A Strong Link Between Oceanographic Conditions and Zooplankton δ¹³C and δ¹⁵N Values in the San Jorge Gulf, Argentina

Maps of (baseline) δ¹³C and δ¹⁵N values of primary producers or consumers near the base of food webs provide crucial information for interpreting patterns in the isotopic composition of consumers that occupy higher trophic levels. In marine systems, understanding how oceanographic variables influence these values enables the creation of dynamic isoscapes across time and space, providing insights into how ecosystems function. The San Jorge Gulf (SJG) in the southwest Atlantic Ocean (45° S-47° S) is an area of particular importance, as it is located on one of the most productive continental shelves in the world, supporting large fisheries and marine mammal and seabird populations. We reconstructed spatial variation in zooplankton δ¹³C and δ¹⁵N values across SJG and investigated their relationship with physical and chemical oceanographic conditions. During cruises in the austral spring of 2016 and 2017, we collected medium-sized copepods whose isotopic composition integrate short-term (days to weeks) variation in oceanographic conditions recorded by phytoplankton at the base of the food web. We also collected data on water column depth, surface and bottom temperatures, water column stability, and macronutrient (nitrate, phosphate, and silicic acid) concentrations. The results revealed significant variation in both δ¹³C and δ¹⁵N values of up to 7-8‱ over a relatively small spatial scale (200-300 km). Copepod δ¹³C values were lower at the center of the SJG, showing an inverse correlation with water column stability, surface nitrate concentration, and water column depth. δ¹⁵N values showed a strong and negative relationship with surface nitrate concentration and water column stability, increasing from south to north in the SJG. δ¹⁵N values also showed a positive relationship with surface silicic acid concentration. These spatial patterns in nutrient dynamics and copepod carbon and nitrogen isotope values are interpreted in the context of the dominant northward current and temporal development of the frontal systems in the SJG.