A zooplankton d13C and d15N isoscape of San Jorge Gulf, Argentina: the potential for tracking movement and habitat use of top predators over small spatial scales

GALVAN DE; NEWSOME SD; FUNES M; PAPARAZZO F

Viña del Mar

Conferencia; 11th International Conference on the Applications of Stable Isotope Techniques to Ecological Studies; 2018

Universidad Andrés Bello

Isoscapes of (baseline) δ13C and δ15N values at the base of food webs provide useful gradients to track animal movements, especially for highly mobile marine top predators that move large distances during their annual life cycle. Besides, quantifying sources of variation in baseline values is necessary to estimate and compare trophic levels and bento-pelagic reliance among species from different sites. In pelagic marine ecosystems, a variety of physicochemical factors influence the δ13C and δ15N of phytoplankton at the base of the food web. Growth rate is one of the primary factors influencing δ13C, while δ15N is influenced by (1) the extent of vertical mixing of the water column that regulates the source and amount of nitrogen available for primary production, and (2) the degree of recycling of nitrogen. Here we reconstruct a δ13C and δ15N zooplankton isoscape for the San Jorge Gulf (40 000 km2) off the southern Patagonian coast of Argentina, a unique ecological hotspot for seabirds and marine mammals that also supports a lucrative prawn fishery (50 000 tons.y-1). In this region, high productivity depends in two oceanographic fronts at the southern and northern margins of the gulf. Isoscapes were reconstructed using zooplankton sized between 200?300μm collected from a regular grid of stations in November of 2016; a series of physical (e.g., temperature, salinity, density) and chemical (e.g., nitrate, phosphate, silicic acid) variables were also measured at each station. Zooplankton δ13C values varied between -24.7? and -17.5?. Lower δ13C values were observed in the center of the gulf where waters are more strongly stratified, while the northern and the southern margins of the gulf where greater mixing occurs had higher δ13C values. δ15N values varied between 5.6? and 13.5? and generally increased from south to north, which closely correlates with the input of nutrients from deep cold waters along the southern margin and higher rates of nutrient recycling along the northern front. These baseline gradients occur over relatively small scales (200?300 km), and may enable the tracking of movement and habitat use of the diverse community of top predators in this region.