Reversing the trendThe Rhine River and North Sea present a good example illustrating how the input of nutrients by rivers into the ocean has evolved through time in European regions, because extensive data are available for both of these water bodies. The first observations were made as early as the mid 19th century. Water samples from the Rhine near the border of Germany and Holland were taken and analysed over several decades. Near the border town of Lobith, researchers documented a strong increase in phosphate and nitrate concentrations from the mid-20th century. Appropriate measures were taken that have succeeded in consistently reducing the concentrations since the mid-1980s. The causes of the increase included a growing input from agriculture and industry as well as the discharge of untreated urban sewage. Laundry detergent with phosphate additives to decalcify the wash water was a significant source of phosphates. As early as the 1970s, a ban on this type of detergent had already begun to reduce the phosphate concentrations in the Rhine. Then, in the 1980s, the nitrogen levels in the river also began to drop. This can be attributed in part to improved fertilizing methods in agriculture that resulted in lesser amounts of nutrients being washed from the fields. Another reason is the improved treatment of industrial and domestic wastewater. In 1987, environmental ministers from the North Sea countries finally agreed to a goal of halving the amounts of phosphate and nitrogen transported by rivers. For phosphates this goal was reached quickly. For the nitrogen compounds it took almost 25 years. Despite decreasing phosphate and nitrogen concentrations in the water, however, the Rhine River still carries large amounts of nutrients to the North Sea, because it flows through a highly developed and intensively used agrarian region. The present nitrate loads are still higher than in the pre-industrial age 150 years ago. Similar situations exist in other European river regions and in the USA.
- 4.4 > Eutrophication in coastal waters is primarily caused by an abundance of nitrates (nitrogen compounds) and phosphates that are washed into the ocean by large rivers. For example, since the middle of last century the concentration of nutrients in the Rhine River near the border town of Lobith has increased enormously. This is largely due to the intensive use of chemical fertilizers in agriculture and inadequate wastewater treatment. Counteractive measures such as a ban on phosphate detergents and improved fertilizing techniques have been successful in significantly reducing the input since the 1980s. But in many other coastal regions of the world the nutrient concentrations continue to increase.
- In some parts of Europe, political decisions have thus led to a reversal of the trends and a reduction of nutrient input into the oceans. But the opposite trend can be observed globally. Computer models indicate that the use of fertilizer is increasing in many regions due to population growth and the intensification of agriculture. Accordingly, in many coastal regions, the amounts of phosphate and nitrogen being washed into the sea by the rivers are increasing. Particularly in Southeast Asia, rivers are carrying more and more nutrients to the sea, and experts expect this trend to continue.
A global problemThe effects of eutrophication have been coming to light since the 1960s. Researchers have noted more abundant algal blooms, oxygen-poor zones in coastal regions, and changes in coastal ecosystems. The causes of eutrophication have been thoroughly analysed in numerous studies, and there is certainly a direct connection between environmental changes and nutrient input. But for a long time researchers were in disagreement as to how the phosphates and nitrates interact as nutrients. Some experts accepted that the “law of the minimum”, formulated by the agronomist Carl Sprengel in 1828, was valid for algal growth. According to this theory, a plant requires several nutrients in order to thrive. If one nutrient is missing, then it cannot grow. This means that the growth of plants would always be limited by the one substance that is not available in sufficient quantity. This would suggest that it is sufficient to remove one nutrient, either phosphate or nitrogen, from the wastewater and rivers in order to stop the growth of algae. This would also significantly reduce the costs of water treatment. This assumption, however, now appears to be too simplistic. Continuing experiments and observations show that multiple factors acting in concert are often responsible for limiting plant growth. Experts call this phenomenon co-limitation. Eutrophication can only be combated successfully if both phosphate and nitrogen are reduced. However, this is fraught with difficulty, primarily be- cause nitrogen released by agricultural activity is not easily contained. This is also true of nitrogen released into the atmosphere by the burning of natural gas, oil or coal. Eutrophication is therefore likely to continue to occur in coastal waters in the future.
- One example of a strongly eutrophic area is the German Bight. In the 1980s the oxygen concentration in its deep waters dropped to alarming levels. At the same time an increase in primary productivity in the form of enhanced algal growth was observed in the Wadden Sea. Seagrass, a plant that is the foundation for a unique habitat in the North Sea and Wadden Sea, disappeared. It was displaced by an excessive proliferation of green algae. All over the world, bays with limited water exchange are affected by eutrophication because nutrients are not effectively dispersed. These include Tokyo Bay, Long Island Sound in the USA, the Baltic Sea, and several of the fjords in Norway. Eutrophication with an excessive growth of phytoplankton has also been observed in some areas in the Mediterranean Sea, such as the north-eastern Adriatic Sea or the bay at Athens. The Gulf of Mexico is a special case: here the Mississippi River discharges such a large volume of nutrients that an extensive low-oxygen area has formed along the coast.
Any chance of recovery?Through systematic measures such as the Water Framework Directive of 2000, or theMarine Strategy Framework Directiveadopted in 2008, the European Union is striving to improve water quality in the European coastal waters. Key parameters for evaluating water quality are sufficient oxygen content, low nutrient levels, and the presence of certain algal species and bottom dwellers. Wherever possible the previously eutrophic waters should be restored to their natural condition, or at least to an only slightly impacted state. Improved monitoring for ongoing assessment should also be carried out in order to identify changes and their causes.Further information on this topic is available here:
- Due to world population growth, eutrophication will continue to be a problem for decades to come. There is presently little hope of a worldwide reduction in the amounts of nutrients being discharged into coastal waters. A true dilemma exists: humankind has a vital need for agriculture and the production of grain, but this results in vast amounts of fertilizers ending up in the rivers and oceans. Often costly abatement measures are therefore required to achieve a balance between the nutrient input from agriculture and the negative impact on the ecosystem. One particular problem is that it is impossible to completely restore a coastal ecosystem affected by eutrophication to its original state. Eutrophication is not fully reversible. Studies in several European coastal systems indicate that a long period of eutrophication produces lasting changes in the ecosystem that cannot simply be reversed by reducing the nutrient input. Nonetheless, the example of the Wadden Sea clearly illustrates that practical measures can be effective in decreasing the amount of nutrients and creating a general improvement in the marine environment. In the north- ern Wadden Sea, for instance, there are indications that the seagrass beds have recovered and are expanding again as a result of the reduction of nutrients and algal blooms.