Current industrial agricultural practices, particularly the overuse of fertilizer and its sloppy management, frequently create a tradeoff between agricultural production and coastal eutrophication. That is increases in agricultural yields have produced low oxygen zones around the world. The UNEP Global Environmental Outlook 2003 maps the location of coastal anoxic zones world wide (somewhat confusingly the worst cases – the persistent ones are coloured yellow, next worst red and orange, and least worst blue).
Global distribution of oxygen-depleted coastal zones. The 146 zones shown are associated with either majorpopulation concentrations or with watersheds that deliver large quantities of nutrients to coastal waters.
- Annual – yearly events related to summer or autumnal stratification
- Episodic – events occurring at irregularintervals greater than one year
- Periodic – events occurring at regular intervals shorter than one year
- Persistent –all-year-round hypoxia
Describing the the figure the report writes:
Oxygen-starved areas in bays and coastal waters have been expanding since the 1960s. The number of known locations around the world has doubled since 1990. While many of these sites are small coastal bays and estuaries, seabed areas in marginal seas of up to 70 000 km2 are also affected. Increased flows of nitrogen from agricultural run-off and the deposition in coastal areas of air-borne nitrogen compounds from fossil-fuel burning stimulate blooms of algae in these waters. The algae sink to the bottom where they are decomposed by micro-organisms that use up most of the oxygen in the system, creating an inhospitable habitat for fish, shellfish, and most other living things. In recent decades, large areas of coastal waters with harmful algal blooms, severely depleted oxygen levels, and disappearing seagrass beds have been identified and clearly linked with increased inputs from the nitrogen cascade.
The primary cause of these oxygen-starved areas varies. For example, the very large ‘dead-zone’ in the Gulf of Mexico is caused primarily by nitrogen from agricultural run-off, whereas the problems in the Baltic Sea, northern Adriatic Sea, Gulf of Thailand, Yellow Sea, and Chesapeake Bay result from a combination of agricultural run-off, nitrogen compounds from fossil-fuel burning being deposited from the air, and discharges of human wastes. Because of these different causal factors, different solutions are required. Severe oxygen depletion of coastal waters has significant negative consequences to economically important fisheries, ecosystem services, and biodiversity. These effects are difficult to quantify because the waters are often simultaneously affected by overfishing and habitat destruction related to extensive coastal development.