Phosphorus accumulation in a free water surface wetland discharging into the Baltic Sea

Sammanfattning: Eutrophication has been an issue of the Baltic Sea for a long time, causing de-creased water clarity due to enhanced growth of algae and oxygen depletion dur-ing the decomposition of algae. The study investigated the free water surface wetland Vilhelmsberg that treats agricultural runoff before the water reaches the Baltic Sea. The area of the wetland Vilhelmsberg (1.6 ha) corresponds to 0.1 % of its catchment area, in which the natural waterbody Maren is located upstream of the wetland. The main objectives of this study were to quantify the phospho-rus (P) accumulation, evaluate the possibility of recycling the sediment and as-sess the risk of internal loading. Therefore, sediment cores were collected in both waterbodies and analysed for (i) total P and P fractions, (ii) metal content, (iii) plant available P and (iv) particle size distribution. Results showed that the wetland Vilhelmsberg had a low sediment accumulation rate of 0.5 cm yr 1 and a low P accumulation of 13 kg ha 1 yr 1. The maximum potential P release rate of the wetland Vilhelmsberg (6.5 mg m 2 d-1) and Maren (5.2 mg m-2 d-1) indicated a small risk of internal loading. However, the proportion of clay particles at the wetland’s inlet was eight times higher than at the outlet, indicating resuspension probably caused by the inflowing Sea water. Thus, it is recommended to change the outlet design to improve the functioning of the wetland. Results also showed that it will be possible to return the sediments of the wetland Vilhelmsberg to the agricultural fields since the metal content was below Swedish legislation limits, whereas the sediment from Maren had too high nickel contents. The sediment represented a good soil amendment regarding the plant available P which was much higher in the wetland (10.5 mg per 100 g DS) and Maren (12.7 mg per 100 g DS), compared to the catchment soil (3.8 mg per 100 g DS). Concluding, the wetland Vilhelmsberg functions as a P trap by decreasing the P load from agricultural land to the Baltic Sea, but it had low particle and P accumulation which could be improved by design changes.