Too much fertiliser can suffocate a lake from the top down
Feed a lake too well and it can die. Fertiliser washing off fields, sewage and exhaust fumes load water with nutrients, algae explode across the surface, and when they rot, bacteria strip the oxygen fish need. The name, eutrophication, comes from a Greek word meaning well-nourished.
The process starts with a surplus, usually of phosphates and nitrates. Algae and other fast-growing plants respond first, spreading into dense blooms that shade out plants rooted on the bottom. When the bloom dies, decomposing bacteria consume dissolved oxygen, sometimes until none is left. Fish and invertebrates suffocate, leaving so-called dead zones that keep spreading until they reach water with enough oxygen to stop them. Food webs rearrange around the few species that thrive in rich water.
Which nutrient matters depends on the setting. Phosphorus usually limits plant growth in fresh water, and because it clings to soil particles and settles in lakes and wetlands, it keeps building up there. Landmark whole-lake experiments at the Experimental Lakes Area in Ontario during the 1970s established that freshwater systems are phosphorus-limited. In the sea, nitrogen is the key constraint, and nitrogen oxides from burning fossil fuels, settling out of the air, have pushed ocean levels up.
Nature does this too, but slowly. Lakes gradually accumulate sediment, dissolved minerals and dead plant matter over geological time. Human-driven, or cultural, eutrophication is far faster, and it became a recognised pollution problem in European and North American lakes in the mid-20th century, after chemical fertilisers spread during the green revolution. Phosphate detergents added to it until they were phased out in the 1970s; sewage and farming are now the main phosphate sources. Shallow lakes and shorelines suffer most, since wind and waves stir nutrients back up from the sediment.
Some reservoirs show the opposite course, called meiotrophication: rich when first filled, they slowly grow leaner as nutrient-poor inflows flush them out. Remedies include cutting pollution from sewage outlets and farmland and introducing shellfish and seaweed, which help pull nitrogen from the water and so restrain the cyanobacteria behind most harmful blooms.
Source: Eutrophication
