Impact of climate change on arctic and alpine lakes: Effects on phenology and community dynamics

Abstract

The ecology of Arctic and Alpine lake communities is heavily influenced by ice and snow, two factors governed by climatic conditions (Rouse et al. 1997). Climate change generates variation in ice and snow cove r phenology, thickness and texture that triggers a broad spectrum of physical,chemical and biotic responses (Quayle et al. 2002). Changes in community structure and dynamics caused by these environmental responses are early symptoms of an ecological impact of climate change. The simple character of Arctic and Alpine lake communities, with a small number of species arranged along few trophic levels, facilitates detection of such structural changes and permits detailed studies of dynamics. The sharp influence of climate and simple community structure, combined with the opportunity of tracking past ecological change using paleoecological records (Smol et al. 2005), make Arctic and Alpine lakes suitable ecosystems for an early, reliable assessment of ecological impacts of global climate change. Global climate change is expected to modify the range and phenology of many terrestrial and aquatic species (Parmesan and Yohe 2003). Projections of the future impact of climate change on biodiversity, based on species-area relationships (Rosenzweig 1995) and the assumption that climate determines present distributions, stress how climate induced range alteration will greatly enhance the risk of extinction for many species (Thomas et al. 2004). Meanwhile, range expansion and changes in phenology consistent with global warming scenarios are being documented for an increasing number of taxa (Root et al. 2003). The above assessments of ecological impact of climate change are mainly based on comparative studies from tropical and temperate regions, the focus of past research efforts. Evaluation of an impact of recent climate change on Arctic and Alpine communities must rely on evidence that is more sparse or indirect. Information needed for the extrapolation of climate impact scenarios in Arctic and Alpine systems is richer, for it includes data from lower latitudes and altitudes that may be assumed to reproduce relevant aspects of future environmental conditions. Arctic and Alpine lakes are characterized by an extended period of ice cover, cold water and low production. Warming results in anticipated ice melt and delayed ice formation (Magnuson et al. 2000), and in increased water temperature (Schindler 1997). The resulting prolonged ice-free season, stronger thermal stratification and enhanced nutrient resuspension are expected to increase lake production (Rouse et al. 1997). Further increases in production may derive from indirect effects of warming on external inputs of nutrients and organic carbon (Quayle et al. 2002). The above changes in environmental conditions may also shift the main contribution to lake production from benthic to pelagic (Korhola et al. 2002). Increased temperature and production will facilitate successful invasion and introduction of species adapted to warmer and more productive waters. The ecological implications of the above environmental changes, however, reach beyond range alteration driven by physiological limitations. Within a lake, a longer productive season protracts the active period of community members increasing the scope for ecological interactions and ecosystem engineering. By relaxing the physical and energetic limitations on demographic and interaction parameters, and by modifying the nature and structure of interactions, warming resets the internal machinery that drives community dynamics (Harrington et al. 1999, Schmitz et al. 2003). The above qualitative generalization is intended as a reference scenario for the following discussion of climatic impact on phenology and community dynamics of plankton, benthos and fish. The scenario invites some general considerations about impact on biodiversity. Medium term projections (year 2100) rank climate change as a main driver of future biodiversity loss in Arctic and Alpine freshwaters (Sala et al. 2000). Many of the cold stenothermal species populating these regions are relictual endemics that are forced into deep, cool waters of lakes in order to survive. Under warming, the deep refuges would gradually become unbearable due to the higher temperatures and lower oxygen concentrations. Loss of even a few endemic species will have a strong impact on regional diversity, considering that high altitudes and latitudes have relatively few species to begin with (Lomolino 2001, Willig et al. 2003). Empirical, hump shaped relations between productivity and species richness in lakes suggest that the expected increase in productivity will be followed by increased species richness (Rosenzweig 1995, Dodson et al. 2000). This does not contradict the expectation of higher exctinction rates given that recipient communities will experience substitutions by exotic species that are expanding their range. The rates of change in species composition and richness may also increase, due to higher propagule pressure. The latter effect is most likely in low latitude Alpine lakes, where the distance between different ecoclimatic regions is short and the species pool at lower altitudes is large. The scenario of a warmer, wetter climate that will increase ice-free season duration, water temperature and productivity of Alpine and Arctic lakes must be treated with caution. The magnitude and even direction of ecological response to climate change will depend on a variety of environmental conditions that were not addressed. Basin morphometry, hydrology, catchment and geographic position are all likely to affect the character of climatic impact and will be considered in more detail below. The literature reviewed includes assessment and prognostic studies of climatic impact on Arctic and Alpine lakes. As mentioned earlier, the evidence of recent ecological changes reported in these studies is seldom direct. Reviews of assessment studies dealing with climatic impact on lakes mainly refer to data collected in temperate regions (Schindler 1997, Straile et al. 2003). In the absence of direct observations, microfossil accumulations in lake sediments provide compelling indication of climate driven ecological change (Smol et al. 2005). Paleolimnological records may even allow the detection of changes in the intensity of ecological interactions, as in the case of fish predation on the planktonic prey Daphnia (Jeppesen et al. 2002).