Impact of internal waves on the spatial distribution of Planktothrix rubescens (cyanobacteria) in an alpine lake

Abstract

The vertical and horizontal distribution of the cyanobacterium, Planktothrix rubescens, was studied in a deep alpine lake (Lac du Bourget) in a 2-year monitoring program with 11 sampling points, and a 24-h survey at one sampling station. This species is known to proliferate in the metalimnic layer of numerous deep mesotrophic lakes in temperate areas, and also to produce hepatotoxins. When looking at the distribution of P. rubescens at the scale of the entire lake, we found large variations (up to 10 m) in the depth of the biomass peak in the water column. These variations were closely correlated to isotherm displacements. We also found significant variations in the distribution of the cyanobacterial biomass in the northern and southern parts of the lake. We used a physical modeling approach to demonstrate that two internal wave modes can explain these variations. Internal waves are generated by wind events, but can still be detected several days after the end of these events. Finally, our 24-h survey at one sampling point demonstrated that the V1H1 sinusoidal motion could evolve into nonlinear fronts. All these findings show that internal waves have a major impact on the distribution of P. rubescens proliferating in the metalimnic layer of a deep lake, and that this process could influence the growth of this species by a direct impact on light availability.

Short‐term displacement of Planktothrix rubescens (cyanobacteria) in a pre‐alpine lake observed using an autonomous sampling platform

“…Our results are in line with other observations of the patchiness in P. rubescens horizontal distribution in pre-and alpine lakes (Cuypers et al 2010;Sotton et al 2011). Large patches (, 500 m long) of dense Planktothrix biomass recurrently formed in the metalimnion during our sampling, with apparent effects on the local physicochemistry.…”

“…Short-term variations in the vertical distribution of P. rubescens due to displacement by seiches have been scarcely documented (Thomas and Mä rki 1949;Thomas 1950;Cuypers et al 2010;Carraro et al 2012), and there is no information on the variations in light conditions that are experienced by P. rubescens in situ. Comprehensive calculations indicated that vertical shifts significantly affect the net photosynthetic production balance because this metalimnetic population is located close to its photosynthetic compensation point (Walsby et al 2001).…”

“…The spatial distribution of cyanobacteria varies vertically as well as horizontally in shallow (Moreno-Ostos et al 2009;Pobel et al 2011) and in deep lakes (Cuypers et al 2010;Salcher et al 2011). In the epilimnion, spatial variations may occur on short time scales (i.e., from hours to days) because most of these changes arise from meteorological conditions.…”

“…Once winds stop, the thermal layers of the lake slide over each other to redistribute a new equilibrium generating an oscillation, called an internal seiche, which produces vertical movements of water at depth, but produces very little motion at the surface (Boegman 2009). These short-term, wind-induced events generate vertical displacements of metalimnetic populations of cyanobacteria, causing episodic changes in the irradiance that reaches the cells, which in turn modify photosynthetic rates with cascading effects on the ecosystem of small and large lakes (Cuypers et al 2010;Pannard et al 2011). …”

Modelling the plankton groups of the deep, peri-alpine Lake Bourget

“…Average surface concentrations of DIP and TP can considered to be very good (r>0.85, Fig.8). The measured DIP concentrations during summer tend to be somewhat higher than the simulations, which might be, to some extent, due to the river uxes that not by internal waves or upwelling events that occur frequently in Lake Bourget (Cuypers et al, 2011), given their low growth rates (Bright and Walsby, 2000) and limited losses imposed by zooplankton (Kurmayer and Jüttner, 1999). On the other hand, their growth at 10-20 m depth (Fig.…”