Sebastian Teichert

Paleoclimatic changes during the late Cenozoic led to substantial losses of tree diversity in Western and Central Europe. A modern synthesis linking which taxa were lost, the timing of these losses, and their implications for the adaptation of the region’s forest ecosystems to ongoing climate change is needed. Here, we compile a dataset of fossil occurrences of tree genera from Western and Central Europe to analyze tree diversity changes during the Cenozoic (66 Ma – present). We find that tree genus richness declined 75% after its peak in the Miocene (23 – 5.3 Ma), with most losses already occurring during the Plio-Pleistocene transition (ca. 2 – 3 Ma). We then use genus-level niche models to explore how these changes unfolded within climate space. Our results show how the current regional tree flora was progressively filtered towards the colder and drier margins of climate space. This coincided with the losses of most heat-tolerant lineages, including many temperate genera that could have profited under modern climate warming (e.g., Castanea , Juglans, Aesculus ). We foresee several ways in which our analyses of paleoecological data can inform forest management to increase regional tree diversity and enhance forest resilience under climate change.

Rhodoliths, formed by crustose coralline algae (CCA), are prominent components in carbonate platform deposits, contributing to the characteristic rhodalgal facies. Their formation is influenced by both biological and physical factors, and under specific conditions, they can accumulate, forming extensive rhodolith beds. This study examines the depositional mechanisms underlying the formation of monospecific ( Sporolithon nodosum ) shallow‐water rhodoliths from the Whangaparaoa Peninsula, North Island, New Zealand. The findings suggest that these rhodoliths originate in subtidal rocky channels at approximately 5 m depth, by the encrustation of nuclei derived from surrounding geological strata. Following their formation, these rhodoliths are transported into intertidal channel systems and eventually deposited in supratidal beach areas. Notably, the structure, morphology and composition of rhodoliths across these three depositional settings exhibit minimal variation, suggesting that the rhodoliths belong to a strongly interconnected coastal system with only brief intervals between translocation events. In this context, the study provides a sedimentological model for the development of rhodoliths in a coastal setting. Furthermore, it challenges the presence of a significant rhodolith bed sensu stricto in deeper offshore waters, as exploratory diving surveys revealed extensive sand deposits that hinder rhodolith development in these areas.

Rhodoliths composed of crustose coralline algae (CCA) are marine calcifiers of global significance. Here, we investigate how floridean starch storage patterns of Arctic rhodoliths from Svalbard are affected by environmental conditions. Quantifying the amount of starch in photomosaic scans of rhodolith slabs via amylopectin–iodine complex formation, we found that shallow water rhodoliths contain significantly higher starch percentages compared to the deeper-water dwellers. We conclude that the observed starch patterns are mainly controlled by water depth because light and rhodolith turnover frequency both decrease in deeper waters. Regarding rhodolith turnover, the occasional burial of turned rhodoliths in deeper waters can result in a dieback of the outer CCA thallus areas, which contain important starch supplies. As rhodoliths are both calcifiers and photoautotrophs, we highlight their relevance in potentially contributing to global blue carbon, that is, their role as a marine carbon sink. In this context, our quantification approach of floridean starch patterns in rhodoliths provides a straightforward basis for further studies on this topic.

The calcareous matrix of rhodoliths can be composed of one or more crustose coralline algae (CCA) taxa as well as a mixed assemblage of various encrusting organisms. Studies on modern and fossil rhodoliths assume such associations to vary with water depth. Our study explores the quantitative biological composition of calcareous rhodolith matrices along a bathymetric gradient at the Arctic Svalbard archipelago. Using a methodological combination of virtual micro‐CT cross‐sections with a modified point counting approach, we found five different taxonomic groups: CCA, bivalves, serpulids, bryozoans and balanids. While water depth does not influence the general taxon richness as well as the abundance of bivalves, it significantly affects the proportional matrix composition of encrusting organisms by a combination of environmental factors and biological interactions. The decrease in CCA skeletal material with increasing water depth is significantly governed by impaired irradiance conditions. Regular rhodolith movement in shallow waters fosters the proportion of CCA, while decreased movement in deeper waters spurs the proportion of other encrusters. This potentially results from post‐mortem fouling of dead rhodolith parts followed by a recolonization with slow‐growing CCA species Boreolithothamnion glaciale. Our results highlight mechanisms controlling the biogenic composition of calcareous rhodolith matrices and underline the potential of matrices compositions for palaeogeography and palaeobathymetry. Our study contributes to an improved understanding of the composition and developmental patterns of rhodoliths. This can add to a better comprehension of the respective ecosystems on a broader scale and be beneficial for conservation purposes.

Rhodoliths built by crustose coralline algae (CCA) are ecosystem engineers of global importance. In the Arctic photic zone, their three‐dimensional growth emulates the habitat complexity of coral reefs but with a far slower growth rate, growing at micrometers per year rather than millimeters. While climate change is known to exert various impacts on the CCA's calcite skeleton, including geochemical and structural alterations, field observations of net growth over decade‐long timescales are lacking. Here, we use a temporally explicit model to show that rising ocean temperatures over nearly 100 years were associated with reduced rhodolith growth at different depths in the Arctic. Over the past 90 years, the median growth rate was 85 μm year⁻¹ but each °C increase in summer seawater temperature decreased growth by a mean of 8.9 μm (95% confidence intervals = 1.32–16.60 μm °C⁻¹, p < .05). The decrease was expressed for rhodolith occurrences in 11 and 27 m water depth but not at 46 m, also having the shortest time series (1991–2015). Although increasing temperatures can spur plant growth, we suggest anthropogenic climate change has either exceeded the population thermal optimum for these CCA, or synergistic effects of warming, ocean acidification, and/or increasing turbidity impair rhodolith growth. Rhodoliths built by calcitic CCA are important habitat providers worldwide, so decreased growth would lead to yet another facet of anthropogenic habitat loss.

