Nutrient Enrichment

Heath, Michael; (2010) Nutrient Enrichment. In: Annual Report Card 2010-11, MCCIP Science Review. Marine Climate Change Impacts Partnership.

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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.