Schubert, C. J. | bioRxiv Science

bioRxiv2021

Isotopic signatures induced by upwelling tag regional fish populations in Lake Tanganyika

Lake Tanganyikas pelagic fish sustain the second largest inland fishery in Africa and are under pressure from heavy fishing and global warming related increases in stratification. Only little is known about whether basin-scale hydrodynamics - including a more stratified north and an upwelling-driven south - induce ecological and genetic differences among populations of highly mobile, pelagic fish inhabiting these different areas. Here, we examine whether the basin-scale dynamics leave distinct isotopic imprints in the pelagic fish of Lake Tanganyika, which may reveal differences in habitat, diet, or lipid content. We conducted two lake-wide campaigns during different seasons and collected physical, nutrient, chlorophyll, phytoplankton and zooplankton data. Additionally, we analyzed the pelagic fish - the clupeids Stolothrissa tanganicae, Limnothrissa miodon and four Lates species - for their isotopic and elemental carbon (C) and nitrogen (N) compositions. The {delta}13C values were significantly higher in the productive south after the upwelling/mixing period across all trophic levels, implying that the fish have regional foraging grounds, and thus record these latitudinal isotope gradients. By combining our isotope data with genetics, we demonstrate that the fish form regional populations on a seasonal to multiannual time scale. Based on {delta}15N and C:N ratios, we found no strong evidence for varying diets or lipid contents between those regional populations. Additional analyses revealed that isotopic variations between specimens from the same location are not linked to genetic differences. We suggest that the development of basinscale ecological differences in response to the prevailing hydrodynamic regimes may be inhibited by lake-wide gene flow on the long term. Our findings show that the pelagic fish species are genetically adapted to the whole lake, but they form regional populations on short time scales. This implies that sustainable management strategies may adopt basin-scale fishing quotas.

ecology↗

bioRxiv2021

Ancient and modern geochemical signatures in the 13,500-year sedimentary record of Lake Cadagno

Although lake sediments are globally important organic carbon sinks and therefore important habitats for deep microbial life, the deep lacustrine biosphere has thus far been little studied compared to its marine counterpart. To investigate the impact of the underexplored deep lacustrine biosphere on the sediment geochemical environment and vice versa, we performed a comprehensive microbiological and geochemical characterization of a sedimentary sequence from Lake Cadagno covering its entire environmental history since formation following glacial retreat. We found that both geochemical gradients and microbial community shifts across the [~]13.5 kyr subsurface sedimentary record reflect redox changes in the lake, going from oxic to anoxic and sulfidic. Most microbial activity occurs within the top 40 cm of sediment, where millimolar sulfate concentrations diffusing in from the bottom water are completely consumed. In deeper sediment layers, organic carbon remineralization is much slower but microorganisms nonetheless subsist on fermentation, sulfur cycling, metal reduction, and methanogenesis. The most surprising finding was the presence of a deep, oxidizing groundwater source. This water source generates an inverse redox gradient at the bottom of the sedimentary sequence and could contribute to the remineralization of organic matter sequestered in the energy-limited deep subsurface.

microbiology↗