On the summertime mixed layer development in the marginal sea-ice zone of the Mawson coast, East Antarctica

G.D. Williams, S. Nicol, B. Raymond, and K. Meiners

Two hydrographic surveys (KACTAS and KAOS) were conducted on the East Antarctic shelf region offshore from the Mawson coast between 61-66°E, in the austral summers of January 2001 and 2003. The variability in timing of sea-ice melt and subsequent mixed layer development is observed to have had a significant impact on the ecology of the region, which encompasses the feeding grounds for Adélie penguins (Pygoscelis adeliae) from nearby Béchervaise Island. The condition of the penguin colony was depressed in 2003 relative to 2001, as indicated by breeding success rates and stomach content records (see Nicol et al., this volume). This coincides with a two-thirds decrease in the observed abundance of Antarctic krill (Euphausia superba). The mixed layer depth was fouind to be significantly different between the surveys. In KACTAS (January 2001), the mean mixed layer depth (0.1 σ_θ condition) was 68.5 ± 12.4 m. In KAOS (January 2003), the mean mixed layer depth was 33.8 ± 11.2, and increased through the remaining 14 days of the survey. The mixed layer in 2003 was warmer, fresher and reduced in both dissolved oxygen content and fluorescence relative to 2001. It is shown to be approximately one month underdeveloped relative to the same time of year in January 2001. This lag is most likely due to a greater quantity of sea-ice persisting in the study region. AVHRR images show persistent sea-ice in 2003, in particular a region of fast-ice along the coast to the south. The delay in the seasonal timing of sea-ice melt for KAOS relative to KACTAS is concluded to have delayed the development of the mixed layer and the phytoplankton development, with negative impacts across the ecosystem.

STATUS UPDATE:

05/24/07: Received final version with requested U.S. GLOBEC contribution number.