Temperature-dependent growth of Thysanoessa macrura: inter-annual and spatial variability

Somatic growth of pelagic invertebrates is controlled by temperature and food, both of which vary in space and time. Because species specific responses in growth rates to environmental variability may affect populations through changes in reproductive potential, measuring spatial and temporal variability in growth rates of highly abundant zooplankton is critical to predict the impact of climate change on pelagic ecosystems.  Here, we used length frequencies from bi-annual surveys conducted one month apart to estimate growth rates of one the most euphausiids in the Southern Ocean, Thysanoessa macrura. We analyzed data from four separate years (1995, 1998, 2001, 2004) that varied widely in temperature and primary production. Stations within the surveys were grouped by water mass (warm Antarctic Circumpolar Current (ACC) water and cold Weddell Sea water) to assess spatial and inter-annual variability in growth. Growth rates of T. macrura varied between years and water masses, ranging from -0.037 mm day^-1 in Weddell Sea water in 2004 to 0.081 mm day^-1 in ACC water in 1995. Growth rates were higher in ACC water than in Weddell Sea water during all years. Growth rates were strongly correlated with temperature but were not significantly correlated with chl-a concentration. These results suggest that the growth rates of T. macrura may increase in regions exhibiting warming trends, such as the Antarctic Peninsula. This contrasts with published data on the growth rates of Euphausia superba, which is predicted to be impacted negatively by climate warming.