Glider and remote sensing observations of the upper ocean response to an extended shallow coastal diversion of wastewater effluent

Abstract The Orange County Sanitation District (OCSD) diverted wastewater discharge (5.3 × 10 8 l d −1 ) from its primary deep (56 m) outfall8 km offshore, to a secondary shallower (16 m) outfall1.6 km offshore for a period of three weeks. It was anticipated that the low salinity and density of the effluent would cause it to rise to the surface with limited dilution, elevating nutrient concentrations in near-surface waters and stimulating phytoplankton blooms in the region. Three Teledyne Webb Slocum glidersand a Liquid Robotics surface wave gliderwere deployed on transects near the outfallsto acquire high spatial and temporal coverage of physical and chemical parameters before, during, and after the wastewater diversion. Combinedautonomous underwater vehicle (AUV) and MODIS-Aqua satellite ocean color data indicated that phytoplankton biomass increased in the upper water column in response to the diversion, but that the magnitude of the response was spatially patchy and significantly less than expected. Little evidence of the plume or its effects was detectable 72 h following the diversion. The effluent plume exhibited high rates of dilution and mixed throughout the upper 20 m and occasionally throughout the upper 40 m during the diversion. Rapid plume advection and dilution appeared to contribute to the mutedimpact of the nutrient-rich effluent on the phytoplankton community in this coastal ecosystem.