Abstract
Many species and clones of Daphnia inhabit ecosystems with permanent algal blooms, and they can develop tolerance to cyanobacterial toxins. In the current study, we examined the spatial differences in the response of Daphnia longispina to the toxic Microcystis aeruginosa in a lowland eutrophic dam reservoir between June (before blooms) and September (during blooms). The reservoir showed a distinct spatial pattern in cyanobacteria abundance resulting from the wind direction: the station closest to the dam was characterised by persistently high Microcystis biomass, whereas the upstream stations had a significantly lower biomass of Microcystis. Microcystin concentrations were closely correlated with the cyanobacteria abundance (r = 0.93). The density of daphniids did not differ among the stations. The main objective of this study was to investigate how the distribution of toxic Microcystis blooms affects the antioxidant system of Daphnia. We examined catalase (CAT) activity, the level of the low molecular weight antioxidant glutathione (GSH), glutathione S-transferase (GST) activity and oxidative stress parameters, such as lipid peroxidation (LPO). We found that the higher the abundance (and toxicity) of the cyanobacteria, the lower the values of the antioxidant parameters. The CAT activity and LPO level were always significantly lower at the station with the highest M. aeruginosa biomass, which indicated the low oxidative stress of D. longispina at the site with the potentially high toxic thread. However, the low concentration of GSH and the highest activity of GST indicated the occurrence of detoxification processes at this site. These results demonstrate that daphniids that have coexisted with a high biomass of toxic cyanobacteria have effective mechanisms that protect them against the toxic effects of microcystins. We also conclude that Daphnia's resistance capacity to Microcystis toxins may differ within an ecosystem, depending on the bloom's spatial distribution.
Highlights
Planktivorous zooplankton are one of the groups most affected by the mass development of toxic cyanobacteria in inland waters [1]
Such resistance results from genetic changes that result in the local co-adaptation of Daphnia to cyanobacterial toxins [11,12]
Two species of Daphnia were identified in the samples from the Sulejow Reservoir: D. longispina
Summary
Planktivorous zooplankton are one of the groups most affected by the mass development of toxic cyanobacteria in inland waters [1]. An increasing number of publications have shown that Daphnia populations can evolve mechanisms that allow them to coexist with toxic cyanobacteria [8,9,10,6]. Such resistance results from genetic changes that result in the local co-adaptation of Daphnia to cyanobacterial toxins [11,12]. The sensitivity of daphniids to cyanotoxins is most striking in species or clones that are isolated from distinctive habitat types, ecosystems with different trophy and abundances of toxic strains of cyanobacteria [13,5,14]. Previous research has focused on asynchrony in the zooplankton – the spatial distribution of cyanobacteria and the formation of the ‘‘refuge sites’’ that allow large grazers to persist during blooms [15,16]
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