiphaea scotiae. It was concluded that the seasonal shifts were indicative of a “type 2” or compromise overwintering strategy whereby metabolism drops without an accompanying dormant state. The intermediary metabolic enzyme malate dehydrogenase was the best predictor of metabolism in the crustaceans examined here, followed closely by citrate synthase.