IMPACTS OF CLIMATE CHANGE ON NUTRIENT ENRICHMENT
David Mills1 and David Hydes2
- Centre for Environment, Fisheries and Aquaculture Science (Cefas), Pakefield, Lowestoft
- National Oceanography Centre, Southampton
Executive Summary
Nutrient supply in the form of nitrate-nitrogen is generally considered to be the key driver of eutrophication in the marine environment. The main source of nitrate is from rivers. Inputs to rivers are complex and largely determined by human activity; legislation should tend to decrease inputs but current world patterns suggests inputs will increase, as do models looking at climate change effects on the UK. Denitrification is the major process removing nitrate from the North Sea. This requires that inputs of ocean waters are critical to maintaining concentrations in shelf sea waters. Microbiological studies and a model 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 supply to shelf seas, but our understanding of the transfer process is poor as hydrographic models do not work well in this region and observations are sparse. 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. They are not adequate for identifying climate change in the way that the Continuous Plankton Recorder surveys may have done for plankton. 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.
Level of Confidence
LOW
Understanding of climate effects on nutrient concentrations and eutrophication 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 (CPR work e.g. Beaugrand et al., 2000). If pulses of flow are occurring (e.g. Reid et al., 2001) and generating regime shifts 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 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 ammonification 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 which 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.
Because of the low availability of historical data and questions about its quality in some cases (e.g. Joint et al., 1997), the past is not the key to the future in this area of research. However, 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.
Key sources of Information
Please acknowledge this document as: Mills, D. and Hydes, D. (2006). Impacts of Climate Change on Nutrient Enrichment in Marine Climate Change Impacts Annual Report Card 2006 (Eds. Buckley, P.J, Dye, S.R. and Baxter, J.M), Online Summary Reports, MCCIP, Lowestoft, www.mccip.org.uk