Erika McPhee-Shaw

Vertical convergence of resuspended sediment and subducted phytoplankton to a persistent detached layer over the southern shelf of Monterey Bay, California

Work done by the present authors and collaborators in the first year of a 2-year field study established the frequent presence of layers of suspended particulate matter detached from the seafloor over the southern shelf of Monterey Bay, California. In this contribution, we document similar findings over a 1-month period in Fall 2012 and investigate physical processes leading to vertical convergence of particles to the observed layers, both from resuspended sediment originating below the layer depth and from phytoplankton originating above the layer depth. Physical and optical vertical structure was measured by an autonomous vertical profiler, thermistor chain, and acoustic Doppler current profiler, and optical sensors fixed to a bottom-mounted frame measured beam attenuation and particle size distribution within the benthic boundary layer (BBL). These data support a conceptual model for layer formation in which (1) bottom material was mobilized into the BBL by semidiurnal internal tidal currents; (2) brief “updraft” events regularly injected particles into the interior water column during the downslope phase of the semidiurnal internal tide; and (3) particles converged at the detached layer due to a measured, but previously unreported, long time scale mean vertical convergence in flow. Subduction and vertical mixing of phytoplankton to the subeuphotic layer were also observed and are presented in two case studies in connection with horizontal convergences of surface water masses.