The maintenance of the subsurface chlorophyll maximum in the stratified western Irish Sea

Abstract

Lay AbstractOne fundamental aim of marine science is to be able to accurately quantify and predict how mixing in the ocean can affect primary production and the global carbon budget. Shelf seas are the boundary between the coastal regions and the deep ocean. They are important areas for fisheries, as well as for the absorption of carbon from the atmosphere by microscopic organisms called phytoplankton. Phytoplankton living in the well lit surface layer of temperate shelf seas (between latitudes 23.5° and 66.5°) during summer rely on the turbulent supply of nutrients to sustain their growth. By using an instrument that is able to measure fine‐scale ocean currents, we are able to quantify mixing rates in the ocean. The chemical analysis of nutrients in seawater combined with these physical measurements allowed us to quantify the turbulent supply of nutrients from the deep ocean to surface water where phytoplankton live and thus estimate the importance of various mixing mechanisms to biological processes. In the western Irish Sea, we found that, for primary production to be maintained during summer, relatively large‐scale mixing events must take place to supply the nutrients required by phytoplankton. The background mixing rate does not supply sufficient nutrients to phytoplankton, and thus storms, enhanced tidal mixing, or both are likely to be vital in sustaining primary production in this marine ecosystem.