Dependence of upwelling-mediated nutrient transport on wind forcing, bottom topography and stratification in the Gulf of Finland : model experiments

Numerical simulation experiments with an eddy-resolving ocean circulation model were performed to study nutrient transport to the surface layer by summer upwelling events in the Gulf of Finland. It is shown that upwelling along the southern coast of the Gulf, produced by easterly winds, brings more nutrients to the surface layer than upwelling along the northern coast produced by westerly winds of identical strength. The different ability of upwelling to transport nutrients to the surface layer along the southern and northern coasts is explained by features in the bottom topography. A steeper bottom slope and greater sea depth along the southern coast causes more nutrients to be transported to the surface layer. Offshore transport of upwelled nutrients is mostly caused by mesoscale structures - filaments and eddies. It is also shown that upwelling events along both coasts transport nutrients into the upper layer with a clear excess of phosphate. This phosphate excess might promote nitrogen-fixing cyanobacteria due to nitrogen limitation for other phytoplankton groups.