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In the next century, models predict average water temperatures will increase by 1 to 7 degrees celsius (IPCC, 2001). Many life processes in of marine animals and plants are dependent on water temperature, and could be significantly altered by a rise of even a few degrees in temperature (Harley, 2006). Higher temperature waters, such as those in the tropics have less primary production in the form of phytoplankton, which almost all fish derive their energy from. . This is mainly due to the fact that a greater temperature gradient causes more intense stratification of the water, thus weakening the upwelling of cool, nutrient-rich water to the surface. A decrease in primary production causes a decrease in the numbers of individuals at higher trophic levels, such as carnivorous fish including fish that depend on phytoplankton for food(Harley, 2006). Therefore, in many cases, it is reasonable to believe that warming of seawater will cause fish stocks to decrease (Harley, 2006).
Changes in the temperature of ocean water have the potential to cause significant changes in water chemistry. Addition of fresh water from melting ice caps decreases the salinity of ocean regions, which can be detrimental to species with low tolerances to changes in salinity (Harley, 2006). Also, as seawater warms, its ability to dissolve gases decreases dramatically. One of these gases is oxygen, which is essential to all animals for respiration (Harley, 2006). Geological records from past global warming events has shown evidence of severe, large-scale hypoxic episodes, sometimes reaching global scales (Bralower, 2002). A significant drop in dissolved oxygen levels would have a detrimental effect on many species. Another critical area of seawater chemistry that will likely be affected by global warming is the carbonate buffering system. The ocean have an enormous capacity to take up carbon dioxide. However, as atmospheric carbon dioxide levels rise, the equilibrium of the carbonate-bicarbonate-carbonic acid cycle will shift towards greater amounts of carbonic acid, lowering the pH of the water (Harley, 2006). Like temperature, there are many species who are sensitive to even small changes in pH. Ocean acidification would have detrimental effects on sea life, especially important calcareous primary producers, such as coccolithophores, and animals that posses carbonate shells. There is also geological data which indicates build up of toxins in the ocean during intense global warming events (Bralower, 2002).
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Climate change will likely change the geographical distribution of many species. For instance as temperature rises, many species will have to shift tohigher to higher latitudes in order to remain under similar environmental conditions (Harley, 2006). These species shifts can introduce alien species to ecosystems where they had not been previously present, which can fundamentally alter these ecosystems. For instance, huge swarms of mauve stingers (Pelagia nocticula), which can devastate populations of fish, are becoming common in the waters off of Britain, where they had not been known until recent years (CNN, 2007). For some species, such as the Antarctic Icefish, there may be no higher latitudes to which they can move. For these species, climate change may well lead to extinction (Pauly 2007).Many biological processes are temperature dependent and would be adversely affected by even a few degrees of temperature change (Harley, 2006). Higher temperature waters, such as those in the tropics tend to have less primary production. This results from the fact that a a strong temperature gradient prevents the upwelling of cool, nutrient rich waters, such as is common off the coasts of Peru and Chile during the negative periods of the ENSO cycle (Harley, 2006). A drop in the amount of primary production causes drops in higher trophic levels as well. Therefore, in many cases, it is reasonable to believe that global warming will cause fish stocks to decrease (Harley, 2006).
Another of the most visible effects of climate change is coral bleaching. When hermatypic corals are stressed by high water temperatures or other effects, they expel their symbiotic zooxanthelle from their tissues. This process deprives corals of the color, as well as of their primary source of nutrition. If corals are without their symbionts for too long they can perish from starvation. The impact of coral death then spreads through the reef ecosystem. Secondary effects are most obvious in fish, especially among those that feed specifically on corals, such as butterfly fish. Studies have indicated that such fish were gradually starving to death and that their decline in numbers resulted from a failure to breed in the months and years following the destruction of their reef. As it stands today, more than 30% of coral reefs throughout the world are already severely degraded and up to 60% of corals may be lost by 2030 due to temperature induced bleaching (ARC, 2007).
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