1.3.4.4 Changes in lakes

Observations indicate that lakes and rivers around the world are warming, with effects on thermal structure and lake chemistry that in turn affect abundance and productivity, community composition, phenology, distribution and migration (see Section 1.3.2.3) (Tables 1.3 and 1.6).

Abundance/productivity

In high-latitude or high-altitude lakes where reduced ice cover has led to a longer growing season and warmer temperatures, many lakes are showing increased algal abundance and productivity over the past century (Schindler et al., 1990; Hambright et al., 1994; Gajewski et al., 1997; Wolfe and Perren, 2001; Battarbee et al., 2002; Korhola et al., 2002; Karst-Riddoch et al., 2005). There have been similar increases in the abundance of zooplankton, correlated with warmer water temperatures and longer growing seasons (Adrian and Deneke, 1996; Straile and Adrian, 2000; Battarbee et al., 2002; Gerten and Adrian, 2002; Carvalho and Kirika, 2003; Winder and Schindler, 2004b; Hampton, 2005; Schindler et al., 2005). For upper trophic levels, rapid increases in water temperature after ice break-up have enhanced fish recruitment in oligotrophic lakes (Nyberg et al., 2001). In contrast to these lakes, some lakes, particularly deep tropical lakes, are experiencing reduced algal abundance and declines in productivity because stronger stratification reduces upwelling of the nutrient-rich deep water (Verburg et al., 2003; O’Reilly, 2007). Primary productivity in Lake Tanganyika may have decreased by up to 20% over the past 200 years (O’Reilly et al., 2003), and for the East African Rift Valley lakes, recent declines in fish abundance have been linked with climatic impacts on lake ecosystems (O’Reilly, 2007).

Community composition

Increases in the length of the ice-free growing season, greater stratification, and changes in relative nutrient availability have generated shifts in community composition. Of potential concern to human health is the increase in relative abundance of cyanobacteria, some of which can be toxic, in some freshwater ecosystems (Carmichael, 2001; Weyhenmeyer, 2001; Briand et al., 2004). Palaeolimnological records have shown widespread changes in phytoplankton species composition since the mid-to-late 1800s due to climate shifts, with increases in chrysophytes and planktonic diatom species and decreases in benthic species (Gajewski et al., 1997; Wolfe and Perren, 2001; Battarbee et al., 2002; Sorvari et al., 2002; Laing and Smol, 2003; Michelutti et al., 2003; Perren et al., 2003; Ruhland et al., 2003; Karst-Riddoch et al., 2005; Smol et al., 2005). These sedimentary records also indicated changes in zooplankton communities (Douglas et al., 1994; Battarbee et al., 2002; Korhola et al., 2002; Brooks and Birks, 2004; Smol et al., 2005). In relatively productive lakes, there was a shift towards more diverse periphytic diatom communities due to increased macrophyte growth (Karst-Riddoch et al., 2005). In lakes where nutrients are becoming limited due to increased stratification, phytoplankton composition shifted to relatively fewer diatoms, potentially reducing food quality for upper trophic levels (Adrian and Deneke, 1996; Verburg et al., 2003; O’Reilly, 2007). Warming has also produced northward shifts in the distribution of aquatic insects and fish in the UK (Hickling et al., 2006).

Phenology

With earlier ice break-up and warmer water temperatures, some species have responded to the earlier commencement of the growing season, often advancing development of spring algal blooms as well as clear-water phases. The spring algal bloom now occurs about 4 weeks earlier in several large lakes (Gerten and Adrian, 2000; Straile and Adrian, 2000; Weyhenmeyer, 2001; Winder and Schindler, 2004b). In many cases where the spring phytoplankton bloom has advanced, zooplankton have not responded similarly, and their populations are declining because their emergence no longer corresponds with high algal abundance (Gerten and Adrian, 2000; Winder and Schindler, 2004a). Zooplankton phenology has also been affected by climate (Gerten and Adrian, 2002; Winder and Schindler, 2004a) and phenological shifts have also been demonstrated for some wild and farmed fish species (Ahas, 1999; Elliott et al., 2000). Because not all organisms respond similarly, differences in the magnitude of phenological responses among species has affected food-web interactions (Winder and Schindler, 2004a).

Table 1.6. Examples of changes in freshwater ecosystems due to climate warming.