Coupled physical, chemical, and microbiological measurements suggest a connection between internal waves and surf zone water quality in the Southern California Bight

Abstract

Internal waves have been implicated in the cross-shore transport of scalars such as larvae, nutrients, and pollutants at locations around the world. The present study combines physical measurements with a comprehensive set of surf zone water quality measurements to evaluate the possible impact of cross-shore internal wave transport on surf zone water quality during two study periods. An array of oceanographic moorings was deployed in the summer of 2005 and 2006 at 10-20 m depth offshore of the beach to observe internal waves. Concurrently, surf zone water quality was assessed twice daily at night at an adjacent station (Huntington State Beach) by measuring concentration of phosphate, dissolved inorganic nitrogen (DIN), silicate, chlorophyll a, fecal indicator bacteria (FIB), and the human-specific fecal DNA marker in Bacteroidales. The baroclinic component accounted for about 30% of the total variance in water column velocity, indicating the importance of density-driven flow during the summer when the water column was stratified. Arrival of cold subthermocline water in the very nearshore (within 1 km of the surf zone) was characterized by strong baroclinic onshore flow near the bottom of the water column. The near bottom, baroclinic, cross-shore current was significantly lag-correlated with the near bottom temperature data along a cross-shore transect towards shore, demonstrating shoreward transport of cold subthermocline water. Wavelet analysis of temperature data showed that non-stationary temperature fluctuations were correlated with buoyancy frequency and the near bottom cross-shore baroclinic current. During periods of large temperature fluctuations, the majority of the variance was within the semi-diurnal band; however, the diurnal and high frequency bands also contained a substantial fraction of total variance. The bottom cross-shore baroclinic current was proposed as a proxy for shoreward transport potential by internal waves and was positively correlated with phosphate concentration in both years, silicate in 2005, and fecal indicator bacteria measurements in 2006. The results suggest internal waves are an important transport mechanism of nutrient-rich subthermocline water to the very nearshore in the Southern California Bight, and may facilitate the transport of FIB into the surf zone or enhance persistence of land-derived FIB.

Connecting wind‐driven upwelling and offshore stratification to nearshore internal bores and oxygen variability

“…[]). We note that the Monterey Bay study site contains steeper bathymetric slopes and a more complex shoreline compared to the gradually sloping bathymetry typically found in Southern California [e.g., Pineda , ; Wong et al ., ] resulting in slightly different bore dynamics [see Walter et al ., ]. Nonetheless, this period of the bore event consistently has the strongest cross‐shelf velocities, and previous observations indicate that elevated levels of diapycnal mixing in the stratified interior are observed during the warm‐water relaxations [ Walter et al ., ].…”

A new offshore transport mechanism for shoreline‐released tracer induced by transient rip currents and stratification

“…Farther offshore on the inner shelf, other potential offshore transport mechanisms may be active. For example, wind [e.g., Connolly et al , ; Shanks and Brink , ], eddies [e.g., Mullaney and Suthers , ], tides [e.g., Luettich et al , ], and internal waves [e.g., Pineda , ; Wong et al , ] may be important physical transport drivers on the shelf. Upwelling‐induced cross‐shelf circulation has been hypothesized to sweep early‐stage larvae offshore [ Roughgarden et al , ; Connolly et al , ].…”

Episodic vertical nutrient fluxes and nearshore phytoplankton blooms in Southern California

“…Crossshore (u) and alongshore (v) currents near M18 were measured with a bottom-mounted Acoustic Doppler Current Profiler (ADCP, 600 kHz, www.rdinstruments. com) from June to October (Wong et al 2012). The ADCP data were averaged to 6-min intervals with 1-m vertical bins.…”