Sea Ice and Plankton Ecology at Palmer Station

One of the tenets of polar oceanography is the dependence of phytoplankton blooms on sea ice retreat and subsequent water column stratification. Palmer LTER commenced nearshore research operations at Palmer Station in 1991, building on preceding research on Adélie penguins by Bill Fraser starting in 1975 and studies of plankton ecology in the Peninsula region during the 1980s. Local observations of sea ice cover, water column structure, phytoplankton biomass and production rates and nutrient concentrations, all made annually at ca. 4-day intervals between October and April reveal the details and complexity of ice-phytoplankton relationships. The period of sea ice cover at Palmer Station is decreasing. Sea ice advance and retreat are occurring later and earlier, respectively. In recent years, the area has been ice-free (defined as 15% ice concentration) from early November to late August so that the period of ice cover is around 75 days (down from ca 200 in the 1990s). On average the water column begins to stratify from the combined effects of warming and freshening in early December, developing a shallow (25 m) mixed layer that persists until March. In contrast, the nearshore phytoplankton bloom usually commences in November. There is no relationship of bloom initiation to the date of ice retreat, at least at the immediate local scale covered by these observations. Bloom amplitude varies by over an order of magnitude. Blooms may commence, and even attain maximum levels before, immediately following, or almost a month after the ice retreat. There is no relationship between the duration or intensities of ice coverage and bloom size/duration. Surface nitrate declines from its winter value of ~30 uM to 10-15 uM in most years. NO3 depletion starts in November and resupply from vertical mixing begins in February. Comparison with silicate concentration reveals that in some years the bloom is dominated by diatoms, while in others, non-diatoms account for most NO3 depletion.