The supply of macro-nutrients (nitrate, ammonia and phosphate) is generally considered to be the key driver of eutrophicationThe undesirable disturbance to ecosystem health and water quality that arises from nutrient enrichment caused by human activity. 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. Conversion of nitrogen species into forms unavailable for biological use, generally into nitrogen gas.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 FerryboxesFerries make repeated journeys and are used as platforms for packages of scientific instruments allowing important cost effective measurements of, for example nutrients and salinity, to be made. 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.

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Understanding of climate effects on nutrient concentrations and eutrophicationThe undesirable disturbance to ecosystem health and water quality that arises from nutrient enrichment caused by human activity. in the North Sea is poor. Insufficient data exists on changes in nutrients with time and over sufficiently large areas to be able to make similar assessments to those done for plankton (CPRDeveloped by Sir Alister Hardy the Continuous Plankton Recorder (CPR) is a towed instrument that takes continuous samples of plankton in the ocean surface, on sections that span ocean basins. CPR surveys have been in operation for 75 years and now form a long, rich and valuable time-series of observations in the many marine ecosystems. See www.sahfos.ac.uk work e.g. Beaugrand et al., 2000). If pulses of flow are occurring (e.g. Reid et al ., 2001) and generating regime shiftsA distinct change in composition of a community in a short space of time. then it would be expected that there may be changes in nutrient loads and that changes in biological activity would be feeding back into changes in nutrient concentration cycles. In the only case where nutrients have been considered as part of an analysis of regime shifts - Weijerman et al . (2005) - there is little evidence of shifts in nutrient concentrations consistent with shifts in salinity.

To better understand the likely impact of climate change on eutrophication of the North Sea the key areas that require research are:- (1) Likely changes in river inputs - this research is underway. (2) Better understanding of the role of Conversion of nitrogen species into forms unavailable for biological use, generally into nitrogen gas.denitrification - little research on this is currently been done. The paper by Brion et al . (2004) shows that estimates of its importance have large uncertainties. The consequences of increasing temperature on the ratio of denitrification to Generation of the ammonia form of nitrogenammonification are only considered in one paper (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. (4) The relative effects of increased storminess and increased stratification have not yet been examined for shelf sea systems.

Two significant time series providing information over decades are available from the CYPRIS site in the Irish Sea and the MBA E1MBA Station E1 (MBA E1) is situated approximately 20 nautical miles from Plymouth. The MBA started collecting data at E1 in 1902 and continued until funding was withdrawn in 1987. Throughout the years various parameters have been measured at these stations (temperature, salinity, nutrients, zooplankton, phytoplankton, chlorophyll, benthos). The length of the Plymouth series makes these data essential for showing environmental change over decadal scales. site in the English Channel. The CYPRIS data clearly records increases in nutrient concentrations before 1974 and levelling off since then overall concentration of nitrate are less than would be expected due to removal by denitrification in the central Irish Sea (Gowen et al ., 2002; Hydes et al ., 2004). The E1 data set has the potential to be more directly influenced by changes in ocean supply. The validity of the inter-decadal changes seen in the data has been questioned (Joint et al ., 1997), however data collected in 2006 shows that concentrations of phosphate are now similar to the relatively low concentrations observed in the 1960s (Kelly-Gerreyn et al ., 2007). Continued collection of time series data is required. This should be done in conjunction with new systems of monitoring using buoys and FerryboxesFerries make repeated journeys and are used as platforms for packages of scientific instruments allowing important cost effective measurements of, for example nutrients and salinity, to be made.. 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.

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David Hydes
National Oceanography Centre, Southampton, University of Southampton Waterfront Campus, European Way, Southampton SO14 3ZH, UK.

David Mills
Centre for Environment,Fisheries & Aquaculture Science (Cefas), Lowestoft Laboratory, Pakefield Road, Lowestoft, Suffolk. NR33 0HT, UK.

Mike Heath
Fisheries Research Services (FRS), Marine Laboratory, 375 Victoria Road, Aberdeen, AB11 9DB, UK.