Spatial variability of inorganic nutrients in the marginal ice zone of the Bellingshausen Sea during the Austral spring
Data on nutrients (nitrate, nitrite, ammonium, phosphate, silicate), biogenic-silica and chlorophyll a concentrations were collected along a south-to-north transect through the marginal ice zone of the Bellingshausen Sea during the Austral spring of 1992. There was a marked gradient in near-surface (≤ 100 m) concentrations from the most southerly occupied station (70°15′S, 85°07′W) to the northernmost (67°30′S, 85°00′W). Nitrate, phosphate and silicate concentrations decreased along the transect from 33 to 21, 2.2 to 1.2 and 76 to 35 mmol m−3 , respectively. Nitrite, ammonium and biogenic-silica levels increased from 0.04 to 0.16, 0.01 to 2.5 and 0.2 to 4 mmol m−3, respectively from south to north. Chlorophyll a concentrations increased from ∼0.1 at the most southerly station to >4.0 mg m-3 in the north. A simple ice-melt model suggests that only a proportion of the previous winter's sea-ice had melted in the study area. The impact of this ice-melt on nutrient concentrations was trivial. Predicted time-scales of nutrient removal by phytoplankton growth varied for the three nutrients. Nitrogen uptake appeared to be underestimated with respect to phosphorous, consistent with measurements of nitrogen preference from the same cruise. Silicate use appeared to have started later than nitrogen or phosphorus uptake, indicative of species succession in the phytoplankton. Hydrographic variability accounted for some of the silicate change but not the nitrate or phosphate anomalies. In the case of both nitrogen and phosphorus utilisation, the duration of uptake for steady-state growth at a rate of 0.1 day−1 appeared greater than that available in the interval between the start of the ice retreat and the time of the study