This is as a result of orthophosphate P being adsorbed in the soil and sediment in the vadose or the saturated zone [ 18 ]. However, wastewater has been reported to cause heavy groundwater contamination leading to P elevation [ 38 , 39 ].
Consequences of eutrophication include excessive plant production, blooms of harmful algae, increased frequency of anoxic events, and death of fish, leading to economic losses and health implications which include costs of water purification for human and industry use, losses of fish and wildlife production and losses of recreational amenities [ 10 , 40 ].
Some of the consequences of eutrophication includes:. High level of Lake Eutrophication has led to suffocating of fish population on a massive scale with a very negative repercussions on the economy [ 41 ]. The total economic loss incurred from algal bloom in the Lake Tai China catchment area was estimated at U.
During winter of —, a severe oxygen deficit induced a fish kill in the eutrophicated two-basin Lake Aimajarvi in southern Finland, which resulted in cascading effects on the lower trophic levels of the lake [ 42 ].
Coastal areas are an important economic source for tourism [ 43 ]. The algal bloom have degraded the investment environment and damaged the hospitality and tourism industry [ 41 ].
Toxin producing algae can cause mass mortalities of fish marine mammals, birds and human illness through consumption of sea food [ 44 ]. It is estimated that 60—80 species of about known phytoplankton are toxin producing and capable of producing harmful algal blooms [ 8 ]. In humans, toxins arising from harmful algal blooms have mainly been from shellfish consumption [ 44 ], bivalve shellfish have been reported to graze on algae and concentrate toxins effectively.
As a result the poisoning can lead to paralytic shellfish poisoning, diarrhetic shellfish poisoning, neurotoxic shellfish poisoning, amnesic shellfish poisoning and azaspiracid shellfish poisoning. In addition there are many respiratory complaints from inhaling contaminated aerosols [ 45 ].
A case reported that in July teenage boys swam in a blue-green algae covered golf course in Dane County, Wisconsin. They all became ill, the most severe symptoms occurred in the boys who swallowed water. Approximately after 48 h one of the boys suffered a seizure and died of heart failure, the coroner identified anatoxin-a as the most likely underlying cause of death [ 46 , 47 ].
In May and June , over California sea lions died and others displayed signs of neurological dysfunction along the central California coast, this was linked to a harmful algal blooms [ 48 ].
Harmful algal blooms is a cause of restriction on drinking water, fisheries and recreation water uses leading to significant economic consequences [ 49 ]. The presence of algal bloom and other species have disrupted the normal supply of drinking water in many parts of the world for example China [ 41 ]. The presence of algal blooms in Lake Tai severely affected industrial and agricultural production as well as lives of the urban dwellers. Whereby in algal bloom forced the shutdown of the entire water supply system and triggered a crisis in water security for the urban population.
The direct economic loss was estimated at about U. Harmful algal blooms present significant challenges for achieving water quality protection and restoration goals especially when these toxins confound interpretation of monitoring results and environmental quality standards implementation efforts for other chemicals and stressors [ 49 ].
There is need to reduce anthropogenic nutrient inputs to aquatic ecosystems in order to reduce the negative effects of eutrophication [ 50 ]. It has been indicated that reducing P input in the water bodies leads to eutrophication mitigation [ 16 , 51 ] Table 1 [ 16 ].
Derived this conclusion from four methods, all long-term studies at ecosystem scales: long-term case histories,. However, [ 6 , 52 ] argued that P based nutrient mitigation commonly regarded as the key tool in eutrophication, in many cases has not yet yielded the desired reductions in water quality and nuisance algal growth in water bodies has not reduced in decades of reducing P input.
Therefore, as a result of these contrasting findings there is need in some cases to consider a combination of different P mitigation strategies for example employing control of nutrient loading, physicochemical and physicomechanical method simultaneously. Control or mitigation of P eutrophication should encompass multiple components which could include; control of pollutant sources, restoration of the damaged ecosystem, and catchment management [ 41 ].
The mitigation strategies includes:. Point source P originating from mines, factories and residence form one of the most important sources of P to water bodies [ 41 ]. For example Lake Tai in China, its catchment area used to be full of heavy industrial polluters, for example, chemical and dye factories.
The township lacked adequate facilities for treating waste water before disposal. Therefore, to mitigate pollution coming from the industry it is important to shut down heavily polluting industries. While as the less polluting plants could be relocated to a designated industrial part to ensure centralization and effectiveness in handling pollution control.
