Aquatic ecosystems

Climate change is an additional threat to aquatic ecosystems and it interacts with other human-caused stresses like pollution and fragmentation of landscape. Aquatic systems are increasingly disconnected and isolated, making adjustment to the changes through animal and plant dispersal difficult. This certainly applies to the riverine and coastal ecosystems (Poff et al., 2002). For aquatic ecosystems, a distinction is made between freshwater systems and saltwater systems. Lakes, streams, wetlands and rivers are part of the freshwater ecosystems; saltwater ecosystems contain the sea, corals and estuaries.

It is expected that freshwater lakes will be influenced through an increase in air and water temperature. Many lake systems as well as rivers and streams are shallow, well mixed, meaning they exchange heat and oxygen easily with the atmosphere. Under rising air temperatures in the coming century, the temperature of the rivers, streams and lakes will also increase. The average global surface temperature is expected to increase by 1.4-5.8°C by 2100 relative to 1990 (IPCC, 2001). If, for example, the water temperature increased by 4°C in a region, the present day ecosystems would have to migrate 680 km to higher latitudes, in order to maintain the same thermal regime (Sweeny et al., 1992). This leads to a reduction in cold water habitats and might cause a decrease in cold water species like trout and salmon and an increase in warm water species like bass (Mohseni et al., 2003). For species in isolated systems, like lakes and wetlands without corridors, the rise in temperature will mean their demise. A study on the effects of CC on lake trout in Alaskan lakes concludes that these species might not survive under future climate circumstances and this will lead to major food-web changes in Arctic lakes (McDonald et al., 1996). Besides the disappearance of invertebrate species from areas, the increase in water temperature has the potential to lead to an increase in algae blooms and pests like botulism (Poff et al., 2002).

The summer of 2003 was very dry in Northwest Europe, the catchment of the Rhine River. During the first half of the year less precipitation fell and this affected the water level of the Rhine. In August, the lowest levels ever were measured. These low levels led to problems with transport along the river and with water temperature, because the drought coincided with record-breaking temperatures. The summer of 2003 was the warmest summer in 500 years in Europe, according to the University of Bern. Fish in the river were affected by these changes. The eel, for instance, migrates during the summer from upstream areas to the sea. In 2003, many were infected by a bacterial plague, which was a result of the high water temperature. The high temperature of the water, sometimes higher than 30°C, caused them severe stress. Their poor condition and the low water level led to the death of tens of thousands of eels, as many of them were injured by the screw propeller of boats and many died as a result of their infections. At the same time, the governments in the Netherlands and Germany gave special permission to energy plants to release cooling water into the Rhine above the legislative temperature of 30°C, which further increased the heat stress on the system.

The sources of many rivers lie in mountainous regions, for example the Alps for the Rhine in Europe and the Tian Shan mountains for the Syr Darya in Central Asia. Changes in hydrology in the upper part of the basin may have implications downstream of the river (Lettenmaier et al., 1999). Aquatic ecosystems and riparian terrestrial ecosystems can be affected indirectly by changes in the mountains induced by CC. It is expected that a greater proportion of precipitation will fall as rain instead of snow and that the melting season will shift to an earlier start in the year. The summer base flow and (spring) peak flow will change as a result (Frederick and Gleick, 1999). Many species, aquatic and riparian, are sensitive to changes in the frequency, duration and timing of extreme events like floods and droughts. Some of them have adapted their reproductive strategies to avoid or to take advantage of the spring floods. An example is the salmon, which depends on the spring floods for its migration to spawning grounds (Poff et al., 2002). Changes in precipitation can lead to a reduction in inflow to lakes. When this decrease in inflow lasts for a longer time, the level of the lakes will drop and run the risk of drying out like the Aral Sea. The opposite is also possible. The level of the Caspian Sea, for example, has been rising for the last few decades, but this fluctuation is probably not driven by CC.

Wetlands are adjusted to their local circumstances with periodical floods and droughts. Water dynamics make these areas very productive. Fluctuations in the current water regime could change the timing and duration of the droughts and floods, and will affect the ecosystem. If the depth or duration of flooding changes, the area will become habitable for other species that are able to cope with the new circumstances. This will be terrestrial species when flooding decreases or aquatic species when flooding increases. Coastal wetlands and mangroves also depend on the inflow of freshwater from the rivers. The height of sea level is another factor of influence on coastal ecosystems. A reduction of inflow of fresh water, or a rise in sea level, will lead to an increase in salinity level. If these changes last for a longer period, species may not endure the strain of the increased salinity level and they will be replaced by other species that can cope with these levels (Blasco et al., 1996; Ellison et al., 2000). If the sea level rise is too fast, ecosystems cannot keep pace with this rise and they will drown. This could be prevented in regions where the supply of sediment is enough to keep up with sea level rise. This is not likely to be the case in many regions, because of the anthropogenic alterations of the rivers that prevent large quantities of sediment being transported downstream. With the disappearance of mangrove forests, the natural defence against the sea disappears. The shoreline will be vulnerable to erosion and the coastal zones will be impacted.

0 0

Post a comment