Abstract
Increased temperature in the epilimnion and hypoxia in the metalimnion of a lake would result in an increase of positive-size-selective fish predation on zooplankton and in turn in a decrease of mean body size in zooplankton populations and communities. We tested this hypothesis in four types of experiments with juvenile rudd (Scardinius erythrophthalmus) foraging on Daphnia longispina in an indoor twin column tank system. In each experiment of the first three types, one column contained one of three types of experimental treatments differing from the control treatment (in the other column) by the following: (i) elevated temperature in the epilimnion, (ii) hypoxia in the metalimnion and (iii) simultaneous elevated temperature in the epilimnion and hypoxia in the metalimnion. In the fourth type of experiment, the gradients of temperature and oxygen concentration in both columns were the same, but prior to the experiments, Daphnia and fish in the control treatment were acclimated to normoxia and, in the experimental treatment, to hypoxia. The results confirmed our hypothesis, since the predation rate of fish was greater in each of the first three experimental treatments than in the control. We did not detect an effect of the acclimation to hypoxia on the predation rate of the fish.
Highlights
Numerous studies have demonstrated that the mean body size in populations and communities across species in a variety of taxonomic groups of aquatic ectotherms, including freshwater zooplankton, is positively correlated with latitude, and inversely correlated with temperature (Poulin, 1995; Gillooly and Dodson, 2000; Havens et al, 2015; see Maszczyk and Brzezinski, 2018 for review)
It could be expected that the increased intensity of positive size selective predation on zooplankton might lead to a reduction of their mean body size at population and community levels (Brooks and Dodson 1965; Gliwicz and Wrzosek 2008) due to both increasing mortality (Brooks and Dodson 1965; Gliwicz and Wrzosek 2008) and the forcing of phenotypic changes to the body size of individuals (Machácek, 1991; Maszczyk and Bartosiewicz, 2012)
One potential mechanism is based on the observation that elevated temperature modifies the shape of the vertical gradients of physical factors, such as temperature and oxygen concentration (Hu and Tessier, 1995; De Stasio et al, 1996), which would result in an increased predation rate of planktivorous fish
Summary
Numerous studies have demonstrated that the mean body size in populations and communities across species in a variety of taxonomic groups of aquatic ectotherms, including freshwater zooplankton, is positively correlated with latitude, and inversely correlated with temperature (Poulin, 1995; Gillooly and Dodson, 2000; Havens et al, 2015; see Maszczyk and Brzezinski, 2018 for review). The negative correlation between temperature and the proportion of large-bodied species has long been recognized in the seasonally variable size structure of the zooplankton community, in which large-bodied species tend to decline during summer (Sommer et al 1986; Dupuis and Hann, 2009) This correlation has been observed in the long-term changes in the mean body size of zooplankton communities (Adrian and Deneke, 1996; Benndorf et al, 2001; Gauthier et al, 2014; Sastri et al, 2014), which suggests that it might be apparent with the increasing mean temperature due to global warming. One potential mechanism is based on the observation that elevated temperature modifies the shape of the vertical gradients of physical factors, such as temperature and oxygen concentration (Hu and Tessier, 1995; De Stasio et al, 1996), which would result in an increased predation rate of planktivorous fish
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