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Oceans of Change: Summaries
- Ocean Warming and Coral Reefs
Ray Hayes, PhD, Howard University
Coral reefs are ancient, majestic and diversity-enriched tropical marine ecosystems. However, they are also fragile, sensitive, and responsive to environmental change. Over the recent two decades, we have witnessed a progressive global decline of robust and dynamic coral reefs into fragmented and marginalized communities. This natural disaster has been driven by impacts of global climate change, exacerbated by pollution, destructive fishing practices, over-harvesting, and human environmental mismanagement. Long term impacts from rising sea level rise and oceanic acidity may soon compound other sources of stress. Analyses of satellite-derived sea surface temperature (SST) at over 200 globally-distributed reef sites indicate that coral reefs have not only been exposed repeatedly to summer high SST anomalies, but also essential relief from thermal stress in the winter has failed. Since reef organisms live very close to their physiological tolerance for temperature, a mere one degree Celsius rise in SST results in a stress response of bleaching in reef-building corals. Bleaching results from uncoupling of the normal symbiosis between unicellular algae (zooxanthellae) and coral soft tissue. Without nutrient-producing algae, starving corals no longer produce a skeleton, defend themselves against predators, or reproduce. Warmer seas also promote microbial growth, responsible for infectious diseases in reef organisms. The same warming energizes intense tropical storms (hurricanes and typhoons) that physically damage reefs and disrupt ecosystem functions. Habitats are destroyed, pelagic organisms are dispersed, biodiversity declines, and water quality is deteriorated for months to years following storm exposure. Reef ecosystem goods and services that coastal and island populations depend upon to sustain tourist economies are at risk as living reefs lose their biodiversity and as reef organisms die. Human health is also at risk from the spread of water-borne diseases and as anxiety mounts over jobs, seafood safety, and recreational security. Economic and societal impacts from global coral reef degradation are now of critical significance to residents of over 100 small island states and continental shores. Any hope for the preservation of coral reefs will require immediate and coordinated funding of restoration programs, reduction of greenhouse gas emissions, and control of pollutant discharges into the sea.
- Coral Reefs: Canaries in the Environmental Coal Mine
Nancy Knowlton, PhD, Scripps Institution of Oceanography
Coral reefs are the most diverse of all marine ecosystems. Estimates range from 1-9 million species globally, although these estimates are based on very tenuous assumptions. Coral reefs are also among the most endangered of all marine ecosystems. In the Caribbean, for example, 80% of all coral cover has been lost in the last three decades. The causes of loss are varied, and they operate on local, regional and global scales, often synergistically. Coral bleaching is a stress reaction that results in the loss of symbiotic algae (zooxanthellae) from coral tissues; if prolonged, corals starve to death. One serious source of stress is global warming; when seawater temperatures exceed 1o C above local seasonal maxima, bleaching occurs. In 1980, massive bleaching was seen around the world; in the Indian Ocean 80% bleaching and 20% mortality occurred. Fortunately, some zooxanthellae are more heat tolerant than others, but climate models suggest that mass bleaching will become more and more common. Another major source of coral loss is disease, about which we still know very little in terms of pathogens. Some diseases have already had catastrophic effects. In the Caribbean, for example, two once dominant corals are now officially listed as endangered. Other stresses include destructive fishing, over-fishing (especially of herbivores), sedimentation (often caused by deforestation), predator explosions, storm damage, and now ocean acidification. Complete reproductive failure due to scarcity of mates is also a possibility. Studies of reefs across a gradient of human disturbance indicate that loss of resilience – the capacity to recover – is one of the first consequences of human impacts. Fortunately, actual extinctions are so far limited. Reefs can be thought of as canaries in the environmental coal mine, because they are so sensitive to a diverse array of human impacts, but all marine ecosystems are suffering. Business as usual is not an option if we are to address these problems.
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