Local hydrodynamic in coastal system affects the coccolithophore community at a short spatial scale

Coccolithophores are an ecologically and biogeochemically important group of calcifying marine plankton, contributing globally to both the organic carbon pump and the carbonate counter pump. Understanding their distribution in today's ecosystems is crucial for both paleoenvironmental reconstruction and for predicting their fate under climate change and environmental pollution. Traditionally regarded as open ocean dwellers, more recent studies have highlighted their importance in coastal systems. Here we describe the coccolithophore community in the Catalan Sea (Northwestern Mediterranean), off the Ebro River Delta, based on an oceanographic survey conducted in autumn 2019. We identify three environmentally distinct regimes: mixed (mostly shallow), transition (upper photic layers offshore), and stratified deep waters (below 75 m). Total coccolithophore density is driven by Emiliania huxleyi and peaks in the mixed zone where nutrient concentrations are low, but nitrite concentrations are high. We propose that peak cell densities are partly explained by a switch from nitrate to nitrite usage similar to the behavior of diatoms. Species diversity peaks in deeper offshore waters due to vertical stratification. Helicosphaera carteri and Algirosphaera robusta benefit from E.huxleyi decrease both in coastal and offshore environment.