Eutrophication, hypoxia, dead zones and
nutrient cycle-threat to coastal ecosystem
H.S.Sen and Dipankar Ghorai
The urban developments are taking up fertile agricultural land and leading to pollution of rivers, estuaries and seas by sewage as well as industrial and agricultural effluents. In turn, this is posing a threat to coastal ecosystems, their biological diversity, environmental regulatory functions and role in generating employment and food. Overuse of fertilizer can result in eutrophication, and in extreme cases, the creation of ‘dead zones’. Dead zones occur when excess nutrients—usually nitrogen and phosphorus—from agriculture or the burning of fossil fuels seep into the water system and fertilize blooms of algae along the coast. As the microscopic plants die and sink to the ocean floor, they feed on bacteria, which consume dissolved oxygen from surrounding waters. This limits oxygen availability for bottom-dwelling organisms and the fish that eat them. In dead zones, huge growths of algae reduce oxygen in the water to levels so low that nothing can live. There are now more than 400 known dead zones in coastal waters worldwide, compared to 305 in the 1990s, according to a study undertaken by the Virginia Institute of Marine Science. Those numbers were up from 162 in the 1980s, 87 in the 1970s, and 49 in the 1960s. In the 1910s, four dead zones had been identified (Minard, 2008).
The occurrence of hypoxia in shallow coastal and
estuarine areas has been increasing worldwide, most likely accelerated by
anthropogenic activities. Hypoxia in the Northern Gulf of
Mexico , commonly named the 'Gulf Dead Zone', has doubled in size
since researchers first mapped it in 1985, leading to very large depletions of
marine life in the affected regions (Portier, 2003). He studied changes in
microbial communities as a result of oxygen depletion, the potential
contribution of increasing hypoxia to marine production and emission of N2O
and CH4, and the effect of hypoxic development on methyl mercury
formation in bottom sediments at the Gulf of Mexico 's
Texas-Louisiana Shelf during the summer months.
The World Resources Institute reported that driven by a
massive increase in the use of fertilizer, the burning of fossil fuels, and a
surge in land clearing and deforestation, the amount of nitrogen available for
uptake at any given time has more than doubled since the 1940s. In other words,
human activities now contribute more to the global supply of fixed nitrogen
each year than natural processes do, with human-generated nitrogen totaling
about 210 million metric tons per year, while natural processes contribute
about 140 million metric tons (Table 1).
This influx of extra
nitrogen has caused serious distortions of the natural nutrient cycle,
especially where intensive agriculture and high fossil fuel use coincide. In
some parts of northern
Literatures cited
Coastal Wiki (2008).
Polyfluorinated compounds - a new class of global pollutants in the coastal
environment (http://www.Polyfluorinated
compounds PFC - pollutants in coiastal water.htm).
Minard
Anne (2008). Dead zones multiplying fast, coastal water study says (http://www.Dead Zones
Multiplying Fast.htm)
Portier Ralph, J. (2003). Trends
in Soil Science, Technology and Legislation in the USA. Journal of Soils and
Sediments 3(4):257.
World Resources Institute
(2006). Environment information portal (http://www.Nutrient Overload
Unbalancing the Global Nitrogen Cycle.htm).
HSSen1
& Dipankar Ghorai2
1Former Director, ICAR-CRIJAF
2 Incharge, KVK
(ICAR-CRIJAF), Budbud, Burdwan
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