Stratigraphic and taxonomic significance of siliceous microfossils collected from the Turiec Basin, Western Carpathians (Slovakia)

This study provides the first biostratigraphic data of siliceous microfossils from Turiec Basin, Slovakia. The fossil diatom flora consists of 42 species and varieties, belonged to 22 genera. The diatom assemblage studied from the Turiec Basin bears a strong resemblance to assemblages from non-marine diatomaceous sediment of Miocene age from Rüdenschwinden, a village of the eastern slope of the Hohe Rhön (Central Germany), non-marine sediments of the early Late Miocene from the village of Szilagy (South Hungary), as well as from Bes Konak Basin, Turkey. The investigated profile is generally dominated by Alveolophora jouseana. The similarities and differences within the taxonomy of certain species belonging to the genera Aulacoseira, Alveolophora and Miosira are discussed. The accompanying species are species of the genus Fragilaria Lyngbye sensu lato from class Fragilariophyceae. The most interesting taxa belong to the genus Staurosirella – S. grunowii, S. leptostauron, S. martyi. Among them are two very unusual taxa identified only to genus. Ecological data for the diatom taxa and the diatom frustules/ chrysophycean stomatocysts ratio are used in an attempt to reconstruct in detail the palaeoecological conditions at the time of sediment deposition

Morphotype variations in subfossil diatom species of Aulacoseira in 24 Michigan Lakes, USA

Diatom assemblages preserved in lake sediment records can provide proxy data of past environmental changes in biological conditions. In order to investigate past changes in the environment of north-central Michigan, diatom assemblages were studied in sediment cores retrieved from 24 lakes. Diatoms were analyzed from the \u27top\u27 and \u27bottom\u27 of each core to reconstruct land-use changes in this area. Aulacoseira taxa were identified and evaluated with light and scanning electron microscopy. Results of these observations showed the presence of some variability of the morphological features within North American species populations. Diatom species composition in surface sediments and differences between tops and bottoms corresponded to changes in land use surrounding the lakes, ranging from predominantly forest and rangeland to urban and agriculturally impacted. Diatom-inferred past conditions revealed that the observed morphotypes probably represent taxa with different ecological preferences. The main factors influencing the variability of these morphotypes are changes in the trophic status of the lakes

Environmental filtering drives assembly of diatom communities over evolutionary time‐scales

Aim Ecological communities are structured through the interplay of deterministic assembly processes such as competition and environmental filtering. Whereas the drivers of spatial community structure are frequently studied in extant communities, little is known about the relative importance of assembly processes in response to environmental factors over evolutionary time-scales. Here, we use an integrative framework to unravel community assembly processes since the inception of a long-lived lake ecosystem. Location Lake Ohrid. Time period From lake formation 1.36 million years ago to the present. Major taxa studied Planktonic diatoms. Methods We constructed a dated phylogeny of extant and extinct diatoms and collected trait data for 380 fossil communities to quantify phylogenetic community structure and functional richness and to determine the relative importance of deterministic assembly processes over time. We then used regression analysis to correlate the phylogenetic community structure with palaeoenvironmental and intrinsic biological predictors and to identify primary drivers of assembly processes. Results Our results suggest a dense packing of niche space with higher species richness and co-occurrence of closely related species. There are only two short episodes in the very recent past dominated by distantly related taxa. We found distinct changes in phylogenetic community structure upon speciation or extinction events and an increase in mean community relatedness over time. Main conclusions Our finding of closely related co-occurring species implies environmental filtering as the primary assembly mechanism, with a minor but increasingly important role of competition towards the present, driven by evolutionary dynamics. Such an increase in the relative contribution of competition to the assembly of communities in relation to the aging of an insular ecosystem, together with a denser packing of morphospace in the early phase of system ontogeny is compatible with ecological predictions according to the theory of island biogeography

Deep drilling reveals massive shifts in evolutionary dynamics after formation of ancient ecosystem

The scarcity of high-resolution empirical data directly tracking diversity over time limits our understanding of speciation and extinction dynamics and the drivers of rate changes. Here, we analyze a continuous species-level fossil record of endemic diatoms from ancient Lake Ohrid, along with environmental and climate indicator time series since lake formation 1.36 million years (Ma) ago. We show that speciation and extinction rates nearly simultaneously decreased in the environmentally dynamic phase after ecosystem formation and stabilized after deep-water conditions established in Lake Ohrid. As the lake deepens, we also see a switch in the macroevolutionary trade-off, resulting in a transition from a volatile assemblage of short-lived endemic species to a stable community of long-lived species. Our results emphasize the importance of the interplay between environmental/climate change, ecosystem stability, and environmental limits to diversity for diversification processes. The study also provides a new understanding of evolutionary dynamics in long-lived ecosystems

