Front‐Eddy Influence on Water Column Properties, Phytoplankton Community Structure, and Cross‐Shelf Exchange of Diatom Taxa in the Shelf‐Slope Area off Concepción (∼36–37°S). Issue 11 (21st November 2017) - Morales, Carmen E.
Abstract: In eastern boundary current systems (EBCSs), submesoscale to mesocale variability contributes to cross‐shore exchanges of water properties, nutrients, and plankton. Data from a short‐term summer survey and satellite time series (January–February 2014) were used to characterize submesoscale variability in oceanographic conditions and phytoplankton distribution across the coastal upwelling and coastal transition zones north of Punta Lavapié, and to explore cross‐shelf exchanges of diatom taxa. A thermohaline front (FRN‐1) flanked by a mesoscale anticyclonic intrathermocline eddy (ITE‐1), or mode‐water eddy, persisted during the time series and the survey was undertaken during a wind relaxation event. At the survey time, ITE‐1 contributed to an onshore intrusion of warm oceanic waters (southern section) and an offshore advection of cold coastal waters (northern section), with the latter forming a cold, high chlorophyll‐a filament. In situ phytoplankton and diatom biomasses were highest at the surface in FRN‐1 and at the subsurface in ITE‐1, whereas values in the coastal zone were lower and dominated by smaller cells. Diatom species typical of the coastal zone and species dominant in oceanic waters were both found in the FRN‐1 and ITE‐1 interaction area, suggesting that this mixture was the result of both offshore and onshore advection. Overall, front‐eddy interactions in EBCSs could enhance cross‐shelf exchanges of coastal and oceanic plankton, as well as sustain phytoplankton growth in the slope area through localized upward injections of nutrients in the frontal zone, combined with ITE‐induced advection and vertical nutrient inputs to the surface layer. Plain Language Summary: The oceans exhibit high submesoscale (a few km in the horizontal axis) variability in physical processes, which influence plakton distribution and ecosystem dynamics. In this study, two common features in the oceans, fronts and eddies, were analyzed in terms of the submesoscale variability generated by their interaction in a region of coastal upwelling. We found that front‐eddy interactions contribute to the transport of phytoplankton between the coast and the oceanic zone. This finding is surprising since, until now, it was thought that eddies in coastal upwelling areas contributed to a mostly offshore transport of plankton but not the other way around. This observation is relevant to understand potential changes in submesoscale dynamics under a climate change scenario and to predict the response of phytoplankton species to these changes. Key Points: Front‐eddy interaction in the early stages of intrathermocline eddy formation in the slope area off Concepción Intense submesoscale variability in nutrients and phytoplankton across the coastal upwelling and coastal transition zones A mixture of coastal and oceanic diatom species in the area of front‐eddy interaction imply both offshore and onshore advection