Nutrient enrichment

The supply of macro-nutrients (nitrate, ammonia and phosphate) is generally considered to be the key driver of eutrophication of shelf seas. Current world patterns suggest inputs are increasing, while inputs to European seas may be decreasing. If summers get drier, these inputs may continue to decrease although sudden summer storms may deliver nutrient pulses with consequences that are difficult to predict. Denitrification is the major process that removes nitrate from the North Sea. Consequently inputs of ocean waters are critical to maintaining concentrations in shelf sea waters. Studies suggest increased temperatures may decrease denitrification. Higher concentrations of nitrate may lead to a switch to phosphate as the limiting nutrient. Increased storminess will increase concentrations of nutrients at the ocean surface and may increase transfer into shelf seas. Our understanding of the transfer process is poor. Models of productivity in the ocean in a warmer climate suggest increased stratification in summer will limit nutrient supply to surface waters during the productive seasons and inhibit mixing due to storms in winter. Similar model scenarios have not yet been run for shelf seas. The few existing long-term data sets have proved useful in identifying the path of eutrophication and relative impacts in different regions of the North Sea. The data record changes in nutrient concentration but it has proved extremely difficult to discriminate between the effects of human discharges and those which may be due to climate change through rainfall and ocean transport. New systems of monitoring using buoys and Ferryboxes have the potential when used with numerical models to improve our ability to deconvolute and quantify the complex set of processes that control nutrient supply and eutrophication