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Dieren en planten

Phaeocystis   Diatoms   Flagellates   Dinoflagellates   Phytoplankton   

Water en land

Mens en Milieu

Ecological processes   Eutrophication   
Phaeocystis, Ecomare

Eutrophication

An ecosystem is geared to a certain supply and transport of building materials. When the supply of these materials is so excessive that clearly visible changes take place in the system, there is talk of eutrophication. The amount of algae in the marine environment increases explosively. Higher-order water plants disappear if the water becomes too turbid from the large amounts of algae, which prevent light to penetrate to the bottom. The water can become temporarily deficient in oxygen when the algae mass dies, resulting in death among aquatic animals.

  • Algae bloom
    Red tide, Marijke de Boer

    Phytoplankton in seawater can be found in enormous amounts. It reproduces rapidly. The smallest species can double in number within an hour, the largest within a day. Under favourable conditions (lots of sunlight in combination with a large supply of nutrients), the number of planktonic plants in the water can increase so much that the water discolours. It can turn green, red or brownish, depending upon the algae species. Natural algae blossoms occur regularly in the relatively shallow North Sea because the average temperature of the water is favourable and it contains huge amounts of nutrients. Of the more than five hundred species of phytoplankton in the North Sea, around fifty species display explosive growth phenomena from time to time.

  • Over-fertilization of the marine environment

    The increasing input of nutrients has led to an increase in algae. In 1985, there was three to four times more phosphate along the North Sea coasts than in 1935 when the North Sea was still relatively 'unused' (data from a study by Dienst Getijdenwateren Rijkswaterstaat). The nutrient concentrations did not noticeably rise further away from the coasts (around thirty to seventy kilometers) in comparison to earlier days. During strong lengthly easterly winds, higher concentrations of nutrients are measured there as well.
    After 1988, the concentration of phosphate in seawater continually decreased as a result of cleaner discharges. Nitrogen concentrations also show a declining trend. Data from the NIOZ on the development of Phaeocystis in the Marsdiep near Texel also show a decline in the algae biomass during blooms since 2000. The effect of the ban on phosphates is particularly noticeably by the less amount of foam on the beach, which forms when Phaeocystis breaks down.
    Noctiluca (an alga that has lost the ability to photosynthesize and therefore falls into a category between plant and animal, as a manner of speaking) is able to flower under certain situations. The sea turns pink from the flowering Noctiluca a few times every summer, particularly in the vicinity of the German Bight.
    These incidences of algae and seaweed blossoms, when the algae and seaweed die in massive numbers, result in an oxygen deficiency in the water and sea bottom. This regularly leads to massive deaths among benthic organisms in the German Bight and the Kattegat, whereby food shortages occur for many species of fish and seabirds.
    In addition, due to eutrophication, undesirable algae such as the toxic dinophysis are growing more common. People who consume mussels contaminated with this alga can have trouble with their intestines. Other harmful algae can cause fish mortality.
    Despite the decrease in phosphates, algae blossoms are still occurring. According to a scientist from IMARES, the real culprits are the toxic materials from households, agriculture and horticulture that are dangerous for the algae consumers: the copepods in salt water and the water fleas in freshwater.

  • Consequences for the fisheries

    The fish stock has been declining enormously since around 1985. Every once in awhile, various scientists suggest that there is a relationship with the decrease in phosphate discharges. This results in fewer algae and consequently less food for fish. The fisheries ector has suggested performing an experiment to see whether or not discharging phosphate into the sea will produce larger fish populations. IMARES, the institute that studies fish species, doubts the effects of such an experiment. IMARES does not believe that the fish will profit by fertilizing the sea as much as other algae feeders, such as mussels. In addition, it is known that toxic algae result from such fertilization and that jellyfish grow more rapidly.