Inhibited vertical mixing and seasonal persistence of a thin cyanobacterial layer in a stratified lake
Inhibited vertical mixing and seasonal persistence of a thin cyanobacterial layer in a stratified lake
Harmful blooms of the filamentous cyanobacteria Planktothrix rubescens have become common in many lakes as they have recovered from eutrophication over the last decades. These cyanobacteria, capable of regulating their vertical position, often flourish at the thermocline to form a deep chlorophyll maximum. In Lake Zurich (Switzerland), they accumulate during stratified season (May–October) as a persistent metalimnetic thin layer (~2 m wide). This study investigated the role of turbulent mixing in springtime layer formation, its persistence over the summer, and its breakdown in autumn. We characterised seasonal variation of turbulence in Lake Zurich with four surveys conducted in April, July and October of 2018 and September of 2019. Surveys included microstructure profiles and high-resolution mooring measurements. In July and October, the thin layer occurred within a strong thermocline (N≳0.05 s−1) and withstood significant turbulence, observed as turbulent kinetic energy dissipation rates (ε≈10−8 W kg−1). Vertical turbulent overturns –monitored by the Thorpe scale– went mostly undetected and on average fell below those estimated by the Ozmidov scale (LO≈1 cm). Consistently, vertical diffusivity was close to molecular values, indicating negligible turbulent fluxes. This reduced metalimnetic mixing explains the persistence of the thin layer, which disappears with the deepening of the surface mixed layer in autumn. Bi-weekly temperature profiles in 2018 and a nighttime microstructure sampling in September 2019 showed that nighttime convection serves as the main mechanism driving the breakdown of the cyanobacterial layer in autumn. These results highlight the importance of light winds and convective mixing in the seasonal cycling of P. rubescens communities within a strongly stratified medium-sized lake.
Convection, Lake Zurich, Metalimnion, Microstructure turbulence, Mixing, Mixing efficiency, Planktothrix rubescens, Thin plankton layer
Castro, Bieito Fernández
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Steiner, Oscar Sepúlveda
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Knapp, Deborah
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Posch, Thomas
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Bouffard, Damien
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Wüest, Alfred
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14 March 2021
Castro, Bieito Fernández
8017e93c-d5ee-4bba-b443-9c72ca512d61
Steiner, Oscar Sepúlveda
ce37b37a-cfdf-47bb-a1f7-91fe9e4feac7
Knapp, Deborah
953429a0-9cb9-4de7-bb94-ffbf9eefcd43
Posch, Thomas
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Bouffard, Damien
a3eb0e77-974e-40b5-a122-388e1c15704b
Wüest, Alfred
1d8766ff-b66d-40df-a01e-fe4062512f08
Castro, Bieito Fernández, Steiner, Oscar Sepúlveda, Knapp, Deborah, Posch, Thomas, Bouffard, Damien and Wüest, Alfred
(2021)
Inhibited vertical mixing and seasonal persistence of a thin cyanobacterial layer in a stratified lake.
Aquatic Sciences, 83 (2), [38].
(doi:10.1007/s00027-021-00785-9).
Abstract
Harmful blooms of the filamentous cyanobacteria Planktothrix rubescens have become common in many lakes as they have recovered from eutrophication over the last decades. These cyanobacteria, capable of regulating their vertical position, often flourish at the thermocline to form a deep chlorophyll maximum. In Lake Zurich (Switzerland), they accumulate during stratified season (May–October) as a persistent metalimnetic thin layer (~2 m wide). This study investigated the role of turbulent mixing in springtime layer formation, its persistence over the summer, and its breakdown in autumn. We characterised seasonal variation of turbulence in Lake Zurich with four surveys conducted in April, July and October of 2018 and September of 2019. Surveys included microstructure profiles and high-resolution mooring measurements. In July and October, the thin layer occurred within a strong thermocline (N≳0.05 s−1) and withstood significant turbulence, observed as turbulent kinetic energy dissipation rates (ε≈10−8 W kg−1). Vertical turbulent overturns –monitored by the Thorpe scale– went mostly undetected and on average fell below those estimated by the Ozmidov scale (LO≈1 cm). Consistently, vertical diffusivity was close to molecular values, indicating negligible turbulent fluxes. This reduced metalimnetic mixing explains the persistence of the thin layer, which disappears with the deepening of the surface mixed layer in autumn. Bi-weekly temperature profiles in 2018 and a nighttime microstructure sampling in September 2019 showed that nighttime convection serves as the main mechanism driving the breakdown of the cyanobacterial layer in autumn. These results highlight the importance of light winds and convective mixing in the seasonal cycling of P. rubescens communities within a strongly stratified medium-sized lake.
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FernándezCastro2021_Article_InhibitedVerticalMixingAndSeas
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Accepted/In Press date: 1 February 2021
e-pub ahead of print date: 14 March 2021
Published date: 14 March 2021
Additional Information:
Funding Information:
We acknowledge Sébastien Lavanchy (EPFL) and Michael Plüss (Eawag) for their contribution to the design of the HR-mooring and participation in the field campaigns. We thank Eugen Loher and Daniel Marty for their contribution to the bi-weekly monitoring program. We are grateful to Tomy Doda, Lucas Serra Moncadas, Adem Topak, Camille Minaudo and Luca Cortese for their assistance during fieldwork and MeteoSwiss for the COSMO-1 weather data. This work was financed by the Swiss National Science Foundation Sinergia grant CRSII2_160726 (A Flexible Underwater Distributed Robotic System for High-Resolution Sensing of Aquatic Ecosystems), and Grant 200021_179123 (Primary Production Under Oligotrophication in Lakes). This study was also supported by the grant SeeWandel: Life in Lake Constance - the past, present and future within the framework of the Interreg V-Program Alpenrhein–Bodensee–Hochrhein (Germany, Austria, Switzerland, Liechtenstein) which funds are provided by the European Regional Development Fund as well as the Swiss Confederation and Cantons. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Publisher Copyright:
© 2021, The Author(s).
Keywords:
Convection, Lake Zurich, Metalimnion, Microstructure turbulence, Mixing, Mixing efficiency, Planktothrix rubescens, Thin plankton layer
Identifiers
Local EPrints ID: 447839
URI: http://eprints.soton.ac.uk/id/eprint/447839
ISSN: 1015-1621
PURE UUID: 6df31d98-8e41-452c-85dc-4a124b1a2121
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Date deposited: 24 Mar 2021 17:34
Last modified: 17 Mar 2024 04:04
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Author:
Oscar Sepúlveda Steiner
Author:
Deborah Knapp
Author:
Thomas Posch
Author:
Damien Bouffard
Author:
Alfred Wüest
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