South Georgia: a key location for linking physiological capacity to distributional changes in response to climate change

Antarctic marine invertebrates from the Western Antarctic Peninsula (WAP) are generally stenothermal, with three-month survival and activity limits above the average maximum summer seawater temperature (1.0 degrees C) of 1-6 degrees C and 1-3 degrees C respectively. For many of these species to survive the warmer maximum temperature at the sub-Antarctic island of South Georgia (5 degrees C), they require either greater thermal flexibility, or must avoid the warmest water-masses. The mean depths and depth range of WAP gastropod and bivalve molluscs were compared with the mean depths of these same species at South Georgia; separated into water masses delimited by the 1 degrees C isotherm at South Georgia, surface Antarctic water (SAW, 90 m), winter water (WW 90-150 m) and circumpolar deep water (CDW > 150m). Bivalves in the SAW and CDW categories at the WAP were centred around the cooler WW (< 1.2 degrees C) at South Georgia, with a narrower mean depth range for CDW bivalves. There was no difference in the average depth of gastropods, but a reduced depth range in the CDW. The apparent temperature limit to bivalve mean depths and not gastropods at South Georgia, suggests that further latitudinal comparisons could yield information on the underlying physiological mechanisms determining the range limits of Southern Ocean fauna.