A two-ship cruise to the Bellingshausen Sea marginal ice zone (MIZ) in the Austral spring-summer 1992 enabled a detailed study of the phytoplankton dynamics during the retreat of the ice sheet. Stations were set up along a transect from within the ice sheet 70°S, 85°W to the open ocean 67°S, 85°W, and the rates of primary production and nitrogen uptake were determined in the ice and water column using in situ 13C and 15N incubation techniques. Nitrogen uptake was subdivided into components of nitrate, ammonium and urea uptake, and fractionated into 20 um size classes. The ice station water column had a very low biomass, with values integrated to 30 m of 3 mg chl m−2, 480 mmol POC m−2, and 25 mmol PON m−2, high nutrient concentrations, and integrated carbon and nitrogen uptake values of 24 mg C m−2 day− and 12 mg N m−2, day−1. Nutrient concentrations decreased and biomass and productivity in the water column increased, towards the northernmost open water station. Here, integrated values of 170 mg chl m−2, 1730 mmol POC m−2, and 190 mmol PON m−2, and carbon and nitrogen uptake values of 690 mg C m−2, day−1 and 104 mg N m−2 day− were obtained. Whilst the 20μm phytoplankton dominated in the open water stations and supported most of the productivity. The dominant nitrogenous source varied between stations: at the Ice Station G, the uptake rate of urea was six times that of nitrate or ammonium; at Open Water Station J, nitrate was the preferred substrate, producing |-ratios (nitrate/nitrate + ammonium + urea) of 0.9; and at the northernmost Station K, ammonium was the dominant substrate giving |-ratios of 0.4. A detailed discussion of methods available for |-ratio calculations using several different data sets from this cruise are given. Varied estimates resulting from this exercise have important implications in terms of global carbon budget calculations. Productivity estimates from 13C and 15N uptake data are compared with 14C data, and nitrate, phosphate, chlorophyll and pCO2 mass balance estimates. These data are discussed in the context of past studies in other areas of the Southern Ocean.
|Number of pages||28|
|Journal||Deep-Sea Research Part II - Topical Studies in Oceanography|
|Publication status||Published - 1 Jan 1995|