Preface: Cladocera crustaceans: sentinels of environmental change

Abstract

Growing awareness of the negative impacts of global change urges scientists to look for adequate means to assess past and ongoing environmental change. The extent and dynamics of natural ecosystem variability is not yet fully clear, though understanding of this variability is crucial for predicting future trends (IPCC, 2007). Paleolimnological records holding valuable proxy information in the form of sediment features, geochemical records and microand macrofossil assemblages, can complement observational data by extending timescales, integrating sub-annual variability and expanding the range of sites that can be studied (Battarbee et al., 2005). Modern limnological time-series, though limited in space and time, can still add crucial information to these surveys. Similarly, in situ and laboratory experiments can complement paleolimnological approaches by improving our mechanistic understanding of the relationship between proxy indicators and their environment. Cladocera (Crustacea: Branchiopoda; water fleas) play a key role in freshwater ecosystems because of their pivotal position in the food web, sandwiched between top-down regulators (fish and invertebrate predators) and bottom-up factors (phytoplankton). The intermediary position occupied by cladocerans underlies their significance for nutrient cycling in freshwaters (Jarvinen & Salonen, 1998; Urabe et al., 2002) and as sentinels of environmental change (Jeppesen et al., 2001; Korhola & Rautio, 2001; Kurek et al., 2010). Subfossil cladocerans (typically Bosminidae, Chydoridae and Daphniidae) appear in lake sediments mainly in disarticulated body parts, such as head shields, carapaces and postabdomens, or as epphipia (a structure that protects diapausing eggs). The taxonomic composition of these remains has been used to track past changes in the environment, including eutrophication (Brodersen et al., 1998; Shumate et al., 2002; de Eylo et al., 2003; Davidson et al., 2007; Chen et al., 2010), acidification (Paterson, 1994; Jeziorski et al., 2008), lake water calcium decline (Jeziorski et al., 2008), lake levels (Korhola et al., 2000, 2005) and climate (Lotter et al., 2000; Kattel et al., 2008). In addition, the taxonomic composition of cladocerans shows a strong response to biotic factors that can vary with lake trophic state, such as fish density (Jeppesen et al., 1996, 2001; Finney et al., 2000) and submerged macrophytes (Davidson et al., 2007). Cladocera-based reconstructions have not remained qualitative only, but the strong link between cladoceran Guest editors: H. Eggermont & K. Martens / Cladocera as indicators of environmental change.