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

Abstract

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.

Environmental Drivers of Benthic-Interior Exchange Events Over the Continental Shelf of Monterey Bay, California

“…The resting position for the profiler was at the bottom of the cable, 9 m above bottom (at 60 m depth). For further details on the operation of the autonomous profiler, see andSevadjian et al, (2015).…”

“…I also performed a spatial analysis of surface wave-induced bed shear stress ( w ) spanning the entire southern bay shelf (20 to 100m isobaths) to illustrate how w varied. I then passed the model outputs through lagged cross-covariance functions and probabilistic statistical classification models (described below) to characterize the relationship between observations of detached SPM layers and frictional bottom shear stress and to determine whether the occurrence of detached SPM layers was directly related to local sediment resuspension events.Optical Peak Identification AlgorithmThe temporal variability and vertical distribution of observed detached SPM layers (first and second questions driving this study) were determined by a MATLAB algorithm designed following methods by), Sullivanet al, (2010 andSevadjian et al, (2015). I designed the algorithm to identify detached optical peaks of SPM (i.e., elevated SPM overlying clearer water) in individual vertical profiles collected by the profiler by examining the fine-scale vertical structure of corrected beam attenuation values.…”