Climate change will affect many marine organisms, either directly through their own physiology, or indirectly through changes in the distribution of prey and/or predator species. Within benthic habitats, rapid changes can lead to reduced survival of previously well-established species, while providing opportunities for other species better adapted to warmer waters. This in turn may change the structure of the community and lead to the decline of particular habitats if important species such as reef-builders are affected. Rising sea levels, coupled with possible changes in storms could cause increased erosion of intertidal sediments. In areas where fixed structures (artificial or otherwise) are present, this can result in habitat loss. Increased acidification of seawater will also adversely affect many marine organisms, particularly those possessing hard external skeletons containing calcium carbonate.

UK marine habitats

Climate change is likely to have a significant impact on coastal and shallow marine habitats, notably through sea level rise and temperature change. Considerable changes to marine communities are likely as water temperature rises, with cold-water species potentially being replaced by southern species shifting their distribution northward. In the longer term, rising CO₂ concentrations in seawater will lead to increased acidification of marine habitats, which is likely to have a negative impact on numerous groups of hard-shelled marine organisms such as corals, molluscs, crustaceans and various plankton species. These organisms are important components of many marine habitats, and changes to them will also affect their associated food webs.

Figure 6.4 Relative change of Calanus helgolandicus (a warm-water species) and Calanus finmarchicus (a coldwater species) since the late 1950s. Note: while the warm-water species is replacing the cold-water species the actual total Calanus abundance is decreasing.

Microbes and plankton

Nearly half the Earth’s basic food supply comes from photosynthesis performed by microbes and phytoplankton in the oceans. This role as the primary producer means that microbes and phytoplankton are of fundamental importance in the marine environment, both through their role as the basis of the marine food web and through the take up of carbon. Without the ability to migrate and with lifecycles that are too long for evolutionary adaptation at the rate of climate change, plankton may be particularly sensitive to acidification and warming and so particularly important in terms of climate change impacts on entire ecosystems. Microbes have lifecycles that are short relative to climate change timescales and so have a higher potential to adapt to warming and acidification. Harmful algal bloom species are thought to be primarily controlled by climate dependent factors such as mixing, temperature and circulation of the UK seas.

The biological component of the uptake of CO₂ is limited by the supply of nutrients and physical factors such as sunlight and mixed layer depth, all of which are potentially sensitive to climate change. If the oceans begin to lose their ability to take up CO₂, the effects of climate change will be felt more quickly and more severely.

The marine growing season has become longer since the late 1980s due to rising sea temperatures. In the mid- to late 1980s a sudden shift in plankton species in waters around the UK affected many aspects of the marine ecosystem. The shift is related to both the coincident pronounced change in the North Atlantic Oscillation and the rising Northern Hemisphere temperature.

Distributions of different types of plankton are likely to shift in response to a warming climate but this may not be smooth. Modelling using the IPCC A2 and B2 greenhouse gas emissions scenarios suggests that the poleward movement of the calanoid copepod Calanus finmarchicus will continue to 2100 at a rate of 1° latitude per decade and that the species will disappear from the North Sea at around that time (see also Figure 6.4). This and other associated changes in the plankton will have profound consequences for the functioning, carrying capacity for living marine resources, and conservation of UK marine ecosystems.

Earlier and longer growing seasons will change the timing of productivity within the year and may affect the lifecycles of predator species. Ocean acidification is likely to have a strong impact on planktonic ecosystems. These ecosystems may be further affected as non-native species enter warmer UK waters and pathways through the opening up of the Arctic. Changing habitats may favour certain microbial populations over others resulting in altered microbial assemblages with unknown impacts on oceanic food webs. The biological component of the ocean uptake of CO₂ is likely to be affected by warming and acidification, but we cannot yet determine plankton variability and its contribution to atmospheric levels of CO₂ at a UK, regional and global scale, and this is one of the key factors missing in global climate change modelling.

Fish

At present the overriding pressure on fish stocks comes from fisheries. However, fish have complex lifecycles with stages that have different sensitivities to climate drivers. Abundance of some warm-temperate species increased in southern UK waters during recent warming periods, and declined during cooling periods. Over the past three decades, distributions of some exploited and non-exploited North Sea fishes appear to have responded markedly to higher sea temperature by moving northward and into deeper waters.

A very different mix of fishes might exist in UK waters in years to come, including some species that are currently not native. However, as yet we have no detailed predictions – in part because of the complexity of the relationship between climate and distribution, and uncertainty about how the key drivers themselves will change.

Seals, turtles and cetaceans

We have only a poor understanding of the impact of climate change on marine mammals and turtles. In the future, climate change could influence them through for example, changes in the distribution of their prey species. It could also affect seal haul-out and turtle nesting sites, where higher incubation temperatures could lead to a female bias in the sex ratio of turtles. However, the details remain uncertain.

Marine birds

Climate change may have direct effects on seabirds through rising sea level, and changes in temperature and weather, as well as indirect effects through changes in the ecosystems that support them – for example by reducing the number and nutritional value of prey fish. There is evidence that populations of waders have redistributed within the UK and in Europe and this can in part be attributed to climate change.

As sea temperatures continue to rise, it is likely that kittiwakes and other seabirds that feed on small shoaling fish will experience poor breeding seasons with increasing frequency. The combination of reduced recruitment and lower adult survival could lead to further very large declines in population size. Rises in sea level, particularly in the Southern North Sea, may wash away coastal nesting habitat of groundnesting seabirds such as terns. Any increase in storminess could also lead to nests being washed away during the summer or to largescale mortality during winter.