Climate Change

Climate change can have an effect on many physical processes that influence physics, biogeochemistry and the lower food web, resulting in changes that are not straightforward to predict (Holt et al., 2016).

Climate change may also have an impact on the level of nutrient enrichment itself. Climate change could lead to changes in circulation patterns and occurrence and in duration of stratification, which could impact nutrient levels and primary production. Expectations about the direction of the effects are uncertain, however (Holt et al., 2016, Schrum et al., 2016).

In general, a freshening of the Greater North Sea is expected owing to increased river discharges, changes in ocean- shelf exchange and increased outflow from the Baltic Sea, but again the uncertainty is large (Schrum et al., 2016; Holt et al., 2016). Conversely, increases in drought conditions are also expected. A short drought in the Kattegat catchment in summer 2018 resulted in reduced crop yield. This left a nutrient excess in farmland that in turn resulted in a higher winter nutrient load. Changes in seasonal rainfall patterns in either direction can change both the intensity and frequency of nutrient inputs where sudden, large events can cause excessive flooding and increased inputs into the marine environment. It is expected that climate change will result in more hydrological extremes and higher river discharges, particularly in the northern parts of the North Sea (Willems and Lloyd-Hughes, 2016). Under future climate change scenarios, generally lower annual mean river flow in Region IV and higher annual mean river flow in Regions I, II and III is expected (see Climate Change TA ). Increased nutrient loading could be expected if river discharges increase, but this also depends to a large extent on future land use and socio-economic developments (Arheimer et al., 2012; Bartosova et al., 2019).

Climate change may also have an impact on the direct and indirect effects of nutrient enrichment. Increased water temperatures have been shown to lead to phenological shifts, biogeographical changes and changes in abundance of plankton (see Brander et al., 2016 for an overview). With changes in phytoplankton composition, changes in chlorophyll concentrations and primary production can be expected.

The indirect effects of eutrophication on oxygen concentrations in the near-bottom layer now show only localised but persistent areas of oxygen deficiency in OSPAR Regions II and IV. Climate change can impact upon dissolved oxygen concentration in many ways, most evidently via the direct effect on solubility, but it can also increase metabolic rates and oxygen demand and increase stratification, which inhibits the supply of oxygenated waters to lower levels. The duration of stratification is expected to increase and regions that show oxygen depletion are expected to become larger (Wakelin et al., 2020).

It is also important to consider Ocean Acidification and its interaction with eutrophication effects. There are likely to be changes in our activities to reduce GHG emissions which could affect the scale of the eutrophication impacts. There is also the need to consider the role of eutrophication in oxygen sags and how we will separate out this response from climate change impacts. Ultimately, managing non-climate-related pressures through programmes of measures to reduce eutrophication will result in a more resilient ecosystem, benefiting both humans and the environment they depend on.