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To investigate the extent to which anthropogenic perturbations such as eutrophication and trace metal pollution (i.e., environmental stress sensu Odum 1985) influence the reproductive modes of cladoceran populations, we analyzed the abundance of subfossils of the chydorids Alonella nana (Baird, 1850) and Alona affinis (Leydig, 1860) in sediment cores from three Finnish lakes. Reconstruction of lakes' pollutant history and the biological response of chydorids indicate that in two of the lakes the proportion of individuals reproducing sexually increased with environmental stressors. More specifically, A. nana responded to eutrophication in Lake Hampträsk with greater production of ephippia, while A. affinis responded to aluminum pollution or acidification in Lake Pieni Majaslampi. In contrast, the reference lake, Lake Iso Lehmälampi, showed no radical changes in sexual reproduction over the twomillennium long sediment record. We conclude that chydorids may use sexual reproduction as a strategy for overcoming unexpected environmental stresses.

Temperature sensitivity of chydorid (Cladocera, Chydoridae) gamogenesis in southern Finland was examined. Chydorid sweep net samples and littoral surface water temperature measurements of seven lakes were taken weekly. Percentage abundances of gamogenetic females were used to estimate the temperature optima and tolerances for gamogenesis of individual chydorid species. The estimations were carried out using a weighted averaging (WA) technique. The results indicated that the optimum temperature of gamogenesis ranged between 7.0 and 9.7 °C and the total tolerances were between 2.0 and 4.7 °C suggesting a species-specific response for gamogenesis. Alonella nana and Chydorus sphaericus s.l. had the lowest optima, which was possibly due to their ubiquitous ecological preference and wide geographical distribution. Camptocercus rectirostris, Alonella excisa and Grabtoleberis testudinaria had high optima for their gamogenesis. These estimations provide first numerical autecological data on gamogenesis of natural and contemporary chydorid populations. Furthermore, the results provide species-specific estimations of the effect of the present climate warming on chydorid reproduction patterns. This information can be useful background data in global change assessments regarding freshwater lakes.

We investigated 2500 years of community succession in Cladocera from the sediments of a mountain lake (Lake Piramide Inferiore) located in the Khumbu Valley close to Mt. Everest in the Nepalese Himalayas. Our objective was to determine late Holocene changes in cladoceran species composition and abundance in a biogeographical context and with respect to previous proxy-based paleolimnological data (algal pigments and organic content). The results suggested that cladoceran fauna of Lake Piramide Inferiore was species-poor and dominated by Chydorus cf. sphaericus throughout the sequence. The sediment profile recorded the occurrence of Alona guttata type individuals, which were attributed to Alona werestschagini Sinev 1999 based on their morphology and the species' current distributional range, and this was the first record of its presence in the Himalayas. In addition, a periodic long-term succession of melanic Daphnia (Ctenodaphnia) fusca Gurney, 1907 and non-melanic D. (Daphnia) dentifera Forbes 1893 was observed in the sediments. The millennia-long cladoceran community changes, although subtle due to the C. cf. sphaericus dominance, were in general agreement with the previous proxy-data of lake productivity following the regional paleoclimatic development and apparently partly driven by bottom-up mechanisms. The periodic occurrence and success of D. fusca and D. dentifera throughout the late Holocene in Lake Piramide Inferiore, combined with the knowledge of their phenotypic properties (i.e. carapace melanization) and previous investigations on their contemporary and past distribution in Khumbu Valley, suggested that they may have responded to altered underwater UV radiation regimes. Furthermore, they may have even periodically excluded each other subsequent to changes in the underwater UV environment. The results indicated the usefulness of fossil cladoceran analysis as a tool in biogeographical research, since the occurrence of species in space and time can be observed through sediment records and taxonomic identity of the remains may be resolved with the help of regional faunal distribution.

In this study, water depth optima, i.e. habitat preferences within a depth gradient, were estimated for mutual midge (Diptera: Nematocera; Ceratopogonidae, Chironomidae) taxa in surface sediment intralake data sets from eastern Finland and Austrian Alps and in a regional data set across Finland. The aim was to investigate how the optima and tolerances differ in these data sets and to discuss whether the possible causal factor for deviance is related to local adaptation, taxa representativeness, or some other factor. A total of 20 mutual taxa were found from the data sets and the estimated optima and tolerances were highly similar, with the exception of three taxa, which had deviating optima in the Austrian lake. The reason for these differences was most likely that the optima were poorly estimated in the Austrian lake due to low abundances and number of occurrences of these taxa. No evidence for intraspecific niche separation or local habitat adaptation was found between the Austrian and Finnish sites, although, interspecific variation in habitats was evident. Therefore, water depth optima estimated from representative number of specimens may be applicable for various ecological, limnological, and paleolimnological purposes. However, when the optima are applied outside the data set’s coverage, the results should be interpreted with caution. In addition, the given optima are not applicable in sites deeper than the sites in the data sets.

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.