Search

Marine community diversity surveys require a reliable assessment to estimate ecosystem functions and their dynamics. For these, non-invasive environmental DNA (eDNA) metabarcoding is increasingly applied in zoological studies to complement or even replace traditional morphological identification methods. However, uncertainties remain about the accuracy of the diversity detected with eDNA to capture the actual diversity in the field. Here, we validate the reliability of eDNA metabarcoding in identifying metazoan biodiversity in highly dynamic marine waters of the North Sea. We analyzed biodiversity from water (eDNA) and zooplankton samples with cytochrome c oxidase subunit 1 (COI) and 18S rRNA (18S) metabarcoding at Helgoland Roads and validated the optimal molecular resolution by morphological and molecular zooplankton identification (metabarcoding) with the result of merely a few false-negative detections. eDNA and zooplankton metabarcoding resolved 354 species from all major and in total 16 metazoan phyla. This molecular genetic species inventory overlapped by 95.9% (COI) and 81.9% (18S) with published inventories of local, morphologically identified species, among them neozoa and rediscovered species. Even though half of all species were detected by both eDNA and zooplankton metabarcoding, the methods differed significantly in their detected diversity. eDNA metabarcoding performed very well in cnidarians and annelids, whereas zooplankton metabarcoding identified higher numbers of fish and malacostraca. Species assemblages significantly differed between the individual sampling events and the cumulative number of identified species increased steadily over the sampling period and did not reach saturation. About a third of the species were detected only once while a core community of 22 species was identified continuously. Our study confirms eDNA metabarcoding to be a powerful tool to identify and analyze North Sea fauna in highly dynamic waters and we recommend inve

Cell size is a master trait in the functional ecology of phytoplankton correlating with numerous morphological, physiological, and life-cycle characteristics of species that constrain their nutrient use, growth, and edibility. In contrast to well-known spatial patterns in cell size at macroecological scales or temporal changes in experimental contexts, few data sets allow testing temporal changes in cell sizes within ecosystems. To analyze the temporal changes of intraspecific and community-wide cell size, we use the phytoplankton data derived from the Lower Saxony Wadden Sea monitoring program, which comprises sample- and species-specific measurements of cell volume from 1710 samples collected over 14 yr. We find significant reductions in both the cell volume of most species and the weighted mean cell size of communities. Mainly diatoms showed this decline, whereas the size of dinoflagellates seemed to be less responsive. The magnitude of the trend indicates that cell volumes are about 30% smaller now than a decade ago. This interannual trend is overlayed by seasonal cycles with smaller cells typically observed in summer. In the subset of samples including environmental conditions, small community cell size was strongly related to high temperatures and low total phosphorus concentration. We conclude that cell size captures ongoing changes in phytoplankton communities beyond the changes in species composition. In addition, based on the changes in species biovolumes revealed by our analysis, we warn that using standard cell size values in phytoplankton assessment will not only miss temporal changes in size, but also lead to systematic errors in biomass estimates over time.

Multiple hypotheses predict how gradients of nutrient availability, plant biomass, and temperature shape trophic pyramids. We aim to disentangle the simultaneous influence of those factors and their indirect effects on trophic structure and individual trophic levels. Location: United States. Time period: 2017. Major taxa studied: Invertebrates. Methods: To examine differences in trophic pyramid shape and abundance within trophic levels and across ecological gradients, we conducted 54 standardized surveys of invertebrate communities in North American grasslands. We tested for the direct and indirect effects of plant biomass, temperature, sodium (Na), other essential elements (e.g. N, P, and K), and toxic heavy metals, (e.g. Ar and Pb) in plant tissue on both individual trophic levels, and trophic pyramid shape, estimated as the community trophic mean (CTM). Results: Plant sodium increased CTM, indicating that high plant sodium concentrations are associated with top-heavy invertebrate trophic pyramids. Sites with higher plant biomass had higher proportions of herbivores compared to higher trophic levels. Finally, increasing temperature resulted in more top-heavy trophic pyramids. Overall, plant biomass, temperature, and plant chemistry directly and indirectly affected the abundances within different trophic levels, highlighting the complexity of factors regulating trophic structure. Main conclusions: Trophic structure of grassland invertebrate communities is strongly influenced by plant sodium, plant biomass, and to a lesser extent, temperature. Grasslands occupy 30% of the Earth’s terrestrial surface and are an imperiled ecosystem due to conversion to row crop agriculture. As biogeochemistry and temperature in the Anthropocene are increasingly modified, our results have considerable implications for the trophic structure of future grassland communities.

