A continuous fish fossil record reveals key insights into adaptive radiation.

Adaptive radiations have been instrumental in generating a considerable amount of life's diversity. Ecological opportunity is thought to be a prerequisite for adaptive radiation1, but little is known about the relative importance of species' ecological versatility versus effects of arrival order in determining which lineage radiates2. Palaeontological records that could help answer this are scarce. In Lake Victoria, a large adaptive radiation of cichlid fishes evolved in an exceptionally short and recent time interval3. We present a rich continuous fossil record extracted from a series of long sediment cores along an onshore-offshore gradient. We reconstruct the temporal sequence of events in the assembly of the fish community from thousands of tooth fossils. We reveal arrival order, relative abundance and habitat occupation of all major fish lineages in the system. We show that all major taxa arrived simultaneously as soon as the modern lake began to form. There is no evidence of the radiating haplochromine cichlid lineage arriving before others, nor of their numerical dominance upon colonization; therefore, there is no support for ecological priority effects. However, although many taxa colonized the lake early and several became abundant, only cichlids persisted in the new deep and open-water habitats once these emerged. Because these habitat gradients are also known to have played a major role in speciation, our findings are consistent with the hypothesis that ecological versatility was key to adaptive radiation, not priority by arrival order nor initial numerical dominance.

Testing alternative hypotheses for the decline of cichlid fish in Lake Victoria using fish tooth time series from sediment cores.

Lake Victoria is well known for its high diversity of endemic fish species and provides livelihoods for millions of people. The lake garnered widespread attention during the twentieth century as major environmental and ecological changes modified the fish community with the extinction of approximately 40% of endemic cichlid species by the 1980s. Suggested causal factors include anthropogenic eutrophication, fishing, and introduced non-native species but their relative importance remains unresolved, partly because monitoring data started in the 1970s when changes were already underway. Here, for the first time, we reconstruct two time series, covering the last approximately 200 years, of fish assemblage using fish teeth preserved in lake sediments. Two sediment cores from the Mwanza Gulf of Lake Victoria, were subsampled continuously at an intra-decadal resolution, and teeth were identified to major taxa: Cyprinoidea, Haplochromini, Mochokidae and Oreochromini. None of the fossils could be confidently assigned to non-native Nile perch. Our data show significant decreases in haplochromine and oreochromine cichlid fish abundances that began long before the arrival of Nile perch. Cyprinoids, on the other hand, have generally been increasing. Our study is the first to reconstruct a time series of any fish assemblage in Lake Victoria extending deeper back in time than the past 50 years, helping shed light on the processes underlying Lake Victoria's biodiversity loss.

Anthropogenic Eutrophication Drives Major Food Web Changes in Mwanza Gulf, Lake Victoria.

Discerning ecosystem change and food web dynamics underlying anthropogenic eutrophication and the introduction of non-native species is necessary for ensuring the long-term sustainability of fisheries and lake biodiversity. Previous studies of eutrophication in Lake Victoria, eastern Africa, have focused on the loss of endemic fish biodiversity over the past several decades, but changes in the plankton communities over this same time remain unclear. To fill this gap, we examined sediment cores from a eutrophic embayment, Mwanza Gulf, to determine the timing and magnitude of changes in the phytoplankton and zooplankton assemblages over the past century. Biogeochemical proxies indicate nutrient enrichment began around ~ 1920 CE and led to rapid increases in primary production, and our analysis of photosynthetic pigments revealed three zones: pre-eutrophication (prior to 1920 CE), onset of eutrophication with increases in all pigments (1920-1990 CE), and sustained eutrophication with cyanobacterial dominance (1990 CE-present). Cladoceran remains indicate an abrupt decline in biomass in ~ 1960 CE, in response to the cumulative effects of eutrophication and lake-level rise, preceding the collapse of haplochromine cichlids in the 1980s. Alona and Chydorus, typically benthic littoral taxa, have remained at relatively low abundances since the 1960s, whereas the abundance of Bosmina, typically a planktonic taxon, increased in the 1990s concurrently with the biomass recovery of haplochromine cichlid fishes. Overall, our results demonstrate substantial changes over the past century in the biomass structure and taxonomic composition of Mwanza Gulf phytoplankton and zooplankton communities, providing a historical food web perspective that can help understand the recent changes and inform future resource management decisions in the Lake Victoria ecosystem. Supplementary Information: The online version contains supplementary material available at 10.1007/s10021-024-00908-x.

Scanning Hyperspectral Imaging for In Situ Biogeochemical Analysis of Lake Sediment Cores: Review of Recent Developments.

