Summertime mixed layer development in the marginal sea ice zone off the Mawson coast, East Antarctica

Two small-scale Antarctic marine ecosystem surveys were conducted offshore from the Mawson coast ( 61 – 66 ∘ E ) , in the austral summers of January 2001 and 2003. Striking differences were observed in the state of the marine ecosystem between the surveys; in particular krill abundance and penguin breeding success were significantly lower in 2003. In this paper we examine the variability in the physical oceanography between the two surveys, and identify the development of the summer mixed layer (SML) as the key physical process influencing the differences in ecological conditions. The mixed layer in 2003 was warmer, fresher and reduced in both dissolved oxygen content and fluorescence relative to 2001. In 2001 the mean mixed-layer depth was 68.5 ± 12.4 m . In 2003, the mean mixed-layer depth was 33.8 ± 11.2 m , and increased through the remaining 14 days of the survey. The SML in 2003 was underdeveloped by over a month relative to the 2001 hydrography and we relate this to the seasonal variability in the pattern and timing of sea-ice melt. AVHRR satellite images show a region of fast ice against the Mawson coast that had greater spatial and temporal extent in 2003. We conclude that delayed mixed layer development due to persistent sea ice is likely to have a negative impact on the marine ecosystem of the Antarctic shelf. This may have important implications for predicting the impact of future variability in the sea-ice growth/melt transition due to climate change.