The loading that has been fueling the eutrophication process in Lake Pyhäjärvi is mainly imported into the lake from elsewhere: from the catchment area, the shore areas or the air. Furthermore, nutrients that have been deposited into the sediments at the bottom of the lake may re-enter the water column and become available to organisms. All in all, 66 % of Lake Pyhäjärvi phosphorus-loading is imported by the Yläneenjoki and Pyhäjoki rivers from the catchment area. Water protection measures carried out on the river banks and in the local catchment area are of utmost importance in decreasing the external loading. Phosphorus exits Lake Pyhäjärvi through the river Eurajoki that flows out of the lake. It is also assimilated by the fish that are then removed from the lake by fishing. As the Pyhäjärvi Restoration Programme progressed, the amount of phosphorus being stored in the lake together with the internal loading reduced substantially. However, mild winters can increase the loading significantly.
The dramatic effects that mild winters have on annual phosphorus and particulate-matter loading can be clearly seen from loading development data. Heavy rainfall during periods without vegetation cover and mild temperatures cause extensive erosion. Snow cover conditions where snow fell and thawed repeatedly, together with an unfrosted ground, rinsed record-breaking amounts of nutrients into waters. These peak loadings cannot be stopped by traditional wetlands and protection zones. Novel methods for the management of winter time loadings need to be invented since climate change scenarios predict an increase in mild winters with less snow cover.
Winter loading is challenging
Annual phosphorus loads imported by the Yläneenjoki and Pyhäjoki rivers were 8,3 tons and 2,1 tons, respectively. On average during the years 1980-2010 the river Yläneenjoki imported 8,8 tons and the river Pyhäjoki 1,8 tons of phosphorus annually. The nutrient load was concentrated in the winter months and showed up as strong peaks during heavy rainfall. Nutrient retention in the catchment area is particularly challenging during the winter months because water protection measures based on biological activity, for example, wetlands do not retain nutrients. Vegetation cover is the most important way of retaining winter time nutrient loading; thus direct sowing, a rather common practice in the Lake Pyhäjärvi area, is an effective water protection method in a changing climate.