Primary productivity off the coast of East Antarctica (80-150 degrees E): January to March 1996

During February and March 1996, an interdisciplinary research project was undertaken off the coast of East Antarctica, south and southwest of Australia, from approximately 63 to 66,25 degrees S and 80 to 150 degrees E, Coastal, continental shelf and open ocean waters were sampled, encompassing the Antarctic Slope Front (ASF) and the Antarctic Divergence CAD). Sea ice coverage during the previous winter exhibited a gradual decline in northwards extent from 80 degrees E, where its edge was located at 57 degrees S, to 150 degrees E, where it reached 62-63 degrees S, Productivity versus irradiance (P-l) experiments were conducted to calculate primary productivity, and measurements of the photochemical quantum efficiency of photosynthesis (F-nu F,IF,) were made using a fast repetition rate (FRR) fluorometer. Phytoplankton pigment (HPLC) and nutrient (nitrate, phosphate and silicate) concentrations also were obtained. Mean chlorophyll a concentrations in the mixed layer were highest ( > 1 1 mu g 1-(1)) on the continental shelf along 93,5 degrees E and in the shelf and shelf-break waters of a meridional transect along 120 degrees E, Integrated production over the mixed layer ranged from greater than 700 mg C m(-2) d(-1) slightly north of the AD along 80 degrees E, to less than 50 mg C m (-2) d(-1) in open ocean water at the northeastern corner of the survey area. The elevated biomass and productivity west of approximately 120 degrees E was attributed to macro- and micro-nutrient release and upper water column stratification following the spring/summer ice melt that preceded and coincided with the sampling period, Macronutrient concentrations were not found to be limiting, and in the upper 75 m of the water column ranged between 20.8 and 33.7 mu M for nitrate, from 0.75 to 2.3 mu M for phosphate, and from 22.6 to 74.9 mu M for silicate. Nutrient uptake rates, inferred From depletion in the mixed layer, indicated that silicate uptake was uncoupled from that of nitrate and phosphate in the northeastern region of the survey area. The mixed layer deepened slightly and critical depths shoaled by a factor of three as the survey moved eastwards from 80 to 150 degrees E, a pattern most likely due to the onset of winter. Daily irradiance also decreased between January and March, and manipulations of the light field used in the production modelling of the eastern transects resulted in productivity estimates similar to the western transects. These observations, together with the analysis of the FRR data, suggest that the phytoplankton community east of approximately 120 degrees E may have been co-limited by iron concentrations and incident irradiance. The lower biomass and productivity in the east also coincided with greater southwards extension of warmer oceanic waters, and a shift from a krill/diatom to salp/picoplankton biological community. (C) 2000 Elsevier Science Ltd. All rights reserved.