Oliver Heiri

Aquatic invertebrate remains in lake sediments can potentially be used to provide information on past changes in water depth based on sediment records. However, only limited data are available on the distribution of invertebrate remains across varying water depths in stratified Central European lowland lakes. We analysed fossil chironomid and cladoceran assemblages from surface sediments sampled along ten transects of six Swiss lowland lakes with the aim of exploring the relationship between the distribution of species assemblages of these indicator groups and water depth. Subfossil assemblages of chironomids and cladocerans in the surface sediments differed markedly between and within the studied lakes of different sizes, depths, and trophic states. For nine of ten transects, relatively strong shifts in the assemblages associated with water depth were detected. However, the strength and pattern of change in assemblages along the depth gradient differed between cladocerans and chironomids. Cladoceran assemblages showed clear changes along the entire surveyed depth gradient, with more pronounced changes in the shallower samples. On the contrary, the chironomid assemblages, which also showed notable changes across the entire depth gradient, showed more pronounced changes across the estimated thermocline position and in the deeper transect parts. These results imply that the choice of indicator group in future studies aiming to reconstruct changes in water depth based on chitinous aquatic invertebrate remains could depend on the expected palaeowater depth, and that both groups analysed together can be expected to react more sensitively over a broader range of water depths than one of the groups analysed individually.

Aim: Theories on ecological stability lack the long‐term observational data necessary to comprehend the dynamics of durable and complex ecosystems like forests; consequently, the influence of biodiversity on ecosystem stability remains predominantly unexplained. This study examines the mechanisms of forest community and tree population change using long‐term palaeoecological data, evaluates responses to diverse effects, assesses stability and identifies the conditions that compelled these ecosystems to transition into alternative states. Furthermore, we aim to test whether higher diversity and larger populations promote stability. Location: Central Europe. Taxon: Tracheophytes (spore and pollen‐producing vascular plants). Methods: We analyse a 12,000‐year lacustrine sedimentary record from a mountain lake Černé jezero, located in a spruce‐dominated temperate mountain ecosystem in the Bohemian Forest of Central Europe, utilizing various proxies such as pollen, charcoal and geochemical data. We examined accumulation rates, ecological indices, and climatic and regression models to evaluate biodiversity, disturbances, rates of change, population stability and climate dynamics to identify the drivers of these changes. Results: Our findings indicate that species richness is negatively correlated with tree population stability. Furthermore, we discovered that tree population growth does not always facilitate stability as theorized. Main Conclusions: As mountain forests encounter escalating pressures from climate change and anthropogenic influences, our findings suggest that competition and turnover among species can undermine tree population‐level resilience. Our results also emphasize the necessity of preserving species diversity while acknowledging the potential emerging threats to forest population stability.

Understanding the long-term drivers of biodiversity change requires datasets that expand the timescale of ecological observations. Using lake sediments as natural archives, we reconstruct plant diversity change at high temporal resolution across the Swiss Plateau, a key region for European land use history. Contiguous pollen records, from three sites, spanning 7.6 millennia, were constrained by 227 radiocarbon dates and partly annually laminated sequences, yielding decadal-scale chronologies. We examine relationships between palynological richness (PRI), a proxy for plant diversity, and reconstructed land use probability (LUP) across six cultural-historic epochs. Vegetation histories are similar across sites: while beech forests persist throughout, other deciduous trees decline and grassland taxa expand from the Neolithic onwards. Plant diversity responds strongly to land use, increasing with openings, cultivation and fire, and declining during crises such as the Migration Period and the Black Death. Medieval and early modern landscapes support the highest richness, while diversity has declined in recent decades, likely due to industrialised agriculture. Our findings show that humans have been major drivers of plant diversity change in European lowlands for more than seven millennia and that sustainable land use legacies from the past may hold the key to reversing today's biodiversity losses.

The analysis of fossil biotic remains in lake sediment records allows for the reconstruction of past ecosystem dynamics and as such forms a powerful tool for understanding ecosystem processes and environmental change on a range of spatial and temporal scales. However, when producing palaeoecological datasets, analysts are often not able to assign all specimens to morphotypes at the highest taxonomic resolution. As a result, datasets containing unprocessed or raw counts usually include categories of identification across multiple different taxonomic levels (e.g. species morphotype, genus, tribe or even family level), also for fossil remains that may actually have originated from the same taxon (e.g. the same species or species morphotype). Whilst different strategies to deal with this problem have emerged over the years, it is rarely described in papers how analysts dealt with this issue, and how datasets were processed from raw counts to a final dataset used for palaeoecological interpretation, numerical analysis or quantitative inference. Using chironomid identifications as an example, we here describe the four main strategies for dealing with such multi-level identifications that can be applied to unprocessed count data with different levels of taxonomic detail: combining, retaining, deleting and assigning. We discuss advantages and disadvantages of each of these strategies and illustrate their impacts on palaeoecological analyses using both theoretical and practical examples. We conclude that there is no one optimal way to deal with the issue of incomplete or multi-level taxonomic identifications of fossils that may have originated from the same taxon, but that analysts will have to determine the strategy that best befits their project on a case-by-case basis, taking into account the type and quality of data they are working with, as well as the overall aims of their research project. We recommend that the choice of strategy (or combined strategies) is clearly described in the form of a brief statement in the Methods section of manuscripts where primary data are presented and that ideally both the unprocessed raw count data and the processed data used for ecological and numerical analyses are archived where publication outlets allow for this. Whilst we use chironomid research as the primary example throughout this manuscript, the proposed strategies and recommendations are relevant to a wide range of microfossil groups.

