The Role of Deep Winter Mixing and Wind‐Driven Surface Ekman Transport in Supplying Oceanic Nitrate to a Temperate Shelf Sea

Abstract

AbstractNutrient availability across temperate continental shelves just before the onset of seasonal stratification is an important control on the spring phytoplankton bloom. However, shelf‐scale quantification of nitrate supply from the open ocean during winter, a major source of shelf nitrate, is lacking. We used an objective analysis of subsurface ocean temperature (2000–2021), a nitrate climatology and an atmosphere reanalysis to quantify: (a) the interannual variability of the winter surface mixed layer depth (MLD) along the North West (NW) European shelf break, (b) oceanic surface nitrate concentration following deep winter mixing (i.e., nitrate recharge), and (c) the pre‐bloom wind‐driven cross‐shelf surface Ekman transport. Our results show clear latitude‐dependent regimes. In the north, across the Rockall‐Malin and Hebrides shelves, winter winds drive an average on‐shelf surface nitrate transport of 6.9 and 13.1 mmol m−1 s−1, respectively. In the south, adjacent to the Celtic and Armorican shelves, large year‐to‐year variability in the MLD across strong subsurface vertical nitrate gradients results in significant year‐to‐year variability (up to ∼6 mmol m−3) in the winter nitrate recharge. However, the winter surface Ekman transport is off‐shelf with an average magnitude of 3.7 and 5.4 mmol m−1 s−1 across the Armorican and Celtic Sea shelves, respectively. Our study provides the first shelf‐scale estimates of interannual variability in the winter wind‐driven surface nitrate supply to the NW European Shelf. The limitations of this study imposed by reliance on a nitrate climatology highlights the need for sustained biogeochemical sampling across the shelf slope to better understand cross‐shelf nitrate transport processes.