Jordbrukets bidrag till föroreningen av Gullmarsfjorden
Karlsson, Ingrid M.
The transport from land of phosphorus and nitrogen to Gullmar Fjord has increased by 25-30% during the past decade. Within the last few years there has been several instances of low oxygen concentrations in the deeper water levels of the fjord, obviously caused by the increasing pollution load. A main cause of the increasing transport of phosphorus and nitrogen to the Gullmar Fjord is the 10-20% increase in precipitation within the drainage basin, which prevailed during the last ten years. The high precipitation increased runoff, which may have increased leaching of nutrients from all landbased sources into the streams. The percentage of nutrient loss from agricultural land was probably over-estimated in the Gullmar Plan (published in 1984), mainly because the cultivation intensity and the use of fertilizer and manure was, and still is, much lower in the Gullmaren area than in the agricultural areas from where the area leaching coefficients were adopted. Agricultural acreage and numbers of domestic animals have decreased during the last ten years, which indicates that the agricultural activities have not caused the last few years' increase in pollution. There is some evidence that the high percentage of rock and shallow soils with no or very limited vegetation (55% of the land area in the county of Göteborgs and Bohus län) increases the risk of erosion, which means that loss of phosphorus may be high from this type of land. The same type of areas also have a limited capacity for storage and uptake of nitrogen in the rootzone, and this means that the nitrogen deposited from the atmosphere (16-17 kg per ha and year) runs a high risk of being leached out. These hypothesises should be tried on shallow soils in the field within the area of interest. Accurate estimations of how much of the pollution is caused by agricultural activities are very difficult to make, since there are no leaching studies on agricultural land from the actual area. A guess based on the facts collected within this study is that agriculture contributes 25-40% of the nitrogen pollution and 15-25% of the phosphorus pollution. An intensified monitoring of runoff volumes and nutrient contents in streams discharging from areas of different land usages into Gullmar Fjord is suggested. Also, field experiments aiming at minimizing the nutrient pollution from agricultural and forested land in the Bohuslän and Dalsland areas should be carried out. These activities could take place along with campaigns to minimize pollution from other sources as weIl. In the report there are also suggestions on which types of agricultural practices are most effective in decreasing the pollution from agricultural land in Bohuslän and Dalsland.
Gullmarsfjorden; Förorening; Jordbruk
Report / Swedish University of Agricultural Sciences, Department of Soil Sciences, Division of Agricultural Hydrotechnics
Publisher: Institutionen för markvetenskap, Sveriges lantbruksuniversitet
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