Massive diatom bloom initiated by high winter sea ice in Admiralty Bay (King George Island, South Shetlands) in relation to nutrient concentrations in the water column during the 2009/2010 summer

Abstract

The South Shetland Islands, including King George Island (KGI), are located in the northern hydrographic zone on the West side of the Antarctic Peninsula (Northern WAP), which is washed mainly by the Antarctic Circumpolar Current (ACC). Continuous vertical mixing of the water column by strong currents, violent tidal water exchanges in deep fjords and frequent strong winds do not favour the seasonal appearance of large phytoplankton blooms. In contrast, the southern WAP is bound by the continental shelf, and is under the influence of the weaker Antarctic Peninsula Coastal Current (APCC), maintaining seasonal winter sea ice, which provides a shallow mixed layer, which in turn supports high levels of biological production in summer. During the 2009 / 2010 summer, however, a distinctive bloom event (chl-a > 20 μgL−1) was documented for in Admiralty Bay (AB), the largest and deepest fjord in KGI. It commenced in early summer (29 December 2009) at the edge of melting sea ice which had persisted since the preceding cold winter of 2009. The phytoplankton group dynamics, determined using photosynthetic pigment data, and associated changes in nutrient concentration during the bloom, have been described in this paper. The area where the phytoplankton bloom developed coincided with the surface of the brash sea ice cover. The sequence and size of biological and chemical changes recorded within the fjord during the bloom resembled the recurring ice edge blooms which occur in the southern WAP, although they had a much shorter lifespan (< two weeks), which may have been due to the slow SE winds and related surface water movement direction, which hindered vertical mixing and kept the surface water within the fjord.