According to Dincer & Sustainability (2020), eutrophication is known as the process of enrichment of water bodies with dissolved nutrients, mainly nitrogen and phosphorus, causing a stimulation of plant growth. When the plants die, the decomposition of organic matter leads to a decrease of dissolved oxygen endangering aerobic organisms in the ecosystem. From 1969 to 2005 a ‘whole ecosystem experiment’ which went for 37 years was performed to detect which nutrient plays the most important role during eutrophication, with the outcome that phosphate plays the most important role (Schindler et al, 2008).

Figure 1 shows the process of eutrophication. Through leaching and runoff of fertilizer, nearby water bodies are enriched with nutrients, causing algal blooms to occur. The massive biomass of algae shadows itself causing a lack of sunlight which leads to the death of many plants. When algae die, the decomposition leads to a high oxygen consumption, causing fish to suffocate.

Fig. 1: Basic scheme of the eutrophication process and the role of agricultural fertilizers.

The process has been known for a long period of time, but it is more present than ever before. Especially developing countries like China and India encounter serious problems regarding drinking water quality. In China alone, water eutrophication occurred in 67 lakes (51,2% of all lakes in China) (Yang et al, 2008). Algal blooms which occured in 2007 in Wuxi China caused a severe drinking water crisis and many drinking water plants had to be shut down, leaving many households without water (Yang et al, 2008).

A typical eutrophicated water body can be seen in figure 2.

Fig. 2: Typical eutrophicated pond, near the city of Gelsenkirchen.

Many industrial countries like Greece or the United states of America also suffer from eutrophication. There is the Pamvotis lake in Greece for example, where the process of eutrophication proceeded in the past decades (Romero et al, 2002). An investigation from the UNEP (United Nation Environmental Protection) showed that 30-40% of all lakes in the world already suffer from eutrophication and with an estimated increase of fertilizer usage, the number is likely to grow (Herschy et al, 2012). Figure 3 shows the estimated global fertilizer use until 2022.

Fig. 3: Global demand for nitrogen, phosphorus and potassium for fertilizer use, 2016 to 2022 (thousand tonnes).
Based on World Fertilizer Trends and Outlook (FAO, 2019).

In general, sources for phosphate pollution can be point or nonpoint pollution, but due to stricter laws in environmental protection like the clean water act from 1972, most of the point pollution sources vanished and nowadays agriculture with its non-point pollution is the biggest source of phosphate pollution (European Commision, 25.10.2020). Since the parameters and circumstances of the water body may differ in every case, there is no master strategy to deal with eutrophication and its consequences. Nevertheless, many approaches were developed over the past decades and showed different effectiveness.

One possible solution could be the addition of predatory fish to an ecosystem, through which the number of prey fish can be reduced. As a result, the population size of cladocerans grows, putting higher pressure on phytoplankton (Shapiro et al, 1975). If the lake reaches a eutrophic or hypertrophic state, this kind of top-down control is no longer possible. Lake Washington showed that phytoplankton in high eutrophic lakes mostly consists of blue algae, which are inedible for plankton (Brendelberger, 2013).

Another natural approach is the use of barley straw, which has shown the ability to reduce phytoplankton blooms through the release of certain phytotoxic compounds. (Everall & Lees 1996). Other approaches suggest the use of metals like iron or zinc to build chelates and make the phosphorus inaccessible for further modifications to phosphate. (Duncan et al., 2012). The use of such algicides can cause the release of toxins, which leave the water undrinkable for several days or even weeks, so that it should only be considered as the ultima ratio (European Commision, 25.10.2020). It has been proven that eutrophic water can be remediated by water plants carrying immobilized nitrogen cycling bacteria, which effectively reduce the concentration of chlorophyll a (Chang et al., 2005).

The simplest method is the harvesting of biomass. Although the concept is very simple, this method is only possible in small ecosystems like ponds because some areas of bigger aquatic bodies are not reachable without huge costs (Duncan, 2012). Additionally, the results do not last for long since the phosphorus remains in the sediment and can be set free later. As a result, phytoplankton starts to reproduce again, and a status quo is reached very fast. The most common methods are sewage interception, sediment dredging, exchange with freshwater or bypass treatment. Artificial aeration and biological approaches like phytoremediation are also commonly used (Chai et al, 2017).

Disadvantages of most of the previously mentioned methods are the high costs or secondary transfer of pollutants. While deep dredging often damages the ecology of river beds, shallow dredging is not able to remove all pollutants. Sewage interception increases the pressure on urban wastewater facilities and is very expensive. Phytoremediation is very dependent on climatic effects like temperature or sunlight (Chai et al, 2017). Artificial aeration has a huge energy consumption and is unsuitable for big water bodies. The probably most widely used method is the activated sludge treatment, in which phosphate can be removed by microbial organisms. Disadvantages of this method are in the unsuitability for in situ habitats and the high energy consumptions. Additionally, the facilities are quite complex and need a complex drainage system (Chai et al., 2017).

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