Why Is There No Life on That Seabed? The Oxygen Ran Out

The sequence begins with nutrients, chiefly nitrogen and phosphorus, arriving in coastal water from rivers. Those nutrients allow an enormous growth of microscopic algae at the surface, far beyond what the water would normally support. The algae are not eaten quickly enough, so they die and sink, and bacteria decomposing them on and near the seabed consume oxygen as they work. Where the water is layered, with warmer fresher water floating on colder saltier water, nothing replaces that oxygen from the surface, so the bottom layer is progressively stripped until almost nothing is left.

The effects are graded rather than uniform:

• •Fish and shrimp leave the area if they can detect it in time
• •Crabs, worms and shellfish cannot move fast enough and die
• •Surviving animals grow more slowly and reproduce less
• •The seabed community shifts towards a few tolerant species
• •Recovery after the water mixes again takes months or years
• •Fisheries lose grounds and catches concentrate elsewhere

The area at the mouth of the Mississippi is the most studied, forming each summer as the river delivers nutrients from an agricultural catchment covering much of the United States, and it has been measured annually since 1985, reaching sizes above twenty thousand square kilometres in bad years. The Baltic Sea holds the largest one, which is now effectively permanent over much of its deep basin, because the sea is nearly enclosed, exchanges water with the North Sea only occasionally, and receives nutrients from a large surrounding population. The northern Adriatic, the Black Sea and Chesapeake Bay all have well-documented versions, and the recorded global count has risen from a handful in the 1960s to several hundred.

Not every oxygen-poor area is caused by people, and separating the two matters for both science and policy. Deep basins that exchange water rarely, including several fjords and the Black Sea below about a hundred and fifty metres, have been oxygen-free for thousands of years for purely physical reasons. Zones where deep currents bring nutrient-rich water to the surface, off Peru, Namibia and parts of the Indian Ocean, support enormous fisheries and also generate natural low-oxygen layers. Those natural zones are expanding measurably as the ocean warms, since warmer water holds less dissolved oxygen and warms the surface more than the depths, which strengthens the layering that prevents mixing.

The cause is diffuse, which makes it far harder to address than pollution from a pipe. Nutrients come from fertiliser applied across millions of individual fields, from livestock manure, from sewage and from the air, and no single party can be identified or regulated in the way a factory can. The catchment can extend across many jurisdictions with no common authority, which is the situation on the Mississippi and on the Danube. Reductions also take years to appear, since nitrogen already in groundwater continues arriving long after application stops. The Black Sea case is the clearest success, where the collapse of subsidised fertiliser use after 1990 reduced inputs sharply and the zone shrank dramatically, which was an accident rather than a policy.

Nutrients from farmland feed an algal growth that sinks and decomposes, and bacteria doing that work strip the oxygen from bottom water that layering prevents being replenished. Animals that cannot swim away die. The Mississippi zone has exceeded twenty thousand square kilometres and the Baltic one is effectively permanent. The cause is millions of individual fields across many jurisdictions, which is why it resists regulation.