Towards a Cleaner Marine Environment
4. Nutrients leave the sea breathless
Nitrogen and phosphorus are nutrients that promote the growth of algae. This leads to vigorous algal growth, oxygen depletion and, in the worst case, fish mortality - especially in the inner Danish waters. Too many nutrients also impoverishes marine flora and fauna, for instance, by massive occurrences of sea lettuce. Nitrogen is the prime cause of such problems.
Agricultural fertiliser runoff is the major source of nitrogen in the marine environment. The nitrogen surplus is leached out of the fields as nitrates and ends up in the aquatic environment. At the same time, ammonia evaporates from the fields and a large part of it becomes dissolved in rain and also finishes in the sea. The combustion of coal and oil, including that consumed by traffic, also contributes to atmospheric fallout. Discharges of waste water also used to contribute large quantities of nitrogen to the sea, in the late 1990s, modern sewage treatment plants brought this contribution down to about 2.5 percent of the total, in contrast to more than 10 percent at the end of the 1980s.
Danish waters plagued by oxygen depletion
A large pool of nutrients accumulates in the sea during the winter, because the algae do not grow, and, therefore, do not consume nutrients at their normal rate, due to the lack of light. As soon as the quantity of light increases in the early spring, this nutrient pool promotes rapid algal growth. Much of the ensuing algal bloom later sinks to the sea-bed, and is degraded by micro-organisms and sea-bed fauna, with the accompanying consumption of oxygen and risk of oxygen depletion. This has major consequences for all life in the sea.
Wind and weather determine oxygen depletion
Whether or not leached nitrogen results in oxygen depletion is highly dependent on the climatic conditions. A mild wet winter causes severe leaching and, thus, lays the foundation for high algal growth in the spring. Without wind, the oxygen-rich water at the top of the water column cannot mix with the depleted water at the bottom. In serious cases, the water's oxygen content can become so low during the summer that sea-bed fauna cannot respire. Some creatures, such as fish, attempt to flee but can be trapped in pockets of water that contain little oxygen, where they suffocate. At this time, there is also a risk of sediment disturbances, in which such toxic gases as hydrogen sulphide bubble up from the sea-bed and kill all life in their path.
Poisonous algae can necessitate bathing bans
Warm and relatively windless summers increase the risk of algal bloom (especially marine blue-green algae, some of which are toxic). Fish are particularly exposed, partly because the toxic algae infest their gills. Some algal species are also toxic to humans. The authorities advise against bathing, when such colonies of algae approach the coast and bathing beaches. Ingestion of or contact with poisonous algae can typically cause vomiting and eczema.
