Zooplankton-based delta C-13 and delta N-15 isoscapes from the outer continental shelf and slope in the subtropical western South Atlantic

Characterizing the patterns of stable carbon (delta C-13) and nitrogen (delta N-15) isotopes at the base of the food webs (baseline) is essential in ecological studies for assessing the feeding habits and migration patterns of marine predators. We analysed delta C-13 and delta N-15 in zooplankton samples collected in spring and autumn (2012-2015), along the south (SCM) and southeast (NCSM) Brazilian oceanic waters. An increase in delta C-13 from the shelf break towards the offshore was associated with the influence of continental water input and upwellings along the shelf break, which introduce C-13-depleted inorganic carbon to the surface, where it will be incorporated into the biological system through phytoplankton growth. An opposite trend was observed in delta N-15, with higher values along the shelf break related to the intrusion of upwelled deep-water nutrients, contrasting with the oligotrophic offshore waters where N-2 fixation takes place, resulting in primary and secondary production that is relatively more depleted in N-15. A latitudinal (north-south) decrease in delta C-13 along the offshore area coincided with known isotopic patterns in inorganic carbon, which were more C-13-depleted towards higher latitudes. On the other hand, an increase in delta N-15 was observed towards the south, reflecting the contrast between the N-2 fixation in the more oligotrophic waters in the NCSM and the higher nutrient availability in the SCM. Additionally, delta C-13 was significantly higher in autumn and delta N-15 was higher in spring. Although inter-annual differences in delta N-15 were non-significant, delta C-13 was significantly lower in 2012 than in the later years. This study provides novel information regarding the patterns of baseline delta C-13 and delta N-15 in the southwestern Atlantic Ocean, which will be useful for future investigation of the ecology of higher trophic-level organisms occurring in this area.