Temperature-dependent growth of Thysanoessa macrura: inter-annual and spatial variability around Elephant Island, Antarctica

Abstract

Somatic growth of pelagic invertebrates is controlled by temperature and food, both of which vary in space and time. Species-specific growth rate responses to environmental variability may affect populations through changes in reproductive potential; therefore, 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 1 month apart to estimate growth rates of one the most abundant euphausiids in the Southern Ocean, Thysanoessa macrura. We analyzed summer data from 4 separate years (1995, 1998, 2001, and 2004) that varied widely in temperature and primary production. Stations within the surveys were grouped by water characteristics: warm, low salinity Antarctic Circumpolar Current (ACC) water, and cold, saline Bransfield Strait and Weddell Sea (MBW) water, to assess inter-annual and spatial variability in cohort growth. Mid-summer cohort growth rates of T. macrura varied between years and water masses, ranging from -0.037 mm d -1 in MBW water in 2004 to 0.081 mm d -1 in ACC water in 1995. Growth rates were faster in ACC water than in MBW water during all years. Growth rates were strongly correlated with temperature (R 2 = 0.82) but weakly correlated with copepod density (R 2 = 0.38), and were not correlated with chl a concentration (R 2 = 0.11). 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.