The law in many parts of the world

The law in many parts of the world increasingly restricts the discharge of agricultural slurry into watercourses . The simplest and often the most economically sound practice returns the material to the land as semisolid manure or as sprayed slurry .

This dilutes its concentration in the environment to what might have occurred in a more primitive and sustainable types of agriculture and converts pollutant into fertilizer . Soil microoganisms decompose the organic components of sewage and slurry and most of the mineral nutrients become available to be absorbed again by the vegetation .

The excess input of nutrients , both nitrogen and phosphorus – based , agricultural runoff ( and human sewage ) has caused many ‘ healthy ‘ oligotrophic lakes ( low nutrient concentrations , low plant productivity with abundant water weeds , and clear water ) to change to eutrophic condition where high nutrient inputs lead to high phytoplankton productivity ( sometimes dominated by bloom – forming toxic species ) . This makes the water turbid , eliminates large plants and , in the worst situations , leads to anoxia and fish kills ; so called cultural eutrophication . Thus , important ecosystem services are lost , including the provisioning service of wild – caught fish and the cultural services associated with recreation .

The process of cultural eutrophication of lakes has been understood for some time . But only recently did scientists notice huge ‘ dead zones ‘ in the oceans near river outlets . , particularly those draining large catchment areas such as the Mississippi in North America and the Yangtze in China . The nutrient – enriched water flows through streams , rivers and lakes , and eventually to the estuary and ocean where the ecological impact may be huge , killing virtually all invertebrates and fish in areas up to 70,000 km2 in extent . More than 150 sea areas worldwide are now regularly starved of oxygen as a result of decomposition of algal blooms , fuelled particularly by nitrogen from agricultural runoff of fertilizers and sewage from large cities . Oceanic dead zones are typically associated with industrialized nations and usually lie off countries that subsidize their agriculture , encouraging farmers to increases productivity and use more fertilizer .

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