An important topic in our research is the cycling of nutrients within lake ecosystems. This is because in a large number of lakes, the internal cycling of nutrients regulates the water quality and prevents or delays the recovery of the ecosystem after the reduction of external nutrient loading. The factors regulating the internal cycling are inadequately known, which prevents efficient lake management and restoration. Therefore, our studies on nutrient cycling have been planned to improve our knowledge on nutrient cycling both in the littoral, pelagic and profundal zones of lakes. The studies have included both biological (fish stocks, macrophytes, microbes) and physical-chemical (wind, turbulence, ice-cover) regulators of nutrient cycling. Both phosphorus and nitrogen cycling has been studied. We have for instance quantified the effects of different macrophyte life forms on resuspension-induced internal phosphorus and nitrogen loading, estimated the effects of fish stocks on phosphorus and nitrogen cycling and studied the effects of macrophyte stands on nitrogen cycling in the littoral zone. With a large lake dataset we have also shown that anoxic conditions do not regulate lake water quality to the extent generally believed.