Potential re-colonisation by cladocerans of an acidic tropical pond

To predict how re-colonisation of acidified lakes will proceed, at least two approaches are possible: (i) to compare the life history traits of candidate species and determine which one has the highest fitness, and (ii) to simulate a more realistic scenario carrying out experiments with the grouping of the candidate species, so that the intrinsic rate of natural increase of each species is integrated with its sensitivity to low pH and its ability to compete with the other candidate populations. The objective of the present study was to investigate the future re-colonisation of such acidified systems, taking as case-study a tropical pond (Lagoa das Dunas, Camaçari, BA, Brazil) and four species of cladocerans occurring in nearby water bodies ( Ceriodaphnia cornuta, Ceriodaphnia silvestrii, Latonopsis australis and Macrothrix elegans), by comparing the two above mentioned approaches. The second approach included two sets of in situ microcosms experiments, one simulating the re-colonisation by immigrating ephippia, thus using neonates of each species as colonisers, and another simulating the immigration of adults. Both these simulations followed nearly the same trends. The integration of the effects of a higher temperature, a different photoperiod and species competition determined differences in the species densities ranking between the two approaches: life history versus microcosms. The densities of C. cornuta in the microcosms matched the biphasic concentration/response hormetic model, in the simultaneous presence of two increasingly intense stressors (interspecific competition and acidity), with a low-dose stimulation and a high-dose inhibition. The present study provided, thus, a further support to the acceptance of hormesis in ecotoxicology, also at the population level in multispecies experiments.