McMurdo Sound summer phytoplankton revisited: a comparison between 1984 and 2010

During January 2010, participants of the 9th International Graduate Training Course in Antarctic Biology (sponsored by the US National Science Foundation) conducted an investigation of the phytoplankton in McMurdo Sound (77°50'S, 166°36'E). Sample sites were established along a transect from open ocean to the inland limit of the annual sea ice at the edge of the Ross Ice Shelf. Hydrographic data (e.g., vertical profiles of temperature and salinity; attenuation of ultraviolet and photosynthetically active radiation (PAR); nutrient concentrations) and biological parameters (e.g., community taxonomic structure, chlorophyll a concentrations) were quantified. The transect corresponded to the December 1984 study area of Palmisano et al. (1986), and a comparison of data across the 25-year period was undertaken. In both studies, the prymesiophyte Phaeocystis was dominant (~99% of cells). Chlorophyll maxima were 12 μg/L in 1984 and 11 μg/L in 2010. Mean nutrient concentrations in 2010 (55.7±2.8 μM silicate and 0.76±0.43 μM phosphate) were also comparable to 1984 means (58.0±11.3 μM silicate and 1.52±0.52 μM phosphate). The 2010 extinction coefficients for PAR ranged from 0.2 in the open water at the ice edge to 0.6 in areas under the annual sea ice, supporting conclusions by Palmisano et al. that Phaeocystis can be productive under a large gradient of light regimes as cells are transported under the ice. The close correlation between data obtained in 1984 and 2010 suggests long-term consistencies in the physical and biological characteristics of McMurdo Sound.