The Response of Freshwater Ecosystems to Climate Variability Associated with the North Atlantic Oscillation

Freshwater ecosystems have been repeatedly shown to respond significantly to large-scale climatic fluctuations [e.g., Strub et al., 1985; Firth and Fisher, 1992; George and Taylor, 1995; Anderson et al., 1996; Schindler, 1997]. Because human demand for high-quality fresh water is continually increasing, the practical relevance of such responses for the large-scale management of inland waters is now gradually beginning to be realized. Specifically, the impact of the North Atlantic Oscillation on lacustrine systems, especially in Europe, is becoming a topic of some importance. Briefly summarizing the information given more extensively in other parts of this volume, the North Atlantic Oscillation (NAO) represents a large-scale fluctuation in the air pressure difference between the Azores High and the Iceland Low [e.g., Hurrell et al., this volume]. It dominates much of the atmospheric behavior in the North Atlantic region, and is known to influence air temperature and precipitation over large areas of the Northern Hemisphere in winter [e.g., Hurrell, 1995; Hurrell and van Loon, 1997]. It is commonly represented in terms of an index based on the difference of the sea-level air pressure measured at a meteorological station close to the center of the Azores High and that measured at a station in Iceland [e.g., Jones et al., this volume]. Positive winter values of an NAO index correspond to a strong meridional pressure gradient that results in strong westerly winds transporting warm, moist maritime air across Europe, giving rise to warm, wet winters there. In contrast, low NAO index values correspond to weak westerlies and cold, dry winters in Europe. The Response of Freshwater Ecosystems to Climate Variability Associated with the North Atlantic Oscillation