Predator–prey interactions in a changing world: humic stress disrupts predator threat evasion in copepods
Increasing inputs of colored dissolved organic matter (cDOM), which is mainly composed of humic substances (HS), are a widespread phenomenon of environmental change in aquatic ecosystems. This process of brownification alters the chemical conditions of the environment, but knowledge is lacking of whether elevated cDOM and HS levels interfere with the ability of prey species to evade chemical predator cues and thus affect predator–prey interactions. We assessed the effects of acute and prolonged exposure to HS at increasing concentrations on the ability of freshwater zooplankton to avoid predator threat (imposed by fish kairomones) in laboratory trials with two calanoid copepods (Eudiaptomus gracilis and Heterocope appendiculata). Populations of both species clearly avoided water containing fish kairomones. However, the avoidance behavior weakened with increasing HS concentration, suggesting that HS affected the ability of copepods to perceive or respond to the predator cue. The behavioral responses of the two copepod populations to increasing HS concentrations differed, with H. appendiculata being more sensitive than E. gracilis in an acute exposure scenario, whereas E. gracilis responded more strongly after prolonged exposure. Both showed similar physiological impairment after prolonged exposure, as revealed by their oxidative balance as a stress indicator, but mortality increased more strongly for H. appendiculata when the HS concentration increased. These results indicate that reduced predator threat evasion in the presence of cDOM could make copepods more susceptible to predation in future, with variation in the strength of responses among populations leading to changes in zooplankton communities and lake food-web structure.
KeywordsBrownification Chemical ecology Global change Zooplankton behavior Humic substances
We thank Thierry Perez for the two-armed choice flume, Michael Sachtleben for technical assistance, Uta Mallok for DOC analyses, and Stella Berger, Thomas Mehner and Jens Nejstgaard for advice and discussion. Special thanks go to Anatole Boiché for his tireless assistance during the experiments and with zooplankton collections. MS received a GDR MediatEC 3658 research Grant (France) and LM and ES were supported by postdoctoral Grants through IGB’s Frontiers in Freshwater Science program. The study also benefitted from support received through the EU project MARS (contract no. 603378) funded under the 7th Framework Programme and the project ILES (SAW-2015-IGB-1) funded by the Leibniz Association.
Author contribution statement
MS originally formulated the idea. MS, MOG, LM and ES conceived and designed the experiments. MS and LM performed the experiments. MS, ES and LM analyzed the data. MS, LM, ES and MOG wrote the manuscript.
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