Initial nutrient retention capacity in a constructed wetland : Evaluating the effectiveness of a newly constructed wetland to reduce eutrophication symptoms in a Baltic Sea bay in northern Sweden

Sammanfattning: Since the turn of the last century, a substantial increase in nutrient load to the Baltic Sea is apparent. Adding the ongoing environmental change with raising temperatures and increased precipitation, this will continue to have a prominent environmental impact on our coastal ecosystems, especially in northern latitudes. Constructed wetlands are becoming more important as a mitigation measure to retain nutrients, however, they are until this day not well studied in northern latitudes. In this paper, nutrient retention in a newly constructed wetland is studied during its first month after activation, as well as potential downstream effects in associated sea bay. An additional literature study compiles information about the current knowledge, use and functionality of wetlands surrounding the Baltic Sea. This is done to widen knowledge regarding effectiveness of wetlands as nutrient traps in general, as well as to compare with the studied wetland. A net retention of 30 % for dissolved organic carbon (DOC) and total phosphorus (TP) was found, as well as 27 % for total nitrogen (TN), 25 % for phosphate (PO43-) and 21 % for nitrate (NO3-). TP was found to be within range of expected retention capacity, when comparing with wetlands included in the synthesis. TN retention, however, seemed to be somewhat greater than in other wetland studies. Furthermore, the retention varied and seemed to be highest during an increased discharge, in the beginning and end of March. This was partly reflected by greater inlet concentrations and transports in most of the parameters during the initial time period. Decreasing temporal trends was seen in concentrations of DOC, total nutrients and NO3- concentrations in the sea bay, indicating an immediate downstream effect of the wetland installation. Findings from the synthesis indicate that there are very few studies in, and thus little knowledge about, wetlands in northern climate. Overall, the results from the pioneer northern wetland in Sörleviken suggest that net retention is possible during its first month post-activation.