'Dead Zones' and Climate Change
Hypoxia in Coastal Ecosystems May Increase As Oceans Warm
Coastal ‘dead zones’ are low-oxygen, or hypoxic, areas that develop on the sea floor when nutrients normally in short supply become plentiful due to nutrient pollution from agricultural fertilizers, sewage treatment plants and the combustion of fossil fuels. Dead zones impact coastal ecosystems by driving fish away from important feeding and nursery habitat and killing the less mobile forms of marine life that fish eat.
‘Dead Zones’ Impact Ecological, Economic and Social Systems
However, the ecological impacts of dead zones can be more permanent. Under sustained hypoxic conditions, the flow of energy through the affected ecosystem’s food web is diverted away from fish and towards microbes, ultimately changing the kinds of organisms that make up the benthic, or bottom-dwelling, community. The longer hypoxic conditions last, the longer it takes for an ecosystem’s community structure to recover once oxygen levels increase.
These ecological changes impact human communities too, especially those that depend on fisheries supported by healthy marine ecosystems for food and income. Many such communities are already suffering from declines in fish populations due to over-fishing and habitat loss. This makes any increase in the number and distribution of coastal ‘dead zones’ a grave concern, but unfortunately this is happening.
Coastal ‘Dead Zones’ are Spreading Worldwide
In their recent paper, “Spreading Dead Zones and Consequences for Marine Ecosystems” (Science, August 15 2008), marine scientists Robert Diaz and Rutger Rosenberg identified 405 coastal dead zones worldwide, covering about 245,000 square kilometres of ocean bottom, an area roughly the size of the United Kingdom. This is a 50% increase in the number of ‘dead zones’ over the last decade and many are lasting longer and occurring more often than ever before.
While these are alarming statistics, perhaps more alarming still is that the number of 405 dead zones is only a minimum estimate. In their study, Diaz and Rosenberg only counted dead zones reported in the scientific literature, which contains information gaps. There is not much information in the literature, for example, on the nutrient status of coastal ecosystems in the tropics.
While reducing nutrient inputs to coastal ecosystems is an obvious step, an additional complication is that climate change may compound the impact of nutrient pollution on coastal ecosystems by affecting the natural factors that influence the formation, intensity and duration of dead zones.
Climate Change May Increase the Spread of ‘Dead Zones’
The General Circulation Models used by the Intergovernmental Panel on Climate Change predict lower dissolved oxygen levels in oceans as climate change progresses for two reasons. First, increased stratification of ocean waters is expected as ocean temperatures rise. Stratification restricts the movement of oxygen between water layers and can intensify and prolong hypoxia. Stratification may be offset in some regions, however, by an increase in violent storms capable of mixing coastal waters.
The second potential effect of climate change is to change rainfall patterns, increasing freshwater runoff from terrestrial ecosystems and thus the input of agricultural fertilizers to coastal waters. A further concern is that climate change may cause existing natural oceanic dead zones to extend beyond their historic range into coastal waters.
Low-oxygen dead zones caused by nutrient pollution are a key stressor of the ecological and socioeconomic components of coastal ecosystems around the world. This problem is spreading and it is possible that climate change will make it worse still. Thus, management strategies aimed at controlling the movement of nutrients from terrestrial to coastal ecosystems must consider possible climate change effects when setting nutrient reduction targets and recovery goals for affected ecosystems.
Dead Zone in the Gulf of Mexico, Satellite Image
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