This study explores changes in cladoceran composition in a high mountain lake of the Retezat (Lake Brazi), the South Carpathian Mountains of Romania, during the Late Glacial-Early Holocene (14,500-11,600 cal. yr. BP) transition using a paleolimnological approach. The lake had a species poor cladoceran community throughout this period. Daphnia longispina, Chydorus sphaericus and Alona affinis were the most common, showing marked fluctuations in their relative abundances through time. Distinct faunal response to warming at the Younger Dryas (YD)/Preboreal transition was recorded by increasing fossil densities and distinct community composition change: Alona affinis became dominant while numbers of Chydorus sphaericus dramatically decreased. In the Early Holocene, the productivity of Lake Brazi seem to have increased as reflected by higher numbers of Cladocera due to appearance of new species (Alona rectangula, A. quadrangularis and A. guttata) which are common in productive waters. Significant negative correlation was found between average dorsal length of daphnid ephippia and the NGRIP d18 O isotope values. Given the absence of fish predation, changes in Daphnia ephippia size were taken to indicate climatic change: larger ephippium size inferred cold conditions during the Late Glacial, while smaller size reflected climate warming during the Early Holocene. We conclude that Cladocera fossils are good indicators of climatic change that happened during the transition from the Late Glacial to the Holocene. We found that climatic conditions can be tracked either by size distribution of Daphnia ephippia (larger ephippium size under colder climate) and/or by community change of cladocerans.
Oxbow lakes are important components of the floodplain systems of lowland rivers. During flood events, oxbows are connected with the main river channel, and behave as lotic systems, while during inter-flood periods, these lakes can be considered as lentic ecosystems. Rivers are generally poor in planktonic organisms and their sediments contain scarce biological remains in comparison to lentic water ecosystems. However, due to their alternating running and standing water regime, sedimentary biological remains of oxbow lakes can be used as proxies for tracking changes of past hydrological regimes. In this study we investigated how cladoceran communities respond to flood events, and whether flood events can be recognized by community analysis of cladoceran remains. A sediment core from Marótzugi-Holt-Tisza oxbow lake was analyzed for identification of past flood events based on changes in the subfossil Cladocera community. Floods were defined based on the proportion of fine sand (50 µm grain size) in the oxbow sediments. If the fine sand portion was <3%, the water regime of the oxbow was considered as lentic, otherwise it was lotic. Both organic and pigment contents were significantly higher in the core sections deposited during lentic stages. Thirty-four Cladocera species were determined in this core, all common to littoral habitats of eutrophic shallow lakes in Hungary. One planktonic (Bosmina longirostris) and four chydorid species (Alona rectangula, Acroperus harpae, Alonella nana and Chydorus sphaericus) were dominant throughout the core and contributed >90% of total remains. Discriminant analysis on cladoceran data confirmed that lotic and lentic hydrological stages were characterized by different Cladocera species associations. Bosmina longirostris, Chydorus sphaericus, Alona rectangula, Acroperus harpae, Leydigia leydigi, A. quadrangularis and A. nana were mainly responsible for the differences between lotic and lentic species assemblages. Our results revealed that Cladocera remains can be used to track changes in the hydrological regime of oxbow lakes.
The body size of aquatic invertebrates is, to a great extent, dependent on ambient temperature, but size distributions are also determined by other factors like food supply and predation. The effect of temperature on organisms is formulated in the temperature–size hypothesis, which predicts a smaller body size with increasing temperature. In this study, the effect of temperature on the subfossil remains of three littoral Cladocera (Alona affnis, A. quadrangularis, and Chydorus cf. sphaericus) was investigated. Exoskeletal remains of these species can be found in large numbers in lacustrine sediments and over a wide north–south range in Europe. The total length of both headshield and postabdomen for A. affinis and A. quadrangularis and carapace length for C. cf. sphaericus were measured to observe their response to changes in latitude and temperature. A different response to ambient temperature in the growth of body parts was observed. The size of the headshields of both Alona species and of the carapace of Chydorus was significantly larger in colder regions as opposed to warm ones. It turned out that the postabdomen was not a good predictor of ambient temperature. While the sizes of all remains increased with latitude, the sizes of the Alona remains was smaller in the mountain lakes of the Southern Carpathians than in other cold lakes, in this case in Finland, a fact indicative of the importance of other factors on size distribution. This study demonstrates that a morphological response to climate is present in littoral cladocerans, and, therefore, changes in the length of headshield and carapace may be used as a proxy for climate changes in paleolimnological records.
Taxonomic, functional and phylogenetic diversity: how subfossil cladocerans mirror contemporary community for ecosystem functioning: a comparative study in two oxbowsBiases of taxa diversity and traits of source (active) and subfossil cladoceran communities with phylogenetic and functional approach were studied in two oxbows of the River Tisza, Hungary. Subfossil cladoceran communities were found to have suffered species loss due to both the degree of heterogeneity in the lake and taphonomical processes. The remains of heavily chitinized species are those most commonly found in the sediment, and therefore bias communities reconstructed on this basis. These biases are mostly caused by: (1) parts of soft bodied filterers (Daphniidae, Moinidae, Sididae (Diaphanosoma)), which are underrepresented in the subfossil remains of cladocerans, and (2) the remains of species that are difficult to collect and, so, underrepresented or absent in contemporary samples; this may have a strong effect on diversity indices. In spite of such biases, the paleocommunity does reflect the functioning of the ecosystem. The subfossil cladoceran community in the Morotva-tó (MT) corresponded to low fish densities and the high macrophyte coverage, indicating weaker top-down forces in this oxbow. The phylogenetic and functional approach to subfossil cladocerans may be useful in paleolimnological reconstruction, since phylogenetic and trait-based diversity indices correctly predicted top-down regulation as a function of the ecosystem.
-Diadesmis fukushimae, a rare oligotraphenic diatom, was found in some high mountain lakes of Romania. Its occurrence in the Parâng and Retezat Mountains is the second European record of the species. To date D. fukushimae has been known only from the type locality (Shenandoah National Park, Virginia, USA) and from a spring (Grotta Guernica, Dolomiti Bellunesi National Park (south-eastern Alps, Italy). Investigation by scanning electron microscopy showed that this species should be transferred to the recently established genus Humidophila. A new combination is proposed, Humidophila fukushimae. The morphological details of the European population are also presented.
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