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Research on chydorid (Crustacea: Cladocera: Chydoridae) reproduction is scarce and very little is known about the dynamics of parthenogenetic (asexual) and gamogenetic (sexual) reproduction modes in these cyclical parthenogens. The paleolimnological approach has a lot to offer to this research since chydorid carapaces (indicative of asexual reproduction) and ephippia (indicative of sexual reproduction) are identifiable in lake sediments. In the current study, we examined historical changes in chydorid reproduction patterns in an Austrian Alpine lake with the objective to explore reproductive shifts under limnological variability and with the hypothesis that sexual reproduction intensifies under unfavourable environmental conditions. For this, we utilised subfossil remains from a sediment sequence covering the past ~300 years in a high Alpine lake, lake Oberer Landschitzsee. The results suggested that in-lake processes likely changed as a response to post-Little Ice Age (LIA) climate warming. Indeed, the benthic quality – indicated by the chironomidbased benthic quality index (BQI) – deteriorated, and the productivity – indicated by organic matter in the sediment – increased. The chydorid community exhibited dramatic shifts in the post-LIA section of the core, around 1850-1950 AD, when the two dominant species Alona (A.) affinis and A. quadrangularis were outcompeted by Acroperus harpae, Alonella excisa, and Chydorus (C.) sphaericus-type and their reproduction gradually shifted towards the dominance of sexual reproduction. The community shift and prevalence of sexual reproduction coincided with the period of unfavourable limnological conditions, evidenced by low BQI values and higher organic content. Therefore, we suggest that the intensified sexual reproduction was a local response to deteriorated benthic conditions and a vigorous adaptive mechanism to ensure successful diapause and genetic variability under environmental change. The results give reason to suggest that fossil chydorid assemblages and ephippia indicate the ecological quality of lake ecosystems and, thus, they can be used as additional proxies in future assessments of the lake ecological status.
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