To succeed in combating lake eutrophication, cooperation of local inhabitants, small factories, and farmers in reducing phosphorus discharge is very important. Here we study the coupled dynamics of people's behavioral choice and the magnitude of eutrophication. Assumptions are: Many players choose between a cooperative and environment-oriented option and an economic alternative. This choice is affected by "social pressure" promoting cooperation. The social pressure is larger when many other players cooperate, and when the lake water is polluted (via an enhanced environmental concern). The model often shows bistability -- having an equilibrium with enhanced cooperation among people and clean water, and the other with low cooperation and polluted water, which are simultaneously stable. It also shows oscillation and chaotic fluctuation. The model may show unexpected parameter dependence -- the improved phosphorus removing efficiency might make water more polluted if it causes the decline in the environmental concern and cooperation among people. We also analyze the case when the limnological dynamics has hysteresis due to nonlinear rate of nutrient recycling from sediment. We conclude that the enhancing conformity is more effective in reducing water pollution than enhancing social concern or reducing the cooperation cost.