Metabolism of Antarctic micronektonic crustacea across a summer ice-edge bloom: respiration, composition, and enzymatic activity

The Antarctic marginal ice zone is an important oceanic front separating the pack-ice and open-water environments. 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 days. 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 Antarctic micronekton similar to that observed in zooplankton species. We also wished to assess the importance of the summer season to speciesʹ life cycles. Two major data sets were collected on 25 species in the following taxonomic groups: amphipods, cephalopods, decapods, euphausiids, isopods, mysids, ostracods, and polychaetes. The first data set described the metabolic rates of individuals in areas of the marginal ice zone with widely different levels of chlorophyll biomass to investigate the effect of the ice-edge bloom on metabolism. Additionally, summer metabolic rates were compared with data from other seasons. The second data set detailed the levels of protein, water, ash, RNA and DNA, and the activities of metabolic enzymes (citrate synthase and malate dehydrogenase) to examine the efficacy of biochemical indices as predictive tools for metabolism. Results suggested that the mobility of the micronektonic species eliminated most direct effects of the bloom on metabolism. Individuals captured in very different productivity regimes showed few significant differences in the metabolic indicators listed above. Isolated cases of changes in body composition and enzyme activity, however, implied that longer-term effects of the bloom may be exhibited. Seasonal increases in metabolism from winter to summer were observed in the euphausiids Euphausia superba, E. triacantha, and Thysanoessa macrura and the amphipod Vibilia stebbingi. 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. Protein content and MDH activity were found to be the best predictors of respiration rate, while nucleic acid measures only correlated with respiration in immature specimens.