Nutrients, primary production and microbial heterotrophy in the southeastern Chukchi Sea:Arctic summer nutrient depletion and heterotrophy

In August 1993, we measured photosynthesis, chlorophyll a, bacterial secondary production, microb~al community respiratory rate, bacterial abundance, dissolved free armno acids, nitrate, phosphate, silicate, and dissolved oxygen in the eastern Chukchi Sea.Our crulse track was mostly in loose pack ice e x c e e d ~n g 50% ice cover, with heavier ice cover near 7 5" N. We sclmpled over the continental shelf and slope, in deep water in the Canadian Basin, and over the Chukchi Cap.Primary production was highest over the upper continental slope, averaging 748 mg C m-' d -' In deep water and heavier ice cover in the Canadian Basin, primary productivity averaged 123 mg C m-' d-l.However.microbial community respiratory rates averaged 840 mg C m ' d-l over the upper slope and 860 mg C m-2 d.' in the Canadian Basin.Nitrate was virtually depleted In the .upperm ~x e d layer, suggesting some nutrient limitation and dependence on regenerated ammonium in late summer.This is supported by f-ratios ranging from 0.05 to 0.38.Estimates of annual prlmary production of organlc carbon, both from our 'v and 13<1 assimilation measurements and from the supersaturation of dissolved oxygen in the upper mixed layer at all stations, suggest that significant primary production occurs well beyond the continental shelves out into the so-called perennial pack ice.Respiratory activity in the upper mixed layer exceeded primary productivity at the deep-water stations, a s it often does in summer oligotrophic conditions at lower latitudes.These observations suggest that rates of both autotrophic and heterotrophic biological activity in the upper mixed layer of the deep waters of the Arctic Ocean may be considerably higher than suspected and should b e incorporated lnto models of polar proccsscs.