Nutrient Enrichment

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.