Phytoplankton growth and new production in the Weddell Sea marginal ice zone in the austral spring and autumn

Rates of primary production, NO3− uptake, and NH4+ uptake by phytoplankton were measured in the marginal ice zone of the Weddell Sea in austral spring 1983 and autumn 1986. In spring Chl concentrations averaged 3 µg liter−1, primary productivity 490 mg C m−2 d−1, and surface phytoplankton growth rates 0.30 doublings d−1. In autumn these rates were all much lower, averaging 0.14 µg liter−1, 126 mg C m−2 d−1, and 0.14 doublings d−1. During both seasons NH4+ was consistently the preferred source of N, but because of the much greater availability of NO3− in the euphotic zone (21–28 µM NO3− vs. ∼0.4 µM NH4+ during both seasons), NO3− uptake rates generally equaled or exceeded those of NH4+. Vertically integrated f-ratios (the ratio of NO3− uptake to the sum of NO3− plus NH4+ uptake) averaged 0.52 (range from 0.35 to 0.70) in spring and 0.72 (range from 0.60 to 0.84) in autumn. In 1983 we observed sharply elevated relative preference indices for NH4+ whenever the ambient NH4+ concentration was <0.3 µM, which we interpret as evidence that Antarctic phytoplankton growing at NH4+ concentrations <0.3 µM have the ability to increase their rate of NH4+ uptake rapidly in response to increased availability. We estimate the new production of the marginal ice zone to be at least 49 g C m−2 yr−1 for the November–March period. This value supports previous findings that the ice edge is the main site of production in the Southern Ocean and that it is likely to dominate the annual cycle of organic matter flux from the euphotic zone.