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Biogeosciences An interactive open-access journal of the European Geosciences Union
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Preprints
https://doi.org/10.5194/bg-2020-214
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2020-214
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  25 Jun 2020

25 Jun 2020

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A revised version of this preprint is currently under review for the journal BG.

Thermocline depth and euphotic zone thickness regulate the abundance of diazotrophic cyanobacteria in Lake Tanganyika

Benedikt Ehrenfels1,2, Maciej Bartosiewicz3, Athanasio S. Mbonde4, Kathrin B. L. Baumann1,2, Christian Dinkel1, Julian Junker5,6, Tumaini Kamulali4, Ismael A. Kimirei4,7, Daniel Odermatt1, Francesco Pomati8, Emmanuel A. Sweke4,9, and Bernhard Wehrli1,2 Benedikt Ehrenfels et al.
  • 1Eawag, Swiss Federal Institute of Aquatic Science and Technology, Department Surface Waters – Research and Management, Kastanienbaum, Switzerland
  • 2ETH Zurich, Institute of Biogeochemistry and Pollutant Dynamics, Zurich, Switzerland
  • 3University of Basel, Department of Environmental Sciences, Basel, Switzerland
  • 4TAFIRI, Tanzania Fisheries Research Institute, Kigoma, Tanzania
  • 5Eawag, Swiss Federal Institute of Aquatic Science and Technology, Department Fish Ecology and Evolution, Kastanienbaum, Switzerland
  • 6University of Bern, Institute of Ecology & Evolution, Bern, Switzerland
  • 7TAFIRI, Tanzania Fisheries Research Institute, Kigoma, Tanzania
  • 8Eawag, Swiss Federal Institute of Aquatic Science and Technology, Department Aquatic Ecology, Dübendorf, Switzerland
  • 9DSFA, Deep Sea Fishing Authority, Zanzibar, Tanzania

Abstract. In spite of the fact that cyanobacterial blooms are classically associated with high nutrient loadings, there is also abundant evidence revealing that nitrogen fixing cyanobacteria (diazotrophs) can prevail under oligotrophic conditions. The mechanisms favouring diazotrophs in oligotrophic water bodies remain, however, poorly resolved. Here we analyse biogeochemical and ecological factors regulating the distribution of nitrogen fixing cyanobacteria in the oligotrophic Lake Tanganyika using sensor profiles of hydrodynamic conditions, nutrient and pigment analyses, as well as phytoplankton community assessment. During periods of stable or re-establishing water column stratification, we find evidence that the location of the thermocline and the euphotic depth can create a functional niche for diazotrophic cyanobacteria: Nitrogen limitation provides an ecological advantage for an apparent mutualistic interaction between diazotrophs and diatoms when the upward transport of nitrate into the euphotic zone is reduced by a subjacent thermocline. Diazotrophs, comprising the filamentous genera Dolichospermum and Anabaenopsis, are key players under these conditions (up to 41.7 % of phytoplankton community), while they are rare otherwise. By contrast, a thermocline located within the euphotic zone allows rapid vertical transport of nitrate for a thriving nitrate assimilating phytoplankton community that evidently outcompetes diazotrophs. Finally, multiple observations of relatively high diazotroph densities in the upwelling region in the South of Lake Tanganyika imply that they may additionally thrive under high nutrient conditions, when nitrogen is heavily deficient with respect to phosphorous. This study highlights that, under nitrogen deficient conditions, cyanobacterial blooms may form in response to reduced nutrient fluxes to the productive surface waters.

Benedikt Ehrenfels et al.

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Benedikt Ehrenfels et al.

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Physical, biogeochemical, and biological data for Lake Tanganyika (2017–2018) B. Ehrenfels, A. S. Mbonde, C. Dinkel, A. Brand, K. Baumann, J. Junker, and B. Wehrli https://doi.org/10.3929/ethz-b-000418479

Benedikt Ehrenfels et al.

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Short summary
Cyanobacterial blooms are a widespread phenomenon, but their formation is poorly understood in nutrient-poor water bodies. During two cruises we collected physical, nutrient, pigment and phytoplankton data to study nitrogen fixing cyanobacteria (diazotrophs) in Lake Tanganyika, Africa. We found that, due to their capability of fixing gaseous nitrogen, diazotrophs thrive when the supply of dissolved inorganic nitrogen to the sun-lit surface waters is low, but phosphorous is still available.
Cyanobacterial blooms are a widespread phenomenon, but their formation is poorly understood in...
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