Zonation of protistan plankton in a productive area of the Patagonian shelf: Potential implications for the anchovy distribution

Abstract

In the southwestern Atlantic Shelf, in northern Patagonia, the coastal-inner shelf area of El Rincón (38°-41°S) is relevant due to retention processes and the occurrence of a mid-shelf front (MSF), which provide favorable habitat for the anchovy Engraulis anchoita, from which other commercial species feed on. Here, we assess the protistan plankton (>5 μm) composition and biomass during spring and summer of different years to explore the community structure at the base of the food web and its potential relation with the heterogenic spatial distribution of the anchovy's life stages previously reported in the literature. Our in situ data together with analyses of surface circulation patterns and satellite chlorophyll a denote a marked zonation of phytoplankton. We find that diatoms are the largest carbon contributors in the coastal sector and dinoflagellates near the MSF, where large copepods and mainly juveniles and adults of E. anchoita are found. In turn, transient small-scale eddies develop in the low chlorophyll central area dominated by nanoflagellates and ciliates, where small copepods and eggs and larvae of the anchovy dominate. Our results provide insights of the potential link between plankton assemblages, hydrological settings and developing stages of a highly exploited fish. Moreover, we contribute to biodiversity records used in global assessments of possible shifts at the population and community levels that might have cascading effects on marine ecosystems' productivity.