Wind‐induced hydrodynamic control of the phytoplankton biomass in a lagoon ecosystem

To provide a closer insight into the relationship between wind‐induced hydrodynamic motion and the biological dynamics of shallow bodies of water, an alternating conditional expectation (ACE) algorithm has been used to examine the nonlinear relationship between the phytoplankton biomass in the Thau lagoon (southern France) and wind intensity. Chlorophyll content (mg m ‒3 ), periodically sampled at diff.erent stations, was used as the dependent variable, and windspeeds (m s ‒3 ) averaged over both short and long intervals preceding each sampling day, were used as predictor variables. Results showed a dome‐shaped relationship between chlorophyll content and mean windspeed. The optimum value of ∼4.0 m s ‒1 was strongly associated with an alternation between the dominant hydrodynamic processes involved, i.e. the beneficial effect of vertical turbulent diffusion, and the detrimental influence of horizontal advective dispersion. Spatiotemporal development of this nonlinear relationship is proposed, based on environmental thresholds, i.e. a depth of <4 m, and a wind‐stress memory of 1–4, 5–8, and 9–13 d. An environmental optimum for phytoplankton enhancement within these time‐space limits is found.