Impact of global climate change on individual ecosystems F 4.2 235 (Fitzharris, 1996). Chapin and Shaver (1996) have proven by experiment a possible change in the composition of species in tundra ecosystems on the basis of increased CO 2 concentration and temperature. Over nine years the mean temperature of a tundra area in Alaska was increased by 3.5 ºC andthe lighting strength and nutrient levels modified. The populations ofthe most common plant species increased, by contrast the lichen and herb species that form the main food source for caribous declined. The international tundra experiment has not yet been able to confirm such a clear reaction to climate change. The different reactions of individual plant species in Sweden, Canada and Alaska observed over many years varied to the point that no general conclusion may be drawn for growth or biotope composition in tundra regions (Henry, 1997). F 4.2.3 Coastal ecosystems Coastal ecosystems are particularly sensitive to climate change, such as for example mangrove forests, coastal marshes, estuaries and river deltas, dune systems, low islands and coral reefs; the latter are discussed in the next section (IPCC, 1996b; Markham et al, 1993). The rate of sea level rise is crucial in that context as is the strength and frequency of storms (Markham et al, 1993). However, the change in the transportation of sediment in the rivers by dam construction and other flood protection measures also influence coastal ecosystems. Over longer periods of time coastlines fall and rise as a result of geological processes. The Mississippi delta accounts for around 41 per cent of total coastal wetlands in the US and in this century an area of around 40 hectares has been lost every day. Climate scenarios (business as usual) predict in this context by the year 2100 an additional loss of 39 per cent ofthe current area (Reid and Trexler, 1991).This jeopardizes numerous animal and plant species and at the same time an effective CO 2 sink is being lost. Mangroves cover an area of 20 million hectares or 25 per cent ofthe tropical coastline. The 34 known tree species from nine families form a unique habitat with typical adaptation such as air roots and other halophyte strategies. Reconstructions of past situations indicate that mangroves can only tolerate sea level rises of up to 12cm a century (Ellison, 1994) – at that rate they would have extreme difficulty adapting to the predicted sea level rises. Mangroves are also at risk, however, by increased water temperatures and changes in salinity and sediment levels. Climatic changes also have far-reaching ecological consequences for other flat sedimental coasts, such as for example the German North Sea coast (Reise, 1993). First of all probably at a warming of 2–4 ºC there will be an influx via the English Channel of species that are similar to the Eem period in the last interglacial and could increase the diversity by 20–40 per cent. With temperature changes however there would not just be an expansion ofthe species, but also a change in their vertical distribution by tidal zone. The frequency of harsh winters has a particular impact on the populations in the Wadden Sea (initially mass extinction and migration of fauna, but then in the subsequent summer above-average settlement of young animals with high biomass). Warming would therefore tend to create high biodiversity, but lower biomass with crucially reduced food supply for fish and sea birds. Higher summer temperatures would probably intensify the impact of hypertrophy (green algae mats, oxygen deficiency in sediment) (Reise, 1993). Overall, the North Sea mudflats are expected to have a reduced capacity to store and remineralize organic materials. F 4.2.4 Coral reefs Coral reefs can exist at water temperatures between 18 and 30°C. They achieve optimum growth between 25 and 29°C, this being just barely below their upper lethal temperature limit. Increased water temperatures triggered by climate change could therefore impair the capacity of reefs to live and function or at least increase their vulnerability to other stress factors. The clearest impact of increased temperature is the bleaching of coral. The coral lose 60–90 per cent oftheir zooxanthels (monocellular algae) that live in symbiosis with them.The zooxanthels also lose 50–80 per cent oftheir photosynthetic pigments (Hoegh- Guldberg and Smith, 1989; Kleppel et al, 1989; Porter et al, 1989). Corals can recover again within a few weeks or months by regenerating zooxanthels if the stress factors are removed (Wilkerson et al, 1988). Immediately after bleaching corals demonstrate reduced skeletal growth and interruption oftheir reproductive cycle. In addition, their resistance, for instance to coverage by algae, is strongly impaired. If external stress factors persist, the coral polyps ultimately die off. In Panama, Colombia and on the Galapagos Islands some rare coral species have already disappeared locally (Glynn and de Weerdt, 1991). When reef-forming corals die reefs are settled by other benthic fauna. Since animals that use coral as a food source are not impaired to the same degree by the stress factors, the feeding pressure on the corals can increase and bring about additional losses (Glynn, 1996).
236 F The biosphere in the Earth System Locations of observed coral bleaching by year 1969–1978 1979–1986 1987–1996 Figure F 4.2-1 Distribution of coral reefs (green) and coral bleaching from 1969 to 1996. Source: adapted from Bryant et al, 1998 In principle, the bleaching ofthe corals is a nonspecific reaction to various stress factors of which too high water temperature and extended summer temperature highs, as occur above all in El-Niño years are the most important. A decline in UV absorbing pigments in the corals was also observed at increased temperatures, thus increasing their sensitivity to UV radiation (Lesser et al, 1990; Glynn, 1993). In rare cases the addition of freshwater contributes to the coral bleaching. Pollutants such as copper or herbicides can also result in local bleaching (Glynn et al, 1984). Coral reef bleaching was first observed in 1963 on the coral reefs off the Southern coast of Jamaica (Goreau, 1964). The large-scale bleaching in the years 1982/83 and 1991/92 was presumably caused by the persistent warming ofthe sea in connection with the El-Niño phenomenon in those years (Glynn, 1993).The large-scale bleaching in 1994 did not however coincide with an El-Niño occurrence (Fig. F 4.2- 1; Glynn, 1996). The El-Niño year 1998 was the warmest in the last 600 years and in some areas resulted in bleaching from 60–100 per cent in the corals. Dead or acutely damaged corals occurred above all in the Caribbean, the Eastern Pacific andthe Indian Ocean (Hoegh-Guldberg, 1999). The accumulated occurrence of coral bleaching in the last few years indicates a connection with climate change. Given the great importance of zooxanthels for the process of forming the calcareous skeleton ofthe reef-forming corals, large areas of reef constructions could become weakened (Glynn, 1996) which would threaten numerous other species of flora and fauna that use the reef for protection, food and reproduction (Wilkinson and Buddemeier, 1994). In addition to an impairment ofthe habitat function of reefs, important use functions ofthe reefs would be affected such as coral reef fisheries, tourism andthe erosion-inhibiting effect of reefs for coastal protection. In the longer term the sea level rise constitutes another potential threat to the coral reefs. Under the climate scenarios that are currently in place, a worldwide rise in sea level of up to 1m for the next 100 years seems possible (IPCC, 1995). Although coral reefs were often impacted in the geological past by fluctuations in sea level and changes in the atmospheric CO 2 concentration (MacCracken et al, 1990) it is questionable whether they would ever be in the position in the foreseeable future to respond to swift changes in the sea level with increased growth. Particularly, the swift-growing shallow water coral species that could adapt well are particularly sensitive to temperature increases and increased feeding pressure (Glynn, 1990). The short time periods available are possibly not sufficient for the corals andtheir zooxanthels to adapt to the new conditions (Jokiel and Coles, 1990; Glynn, 1993).