On the Phytoplankton Bloom in Coastal Waters of Southern King George Island (Antarctica) in January 2010: An Exceptional Feature?

Abstract

Since the early 1990s phytoplankton has been studied and monitored in Potter Cove (PC) and Admiralty Bay (AB), King George/25 de Mayo Island, South Shetlands. Phytoplankton biomass is typically low compared to other Antarctic shelf environments, with average spring - summer values below 1 mg chlorophyll a m-3. The physical conditions in the area (reduced irradiance induced by particles originated from the land, intense winds) limit the coastal productivity at King George Island (KGI), as a result of shallow Sverdrup’s critical depths (Zc) and large turbulent mixing depths (Zt). In January 2010 a large phytoplankton bloom with a maximum of around 20, and monthly averages of 4 (PC) and 6 (AB) mg chlorophyll a m-3, was observed in the area, making it by far the largest recorded bloom over the last 20 years. Dominant phytoplankton species were the typical bloom-forming diatoms that are usually found in the western Antarctic Peninsula area. Anomalously cold air temperature and dominant winds from the eastern sector seem to explain adequate light:mixing environment. Local physical conditions were analyzed by means of the relation between Zc and Zt and conditions were found adequate for allowing phytoplankton development. However, a multi-year analysis indicates that these conditions may be necessary but not sufficient to guarantee phytoplankton accumulation. The relation between maximum chlorophyll a values and air temperature suggests that bottom-up control would render such large blooms even less frequent in KGI under the warmer climate expected in the area during the second half of the present century.