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) m23. 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 m23, and monthly averages of 4 (PC) and 6 (AB) mg Chl a m23, 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