you also have to take in to account stratification of the water, during the rough weather of winter the water column gets completely mixed up are as near as damm it, so there's lots of nutrients in the water, then as the spring/summer arrives the weather calms down, the surface of the water warms stratification begins helping trap phytoplankton in the surface where because of the conditions of warmth, sunlight and availble food begins to bloom, but because your now not getting any mixing of the water, once all the nutrients have been used up, no more food for the phytoplankton to grow so it dies off.
kev