Status: this preprint was under review for the journal BG but the revision was not accepted.
Thermocline depth and euphotic zone thickness regulate the abundance of diazotrophic cyanobacteria in Lake Tanganyika
Benedikt Ehrenfels,Maciej Bartosiewicz,Athanasio S. Mbonde,Kathrin B. L. Baumann,Christian Dinkel,Julian Junker,Tumaini Kamulali,Ismael A. Kimirei,Daniel Odermatt,Francesco Pomati,Emmanuel A. Sweke,and Bernhard Wehrli
Eawag, Swiss Federal Institute of Aquatic Science and Technology, Department Surface Waters – Research and Management, Kastanienbaum, Switzerland
ETH Zurich, Institute of Biogeochemistry and Pollutant Dynamics, Zurich, Switzerland
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.
How to cite. Ehrenfels, B., Bartosiewicz, M., Mbonde, A. S., Baumann, K. B. L., Dinkel, C., Junker, J., Kamulali, T., Kimirei, I. A., Odermatt, D., Pomati, F., Sweke, E. A., and Wehrli, B.: Thermocline depth and euphotic zone thickness regulate the abundance of diazotrophic cyanobacteria in Lake Tanganyika, Biogeosciences Discuss. [preprint], https://doi.org/10.5194/bg-2020-214, 2020.
Received: 09 Jun 2020 – Discussion started: 25 Jun 2020
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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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...