Hyperspectral imaging (HSI) in situ core scanning has emerged as a valuable and novel tool for rapid and non-destructive biogeochemical analysis of lake sediment cores. Variations in sediment composition can be assessed directly from fresh sediment surfaces at ultra-high-resolution (40−300 µm measurement resolution) based on spectral profiles of light reflected from sediments in visible, near infrared, and short-wave infrared wavelengths (400−2500 nm). Here, we review recent methodological developments in this new and growing field of research, as well as applications of this technique for paleoclimate and paleoenvironmental studies. Hyperspectral imaging of sediment cores has been demonstrated to effectively track variations in sedimentary pigments, organic matter, grain size, minerogenic components, and other sedimentary features. These biogeochemical variables record information about past climatic conditions, paleoproductivity, past hypolimnetic anoxia, aeolian input, volcanic eruptions, earthquake and flood frequencies, and other variables of environmental relevance. HSI has been applied to study seasonal and inter-annual environmental variability as recorded in individual varves (annually laminated sediments) or to study sedimentary records covering long glacial−interglacial time-scales (>10,000 years).

Variations of sedimentary Fe and Mn fractions under changing lake mixing regimes, oxygenation and land surface processes during Late-glacial and Holocene times.

Global spread of anoxia in aquatic ecosystems has become a major issue that may potentially worsen due to global warming. The reconstruction of long-term hypolimnetic anoxia records can be challenging due to lack of valid and easily measurable proxies. The sedimentary Mn/Fe ratio measured by X-ray fluorescence (XRF) is often used as a proxy for past lake redox conditions. Yet the interpretation of this ratio can be problematic when Fe and Mn accumulation is not solely redox driven. We used the varved sediments of Lake Moossee (Switzerland) to examine the partitioning of Fe and Mn in seven fractions by sequential extraction under various oxygen conditions over the last 15,000 years. We combined this data with XRF scans and an independent diagnostic proxy for anoxia given by a hyperspectral imaging (HSI)-inferred record of bacteriopheophytin, to validate the use of the XRF-Mn/Fe ratio as redox proxy. In the 15,000-year long record, Fe was bound to humins and amorphous, crystalline, sulfide and residual forms. Mn was mainly present in carbonate and amorphous forms. Higher erosion, prolonged anoxia, diagenesis and humic matter input affected Fe and Mn accumulation. Under holomixis the XRF-Mn/Fe ratio successfully reflected lake redox conditions. Periods with higher detrital Fe input obscured the applicability of the ratio. During phases of permanent anoxia, intensified early diagenetic processes trapped Mn in the sediments in carbonate, crystalline oxide and humic forms. Our study shows that the single use of the XRF-Mn/Fe ratio is often not conclusive for inferring past lake redox conditions. The application of the XRF-Mn/Fe as a proxy for anoxia requires taking into account the individual lake characteristics and changes in lake environmental conditions, which affect the accumulation of Fe and Mn in the sediments.

250-year records of mercury and trace element deposition in two lakes from Cajas National Park, SW Ecuadorian Andes.

Historical records of trace elements in lake sediments provide source-to-sink information about potentially toxic pollutants across space and time. We investigated two lakes located at different elevations in the Ecuadorian Andes to understand how trace element fluxes are related to (i) geology, (ii) erosion in the watersheds, and (iii) local point sources and atmospheric loads. In remote Lake Fondococha (4150 m a.s.l.), total Hg fluxes stay constant between ca. 1760 and 1950 and show an approximately 4.4-fold increase between pre-1950 and post-1950 values. The post-1950 increase in fluxes of other trace elements (V, Cr, Co, Ni, Cu, Zn, As, Cd, and Pb) is lower (2.1-3.0-fold) than for Hg. Mostly lithogenic sources and enhanced soil erosion contribute to their post-1950 increase (lithogenic contribution: > 85%, Hg: ~ 58%). Average post-1950 Hg fluxes are approximately 4.3 times higher in peri-urban Lake Llaviucu (3150 m a.s.l.) than in the remote Lake Fondococha. Post-1950 fluxes of the other trace elements showed larger differences between Lakes Fondococha and Llaviucu (5.2 < 25-29.5-fold increase; Ni < Pb-Cd). The comparison of the post-1950 average trace element fluxes that are derived from point and airborne sources revealed 5-687 (Hg-Pb) times higher values in Lake Llaviucu than in Lake Fondococha suggesting that Lake Llaviucu's proximity to the city of Cuenca strongly influences its deposition record (industrial emissions, traffic, caged fishery). Both lakes responded with temporary drops in trace element accumulations to park regulations in the 1970s and 1990s, but show again increasing trends in recent times, most likely caused by increase in vehicular traffic and openings of copper and gold mines around Cajas National Park.