|2.||Causes and consequences of degradation|
|4.||The future ocean|
The synergistic effects of overfishing, pollution, and climate change pose a grave threat to all marine ecosystems. Complex food webs with abundant sharks, fishes, sea turtles, and whales are being replaced by greatly simplified ecosystems dominated by microbes, jellyfish, and disease. Runoff of excess nutrients from agricultural fields and animal wastes is causing eutrophication and the worldwide growth of anoxic coastal dead zones. The rise of carbon dioxide and other greenhouse gases is warming the ocean and making it more acidic, posing grave threats to coral reefs, polar ecosystems, and any marine organisms with calcareous skeletons. Ecosystem degradation is potentially reversible but there is very little time to act. Cessation of fishing or pollution does not always result in improved fish stocks and water quality, and there is increasing uncertainty about the potential for ecosystem recovery.
benthic. Environments or organisms on the sea floor.
bottom-up control. Regulation of ecosystem structure and function by factors such as nutrient supply and primary production at the base of the food chain, as opposed to “top-down” control by consumers.
by-catch. Nontarget species or juveniles of target species caught in a fishery that are not the intended target of the fishery. By-catch is commonly discarded dead.
dead zone. Area of the ocean with very low or no dissolved oxygen (hypoxic or anoxic) that forms in areas with low circulation and excess primary production (eutrophication).
ecological extinction. Reduction of a species distribution and abundance to the point that it no longer significantly affects the distribution and abundance of other species in the ecosystem.
El Niño-Southern Oscillation. Sustained sea surface temperature anomalies across the central tropical Pacific that are associated with the spread of warm waters from the Indian Ocean and Western Pacific to the Eastern Pacific and are a major influence on global climate, especially in the southern hemisphere. First recognized from the occurrence of warmer surface waters off the coast of Peru every 2–7 years that shuts down coastal upwelling and the anchoveta fishery.
eutrophication. Increase in chemical nutrients, most commonly nitrogen and phosphorus, and primary productivity in excess of the capacity of grazers to consume excess plant material; it is a major factor in the formation of dead zones.
exclusive economic zone (EEZ). The area bordering a nation’s coast where it has special rights over the exploitation of natural resources, including fish, minerals, and petroleum. Except in areas of overlap, the EEZ extends 200 miles offshore.
fishing down the food web. The hypothesis that the observed decline in the mean trophic level of fisheries catches is caused by the selective removal and serial replacement of preferred high-trophic-level species such as swordfish, tuna, and cod by lowerlevel species.
fishing through the food web. The hypothesis that the observed decline in the mean trophic level of fisheries catches is caused by the serial addition of lower-trophic-level species in the presence of decreased predation by selectively removed top predatory species.
keystone species. A species that has a disproportionately large impact on ecosystem structure and function relative to its own abundance.
multiple stable state. The existence of one or more alternative ecological communities in a given habitat that persist over more than a single generation of the