The potential effect of paper and pulp mill effluents on brownification in Hanö Bay, Baltic Sea

Sammanfattning

The Baltic Sea is a semi-enclosed basin characterized by its brackish water and unique biodiversity. However, it is also one of the most polluted seas in the world and is exposed to multiple environmental stressors, including eutrophication, overfishing, and climate change. Freshwater and coastal browning have been observed across the Northern Hemisphere, although the cause and dynamics of brownification in coast and land is debated. Brownification is the shift in water colour and is often associated with increased organic matter which affects the optical properties of water. Hanö Bay has been under discussion due to eutrophication, dead fish, and observed browning of water. Two paper and pulp mills, Nymölla and Södra Cell Mörrum, are located along the Hanö Bay coast, and discharge wastewater into the bay through pipes. The aim of this thesis is to investigate the potential influence of paper and pulp and effluents on possible brownification in Hanö Bay. Total organic carbon and absorbance at 420 nm of water samples were analysed at monitoring stations located in the five major river basins that drain into the bay. In addition, the chlorophyll a concentration, dissolved oxygen content, and Secchi depth were examined at six coastal monitoring stations close to the two mills. Both parametric and non-parametric statistical tests were used to assess if there are any significant trends. Monthly mean secchi depth created from ice-ocean model NEMO and biogeochemical model ERGOM, was analysed in ArcGIS Pro to examine spatial and temporal patterns. The results indicate a significant increase in total organic carbon and high absorbance values in the river basins draining into the bay. However, the coastal monitoring data for Hanö Bay is insufficient to conclude the extent to which paper and pulp mill effluents contribute to brownification. The limited availability of long-term coastal water data, particularly from the 1970s makes it difficult to distinguish between the effects of paper and pulp mills from drivers such as land-use change and climate change. Modelling, past, present and future may fill in the gaps for the temporal data. Previous studies have used proxies for organic matter and pollution through total organic matter, coloured dissolved organic matter, and absorbance. Further projects should focus on water sampling near the mills and along the coast to obtain an overview of the water properties. Future research on brownification and the interactions between land and coastal ecosystems is crucial, especially under the ongoing climate change.