Ponds and small lakes: Microorganisms and freshwater ecology (Naturalists' Handbooks Book 32) by Brian Moss

Author:Brian Moss [Moss, Brian]
Language: eng
Format: azw3
Publisher: Pelagic Publishing
Published: 2017-04-30T16:00:00+00:00

5.3 Eutrophication

In the remote past, before the landscape was converted from natural vegetation to agriculture and pasturage, it is likely that both nitrogen and phosphorus simultaneously limited growth of algae and plants in freshwaters, a situation that still pertains in areas of the ocean. But from the start of the twentieth century, development of the Haber-Bosch process produced ammonia industrially for subsequent conversion to nitrate fertiliser. This process is now releasing about as much combined nitrogen to the biosphere as nitrogen fixed by natural processes. In consequence there is a surfeit of nitrogen in agricultural areas, and because both ammonium and nitrate are very soluble in water, this places greater emphasis on the scarcity of phosphorus in freshwaters. But phosphorus concentrations have also risen as a result of human activities, because of use of superphosphate fertiliser and because human and farm animal sewage is rich in phosphorus and nitrogen. When human populations were lower and most sewage was redistributed to the land, much of this phosphorus was refixed in the soils. Distribution to the land in densely populated areas is no longer acceptable and the sewage is treated to remove organic matter, leaving an inorganic solution of nitrogen and phosphorus compounds and a large array of other substances that pass through the works. As a result of both fertiliser use and large human populations, there is now a severe problem of nutrient pollution, or eutrophication, worldwide and concentrations in rivers, ponds and lakes are, on average, about ten times as high as they would be were the land undisturbed and lightly populated. Nonetheless, experiments such as those described above will still show stimulation of growth by these nutrients in many waters. It is worth trying to emulate natural eutrophicated conditions by doing your experiments in low light. Below the surface, light is scarce in eutrophicated ponds because of absorption by algae and the large amounts of dissolved organic matter that reach such waters. Do you get different results from the same addition of nutrients in normal daylight compared with shaded and deeply shaded conditions?

Garden ponds, and ponds on farms where stock is kept, are usually highly eutrophicated. Fertiliser is liberally used in gardens, be it from an expensive bag or the local stables or chicken farm. Mixed and stock farms can hardly avoid a liberal dressing of manure on their fields and although manure piles from cowsheds and stables are supposed to be covered from rain, it is rare for there not to be a dribble of drainage that does not find its way into a ditch or culvert and down to some of the ponds on the farm. If the garden pond is built up above ground level and lined, it will not be so nutrient enriched for its catchment is limited to its edge and its nutrient supply will come from rain and from topped up tap water in dry periods. Rain however is now relatively rich in nitrate from oxidation in the atmosphere of ammonia volatilised from

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