Changes in nutrient load and lake water chemistry in Lake Ringsjon, southern Sweden, from 1966 to 1996

Monitoring Lake Ringsjon's water chemistry, i.e. of pH, PO4-phosphorus, total phosphorus, Kjeldahl nitrogen, NO2+NO3-nitrogen, total nitrogen, chlorophyll a concentration and Secchi depth, has been conducted on a relatively continuous basis since the mid 1960s. Here, I analyze this data by graphical methods and discriminant analysis. The results show that there was a pronounced decrease in Secchi depth and an increase in phosphorus and chlorophyll a concentrations during the late 1960s and early 1970s. To improve conditions in the lake, new sewage water treatment techniques, using chemical precipitation techniques with post-filtration, were put into operation in the late 1970s in Horby and Hoor. A few years later, new recommendations were given to the farmers about use of fertilizers to further decrease the nutrient load to Lake Ringsjon. The concentrations of phosphorus and chlorophyll a started to decrease in the early 1980s, but there was no increase in Secchi depth. From 1989 to 1991, Secchi depth increased in the three basins of Lake Ringsjon. This increase in Secchi depth co-occurred in time with: (a) a fish-kill in Eastern Basin (1988), fish reductions in Satofta Basin (1990) and Western Basin (1992); (b) a very low phosphorus loading in 1989 (4.7 tons); and (c) phosphorus concentrations in the lake water below 100 mu g l(-1).