Temperature stratification over the NW European shelf seas is showing evidence of beginning slightly earlier in the year, on average. There is no suggestion of strengthening of stratification beyond the normal inter-annual variability. Trends in stratification in regions influenced by fresh water inputs are also not apparent against the background of natural variability.

Predictions for the end of this century suggest that thermal stratification will begin typically 1 week earlier than at present, and end 5 - 10 days later. The strength of the stratification over the whole NW European shelf seas is projected to increase in response to changes in the seasonal heating cycle. Changes to coastal stratification caused by inputs of fresh water cannot yet be predicted.

What is already happening: Medium

There is good 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 UKCP09 predictions are the first attempt at regional-scale assessment of changes in the marine climate over the next century. There is broad confidence in the ability of the model to predict changes over the open shelf seas, though with some local uncertainties close to the shelf edge and in regions influenced by estuaries. The lack of confidence in the results arise from (1) this is the first and only prediction available, and (2) predictions particularly of the strength of stratification are determined by changes in regional meteorology, which is a challenging aspect of future climate projections.

The top priority knowledge gaps that need to be addressed in the short term to provide better advice to be given to policy makers are:

1. We need to improve knowledge of how regional patterns in rainfall and winds will change over the next century.
2. Assessing present changes and trends is hampered by a lack of suitable data: there is a need to maintain the recent efforts in detailed coastal observing in order that (over the next 1 or 2 decades) we reach a position of being able to provide more confident assessments of what is already happening.
3. Model skills in salinity, at the edge of the continental shelf and in coastal regions influenced by river inflows or are intermittently thermally stratified need to be improved. Confidence in the modelling of shelf sea salinity in general is much lower than in temperature

The socio-economic impacts of changes in shelf sea stratification are most likely to be felt through its role within ecosystem processes that may change the biological productivity of the ocean. This could further impact society if the changes associated with nutrient cycling were to require alteration of the monitoring and management of inputs to the marine environment or lead to difficulty meeting legislative requirements on ecological or environmental status.

1. Changes in the timing of stratification. Some fish species rely on timing their spawning with the spring bloom, in order to provide first-feeding larvae with a supply of food (Platt et al., 2003). There are known links between changes in bloom timing and changes in seabird breeding success (Scott et al., 2006), mediated by the role of fish larvae and young fish as food for chicks. Intermittent stratification in shelf seas (e.g. by the spring-neap changes in tidal turbulence) can alter the amount of phytoplankton growth (Sharples, 2008). There is large potential for changes in the intermittent nature of coastal stratification (either thermal or due to fresh water) altering primary production.
2. Changes in the length of time of stratification. Changes in the amount of time that a region spends stratified could have implications for annual primary production budgets and subsequently food availability to fish and seabirds, via the effects on growth of phytoplankton within the thermocline. Increasing the amount of time that a system remains stratified also impacts on conditions in the deep water. Stratification prevents efficient mixing of oxygen between layers. Utilisation of oxygen in the bottom layer (e.g. via the degradation of organic material by bacteria) reduces bottom water oxygen concentrations; increasing the amount of time that the bottom water is isolated from the sea surface will lead to lower oxygen concentrations, with the potential for negative impacts on benthic organisms (including important commercial shellfish); these effects could be pronounced, though limited to shallow coastal regions (Greenwood et al., 2009).
3. Changes in the strength of stratification. Stratification inhibits the transfer of substances between layers of the ocean. Stronger stratification will be more efficient at preventing these vertical transfers. On the shelf this could impact the growth of phytoplankton within the thermocline (by limiting the nutrients mixed into the thermocline), affecting the annual budget of primary production and ultimately the supply of food to the rest of the marine food chain. It will also impact bottom water oxygen concentrations (see point 2 above). This question is amenable to the work ongoing beyond UKCP09. Mixing at the shelf edge has recently been implicated as a driver of key commercial fish stocks via its impact on phytoplankton communities (Sharples et al., 2009). Mixing at the shelf edge could be very sensitive to the strength of the stratification (though in this case it is not clear whether stronger stratification will lead to more of less mixing).

Sharples, J., J. Holt, and S. Dye (2010) Shelf Sea Salinity in MCCIP Annual Report Card 2010-11, MCCIP Science Review, 6pp. www.mccip.org.uk/arc