Summer plankton production and nutrient consumption patterns in the Mertz Glacier Region of East Antarctica

We report on a biological and nutrient survey conducted off the eastern Adélie and George V Coasts of East Antarctica in late December 2000 and January 2001. The region was dominated by the Mertz Glacier Tongue (MGT) and its associated polynya that controlled the amount of open water through the season and influenced stratification near forming and melting ice. As in other regions of the Southern Ocean, large concentrations of nitrate, phosphate and silicic acid were found. Chlorophyll levels varied from 0.3 mg m−3 in pack-ice and slope regions to greater than 10 mg m−3 in a Phaeocystis bloom encountered among grounded ice northeast of the MGT. Net utilization rates of nutrients were derived from the deficit between measured concentrations and estimated winter values. Nitrate (N+N) and silicic acid utilization rates ranged from 2 to 12 and 14 to 30 mmol m−2 d−1, respectively, and were greatest in Commonwealth Bay. The ratio of nitrate to phosphate utilization was low (<10), particularly along the west side of the MGT in the core of the polynya associated with ice-shelf water. The silicic acid to N+N utilization ratio also varied significantly from <2 to >10. Lower silicic acid to N+N ratios were found in regions with substantial populations of Phaeocystis sp. in the eastern, grounded ice region. However, there was significant variation in the silicic acid to N+N utilization that may be induced by mixed-layer iron levels that varied from <0.1 to 0.5 nM at 12 stations. The silicic acid to N+N utilization ratio explained over 30% of the variability in the distribution of surface water pCO2, suggesting a strong link between iron availability, phytoplankton production, and carbon absorption. Possible sources of iron to local surface waters include aeolian transport, release from melting ice, coastal sediments, and the transport of upper circumpolar deep water across the shelf. The combination of early open water in the polynya, variations in surface mixing, and iron supply near such ice tongues may bring about similar biogeochemical distributions in other East Antarctic polynya regions.