... kilograms per hectare is deposited onto forest soil in the Netherlands. (5) This amount is roughly as much as is spread on intensively farmed fields and shows that not only agriculture is a main source but also forestry. A build up of large quantities of these nutrients in the rivers and lakes causes an accumulation of nutrients at a rate greater than they can be used through the natural processes of decomposition and photosynthesis from the living organisms in the water. Mainly in still, stagnant water the build up of nutrients encourages the growth of algae on the surface and this can cause blue-green algae to grow which can be hazardous to animals and humans in that it produces toxins. High levels of phosphorus can cause these algae to grow. (1) Because light oxygen and heat are unable to penetrate through the surface, the algal cells die off from the surface, ...

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