DYSTROPHY AND EUTROPHY IN LAKE ECOSYSTEMS - IMPLICATIONS OF FLUCTUATING INPUTS

Eutrophic and dystrophic states of lake ecosystems are associated with distinct differences in phosphorus (P) input, refractory dissolved or ganic carbon (RDOC) input, and certain internal processes. Eutrophic l akes have high P input and high primary production. Dystrophic lakes h ave high RDOC input. In both types of lake, bacterial metabolism may h elp reduce RDOC levels. In dystrophic lakes, bacterial metabolism may be suppressed by low pH, and primary production is reduced due to ligh t attenuation by RDOC. We analyzed several simple models to ask whethe r eutrophy and dystrophy are alternative stable states of lake ecosyst ems. In-lake processes could create alternative states under certain c ircumstances, but more likely watershed processes maintain eutrophy or dystrophy through contrasts in inputs of P and RDOC. Simulations sugg est that pulses of RDOC result in dystrophic conditions that reverse v ery slowly. Land-use changes or climate fluctuations that change RDOC input rates may have long-lasting effects on trophic state of temperat e and boreal lakes. Lack of information on microbial degradation of RD OC, and the dependency of degradation rate on RDOC levels, primary pro duction, and pH, are major sources of uncertainty in our analysis and are suggested as priorities for further research.