Crustose coralline algae (CCA) are important ecosystem engineers and carbonate producers today and in the geological past. While there is an increasing number of publications on CCA every year, it is evident that there are many misunderstandings and inconsistencies in the assignment of CCA to taxonomic and functional groups. This is partly because CCA are treated by biologists, ecologists and palaeontologists as well as covered by studies published in journals ranging from geo- to biosciences, so that there is often a mixture of terminology used and differing scientific focus. In this review, a comprehensive overview is given on what is known about CCA, their functional traits and their roles in environments from the present and the past. In this context, some bridges are built between the commonly different viewpoints of ecologists and palaeontologists, including suggesting a common and straightforward terminology, highlighting and partially merging different taxonomic viewpoints as well as summarizing the most important functional traits of CCA. Ideally, future studies should seek to quantitatively analyse potential implications for CCA and their associated organisms under ongoing global change.

Knowledge of spatial biodiversity patterns is important for ecosystem assessment. Rhodoliths, free-living calcareous algae, are biotic components that structure the sea floor through their complex calcareous skeletons and their tendency to accumulate in an area to form rhodolith beds. Thereby, rhodoliths are considered to act as ecosystem engineers promoting local biodiversity. In this study, the biodiversity of rhodolith beds in Mosselbukta, Svalbard, was investigated to analyse the proposed link between local biodiversity and the presence of rhodoliths by evaluating beam trawl and underwater video transect data. The comparative analysis of two sampling methods addressing the same research question allowed us to assess the suitability of these two methods. To test our hypothesis and the utility of the two methods, evaluations of the two data sets were carried out separately by using Bayesian statistics. The results confirm a positive relationship between the presence of rhodoliths and local biodiversity with a posterior probability of 70% for the video transects and 85% for the beam trawl data. The similarity of the results of the two methods suggests that both methods are well-suited for the analysis of local biodiversity patterns. The combination of the two methods, with their individual strengths and weaknesses, has provided stronger support for the results and a broader view on different components of the biodiversity in the Svalbard rhodolith beds.

Crustose coralline algae (CCA) occur from the tropics to the poles in photic benthic environments. Here, we report on some of the world’s southernmost and coldest CCA sites in Terra Nova, Ross Sea, Antarctica at 74°41′ S. The recently described red alga Tethysphytum antarticum is investigated for its skeletal architecture, its mineralogical and geochemical composition, as well as for its taxonomic classification. A phylogenetic analysis based on molecular genetics and the sequencing of the photosystem II protein D1 (psbA) gave a perfect match with T. antarcticum. Histological sections and micro-CT-scans provide new diagnostic details for the conceptacles (the reproductive organs of the alga). X-ray diffractometry and electron-microprobe measurements yielded a clear high-Mg calcite (~8 mol%) composition of the skeletal parts. Detailed back-scattered electron imaging of polished petrographic thin sections revealed a two-layered thallus (vegetative plant tissue), comprising an organic-rich irregularly calcified basal layer with rectangular cells, overlain by the main thallus. Elemental maps show relatively increased sulphur in the basal layer, clearly tied to organic cell walls. MgCO₃ and SrCO₃ were targeted with semiquantitative elemental mappings and in an ontogenetic quantitative spot transect. Compared with temperature (−1.95 °C to +1.08 °C), the MgCO₃ (mol%) reflects this world’s coldest CCA site temperature with the lowest MgCO₃ content of 7.9 ± 1.6 mol%. The along transect variability, however, shows with ~6 mol% a larger MgCO₃ variability than expected for the 3 °C intra-annual temperature amplitude in Terra Nova Bay. This implies that in low amplitude settings the biomineralisation control on Mg/Ca ratios can outcompete its temperature sensitivity. Mark-recapture studies, next to the environmental logger station La Zecca are suggested, to perform a detailed growth rate and biomineralisation quantification.

There is an increasing number of studies reporting microplastic (MP) contamination in the Arctic environment. We analysed MP abundance in samples from a marine Arctic ecosystem that has not been investigated in this context and that features a high biodiversity: hollow rhodoliths gouged by the bivalve Hiatella arctica. This bivalve is a filter feeder that potentially accumulates MPs and may therefore reflect MP contamination of the rhodolith ecosystem at northern Svalbard. Our analyses revealed that 100% of the examined specimens were contaminated with MP, ranging between one and 184 MP particles per bivalve in samples from two water depths. Polymer composition and abundance differed strongly between both water depths: samples from 40 m water depth showed a generally higher concentration of MPs and were clearly dominated by polystyrene, samples from 27 m water depth were more balanced in composition, mainly consisting of polyethylene, polyethylene terephthalate, and polypropylene. Long-term consequences of MP contamination in the investigated bivalve species and for the rhodolith bed ecosystem are yet unclear. However, the uptake of MPs may potentially impact H. arctica and consequently its functioning as ecosystem engineers in Arctic rhodolith beds.