Like many other bodies of water, the Baltic Sea is facing a severe eutrophication problem, the excessive nutrient load currently originating primarily from agriculture. Gypsum treatment has shown promising results in reducing phosphorus and suspended sediment loads, especially in fine-textured soils, but its efficacy has not been investigated on catchment scale to any wide extent. This study examines the functionality of the gypsum treatment in an agriculture-dominated boreal catchment in the Archipelago Sea basin. The relation between turbidity and hydro-climatological factors was analysed during different years and seasons and the effect of stream erosion during different flow stages was investigated by applying 2D hydraulic modeling. A special focus was given to deviating winter conditions and freeze-thaw cycles. The results highlight the dependency of stream water turbidity on the discharge and the potential negative impact of the climate change-altered winter conditions of the hemiboreal climate zone on gypsum treatment effectiveness. The turbidity increased even more during high winter discharges, compared to other seasons suggesting that the more frequent freeze-thaw cycles and precipitation coming as rain during the unvegetated period increase erosion challenging the gypsum efficacy. Modelling results revealed that the increased erosional power during very high discharges caused settled material resuspension or stream bed or bank erosion leading to a cumulative increase in turbidity at downstream, an issue needing further research. Demonstrating the real and long-term effects of nutrient reduction measures requires catchment scale investigations over years, considering climatological factors. Based on this study, local and short-term investigations can potentially lead to over-simplified interpretations.