Vertical distribution of Daphnia in Lake Berendonck (The Netherlands) during progressive hypolimnion oxygen depletion

摘要

Large-bodied cladocerans, such as Daphnia, occupy a central position in typical freshwater pelagic food chains. These grazers are capable of consuming relatively large amounts of algae and are prey items for numerous invertebrate and vertebrate predators. Being eaten by a predator is the greatest danger daph-nids face and has the most dramatic effect on their fitness. To cope with the variety of predators, Daphnia has evolved morphological and behavioural strategies that reduce predation (O'Brien 1987). A common anti-predator behaviour of Daphnia is diel vertical migration (DVM) as an escape from light-dependent mortality, mainly fish predation, during the day (Stich Lampert 1981, Van Gool Ringel-berg 2002). Daphnia migrate into lower, colder and darker water strata before dawn, and rise to upper strata during dusk. The ecological cost of lower reproduction due to less food and lower temperature in the hypolimnion may have prevented Daphnia from continuously staying in deep water (Van Gool Ringelberg 2002). In addition, in many systems the hypolimnion shows a progressive depletion of oxygen through spring and summer, which may first provide an oxygen refuge for Daphnia (Hanazato 1992), but becomes a hostile environment when oxygen concentration drops to very low levels (Wright Shapiro 1990, Lass et al. 2000). In this study, we examined the vertical distribution of Daphnia in the small (surface area 20.000 m~2), deep (maximum depth 17 m), mesotrophic Lake Berendonck, The Netherlands (Table 1; Fig. 1). The hypolimnion of Lake Berendonck becomes anoxic during the summer (Fig. 2), therefore we hypothesize that Daphnia avoids the anoxic region with a trade off of increased predation mortality. In a laboratory experiment we examined the effect of reduced oxygen concentration on survival of D. galeata x hyalina.