A fresh (water) perspective on the impacts of the NAO on North Atlantic ecology

North Atlantic climate variability does not only have an influence on North Atlantic ecosystems as described in the previous chapters and by Drinkwater et al. (2003), but influences terrestrial and freshwater ecosystems across the Northern Hemisphere (Ottersen et al. 2001; Mysterud et al. 2003; Stenseth et al. 2002; Straile et al. 2003b). Its impact on freshwater ecosystems has been documented on an area ranging from Lake Mendota in the United States to Lake Baikal in Russia, from Lake Kallavesi in Finland to the Caspian Sea (Straile et al. 2003b). In an increasing number of studies, the North Atlantic Oscillation (NAO) is shown to influence the physics, chemistry, as well as the biology of rivers and lakes, with biological impacts encompassing various trophic levels. A review on NAO impacts was recently provided by Straile et al. (2003b) and will not be repeated here. Instead, this chapter is an essay on selected aspects of NAO impacts on freshwater ecosystems, which might shed some light or pose new questions on the NAO impacts on the North Atlantic. There is one aspect of lake ecosystems, which makes them easier to study compared to marine ecosystems: their size. Because of their smaller size, the physical environment of most freshwater plankton ecosystems may be considered to some extant as simplified because large current systems do not exist as in the ocean and hence cannot influence the abundance of plankton organisms at distinct sampling sites. Horizontal transport is a major topic in this book's chapters of climate impacts on phyto-plankton (Chapter 4), zooplankton (Chapter 5), as well as fish populations (Chapter 6) in the North Atlantic. The impact of horizontal transport on plankton abundance is of such pronounced importance that the term 'translation' has been coined to account for this effect (Chapter 5). Ecological consequences of changes in horizontal transport are many-fold. Direct and indirect effects may be distinguished: the direct effect of changes in horizontal transport is that a distinct species or community will be transported to the site A instead of site B. This, however, might imply important secondary and indirect effects for the transported species/community but also for the local communities at both sites A and B. Site A might be less or more suitable for the growth and reproduction of the transported species than site B because of local differences in temperature and/or food and/or predator abundance (spatial match or mismatch; Cushing 1990). …