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 variability in abiotic site conditions and organism population dynamics in both terrestrial and aquatic ecosystems. Here we demonstrate that the influence of the NAO on lake water temperatures-one of the major factors controlling ecological processes in lakes-differs substantially among lake types of different thermal structures and mixing regimes, even under identical climatic forcing. A frequently circulating polymictic lake was found to be least influenced by the winter effects of the NAO, with an effect lasting only into early spring. In contrast, in a deep dimictic lake with stable summer stratification, the NAO signal persisted in the hypolimnion until the following winter. A shallow dimictic lake revealed an intermediate response, as weather conditions both in April and midsummer probably modified the strength and persistency of the NAO signal in the hypolimnion of that lake. Based on these results, it is to be expected that NAO effects on ecological processes vary significantly among lakes. Because the study period (1979-1998) includes a series of uncommonly warm winter and spring seasons, our findings also suggest that the influence of anticipated climate warming will vary substantially among lake types.