It has been much easier to control point source P, therefore making nutrient discharge from agricultural fields the chief source of pollution [ 41 ]. As a result nutrient discharge from agricultural fields could be addressed through farm, field and catchment management or rationalization of land use [ 41 ]. In the farm scale environmentally sound fertilizer application and nutrient handing is important this would be achieved through appropriate placement and proper timing of application.
This would result in moderate P levels in the soils. In Addition P input could be reduced through increasing digestible P in fodder and reducing total P [ 51 ] Table 2. Examples of fresh waters in some countries where eutrophication decreased following the control of phosphorus inputs.
Latitudes and longitudes are given. Lakes recovered by using chemicals to precipitate phosphorus are not included, modified from [ 16 ].
Mitigation strategies for nutrient management at farm scale. Modified from [ 51 ]. To avoid transport of particulate P and leaching of P, increase soil storage there is need to change soil management. In addition there is need to change crop management in order to reduce run off and reduce leaching [ 51 ] Table 3. Mitigation strategies at field scale modified from [ 51 ]. In the catchment scale eutrophication mitigation strategy would involve water management, land use management and landscape management [ 51 ].
Water management could be achieved through reducing runoff flow and avoiding subsurface leaching. Land use management would involve protecting vulnerable areas and improving sink and sources of P by changing agricultural use patterns. Land scape management would include reducing direct sources of P from farmyard, livestock and reducing surface runoff and erosion from field to field within the catchment [ 51 ] Table 4.
Mitigation strategies at catchment scale modified from [ 51 ]. It has been demonstrated that often the nutrient load and algal blooms in water bodies respond slowly to interventions aimed at controlling external nutrient sources because of the nutrients replenished from waterbodies deposits [ 54 ].
As a result P could be reduced through physiochemical and physicomechanical methods. Whereby the P is trapped and removed from the system or trapped on farm and its mobility to aquatic system reduced. Reduction in the external P loading for control of algal biomass in the water reservoirs can be achieved by the use of ferric dosing.
Ferric dosing technique is a physiochemical method and involves the addition of ferric sulfate or alternatives to the pumped input, to precipitate dissolved particulate and orthophosphate in the coming water. Resulting in a significant reduction of P in the pumped inputs to the aquatic system. Flushing and dredging of floor deposits is a physicomechanical methods meant to remove the already accumulated P from the aquatic system floor [ 55 ].
Threshold concentrations at which action is taken to reduce nutrient loadings thus depend on economic factors, as well as wildlife conservation objectives. Reduce the source of nutrients e. Reduce the availability of nutrients currently in the system e.
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All rights reserved. Control In the past, the traditional eutrophication reduction strategies, including the alteration of excess nutrients, physical mixing of the water, application of powerful herbicides and algaecides, have proven ineffective, expensive and impractical for large ecosystems Michael F.
Chislock, Today, the main control mechanism of the eutrophic process is based on prevention techniques, namely removal of the nutrients that are introduced into water bodies from the water. It would be sufficient to reduce the concentrations of one of the two main nutrients nitrogen and phosphorus , in particular phosphorus which is considered to be the limiting factor for the growth of algae, acting on localised loads loads associated with waste water and widespread loads phosphorus loads determined by diffuse sources such as land and rain.
The load is the quantity milligrams, kilograms, tons, etc. The possible activities to be undertaken to prevent the introduction of nutrients and to limit phosphorus loads can be summarised as follows www3. In cases where water quality is already so compromised as to render any preventive initiative ineffective, "curative" procedures can be implemented, such as: removal and treatment of hypolimnetic water deep water in contact with the sediments rich in nutrients since in direct contact with the release source; drainage of the first cm of sediment subject to biological reactions and with high phosphorus concentrations; oxygenation of water for restore the ecological conditions, reducing the negative effects of the eutrophic process, such as scarcity of oxygen and formation of toxic compounds deriving from the anaerobic metabolism; chemical precipitation of phosphorous by the addition of iron or aluminium salts or calcium carbonate to the water, which give rise to the precipitation of the respective iron, aluminium or calcium orthophosphates, thereby reducing the negative effects related to the excessive presence of phosphorus in the sediments.
Conclusions Water is not a commercial product like any other but rather a heritage which must be defended and protected, especially in the presence of a global decline in the availability of drinking water and increase in its demand.
Despite the considerable efforts made to improve the water quality by limiting nutrient enrichment, cultural eutrophication and the resulting algal blooms continue to be the main cause of water pollution. The prevention and protection action that countries must adopt to safeguard the quality of surface water as requested not only by the scientific community and other experts, but to an increasing extent also by citizens and environmental organisations, is therefore increasingly important ec.
Management of the eutrophic process is a complex issue that will require the collective efforts of scientists, policy makers and citizens.
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