Thermocline depth and euphotic zone thickness regulate the abundance of diazotrophic cyanobacteria in Lake Tanganyika
- 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
- 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.
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Benedikt Ehrenfels et al.


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RC1: 'Review of BG-2020-214', Anonymous Referee #1, 30 Jun 2020
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AC1: 'Response to Anonymous Referee #1', Benedikt Ehrenfels, 16 Jul 2020
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AC1: 'Response to Anonymous Referee #1', Benedikt Ehrenfels, 16 Jul 2020
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RC2: 'Manucript review', Anonymous Referee #2, 06 Sep 2020
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AC2: 'Response to Reviewer #2', Benedikt Ehrenfels, 27 Sep 2020
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AC2: 'Response to Reviewer #2', Benedikt Ehrenfels, 27 Sep 2020


-
RC1: 'Review of BG-2020-214', Anonymous Referee #1, 30 Jun 2020
-
AC1: 'Response to Anonymous Referee #1', Benedikt Ehrenfels, 16 Jul 2020
-
AC1: 'Response to Anonymous Referee #1', Benedikt Ehrenfels, 16 Jul 2020
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RC2: 'Manucript review', Anonymous Referee #2, 06 Sep 2020
-
AC2: 'Response to Reviewer #2', Benedikt Ehrenfels, 27 Sep 2020
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AC2: 'Response to Reviewer #2', Benedikt Ehrenfels, 27 Sep 2020
Benedikt Ehrenfels et al.
Data sets
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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