Dagvattenhanteringens effekter på förutsättningarna för biologisk mångfald av kransalger i Lillsjön, Jämtland / Urban stormwater management and its implications for biodiversity of Chara species – an example from Northern Sweden
Release and load of urban stormwater can affect biodiversity in recipients in both a short and long term perspective. For shallow lakes where natural shifts can occur between clear water and turbid water states due to asymmetric competition between different groups of algae, the role of urban stormwater load over the long term is of high importance. In Jämtland, Northern Sweden, a study was conducted to investigate and identify possible impacts of urban stormwater on biodiversity in general, and specifically on the interspecific competition between redlisted Chara-species (benthic algae) and pelagic phytoplankton. The study was performed in Lake Lillsjön, a small lake with high recreational and nature values in the vicinity of the city of Östersund. Approximately 10 % of Lake Lillsjön’s catchment area is made up of hardstanding, urban surface. In addition to stormwater, the lake is also a recipient for meltwater from the central municipal snowdump. Water and sediment sampling were performed along with the calculation of mass transport of metals and nutrients in water ecosystems. Water flows were derived from the S-HYPE model. The results indicate that the historical load of nutrients and the ongoing load of metals from urban stormwater have played a significant role in a shift in water state, resulting in Lake Lillsjön becoming a more turbid, as opposed to it original clear water state. Consequently, this could reduce production of clear water species such as the benthic algae Chara, and enhance phytoplankton growth. Arguably, in the case of Lake Lillsjön, urban stormwater management has implications for the biodiversity of redlisted Chara-species.