Effects of the magnitude and persistence of turbulence on phytoplankton in Lake Taihu during a summer cyanobacterial bloom

Abstract

Turbulence, a ubiquitous feature of geophysical fluids and highly variable both in space and time, is a major driver of change in both physicochemical characteristics and phytoplankton communities of the water column. The effects, however, of the magnitude and persistence of turbulence on phytoplankton community structure and dynamics of harmful algal blooms are still poorly understood in lakes. To explore the importance of the kinetic energy and duration of turbulence in phytoplankton, a 15-day mesocosm experiment was carried out and tested under four turbulence regimes namely calm water (control), low (energy dissipation rate, ɛ = 1.12 × 10−6), medium (ɛ = 2.95 × 10−5 m2 s−3) and high (ɛ = 1.48 × 10−4 m2 s−3) turbulence, which are comparable to the natural hydrodynamic conditions in Lake Taihu. Results showed that turbulences promoted the growth of phytoplankton and shifted the phytoplankton community structure from being cyanobacteria-dominated (Microcystis spp.) to diatom-dominated (Fragilaria spp.) community after a lag phase of 8–11 days. However, before the 6th day, the biomass of Microcystis spp. was dramatically promoted under turbulent conditions. These findings suggest that the dominance of diatom may be independent within a certain range of turbulent kinetic energy under constant turbulence conditions. This study suggests that short-term (<6 days) turbulence, regardless of its kinetic energy, is beneficial for the cyanobacterial harmful algal blooms (CyanoHABs) in eutrophic lakes.