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 phytoplanktonPlanktonic marine plants. Click for link to the SAHFOS Marine Climate Change Encyclopaedia 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 thermoclineA layer of water characterised by rapid changes in temperature  - separating the upper ‘mixed-layer’ of the ocean from the cooler deep water layer - both of which have generally little relative change in temperature with depth.) 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.

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What is already happening - Medium

Overall based upon a high level of agreement between the observation and modelling studies and good understanding of the basic controls of stratification, but only a moderate amount of evidence being available (there are no long-term time-series of direct observations of stratification).

What could happen - Low

The amount of understanding is probably moderate given that we have a fairly good knowledge of how the process of stratification is driven but a low level of certainty in how the drivers will change and how they will interact.

The limited work that has taken place modelling the impact of potential future changes in the climate on stratification is currently being improved, and will be a part of the next UKCIPThe UK Climate Impacts Programme (UKCIP) provides scenarios that show how our climate might change and co-ordinates research on dealing with our future climate. report in 2008UKIP08 is the next climate change information package for the UK, consisting of five reports and an interactive website. The project brings together climate science from the Met Office to provide information to decision makers, academics and others, on the current climate, and possible future changes. Its interactive website will provide customisable climate change projections when it is launched in late 2008..

Our understanding of the climatic influence in the physics of thermoclineA layer of water characterised by rapid changes in temperature  - separating the upper ‘mixed-layer’ of the ocean from the cooler deep water layer - both of which have generally little relative change in temperature with depth. mixing and the response of associated subsurface productivity is an area of active study but is not yet able to assess the potential for future change.

The  most  direct  effect   is  that  changes   in   stratification   may  change   the  occurrence   of harmful phytoplanktonPlanktonic marine plants. Click for link to the SAHFOS Marine Climate Change Encyclopaedia that can cause the closure of shell-fisheries. Other commercial effects would be seen mainly through modification of ecosystem function leading eventually to fisheries impacts. One could also speculate that the influence of stratification on sediment transport may impact commercial interests.

Jonathan Sharples
Proudman Oceanographic Laboratory, 6 Brownlow Street, Liverpool, L3 5DA

Stephen Dye
Centre for Environment, Fisheries & Aquaculture Science (Cefas), Lowestoft Laboratory, Pakefield Road, Lowestoft, Suffolk. NR33 0HT, UK