In the North Atlantic domain, the Late Glacial period (19-11.8 cal ka BP) is characterized by rapid and high amplitude climatic changes. The amplitude of temperature change in the Central Mediterranean region remains debated because reconstructions are limited and often contradictory. We present a chironomid-inferred temperature record from Lago di Mezzano, Central Italy, covering the Late Glacial. The chironomid record is contextualized with high-resolution multi-proxy data to elucidate on the effects of changing primary production, sediment composition, and oxygen availability on chironomid assemblages.During the Oldest Dryas (>14.8 cal ka BP), reconstructed July air temperatures averaged ~12°C. Temperatures increased in two steps, reaching ~16.8 °C (14.5–14.1 cal ka BP) and, finally, a plateau at ~17.5 °C during the Allerød and the Younger Dryas (13.9–12.0 cal ka BP). This warming is gradual and is paralleled by increasing primary production and gradual extension of hypolimnetic anoxia. During the Younger Dryas, we observe low primary production, an increase in the siliciclastic fraction, and a shift in chironomid assemblages. This chironomid assemblage shift does not translate into significant summer cooling suggesting that, while the Younger Dryas cooling affected catchment hydrology and erosion (Ti, K), the impact of the associated AMOC-slowdown on Central Italian summer temperatures was minimal. Reconstructed absolute temperatures appeared "too cold" considering the geographical location of Mezzano. This discrepancy possibly stems from extensive cold deep lake habitats in Mezzano, and edge-effects of the applied transfer function. Habitat preferences of most chironomid taxa matched trophic and anoxic gradients during the Late Glacial.

Many European lakes have suffered oxygen declines from recent eutrophication raising the question as to which conditions are representative of their pre-impact state. Here we present a palaeoecological record revealing that deepwater oxygen in small lakes can be variable over long timescales and subject to human influence for millennia. Remains of chironomids and other invertebrates indicate that Bichelsee (Switzerland) shifted from a stable, oxygen-rich to a dynamic, oxygen-poor state before major human activity. Initially, increased human activity and clearcutting coincided with improved oxygen conditions for deepwater midge larvae. However, from the Roman Age onwards, similar disturbances were associated with low oxygen and reduced deepwater invertebrate populations. These results demonstrate that small European lakes can have multiple, contrasting pre-impact reference states, and that ecological effects of human activity, even their directionality, may shift over time. Furthermore, repeated cycles of impact and recovery can occur over millennia, complicating assumptions about their baseline conditions.

Black carbon is a paleofire proxy that has been measured from glacial ice, snow, soils and lake sediments, though relatively few comparisons have been made with other fire indicators in sedimentary geoarchives. Microscopic charcoal, quantified from palynological microscope slides and macroscopic charcoal, quantified from wet-sieved deposits, are the most commonly applied methods for paleofire interpretation of Quaternary sediments. This research explores the down-profile patterns across three paleofire proxies (refractory black carbon, microscopic and macroscopic charcoal) and potential paleofire interpretations from a sediment core dating to the last centuries from Speke Gulf, Lake Victoria, and a young soil profile from a kopje located in the surrounding watershed in Serengeti National Park, Tanzania. The results of three paleofire metrics show similar trends within each site, with a positive trend across all metrics and increasing variability with increased measurement values (heteroscedastic). Notably, refractory black carbon (rBC) concentrations are two orders of magnitude higher in lake sediment samples compared to soil samples. rBC is positively correlated with both microscopic and macroscopic charcoal values and the overall profile patterns down the sediment core are similar, with the exception of the rBC increases from 2.5 to 0 cm depth that may result from increased fossil fuel combustion. The Speke Gulf rBC measurements are in an intermediate range between those published from glacial ice and other lake sediments. New rBC records from different ecosystems and temporal scales will provide paleofire insights and potential to interpret source areas and depositional patterns. The exploration of soil archives offers the potential to exploit semi-arid ecosystems and archaeological sites that have no nearby traditional paleoenvironmental study site targets.

Adaptive radiation is an important process for the origin of functional and ecological biodiversity. Understanding how, when, and why adaptive radiations occur is a long-standing interest in evolutionary ecology. Although many adaptive radiations have been studied, few studies resolved the temporal sequence of events during adaptive radiation. Here, we assembled a continuous record of tooth fossils of Lake Victoria’s haplochromine cichlid fish, the fastest and youngest of the classical adaptive radiations, from sediment cores extending from lake refilling to the present. We use these fossil records to reconstruct temporal patterns in the unfolding of this adaptive radiation. Our results reveal an extremely fast expansion in morphospace, from an undiverse ancestral condition, within the first three millennia after the onset of the modern lake. Comparison with modern cichlid teeth suggests that large-scale diversification across the food web emerged within these first three millennia. We detect a clear signal of an evolutionary trend from trophic generalists to specialists, but we also show that generalists persisted amid the growing radiation of specialists. Altogether, this pattern confirms the unusual evolutionary potential of the Lake Victoria hybrid lineage of haplochromine cichlids that seeded the radiation and the unusual speed with which the adaptive radiation occurred.