Early warning indicators of decadal shifts in the planktonic assemblage of the Cabo Frio upwelling ecosystem

A temporal regime shift in the ecosystem is characterized by changing temperature and salinity, followed by a shift in the ratio of diatom-to-dinoflagellate and copepod-to-cladoceran. • A planktonic community change occurred associated with a decadal environmental shift. • The proportion of copepods and diatoms relative to cladocerans and dinoflagellates is being diminished in the CFUS. • Long temporal trends of temperature and salinity are likely promoting dinoflagellates and cladocerans. • A cascade effect is expected in the local food web in the next years. Long-term monitoring of coupled environmental and biological components in upwelling ecosystems is critical for early warning under the global warming context. Temperature, salinity, nutrients, and plankton populations are promising indicators of the ecosystem state that help us to address the current status of the oceans and construct better predictions for the future. The Cabo Frio Upwelling System (CFUS) is a regionally productive wind-driven coastal upwelling ecosystem on the northern boundary of the South Brazilian Shelf sustaining diverse marine life including large stocks of fish and squid. Like other cold marine ecosystems, most of its functioning is likely threatened by ocean warming which emphasizes the need for ecological indicators. This study aimed to analyze the causal relationships between the temporal changes in the physical and chemical properties and the dominant planktonic communities leveraging long-term observations (20 years). The results suggested a link between the temporal changes in the ecosystem conditions and the composition of the plankton assemblage, notably an increasing proportion of dinoflagellates relative to diatoms and cladocerans relative to copepods. This increase in the proportional abundance of dinoflagellates and cladocerans over time suggests a regime shift in the plankton assemblages during the 2000s, highlighting some large phytoplankton and zooplankton groups as early indicators of productivity shifts in upwelling ecosystems.