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 u ndertaken off the coast of East Antarctica, south and southwest of Australi a, from approximately 63 to 66,25 degrees S and 80 to 150 degrees E, Coasta l, continental shelf and open ocean waters were sampled, encompassing the A ntarctic Slope Front (ASF) and the Antarctic Divergence CAD). Sea ice cover age during the previous winter exhibited a gradual decline in northwards ex tent 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 me asurements of the photochemical quantum efficiency of photosynthesis (F-nu F,IF,) were made using a fast repetition rate (FRR) fluorometer. Phytoplank ton pigment (HPLC) and nutrient (nitrate, phosphate and silicate) concentra tions also were obtained. Mean chlorophyll a concentrations in the mixed la yer were highest ( > 1 1 mu g 1-(1)) on the continental shelf along 93,5 de grees E and in the shelf and shelf-break waters of a meridional transect al ong 120 degrees E, Integrated production over the mixed layer ranged from g reater 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 northeaster n corner of the survey area. The elevated biomass and productivity west of approximately 120 degrees E was attributed to macro- and micro-nutrient rel ease and upper water column stratification following the spring/summer ice melt that preceded and coincided with the sampling period, Macronutrient co ncentrations were not found to be limiting, and in the upper 75 m of the wa ter 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 uptak e rates, inferred From depletion in the mixed layer, indicated that silicat e uptake was uncoupled from that of nitrate and phosphate in the northeaste rn region of the survey area. The mixed layer deepened slightly and critica l 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 t he light field used in the production modelling of the eastern transects re sulted in productivity estimates similar to the western transects. These ob servations, together with the analysis of the FRR data, suggest that the ph ytoplankton community east of approximately 120 degrees E may have been co- limited by iron concentrations and incident irradiance. The lower biomass a nd productivity in the east also coincided with greater southwards extensio n of warmer oceanic waters, and a shift from a krill/diatom to salp/picopla nkton biological community. (C) 2000 Elsevier Science Ltd. All rights reser ved.