For example, freshwater lakes have been predicted to increase in productivity across trophic levels as a consequence of the effect of warming on metabolism and production (Regier and Meisner, 1990). However, the warming effect is often associated with other physical changes which can have countervailing effects, such as thermal stratification resulting from lighter winds. These may result in anoxic conditions or less intense upwelling and/or seasonal mixing, which may cause fish mortality or reduced primary productivity.