Eutrophication

Eutrophication is a process by which bodies of water become highly charged with nutrients. Typically such nutrients are dominated by nitrates and phosphates, which in themselves are relatively simple, non-toxic compounds. Natural, or background, levels of such nutrients vary considerably from place to place, and of course some places are naturally ? eutrophic?, particular in areas close to river or near ocean upwellings. By contrast, most of the open oceans are very low in nutrients, or ?oligotrophic?, and most of the species and ecosystems which we find in the oceans have adapted to survive in these low-nutrient conditions.

Eutrophication can result from augmentation of nutrient inputs to coastal and marine areas as a consequence of human activities, including sewage discharge or agricultural runoff . In general, such eutrophication is confined to the vicinity of coastal discharges but, because of the multiplicity of such discharges and the massive quantities in some coastal regions, combined with some regional atmospheric transport of nutrients, some very large coastal areas are now affected. Nutrient levels are high over wide parts of the Mediterranean and the North Sea and eutrophication is a particular problem in the Gulf of Lyon and the northern areas of the Adriatic.

One of the principal effects of the enhanced concentrations of nutrients is a massive growth in primary productivity. This may be shown in the growth of macroalgae, but more typically in increases in phytoplankton abundance. Such changes are associated with changes in species diversity, the occurrence of toxic algal blooms, and often the growth of other species such as bacteria and algae, leading to reductions in dissolved oxygen and associated fish kills.