(Table 1) Characteristics of air temperature, sea surface temperature, sea surface salinity, nutrients, and total particulate matter for Admiralty Bay and Potter Cove, King George Island, Antarctica

Since the early 1990s, phytoplankton has been studied and monitored in Potter Cove (PC) and Admiralty Bay (AB), King George/25 de Mayo Island (KGI), South Shetlands. Phytoplankton biomass is typically low compared to other Antarctic shelf environments, with average spring - summer values below 1 mg chlorophyll a (Chl 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 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 mg Chl a/m**3, and monthly averages of 4 (PC) and 6 (AB) mg Chl a/m**3, was observed in the area, making it by far the largest recorded bloom over the last 20 yr. 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 relationship between Zc and Zt, and conditions were found adequate for allowing phytoplankton development. However, a multiyear analysis indicates that these conditions may be necessary but not sufficient to guarantee phytoplankton accumulation. The relation between maximum Chl 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.

Gene expression and physiological changes of the Antarctic krill Euphausia superba under different hypoxia intensities

Antarctic krill (Euphausia superba) from South Georgia comprise one of the most northern and abundant krill stocks. South Georgia waters are undergoing rapid warming, as a result of climate change, which in turn could alter the oxygen concentration of the water. We investigated gene expression in Antarctic krill related to aerobic metabolism, antioxidant defence, and heat-shock response under severe (2.5% O2 saturation or 0.6 kPa) and threshold (20% O2 saturation or 4 kPa) hypoxia exposure compared to in situ levels (normoxic; 100% O2 saturation or 21 kPa). Biochemical metabolic and oxidative stress indicators complemented the genic expression analysis to detect in vivo signs of stress during the hypoxia treatments. Expression levels of the genes citrate synthase (CS), mitochondrial manganese superoxide dismutase (SODMn-m) and one heat-shock protein isoform (E) were higher in euphausiids incubated 6 h at 20% O2 saturation than in animals exposed to control (normoxic) conditions. All biochemical antioxidant defence parameters remained unchanged among treatments. Levels of lipid peroxidation were raised after 6 h of severe hypoxia. Overall, short-term exposure to hypoxia altered mitochondrial metabolic and antioxidant capacity, but did not induce anaerobic metabolism. Antarctic krill are swarming organisms and may experience short periods of hypoxia when present in dense swarms. A future, warmer Southern ocean, where oxygen saturation levels are decreased, may result in smaller, less dense swarms as they act to avoid greater levels of hypoxia.