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

• Published in: Deep Sea Research Part II: Topical Studies in Oceanography
• Main Authors: Strutton, PG, Griffiths, FB, Waters, RL, Wright, SW, Bindoff, NL
• Format: Article in Journal/Newspaper
• Language: English
• Published: Pergamon 2000
• Subjects: Earth Sciences; Oceanography; Physical Oceanography; Antarctic; East Antarctica; The Antarctic; Antarc*; Antarctica; Sea ice
• Online Access: https://doi.org/10.1016/S0967-0645(00)00028-X; http://ecite.utas.edu.au/48120

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 66.25S and 80 to 150E. Coastal, continental shelf and open ocean waters were sampled, encompassing the Antarctic Slope Front (ASF) and the Antarctic Divergence (AD). Sea ice coverage during the previous winter exhibited a gradual decline in northwards extent from 80E, where its edge was located at 57S, to 150E, where it reached 6263S. Productivity versus irradiance ( P I ) experiments were conducted to calculate primary productivity, and measurements of the photochemical quantum efficiency of photosynthesis ( F v / F m ) 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 (>1μgl −1 ) on the continental shelf along 93.5E and in the shelf and shelf-break waters of a meridional transect along 120E. Integrated production over the mixed layer ranged from greater than 700mgC m −2 d −1 slightly north of the AD along 80E, to less than 50mgCm −2 d −1 in open ocean water at the northeastern corner of the survey area. The elevated biomass and productivity west of approximately 120E 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 75m of the water column ranged between 20.8 and 32.7μM for nitrate, from 0.75 to 2.3μM for phosphate, and from 22.6 to 74.9μM 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 150E, 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 120E 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.