Over-fertilization of the seas
Rivers – the lifeblood of coastal waters
Coastal waters are among the most productive regions of the oceans. The greatest numbers of fish, shellfish and seafood in general are caught here. The high productivity is a result of nutrients that are transported by rivers from the land into the sea. These mainly comprise phosphate and nitrogen compounds, which plants require for growth. Phytoplankton in the ocean, microscopically small algae in particular, also utilizes these substances. Because of the high availability of nutrients, phytoplankton grows exceptionally well in coastal regions. It is consumed by zooplankton, small crustaceans, fish larvae, and other creatures, and thus forms the base of the food web in the ocean.
The high productivity of coastal waters also makes them increasingly attractive areas for aquaculture. The output of the aquaculture industry increased worldwide by a factor of fifteen between 1970 and 2005. But rivers are not the only source of nutrients for coastal areas. On the west coast of Africa, for instance, ocean currents from greater depths bring nutrient-rich water up to the surface, where light can penetrate. In these upwelling regions, the nutrients also promote a rich growth of algae, increase productivity through the entire food web, and ultimately produce a greater yield for fisheries. A natural level of nutrients is therefore a positive factor and is essential for marine organisms in the coastal waters.
- 4.1 > Eutrophication stimulates the growth of algae, which are sometimes pounded to foam in the surf, as seen here on the German North Sea coast.
Too much of a good thing
In many densely populated regions of the Earth, however, excessive amounts of nutrients are finding their way into the coastal waters. A large proportion of these nutrients come from the intensive agricultural application of chemical fertilizers, which are washed by rain into the rivers.
Between 1970 and 2005 the amount of nitrogen fertilizer alone, applied globally, increased by almost a factor of three. Nitrogen and phosphate compounds are also transported to the sea by untreated wastewater, and via the atmosphere from the burning of fossil fuels. The production and decay of organic material are unnaturally intensified by the huge amounts of nutrients in coastal waters. Scientists call this process eutrophication. The availability of nutrients is so great that the phytoplankton population grows beyond normal levels, producing a classic algal bloom. In the North Sea and in the Wadden Sea, massive algal occurrences are occasionally whipped into a foam by the surf. These sometimes form piles up to a metre high, resembling giant meringues. A serious threat is presented by the propagation of toxic algae.
- 4.2 > Over-fertilization of the seas usually first becomes apparent with the appearance of copious amounts of green algae. Prior to the start of the Olympic sailing competition in Qingdao in 2008, the algae had to be removed from the water surface by hand.
4.3 > When conditions are favourable for phytoplankton growth, algal blooms occur in the oceans, as here in the Baltic Sea. Through the massive reproduction of cyanobacteria, formerly called blue-green algae, the water in these areas turns green. Such phenomena are completely natural, but because of over-fertilization these blooms are occurring with unusually high frequency today.
- These are poisonous to various organisms in the sea, such as fish and clams and if they enter the food chain, they may also be ingested by humans. Numerous cases have been reported of people dying after eating poisoned shellfish. Scientists have also verified the deaths of marine mammals from algal toxins that they ingested with their food. These toxic algal blooms occur along the coast of Texas, for example. Because they discolour the water they are commonly called “red tides” or “brown tides”.
The blooms of non-toxic algae can also create problems when the algae die. The dead algae sink to the bottom where they are broken down by microorganisms through a process that depletes oxygen in the seawater. Low oxygen concentrations in the water can lead to large-scale mortality of fish and crustaceans. When the oxygen levels begin to drop, the animals that can actively move, such as fish and crabs, leave the area first. Within the sea floor, the population of animals that require a healthy oxygen supply diminishes at the same time. If the oxygen concentration continues to drop, then most of the other species living in the sea floor also disappear. Only a few species that can tolerate low oxygen levels remain. If the bottom water finally becomes completely depleted of oxygen, even these organisms will die off.
But eutrophication also causes blooms of other organisms besides phytoplankton. It has a significant effect on larger plants, and can often change entire coastal ecosystems. One example of this was the formation of a vast carpet of green algae on the Chinese coast at Qingdao in 2008, which disrupted the Olympic sailing competition. In other cases, eutrophication leads to the disappearance of seagrass beds (Chapter 5) or to changes in the species composition in certain habitats. In short, eutrophication is an illustration of how changes onshore can impact the ocean, because the oceans are connected to the land masses by rivers and the atmosphere. To counteract the negative effects of eutrophication, serious efforts are being made to reduce the input of phosphate and nitrogen compounds into coastal waters. >