The response of freshwater ecosystems to climate variability associated with the North Atlantic Oscillation

The North Atlantic Oscillation (NAO) affects the physics, hydrology, chemistry and biology of freshwater ecosystems over a large part of the Northern Hemis- phere. Physical impacts of the NAO include effects on lake temperature profiles, lake ice phenology, river runoff and lake water levels. These physical and hydrological responses influence the chemistry and biology of fresh waters by affecting the leaching of nutrients from the soil and by altering the distribution of nutrients and oxygen in lakes. Finally, the population dynamics of freshwater organisms on several trophic levels including autotrophs, herbivores and vertebrate predators are directly and indirectly linked to the NAO via food-web interactions. As a result, the effects of mild winters associated with the positive index phase of the NAO can influence the food-web characteristics of lakes in summer. A considerable body of evidence documents the importance of these indirect and food-web mediated effects of the NAO, which might even result in ecosystem regime shifts. Owing to the large-scale impact of the NAO, lakes exhibit spatial coherence over large areas with respect to both physical and biological properties. This coherence is modified by geographical factors such as altitude and latitude, and by lake-specific characteristics such as depth and trophic status.