MCCIP Annual Report Card 2007-2008 Scientific Review - Shelf Sea Stratification and the Spring Bloom Topic Shelf Sea Stratification and the Spring Bloom Author(s) Jonathan Sharples 1 , Stephen Dye 2 Organisation(s) represented 1 Proudman Oceanographic Laboratory, 6 Brownlow Street, Liverpool, L3 5DA 2 Centre for Environment,Fisheries & Aquaculture Science (Cefas), Lowestoft Laboratory, Pakefield Road, Lowestoft, Suffolk. NR33 0HT, UK Executive summary Freshwater Stratification: As stratification of the regions of freshwater influence in Liverpool Bay and the southern North Sea depends on the balance between the rate of supply of the estuarine water and the strength of the mixing processes, changes in winds and rainfall will modify this balance. Onset of Thermal Stratification and the Spring Bloom: Away from sources of freshwater, large areas of the UK shelf seas stratify in response to sunlight in spring and summer (e.g. the Celtic Sea, the North Sea north of Dogger Bank, the Malin Sea). The onset of thermal stratification in spring locks phytoplankton in the surface layer, where they receive lots of sunlight and grow rapidly. Following limited growth through the winter this “spring bloom” is the year’s first appearance of significant concentrations of organic fuel to feed the rest of the ecosystem. There is evidence of a recent trend to earlier stratification and blooms largely in response to warming air temperatures. Our understanding of how shallow seas respond to meteorology suggest that stratification and the associated spring bloom will, on average, occur earlier in a warmer climate. Sub-surface Production The interface between the warm surface layer and the deeper cold water (the thermocline ) becomes a layer of significant growth for phytoplankton once the spring bloom has decayed. Growth here is a response to the supply of sunlight from the surface and weak flow of nutrients mixing up from deeper water. These controlling factors are likely to be influenced by changing climate but exactly how remains an area of limited understanding. 1