Effects of global climate change on European marine biodiversity

Global warming has a range of effects on marine systems. The effects may be related to changing water temperatures, changing water circulation or changing habitat; as a consequence of these changes, altered pathways within biogeochemical cycles and food webs are detected as well. In the first case, the biological responses and impacts result from the physical effects.

Even without human-induced climate change, the biodiversity and biogeography of species is continuously changing (seasonal and yearly changes). Consequently, long term monitoring is necessary in order to evaluate these processes. The marine systems however may become more dynamic and variable due to climate change.

Europe may be less threatened by sea-level rise than many developing country regions. However, coastal ecosystems do appear to be threatened, especially enclosed seas such as the Baltic, the Mediterranean and the Black Sea. These seas have only small and primarily east-west orientated movement corridors, which may restrict northward displacement of organisms in these areas.

Effects on primary production

Higher temperatures and enhanced stratification could affect the productivity of phytoplankton. A number of models predict an increase in global primary production of between 1% and 8% by 2050, when compared to pre-industrial times (SAMIENTO 2004 cit in 1) Because phytoplankton is an important basis of the marine food web, any change in the timing, abundance or species composition of the phytoplankton will have an effect on the whole food web.

Effects on the recruitment process

The population dynamics of a lot of marine vertebrates and fish are driven by recruitment processes. The recruitment of cold temperate species is often synchronized with seasonal production cycles of phytoplankton. Increasing sea water temperatures may advance the timing of reproduction of these fish species; this may result in a mismatch with their food source (phytoplankton) (match/mismatch hypothesis). A change in recruitment success will lead to shifts in species composition.

Example: cod recruitment in the North Sea

The Atlantic cod (Gadus morhua) recruitment in the North Sea, in the past 40 years, was influenced by changes at the base of the food web (bottom-up-control), induced by the rise of temperature. Cod recruitment decreased from the mid-1980s, coincident with unfavorable changes in the plankton ecosystem. Fig 1

Effects on the biogeography

The species movement in a warming area is towards the poles in general. Since global warming accelerated in the late 1980s, pole ward advances of southern species and retreats of northern species have been recorded in zooplankton, fish and benthic species (Brander et al. 2003; Southward et al. 2005 cit in 1). The species distribution is not always northwards. For example when the stock of the main prey of the harp seals in the Barents Sea collapsed, these seals migrated southwards along the coast of Norway and into the North Sea in search of food.

Example: Study of barnacles in the Celtic-Biscay shelf

The Celtic-Biscay shelf was liable to warming in the 1930; in the 1960 there was a switch back to colder. Changes in species assemblages were described on rocky shores, using barnacles as a sensitive indicator of wider changes in marine life. (Southward et al. 1995, 2005). These showed switches between warm water barnacles in the 1950s (Chthamalus spp.) to greater dominance by the cold water barnacle Semibalanus balanoides in the 1960s and 1970s. On rocky shores warm water barnacles now exceed the levels found in the 1950s. (Fig 2)

Effects on the phenological relationships and community structure

The response to climate changes differs between the species, inducing a decoupling of phenological relationships (relative timing of life cycle events). The decoupling may affect the community structure and food webs by altering the interactions between a species and its competitors, mutualists, predators, prey or pathogens. For example, in the case of seabirds, chick diet composition during development is likely to be an important mechanistic link between climate variability and the observed decline in seabird populations (Kitaysky et al. 2005 cit in 1)

Effects on the establishment on invasive species

The establishment of non-indigenous species can be accelerated by rapid warming. For example, the recent warming has accelerated the adaptation of the Pacific oyster (Crassostrea gigas) on the local circumstances in the Netherlands and the UK (Reise et al. 2004 cit. in 1).