Seasonal and spatial variations in macrozooplankton quality as food for fish in a Southwest Atlantic Ocean gulf – the role of lipids and fatty acids

Understanding the nutritional quality of macrozooplankton is crucial for elucidating energy and matter fluxes in marine food webs and their value as fish prey. This study examines the seasonal (winter, spring, summer) and spatial variability in three quality indicators -energy density (ED), polyunsaturated fatty acids (PUFA) content, and DHA/EPA ratio- of Euphausia spp. and Grimothea gregaria. These indicators were analyzed in relation to size and environmental variables (temperature, salinity, and chlorophyll-a concentration) in the San Jorge Gulf (45°–47°S, 65°30′-67°30ʹW; Southwest Atlantic Ocean), a key feeding ground for commercially important fish on the Argentine shelf. Results showed that ED increased with size in Euphausia spp. but decreased in G. gregaria. Seasonal changes were the primary drivers of variations in the nutritional quality, with PUFA content being the index most contributing. For both species, the highest mean PUFA content occurred in winter and the lowest in summer, an inverse pattern to ED. The DHA/EPA ratio also peaked in winter and dropped in spring. Species differences significantly affected PUFA content and the DHA/EPA ratio, with Euphausia spp. exhibiting higher values. Although environmental variables, particularly bottom temperature and chlorophyll-a, varied significantly across sampling areas, no clear spatial patterns emerged in the quality indicators, likely due to the limited sample size. These results provide new and baseline information of zooplankton that can clarify the trophic interactions of many fish and their respective predators along the Argentinean Shelf, and how these relationships may change with environmental variability in the current global change context.