On the summertime mixed layer development
in the marginal sea-ice zone of the Mawson coast, East Antarctica
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 σθ 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.
05/24/07: Received final version with requested
U.S. GLOBEC contribution number.