The supply of macro-nutrients (nitrate, ammonia, phosphate and silicate) is the key driver of nutrient conditions in shelf seas. Increases in nutrient inputs above normal levels for an area can lead to a variety of deleterious effects, including oxygen depletion and mortalities of benthos and fish. Changes in the ratio of nitrogen or phosphorus to silicate in nutrient inputs can also affect the marine food web by altering the balance between diatom and other taxa in the phytoplankton community.

Nutrient inputs to shelf seas come from river inflows, rainfall and particulate deposition from the atmosphere, direct discharges of effluent to the sea, and from the open ocean as a result of currents and mixing. In some of these inputs the nutrient is essentially a natural component, and in others an anthropogenic load. Natural components include land erosion, global volcanic activity, lightning in the atmosphere, and ocean upwelling. Anthropogenic loads derive from urban waste water, agriculture, industry and fossil fuel combustion. Nitrogen and phosphorus inputs originate from both natural and anthropogenic sources, whilst silicate inputs are almost exclusively from natural processes. Current world patterns suggest that anthropogenic nutrient inputs are increasing, while inputs to European seas may be decreasing due to legislation designed to reduce emissions.

The waters around the UK are subject to a wide variety of terrestrial and anthropogenic nutrient inputs, and a range of exposures to oceanic exchange. In general, nutrient conditions in northern shelf waters are most influenced by ocean exchange, whilst terrestrial and anthropogenic inputs are more important in southern UK waters.

Climate change may affect the magnitude of natural inputs due to changing ocean upwelling and currents, and changing patterns of rainfall over the land catchments. Climate change may also affect the patterns of anthropogenic inputs, primarily through rainfall patterns and the effect on river flows. Disentangling trends in nutrient concentrations due to changing climate, human populations and industrialisation, and relating these to eutrophication status which is the major policy issue relating to nutrients, is a major scientific challenge.

What is already happening: Low

What could happen: Low

We are not really able to say with any confidence whether climate change will lead to an improvement or degradation of nutrient status. Partly this is because in most areas nutrient concentrations are a product of anthropogenic inputs, not natural climate effects.

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. Likely changes in river inputs - this research is underway.
2. Better understanding of the role and temperature dependence of denitrification and anammox - the little research on this that has been done highlights large uncertainties (Brion et al. 2004; Kelly-Gerreyn et al. 2001).
3. Changes in the flow of Atlantic water may be an important control of the North Sea ecosystem (Reid et al., 2001) but numerical models that might be used to assess these changes with climate change have only a poor skill level when determining cross-shelf exchange (Huthnance 1995, 1997).
4. The relative effects of increased storminess and increased stratification have not yet been examined for shelf sea systems.

With respect to river inputs, the current knowledge of nutrient fluxes could be improved by the inclusion of discharge estimates from un-monitored parts of the catchments (which constitute more than 25% in some areas), and ground water seepage. Using catchment models such as applied by Cugier et al. (2005b) and Lancelot et al. (2007), and with a better understanding of denitrification and anammox, it should then be possible to predict the consequences of changing rainfall and temperature for nutrient discharge to the sea.

Continued collection of time series data is required. This should be done in conjunction with new systems of monitoring using buoys and Ferryboxes. They provide the high resolution data required to deconvolute and quantify the complex set of processes that control nutrient supply and eutrophication by the validation and calibration of numerical models.

Nutrient enrichment beyond normal concentrations may lead to the symptoms of eutrophication which degrade water quality for recreational and fisheries use, and become damaging to benthos, as a result of algal blooms and de-oxygenation. Identification of a water body as eutrophic due to anthropogenic nutrient enrichment results in significant additional costs for water treatment authorities, and for agriculture in relation to measures for the reduction of nutrient leaching.

Heath, M. (2010) Nutrient Enrichment in MCCIP Annual Report Card 2010-11, MCCIP Science Review, 18pp. www.mccip.org.uk/arc