How islands cause phytoplankton to bloom in their wakes

The development of phytoplankton blooms in Von Kàrmàn vortex streets in island wakes has been investigated using a coupled bio‐physical model which evolves a geostrophically balanced flow past a relatively small (L = 10 km) oceanic island. Three major processes associated with the “island mass effect” are found to occur in series in our numerical experiment. First, increases in phytoplankton were observed in the lee due solely to passive advection from the deep maximum. Second, following the shedding of cyclonic eddies, upwelled nitrate enhances local primary production in the surface euphotic layer resulting in a phytoplankton bloom. The bloom peaks appeared about two weeks downstream from the island. Third, a weaker bloom is also found in the immediate lee of the island, associated with nutrient injection and longer residence time of upwelled water. The demonstrated detailed features of island mass effect are remarkably consonant with previously observed phenomena in the real ocean.