Trophic structure of benthic communities in the Cabo Frio upwelling system (southeastern Brazilian shelf): a temporal study using stable isotope analysis

In Brazil, coastal upwelling is observed in 7 areas along the southeastern/southern region and is most pronounced near Cabo Frio, Rio de Janeiro. This region is exposed to moderate seasonal Ekman-driven upwelling that brings cold water with increased nutrient levels nearshore and is more frequent and intense during the austral spring and summer, primarily due to the prevalence of northeasterly winds. Our aim was to verify the influence of this upwelling on the benthic trophic structure of the inner and outer shelf off Cabo Frio through measurements of sta- ble nitrogen (δ 15 N) and carbon (δ 13 C) isotopes. We identified 1 main pathway of organic matter (OM) transfer from the base to the top of the food web, as observed from the tendencies of the δ 13 C and δ 15 N values. The isotopic signatures of benthic consumers exhibited temporal and spatial vari- ability, with no interaction between them. As the result of a time lag, only consumers (mostly deca- pod carnivores) appeared to reflect the assimilation of 15 N-depleted and 13 C-enriched OM pro- duced and deposited during strong upwelling that occurred 4 mo earlier. Therefore, the intensity and period of the upwelling phenomenon were important for detecting upwelling in benthic food webs. Lighter nitrogen and heavier carbon isotopes were found on the inner shelf. Consumers may have exploited OM of different quality on the inner and outer shelf due to differences in sed- iment, hydrodynamics, mineralisation and assimilation of 13 C-enriched microphytobenthos. Nev- ertheless, 4 trophic levels were estimated in the benthic communities of the continental shelf off the Cabo Frio upwelling system, independent of period or area.