We've Moved!
Visit SDSU’s new digital collections website at https://digitalcollections.sdsu.edu
Description
Euphausiids are a major component of the pelagic ecosystem in the Southern Ocean where they function as a key link between primary production and upper trophic levels. As such, understanding their life history is critical to predicting the impact of climate change on the Antarctic ecosystem. Most research has focused on Euphausia superba, which has exceptionally high biomass, and is prey for many species. However, other euphausiids like Thysanoessa macrura can be numerically dominant in many regions, feeds at a higher trophic level, and could be important for understanding the carbon flow within the Southern ocean. In order to better understand the role of this species, we developed a 17 year time series from plankton samples collected as part of the US Antarctic Marine Living Resource Program to analyze patterns of growth, variability in length-at-age, and response to environmental variability (water temperature and primary production) on seasonal and annual scales. We examine the seasonal and interannual variability in growth to examine the plasticity of growth in relation to water temperature and primary production. We examined the seasonal shift in the length frequency distribution of T. macrura during four years (1995, 1998, 2001 and 2004) to estimate daily growth rates. These four years differed in their environmental conditions, and stations within each year were further grouped by water mass (warm Antarctic Circumpolar Current (ACC) water and cold Weddell Sea water) to assess both inter-annual and spatial 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 faster 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. We then use the time series of length frequency distributions to examine length-at-age and to understand basic life history characteristics of this species. We compare our results to published estimates of length-at-age, growth, and to parameters of growth models. Our analysis suggests that current models of growth in T. macrura predict an annual growth rate that is too slow. We propose a model, based on the observed variability in summer growth rates and fit to the length frequency data using modal analysis that reclassifies age class 1 and age class 2 into a single age class that was spawned over a broad period. The length of each cohort was highly correlated with water temperature in the sampling year, supporting our hypothesis. Our model provides a more logical growth curve and a better understanding of length-at-age. Our study of the growth of T. macrura shows that the constructed length frequency distribution here and in other published studies primarily represents 1 year old animals, and thus is a good index of recruitment but not population size. If so, it suggests that larger, perhaps more fecund individuals are not effectively sampled by the current methods. Given the sensitivity of T. macrura to water temperature variability, and the fact that water temperatures are predicted to increase in the Antarctic Peninsula area, our results suggest that the growth rate and mean length-at-age may increase for T. macrura one of the most abundant euphausiids in the Southern Ocean.
