Hydrobiologia 404: 9–18, 1999. L.-A. Hansson & E. Bergman (eds), Nutrient Reduction and Biomanipulation as Tools to Improve Water Quality: The Lake Ringsjön Story. © 1999 Kluwer Academic Publishers. Printed in the Netherlands. 9 Changes in nutrient load and lake water chemistry in Lake Ringsjön, southern Sweden, from 1966 to 1996 Eva Bergman Department of Limnology, Insitute of Ecology, Ecology Building, University of Lund, SE-223 62 Lund, Sweden. Present address: Department of Biology, Karlstad University, S-65188 Karlstad, Sweden Key words: P-loading, P concentration, sewage water treatment plant, agricultural fertilization control program, fish reduction, Secchi depth Abstract Monitoring Lake Ringsjön’s water chemistry, i.e. of pH, PO 4 -phosphorus, total phosphorus, Kjeldahl nitrogen, NO 2 +NO 3 -nitrogen, total nitrogen, chlorophyll a concentration and Secchi depth, has been conducted on a re- latively 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 Hörby and Höör. A few years later, new recommendations were given to the farmers about use of fertilizers to further decrease the nutrient load to Lake Ringsjön. 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 Ringsjön. This increase in Secchi depth co-occurred in time with: (a) a fish-kill in Eastern Basin (1988), fish reductions in Sätofta 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 μgl −1 . Introduction Lake Ringsjön has been of interest for limnological studies for more than a century. The first reported in- vestigation was performed by Trybom between 1882 and 1885, and dealt with phytoplankton and vegetation (Trybom, 1893). Additional phytoplankton studies were done during the 1920s by professor E. Naumann, and in 1946–1947, studies on macrophytes, phyto- plankton and hydrochemistry were performed (An- dersson, 1948; Almestrand & Lundh, 1951; Lundh, 1951a,b). In the 1960s, physical and chemical vari- ables were also included in the investigations (Björk & Lettervall, 1968), and from the early 1970s several different investigations were conducted. In 1975, Lake Ringsjön was included in the Swedish Environmental Protection Agency’s project ‘Sewage Water Treatment Plant–Recipient’ (‘RR-investigation’) (Ryding, 1983). The purpose of this project was to investigate how fast, and to what extent, nutrient enriched lakes improved following a decrease in the external nutrient load from municipalities, due to installation of chemical and bio- logical treatment at sewage water treatment plants. In 1983, the control program was modified, and was con- ducted by The Department of Limnology, University of Lund in 1983–84 (Enell, 1985). From 1985 to 1996, the same program was conducted by the Swedish En- vironmental Research Institute in Stockholm (Enell, 1986; Enell & Eriksson, 1987, 1988; Enell & Hen- riksson, 1989, 1990, 1991, 1992; Enell et al., 1993, 1994; Enell & Fejes, 1996). In 1987, a biomanipula- tion project started, conducted by The Department of Limnology, University of Lund (Hamrin et al., 1991, 1993; Bergman et al., 1994; Bergman, 1997). In this paper, I focus on the period 1966–1996 because data on physical and chemical variables are available, and because the lake became eutrophic- ated during this period, and efforts were made to improve lake water quality. In the 1970s, sewage water