Phytoplankton bloom phenomena in the North Atlantic Ocean and Arabian Sea

We review bio-optical and physical data from three mooring experiments, the Marine Light-Mixed Layers programme in spring 1989 and 1991 in the Iceland Basin (59 degrees N/21 degrees W), and the Forced Upper Ocean Dynamics Experiment in the central Arabian Sea from October 1994 to 1995 (15.5 degrees N/61.5 degrees E). In the Iceland Basin, from mid-April to mid-June in 1989, chlorophyll-a concentrations are sensitive to small changes in stratification, with intermittent increases early in the record. The spring increase occurs after 20 May, coincident with persistent water column stratification. In 1991, the bloom occurs 2 weeks earlier than in 1989, with a background of strong short-term and diurnal variability in mixed layer depth and minimal horizontal advection. In the Arabian Sea, the mixing response to the northeast and southwest monsoons, plus the response to mesoscale eddies, produces four blooms over the annual cycle. The mixed layer depth in the Arabian Sea never exceeds the euphotic zone, allowing interactions between phytoplankton and grazer populations to become important. For all three mooring experiments, change in water column stratification is key in producing phytoplankton blooms.