Boundary influences on HAB phytoplankton ecology in a stratification-enhanced upwelling shadow

Coastal marine ecosystems are profoundly influenced by processes that originate from their boundaries. These include fluid boundaries—with the atmosphere, oceanic boundary currents and terrestrial aquatic systems, as well as solid boundaries—with the seafloor and coast. Phytoplankton populations transfer complexly interacting boundary influences into the biosphere. In this contribution, we apply data from an ocean observing and modeling system to examine boundary influences driving phytoplankton ecology in Monterey Bay, CA, USA. The study was focused on species that may cause harmful algal blooms (HABs). During September–October 2010, autonomous molecular analytical devices were moored at two locations characterized by different degrees of stratification and exposure to upwelling dynamics. The time-series revealed multiple transitions in local HAB phytoplankton communities, involving diatoms (Pseudo-nitzschia spp.), dinoflagellates (Alexandrium catenella), and raphidophytes (Heterosigma akashiwo). Observational and model results showed that the biological transitions were closely related to environmental changes that resulted from a variety of boundary processes—responses of oceanic circulation to wind forcing, influxes of different water types that originated outside the bay, and emergence of strongly stratified nearshore water into the greater bay. Boundary processes were further implicated at patch scales. High-resolution mapping and sampling of a phytoplankton-enriched patch were conducted in a Lagrangian framework using autonomous underwater vehicles. These highly resolved measurements showed that small-scale spatial patterns in the toxicity of Pseudo-nitzschia populations were related to the coupling of resuspended sediments from the bottom boundary layer to the surface mixed layer.