Differences in the persistency of the North Atlantic Oscillation signal among lakes

Large-scale climatic fluctuations, such as the El Nino Southern Oscillation and the North Atlantic Oscillation (NAO), are known to influence variabili ty in abiotic site conditions and organism population dynamics in both terr estrial and aquatic ecosystems. Here we demonstrate that the influence of t he NAO on lake water temperatures-one of the major factors controlling ecol ogical processes in lakes-differs substantially among lake types of differe nt thermal structures and mixing regimes, even under identical climatic for cing. A frequently circulating polymictic lake was found to be least influe nced by the winter effects of the NAO, with an effect lasting only into ear ly spring. In contrast, in a deep dimictic lake with stable summer stratifi cation, the NAO signal persisted in the hypolimnion until the following win ter. A shallow dimictic lake revealed an intermediate response, as weather conditions both in April and midsummer probably modified the strength and p ersistency of the NAO signal in the hypolimnion of that lake. Based on thes e results, it is to be expected that NAO effects on ecological processes va ry significantly among lakes. Because the study period (1979-1998) includes a series of uncommonly warm winter and spring seasons, our findings also s uggest that the influence of anticipated climate warming will vary substant ially among lake types.