Academic journal article Estonian Journal of Ecology

Climate Change Effects on Shallow Lakes: Design and Preliminary Results of a Cross-European Climate Gradient Mesocosm Experiment

Academic journal article Estonian Journal of Ecology

Climate Change Effects on Shallow Lakes: Design and Preliminary Results of a Cross-European Climate Gradient Mesocosm Experiment

Article excerpt

INTRODUCTION

Climate change is expected to significantly change temperature regimes and precipitation patterns across the world (IPCC, 2007; Bates et al., 2008), with implications for the nutrient status of lakes. In the northern temperate zone, loadings of phosphorus and nitrogen are expected to increase due to elevated precipitation and soil decomposition levels, leading to higher nutrient loss from land to lakes (Schindler and Vallentyne, 2008; Adrian et al., 2009; Jeppesen et al., 2009, 2011). Moreover, internal nutrient loading tends to increase in eutrophic lakes due to higher temperature and increased mineralization and as an indirect warming effect of prolonged or temporary thermal stratification (Jensen and Andersen, 1992; Sondergaard et al., 2003; Mooij et al., 2005; Wilhelm and Adrian, 2008). These changes favour the outbreak of algal blooms, often of toxic cyanobacteria (Huisman et al., 2004; Wagner and Adrian, 2009). However, there are also examples of shallow lakes showing lower nutrient concentrations in warm years due to higher water levels. For example, in Estonian lakes higher winter North Atlantic Oscillation (NAO) years, which are characterized by warmer and wetter winters, are associated with higher water temperatures but also with higher lake water levels (Noges, 2004). In shallow Lake Vortsjarv, high water level years are characterized by a lower P concentration due to weaker resuspension and leakage from bottom sediments, while the N concentration is higher as a result of lower denitrification rates in the deeper water column, leading to a higher N : P ratio and less favourable conditions for N-fixing cyanobacteria (Noges et al., 2003).

In Europe global climate change is predicted to result in an approximately 25-30% decrease in precipitation and enhanced evaporation in the Mediterranean region. Pronounced interannual variation can be expected due to increasing frequencies and magnitudes of extreme drought events (Giorgi, 2006; Giorgi and Lionello, 2008). Droughts may reduce runoff, cause lower external nutrient loading, and potentially increase water clarity. However, such effects may be context dependent. For instance, in eutrophic lakes, higher evaporation, higher internal nutrient loading, and, possibly, reduced nitrification under low-oxygen conditions can lead to higher nutrient concentrations and lower water clarity (Jeppesen et al., 2009, 2011; Ozen et al., 2010; Papastergiadou et al., 2010).

Climate warming may also affect trophic structure and dynamics. In warm lakes, top-down control by fish is strong due to the dominance of small and abundant planktivorous and benthivorous fish (Jeppesen et al., 2010a, 2012), enhancing zooplankton predator control and reducing grazing on phytoplankton (Gyllstrom et al., 2005; Meerhoff et al., 2007; Stefanidis and Papastergiadou, 2010).

Such changes may have an adverse influence on submerged macrophytes due to the deteriorating light conditions. Conversely, a lower water level can improve the light climate for macrophyte growth (Blindow, 1992; Noges and Noges, 1999; Coops et al., 2003; Beklioglu et al., 2006), depending on lake morphology (Beklioglu et al., 2006). A mesocosm study in a Turkish shallow lake showed that macrophytes resist increased nutrient loading, perhaps as a result of evaporation-triggered water level reduction, overriding the deleterious effect of periphyton- and phytoplankton-induced turbidity (Ozkan et al., 2010). A similar outcome was observed in a recent experiment where water level and the presence or absence of fish were manipulated in a warm eutrophic shallow lake (Central Anatolia, Ankara) (Bucak et al., 2012). Results from both studies suggest that increased evaporation during summer in this region may help maintain the growth of submerged macrophytes in eutrophic shallow lakes despite the reduction in water clarity caused by fish predation. Bucak et al. (2012) concluded that the adverse effects of climate-driven eutrophication on water clarity may be counteracted by reduced water levels, provided that physical disturbance is not severe in the shallow margins of the lake. …

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