La description des gradients spatiaux ainsi que la documentation des dynamiques temporelles de la biodiversité sont des piliers centraux de l'écologie moderne, en particulier dans le contexte actuel des changements globaux auquel l'intérêt porté est croissant depuis les dernières décennies. Les communautés écologiques sont désormais reconnues comme des entités issues de la sélection d'espèces à partir d'un pool régional, sélection déterminée par différents processus appelés règles d'assemblage. Les deux règles d'assemblage principalement reconnues sont les filtres environnementaux, sélectionnant des espèces similaires adaptées à un milieu, et la limite à la ressemblance, sélectionnant des espèces dont les caractéristiques n'induisent pas de compétition trop forte et permettent la coexistence. Les objectifs de ma thèse ont été de (i) comprendre les patrons spatiaux de la diversité des communautés piscicoles à large échelle, (ii) comprendre la dynamique temporelle de cette diversité et (iii) d'appréhender la dynamique temporelle des processus sous-jacents à la structuration des communautés. Pour ce faire, des données de communautés piscicoles ont été analysées, d'une part à l'échelle continentale (i.e. européenne), et d'autre part à l'échelle nationale (i.e. France) pour laquelle la dimension temporelle est également disponible (i.e. séries temporelles depuis 1966 jusqu'à 2012). La description de la diversité piscicole européenne a permis de mettre en évidence la complémentarité des facettes phylogénétique, morphologique et écologique de la diversité. De plus, le climat, et en particulier la saisonnalité des conditions climatiques, est apparu comme un déterminant majeur de la distribution spatiale de la diversité. A une échelle plus locale et en tenant compte de la temporalité des communautés, il a été mis en évidence que les communautés piscicoles françaises connaissaient actuellement une réorganisation taxonomique due aux déclins de populations et menant à une homog, The description of spatial patterns as well as temporal dynamics of diversity has been a major cornerstone in modern ecology, especially in the contemporary context of global changes for which a growing concern is notable during last decades. Ecological communities are now acknowledged as the result of species that are sorted by selection from the regional pool. This selection of species is determined by several processes, namely the assembly rules. The two main assembly rules are the habitat filtering, selecting species that are able to inhabit in a given set of abiotic conditions, and the limiting similarity, selecting species for which ecological features are dissimilar in order to avoid a too strong competition and to coexist. The goals of my thesis were to (i) understand spatial patterns of freshwater fish community diversity at large scale, (ii) understand temporal dynamic of this diversity and (iii) describe temporal dynamics of the processes underlying the structuration of communities. In order to do this, data of freshwater fish communities has been analyzed first at continental scale (i.e. European) and then at national scale (i.e. France) for which temporal dimension of the data was also available (time series from 1966 to 2012). Based on the European freshwater diversity, we highlighted that phylogenetic, morphological and ecological facets were complementary. Moreover, climatic conditions, and especially their seasonality, seem to be a major driver of the spatial distribution of diversity. At finer scale and by taking into account the temporality of communities, we found that French freshwater communities experienced taxonomic reorganization due to population declines that ultimately leads to taxonomic homogenization of freshwater diversity in France. Although functional homogenization has been observed due to demographic fluctuations of warm water-dweller and low trophic level species, consequences at large scale remained to be explored. Moreover, sinc