Eutrophication is described as the accumulation of excessive algal and plantgrowth due to increased availability of one or more limiting growth factors required for photosynthesis such as nutrient fertilizers, carbon dioxide and sunlight. The accumulation of phosphorus and nitrogen in a water bodies such as lakes and seas leads to the excessive growth of phytoplankton and short-lived macro algae(Chislock, et al., 2013). The excess nutrients reaching water bodies are attributed to anthropogenic activities such as agricultural activities, water treatment plants and effluent from industries. The nutrients that causes eutrophication originate from diffuse sources such as agriculture and point sources such as water treatment and industries discharging waste water into rivers that leads to the lake or seas.
In the case study of Baltic Sea, the findings indicated that nitrogen contributes to 71 % of the total nutrient load and originate from diffuse sources mainly agriculture which contributes to 80 % of the nitrogen load. On the hand, the major sources of phosphorus are the municipalities which contributes 90 % of the total point source discharge. The nitrogen and phosphorus arrives into the lakes and seas through direct discharge or through rivers(Chislock, et al., 2013). In some cases, pond managers and aquaculture scientists intentionally add fertilizers into the water body to increase primary productivity, hence enhancing biomass and density and increases fish productivity. Naturally, eutrophication occurs when lakes or seas are filled with sediments. The natural eutrophication process occurs over a long period of time as compared to the human causes. In addition, natural causes of eutrophication takes place in a very slow process since it requires enough sediments to settle at the bottom of the sea or lake to supply enough nutrients for phytoplankton and algae to grow.
Effects of eutrophication
The notable consequence of eutrophication is the emergence of noxious dense algal bloom, foul-smelling phytoplankton which leads to the reduction of water quality and clarity. The dense growth of algal bloom reduces the penetration of light which is needed for photosynthesis process to take place. As a result, it reduces the growth and subsequently leads to die-offs of plants in the littoral zones(Chislock, et al., 2013). Limited penetration of light into the littoral zones also reduces the success rate of predators to catch the prey. The other effect of eutrophication is the elevation of pH due to the depletion of dissolved inorganic carbon caused by increased rate of photosynthesis. Increased pH limits the chemosensory abilities of the aquatic life that depends on perception of dissolved chemical cues to go about their daily activities. This phenomenon is described as blinding.
Eventually, the dense algal blooms dies and the microbial decomposition process begins thus significantly depleting dissolve oxygen. This condition creates anoxic or hypoxic dead zone with limited dissolve oxygen hence leading to death of fish. Studies also indicated that some algal bloom produce noxious toxins which causes public health risks(Chislock, et al., 2013). In addition, toxigenic cyanobacteria such as Cylindrospermopsis and Anabaena dominate nutrient-rich water bodies. This can lead to poisoning of humans, wildlife and domesticated animals. Further findings indicated eutrophication leads to the accumulation of off-flavor compounds such as geosmin and methylisoborneal which is attributed to cyanobacteria. This affects the quality of fish and drinking water especially the municipal drinking water.
Eutrophication leads to changes in aquatic community structure. For instance, cyanobacterial blooms favor the growth of small-bodied zooplankton which in turn dominates the plankton communities. This condition is attributed to the fact that cyanobacteria possesses anti-herbivore characteristics such as poor food quality, morphology and toxicity(Minaudo, et al., 2015). It also affects the fish composition in the affected waters because planktivorous fishes such as bream and shad prefer nutrient rich waters while piscivorous fishes such as pike and bass prefer oligotrophic lakes which is nutrient-poor.
Considering the threats associated with eutrophication in terms of the quality of recreational, fisheries and potable drinking water sources. Despite enactment of legislation to control point-source of pollution that leads to eutrophication, algal bloom and cyanobacteria remain one of the challenge of surface water globally. Five major methods can be used to control eutrophication:
- Application of potent herbicides and algaecides
- Covering water bodies with water-based stain or opaque liners
- Mechanical mixing
- Changing nutrients ratios
- Diversion of excess nutrients
However, these control strategies are costly and ineffective especially in large complex ecosystem. Therefore, the most appropriate control method is to reduce the amount of nutrient reaching water bodies especially those from point-sources. Bio-manipulation is used to control nutrient loading originating from diffuse sources. Some of the legislation enacted include urban waste treatment directive and nitrates directive(Haahti, et al., 2010). Education serves as the most effective control method. Municipalities and farmers are educated on methods of reducing nutrients loading in water bodies. The municipalities are educated on the importance of collecting, treating and discharge of waste water. In the other hand, farmers are educated on good farming practices that protect both surface and ground water from fertilizers pollutions. It is important to integrate the control methods in order to ensure that eutrophication is reduced in surface water.