Pushing and pulling an algal bloom: Physical controls of diel variability in nearshore phytoplankton biomass

High-biomass microalgal blooms frequently occur in littoral environments worldwide, often causing noxious effects on aquatic ecosystems and coastal communities. Here, we combine field observations and a simple retention–dispersion model to disentangle the short-term ( ∼ hours) environmental drivers shaping the nearshore dynamics of such outbreaks. Temperature, salinity, fluorescence, current velocities, and meteorological variables were measured in the nearshore waters of a coastal location in Mallorca (Balearic Islands) during the summer of 2018. Daily averages from field data were used to adjust wind and buoyancy flow variations into a one-dimensional advection–diffusion model in the cross-shore direction. Results reveal that the interplay between wind forcing and cross-shore density gradients drives an alternating retention–dispersion mechanism, effectively explaining the observed diel chlorophyll variability within the nearshore boundary. This simplified model captures the primary dynamics of the bloom, isolating key factors that influence its behavior and offering practical insights for coastal water quality monitoring and management. • Nearshore microalgae undergo fast fluctuations unexplained by cell growth rates. • Wind and buoyancy driven flows governed the observed chlorophyll fluctuations. • A 1D advection–diffusion model reproduces the observed phytoplankton dynamics. • The transitional circulation pattern is defined by counterrotating convection cells.