Inhibited vertical mixing and seasonal persistence of a thin cyanobacterial layer in a stratified lake

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 ($$N \gtrsim 0.05$$ N ≳ 0.05 s$$^{-1}$$ - 1 ) and withstood significant turbulence, observed as turbulent kinetic energy dissipation rates ($$\varepsilon \approx 10^{-8}$$ ε ≈ 10 - 8 W kg$$^{-1}$$ - 1 ). Vertical turbulent overturns –monitored by the Thorpe scale– went mostly undetected and on average fell below those estimated by the Ozmidov scale ($$L_O \approx 1$$ L O ≈ 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.

The emergence and dominance of Planktothrix rubescens as an hypolimnetic cyanobacterium in response to re‐oligotrophication of a deep peri‐alpine lake

“…Other studies on the microstructure of P . rubescens DCM in Lake Zurich found similar values (same order of magnitude) of the Ozmidov scale at the Planktothrix peak (Castro et al 2021). However, in Lake Zurich the population of P .…”

“…However, in Lake Zurich the population of P . rubescens is still found in the metalimnion (Castro et al 2021). In Lake Hallwil, the deepening of the DCM—caused by finding neutral buoyancy at greater depth, linked to improved light conditions and P‐limitation— resulting from re‐oligotrophication, has not yet led to a loss of buoyancy‐controlled depth positioning of P .…”

“…If the DCM shifts below the metalimnion, buoyancy regulation will be restricted for two main reasons: (1) reduced metabolic activity at low temperature, as seen in experiments with other cyanobacteria, slows down ballast adjustments (You et al 2018) and (2) increasing vertical turbulent diffusivity with depth in the hypolimnion, enhanced by higher shear stress, hinders P . rubescens buoyancy‐controlled positioning (Castro et al 2021). Thus, the likelihood of stronger turbulent diffusivity resulting in the vertical spread of P .…”

The Red Harmful Plague in Times of Climate Change: Blooms of the Cyanobacterium Planktothrix rubescens Triggered by Stratification Dynamics and Irradiance

“…Therefore, epilimnetic entrainment in autumn may not entirely be forced by deepening z mix but also driven by light-dependent buoyancy regulation. Recently, the role of convection-induced turbulences was highlighted, which may break up the strong stratification in autumn, allowing P. rubescens to escape the metalimnion and to follow specific irradiances ( Fernández Castro et al, 2021 ).…”

Is Toxin-Producing Planktothrix sp. an Emerging Species in Lake Constance?

“…populations. Indeed, light winds and convective mixing are highly important in the seasonal cycling of P. rubescens communities within a strongly stratified medium-sized lake [ 70 ]. In consequence, this means that toxin-producing Planktothrix spp.…”