Eco-evolutionary responses and feedbacks of a key herbivore to lake oligotrophication
Final Report Abstract
This project examined changes in abundances and traits of the crustacean zooplankton, especially Daphnia during the last century in Lake Constance. This period was characterized by large changes in nutrient concentrations (eutrophication and re-oligotrophication) during which Lake Constance developed from an ultraoligotrophic lake to an eutrophic lake and back to an oligotrophic lake. The presence of zooplankton data for the different trophic states of Lake Constance allowed to test the PEG model which describes the effect of trophic state on plankton succession. This test suggests that the PEG model overestimates the role of nutrients and underestimates the role of temperature for spring dynamics of zooplankton. Zooplankton data from lake Constance showed that on annual average almost all crustacean taxa declined with oligotrophication. Notable exceptions include Daphnia longispina and Diaphanosoma brachyurum, the latter reappeared in the lake in the 2000s. The data show additionally that Daphnia galeata which invaded the lake with eutrophication strongly declined in recent years. Based on the dominant role of temperature as a driver of zooplankton dynamics during spring we used a hydrodynamical model to predict the effects of global warming on spring phenology of Daphnia and the potential of warming to result in a mismatch of Daphnia dynamics with both its food (algae) and its predators (whitefish). Contrary to previous studies, we predict that with seasonally constant warming all three phenologies (algae, fish and Daphnia) will shift synchronously with warming. However, with seasonally heterogeneous warming, phenologies can shift asynchronously. The risk of mismatch of food-web interactions in lake ecosystems with warming thus will depend on how seasonally synchronous global warming will proceed. Oligotrophication of a lake will not only influence the food availability for Daphnia, but also change the overall food web. Using measurements of archived samples, we show that the morphology, i.e. the spina length of Daphnia increased with oligotrophication. As spina length often increases in the presence of invertebrate predators, this suggests that the role of invertebrate predators for Daphnia dynamics also changed with oligotrophication. Using induction experiments, we show that the two most important invertebrate predators in Lake Constance, Leptodora kindtii and Bythotrephes longimanus, can induce morphological changes in Daphnia galeata clones. We show a) that there is clone-specificity, but no predator specificity in respect to the induction of spina size, and b) that there is clone specificity and predator-specificity in the induction of helmet size of Daphnia. Hence, clonal variability in induced spina size suggest the possibility that the observed changes in spina size during oligotrophication result from microevolution of Daphnia. The analyses of Daphnia remains from a dated sediment core finally suggests that also fish predation pressure on Daphnia had changed during the last century with maximum predation pressure during the mid of the 20th century and during the most recent years. The latter suggests that possibly increased fish predation contributed to the decline of D. galeata in Lake Constance during recent years. Paleolimnological analyses suggest also a striking change in Daphnia life history, i.e. allocation of Daphnia to sexual reproduction: The ratio of ephippia to abdominal claws declined during the last century by two orders of magnitude. In summary, we provide evidence that abundances and species composition of Daphnia almost returned to a status prior to massive eutrophication. However, these changes are very likely also influenced by changes in invertebrate and vertebrate predation pressure on Daphnia and Daphnia life history strategy.
Publications
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(2015). Rapid and highly variable warming of lake surface waters around the globe. Geophysical Research Letters, 42, 10773–10781
O’Reilly C M, Sharma S, Gray D K, Hampton S E, …, Straile D, …, Zhang G
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(2015). Taxonomic aggregation does not alleviate the lack of consistency in analysing diversity in long-term phytoplankton monitoring data : a rejoinder to Pomati et al. (2015). Freshwater Biology, 60, 1060–1067
Straile D, Jochimsen M C & R Kümmerlin
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(2015). Trophic mismatch requires seasonal heterogeneity of warming. Ecology, 96, 2794–2805
Straile D, Kerimoglu O & F Peeters
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(2015). Zooplankton biomass dynamics in oligotrophic versus eutrophic conditions: a test of the PEG model. Freshwater Biology, 60, 174–183
Straile D