Late Glacial and Holocene environmental history of the Pirin Mountains (SW Bulgaria): a paleolimnological study of Lake Dalgoto (2310 m)

Diatoms, Cladocera, and chironomids preserved in the sediments of Lake Dalgoto were studied to reconstruct the history of the lake ecosystem in the context of the vegetation history as represented by the pollen stratigraphy. Younger Dryas silty sediments at the base of the core are characterized by low diversity of aquatic organisms. The transition to the Holocene is indicated by a sharp change from silt to clay-gyttja. The migration and expansion of trees at lower elevations between 10200 and 8500 14C-yr BP, along with higher diversities and concentrations of aquatic organisms and the decreased proportion of north-alpine diatoms, point to rapidly rising summer temperatures. After 6500 14C-yr BP the expansion of Pinus mugo in the catchment coincides with signs of natural eutrophication as recorded by an increase of planktonic diatoms. In the late Holocene (4000–0 14C-yr BP) Pinus peuce and Abies are reduced and Picea expands. Cereal grains and disturbance indicators suggest late-Holocene human modification of the vegetation

Morphotype variations in subfossil diatom species of Aulacoseira in 24 Michigan Lakes, USA

Diatom assemblages preserved in lake sediment records can provide proxy data of past environmental changes in biological conditions. In order to investigate past changes in the environment of north-central Michigan, diatom assemblages were studied in sediment cores retrieved from 24 lakes. Diatoms were analyzed from the \u27top\u27 and \u27bottom\u27 of each core to reconstruct land-use changes in this area. Aulacoseira taxa were identified and evaluated with light and scanning electron microscopy. Results of these observations showed the presence of some variability of the morphological features within North American species populations. Diatom species composition in surface sediments and differences between tops and bottoms corresponded to changes in land use surrounding the lakes, ranging from predominantly forest and rangeland to urban and agriculturally impacted. Diatom-inferred past conditions revealed that the observed morphotypes probably represent taxa with different ecological preferences. The main factors influencing the variability of these morphotypes are changes in the trophic status of the lakes

Tertiarius minutulus sp. nov. (Stephanodiscaceae, Bacillariophyta) – a new fossil diatom species from Lake Ohrid

A new fossil diatom species, Tertiarius minutulus sp. nov., is described from a sediment sequence DEEP-5045-1 of Lake Ohrid. The species is characterized by small valves (3.0–8.0 µm) with a round shape, a marginal area with radially arranged costae and a central uneven area with scattered areolae. Externally, the alveoli are occluded by cribra perforated by irregularly arranged pores. Internally, the alveoli are simple and areolae are occluded with domed cribra. The marginal fultoportulae are situated on costae close to the valve margin, located on every 5th or 7th thick internal costa. One to three fultoportulae are present on the valve face, each surrounded by two to three satellite pores. One rimoportula is present, positioned on a costa at the valve face / mantle junction. The species is compared with morphologically similar taxa and a detailed differential diagnosis is provided. Tertiarius minutulus sp. nov. is known only as a fossil taxon from Lake Ohrid, with a first occurrence during the early stages of lake basin development

Revision of Navicula striolata (Grunow) Lange-Bertalot and N. rumaniensis Hustedt with the description of N. friedelhinziae sp. nov.

Navicula striolata was originally described as N. digitoradiata var. striolata from modern material collected in Sweden. After examination of a sample collected from Belgium, the variety was transferred to N. reinhardtii as N. reinhardti var. gracilior. From this time a large mix up of these and related taxa was observed in the literature. A similar species, Navicula rumaniensis had also been established in 1934 from Neogene Romanian materials but there has been much confusion regarding the status of these taxa, leading to a poor understanding of their distribution. In this study, type material of Navicula digitoradiata var. striolata, N. reinhardtii var. gracilior and N. rumaniensis are revised using light and scanning electron microscopy in order to clarify their identity and to investigate possible conspecificity. The results indicate that these species are not synonyms. Conspecificity of the modern N. digitoradiata var. striolata and N. reinhardti var. gracilior was confirmed and lectotypes of both varieties have been designated whereas N. rumaniensis proved to be a separate species. In addition, the study of Neogene material from Bulgaria revealed the presence of a new Navicula taxon—N. friedelhinziae. The morphology of these and similar taxa is discussed