Aspects of water column primary productivity in the Chukchi Sea during summer

Measurements of primary productivity, chlorophyll a, incident solar radiation, phosphate-P, silicate-Si, nitrate-N, nitrite-N, ammonium-N, temperature and salinity were made in the Marginal Ice Zone of the Chukchi Sea in summer 1974. Low to moderate levels of primary productivity (0.07 to 0.97 g C m^-2 half-day^-1) were observed; primary productivity exceeded 3 g C m^-2 half-day^-1 at two stations. Surface primary productivity was nitrogen-limited at most stations. Mean chlorophyll a concentration in the photic zone varied from 0.4 to 17.8 mg m^-3. Higher concentrations and significant subsurface accumulation of chlorophyll a, reaching 40 mg m^-3, were observed in July at stations near the ice-edge than those in open water. No chlorophyll maximum was noted in September, when values ranged from 0.4 to 2.2 mg m^-3. It is postulated that the contribution of sea-ice algae to the total chlorophyll content can be substantial, but that the stay of these cells in the water column may not be long. Non-linear regression estimates from solar radiation and chlorophyll-specific primary productivity data showed a maximal photosynthetic rate of 18 mg C mg chlorophyll a ^-1 half-day^-1, an optimal light intensity of 54 langleys half-day^-1, and markedly reduced primary productivity at moderately higher light intensities. These features indicate that phytoplankton was shade-adapted.