Kan turbiditetsmätningar användas för att uppskatta fosforkoncentrationer i vattendrag? En fallstudie från Ringsjöns avrinningsområde

Like many other lakes, Lake Ringsjön in Sweden has a long history of eutrophication problems. Eutrophication of lakes is mainly due to surplus of phosphorus. Most phosphorus reaches Lake Ringsjön from adjacent watercourses. In order to initiate action plans to reduce phosphorus concentrations in the lake, it is of great importance to know which watercourses have high phosphorus concentrations. Phosphorus analysis, however, is expensive and therefore a simple and cost efficient method of measurement is of great interest. In this study, the relationship between turbidity and total phosphorus concentration was investigated in Kvesarumsån and Hörbyån, two different watercourses in the catchment area of Lake Ringsjön. A positive relationship between turbidity and phosphorus concentration is expected, as phosphorus often is bound to particles in water. Water samples were collected at two occasions, at 5 respectively 10 sites in Kvesarumsån and Hörbyån. A significant positive relationship between turbidity and phosphorus concentration was found in both watercourses. However, it was found that the same value of turbidity corresponds to higher phosphorus concentrations in Hörbyån, as compared to Kvesarumsån. This is likely a result of the large proportion of agricultural land in the catchment of Hörbyån in comparison to that of Kvesarumsån, which is dominated by forest. Equations obtained from the positive relationships were used to estimate phosphorus concentrations from turbidity measurements in watercourses throughout entire catchment area. In total, turbidity was measured at 72 sites. GIS was useful to visualize and interpolate the results. The most turbid watercourses are located in agricultural areas and a significant relationship between turbidity and the proportion of agricultural land upstream each test point was found. Other factors, such as proportion of drained land, water flow and soil phosphorus concentrations, were also tested for relationship with turbidity. These variables, however, did not show any significant relationship with turbidity. Hörbyån and the small watercourses south of Ringsjön were estimated to have the highest phosphorus concentrations. It is therefore suggested that these watercourses should be prioritized in remediation efforts – especially Hörbyån, which has a considerable water flow and an estimated external phosphorus loading of about 50 % of the total external loading in the catchment