From the Danube to the Black Sea: Excess Nitrogen and Phosphorus Choking the Seabed

How can fertilizers spread hundreds of kilometers from the coast end up choking the sea? Behind the Danube’s calm waters lies a dramatic chain reaction capable of feeding algae, draining oxygen, and upending an entire ecosystem.

In the Farmlands of the Danube Basin, an Invisible Journey Begins

It all starts far from the beaches, in the middle of the fields. Nitrogen and phosphorus promote plant growth, but some of these nutrients slip away from the roots. Rain after rain, they make their way into ditches, streams, and rivers. The Danube eventually gathers them across its vast watershed before carrying them to the Black Sea.

This transport is not solely agricultural. Wastewater and certain industrial activities have also contributed to the nutrient inputs. According to the International Commission for the Protection of the Danube, the river remains the primary riverborne contributor to nutrient pollution of the Black Sea, even though the amounts carried have fallen sharply from their historical highs.

When the Sea Receives Too Much Food, a Burst of Life Sets the Stage for Oxygen Depletion

Once in the sea, these nutrients act like a giant liquid fertilizer. Phytoplankton suddenly enjoys an almost ideal feast and can proliferate rapidly. This explosive growth is called eutrophication. On the surface, the water may seem incredibly productive. Beneath this apparent abundance, a far less cheerful scenario is unfolding.

The microscopic algae eventually die and descend through the water column. Bacteria then undertake the task of breaking down this accumulated organic matter. Their work consumes dissolved oxygen. When the water is warm and stratified, the renewal of oxygen becomes harder, while bacterial respiration continues relentlessly.

Below 100 Meters, the Black Sea Already Hides a World Almost Devoid of Oxygen

The Black Sea has a peculiarity that makes this story even more striking. Its deep waters mix very little with surface waters. At depths of roughly 100 to 150 meters, a large portion of the basin is naturally devoid of oxygen and contains hydrogen sulfide. This deep environment is incompatible with most marine animals.

Anthropogenic eutrophication thus does not create this deep anoxia, a crucial nuance. It can, however, trigger seasonal hypoxia on the coastal shelf, where fish, mollusks, and bottom-dwelling organisms live. European surveys show that the Black Sea remains one of the European regions most affected by low oxygen concentrations.

The Surprise: Oxygen Returns, Proof That This Story Can Still Change

Not everything has worsened, however. Since the 1990s, nutrient inputs have declined considerably. The Danube Commission estimates emissions have fallen by about 30% for nitrogen and 50% for phosphorus over fifteen years. With less fuel for algal blooms, some coastal zones have begun to recover.

A study published in 2026 in Biogeosciences confirms that the maximum eutrophication on the western shelf occurred during the 1980s and 1990s, before a gradual decline linked to economic transformations and policies aimed at reducing discharges. Recent work even observes nutrient concentrations approaching levels seen before major eutrophication events.

The problem has not disappeared. Agriculture, wastewater, warming, and stratification continue to exert pressure on marine oxygen, while the European Environment Agency still regards eutrophication as a major issue for the Black Sea. But its story offers a rare insight: when a continent reduces its discharges, a sea can truly begin to breathe again.