The role of surface vertical mixing in phytoplankton distribution in a stratified reservoir

Abstract

We investigated convection caused by surface cooling and mixing attributable to wind shear stress and their roles as agents for the transport of phytoplankton cells in the water column by carrying out two daily surveys during the stratified period of the Sau reservoir. Green algae, diatoms, and cryptophyceae were the dominant phytoplankton communities during the surveys carried out in the middle (July) and end (September) of the stratified period. We show that a system with a linear stratification and that is subject to weak surface forcing, with weak winds ,4 m s 21 and low energy dissipation rate values of the order of 10 28 m 2 s 23 or lower, enables the formation of thin phytoplankton layers. These layers quickly disappear when water parcels mix because there is a medium external forcing (convection) induced by the night surface cooling, which is characterized by energy dissipation rates on the order of ,5 3 10 28 m 2 s 23 . During both surveys the wind generated internal waves during the entire diurnal cycle. During the day, and because of the weak winds, phytoplankton layers rise in the water column up to a depth determined by both solar heating and internal waves. In contrast, during the night phytoplankton mixes down to a depth determined by both convection and internal waves. These internal waves, together with the wind-driven current generated at the surface, seem to be the agents responsible for the horizontal transport of phytoplankton across the reservoir.

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

“…Similar findings have been already reported for cyanobacteria living in the top few meters of the water column (Olsen et al, 2000;Welker et al, 2003) and more generally, several papers have highlighted horizontal mass transfers occurring in phytoplankton and zooplankton communities living in the epilimnetic layer (Verhagen, 1994;Rinke et al, 2007;Serra et al, 2007). However, the main difference between this direct impact of the wind on phytoplankton and its indirect impact through internal waves is that the impact of internal waves persists for several days after the end of the wind event.…”

Lake‐wide distributions of temperature, phytoplankton, zooplankton, and fish in the pelagic zone of a large lake

“…By extrapolating the water current velocities measured by the drifters, it can be estimated that during the 2 d of its highest intensity the storm provoked horizontal displacements over distances of more than 25 km, which fits with our observations. A steering role of hydrodynamic processes on the distribution of pelagic organisms is well documented (George and Winfield 2000;Marce et al 2007;Serra et al 2007). An indirect effect from wind forcing is mediated by internal wave activity inducing local upwelling (Fig.…”

“…In line with the MDFH, however, besides these strong biotic control factors, abiotic processes also act simultaneously on emerging vertical distribution patterns, for instance, windinduced vertical mixing (Webster and Hutchinson 1994;Serra et al 2007) or internal seiching (Marce et al 2007;Rinke et al 2007). Finally, it may also appear that the time scales of large-scale transport processes overlap with ecological timescales of phytoplankton growth and community dynamics, i.e., dynamics in spatial distribution of organisms possibly affect population dynamics (Reynolds 1990;Rojo and Alvarez-Cobelas 2001).…”

The effects of river inflow and retention time on the spatial heterogeneity of chlorophyll and water–air CO<sub>2</sub> fluxes in a tropical hydropower reservoir

“…As sedimentation and light availability increase along a reservoir, the biomass of the primary producers may increase. Phytoplankton is distributed in patches along the reservoir due to differences in the nutrient distribution, light availability and stratification (Serra et al, 2007). Additionally, hydrodynamics factors, such as retention time and river inflow, may influence the phytoplankton communities and their growth (Vidal et al, 2012;Soares et al, 2008).…”