In autumn 2012, Orange County Sanitation District (OCSD) will divert ~150 million gallons/day of secondarily-treated effluent to a nearshore (1 mile offshore) outfall pipe over a period of ~4 weeks. No discharges of this magnitude have been conducted in decades. The planned diversion is expected to create a buoyant surface plume that will spread over much of the coastal region. Because OCSD plans to "super-chlorinate" and then dechlorinate the discharge, the effect of the plume should be predominantly a nutrient addition rather than direct addition of intact microbial populations. The PIs propose to address two broad questions through a study of the plume. First, what happens ecologically and physiologically to the phytoplankton assemblage when nutrients are discharged in the surface ocean for extended periods of time? Second, can this dynamic and shifting environment be sampled by deploying multiple technologies to identify the physical/chemical drivers of the biological response at ecologically relevant space and time scales? They will test two hypotheses: H1: Continual discharge of nutrients to the surface ocean results in a dinoflagellate-dominated bloom which leads to dampening or cessation of vertical migration of the dinoflagellates and drives a shift to net heterotrophy. H2: The bloom will initially result in a strong local sink for carbon dioxide which gradually develops into a strong source as heterotrophy develops.
The study is expected to provide a time-evolving picture of interactions within and between autotrophic and heterotrophic communities and will illustrate the short-term biogeochemical and ecological consequences of sustained nutrient discharge to a shallow coastal site. The planned diversion provides an unprecedented opportunity to study the ecophysiological response in a natural setting over a period of weeks, including the interaction of biology, chemistry, and physics, and it will contribute to basic understanding of anthropogenic nutrient loading to the coastal ocean. Undergraduate and graduate education and training will be furthered through active participation in lab, field, and data synthesis activities involving academic, government, and industry partners.
In September 2012, the Orange County Sanitation District (OCSD) conducted a planned wastewater diversion in order to make repairs to their primary outfall. The diversion, lasting from 11-Sept. to 3-Oct. 2012, resulted in the discharge of 5.3x108 liters per day (138 million gallons per day) of highly treated effluent through their secondary outfall much closer to shore. The secondary outfall has both a diffuser and an open end; the result was that the highly buoyant plume rose to the surface quickly and violently, entraining adjacent oceanic water and leading to a turbulent surface ``boil" 50-100m in diameter, which was visible throughout the diversion. Plume monitoring during the OCSD diversion included shipboard, mooring, and autonomous assets. Surveys were designed to map the extent and position of the plume as well as its chemical, biological, and physical characteristics. Shipboard operations provided near-synoptic measurements over scales >10 km, although the sampling locations were necessarily sparse in space. Anticipating strong stratification during the 2012 Diversion, and given the demonstrated importance of small-scale physical flows on the fate and dispersion of the effluent plume in the area, we deployed a set of three wave-powered profiling moorings (the Wirewalker, a product of the Ocean Physics Group, Scripps Institution of Oceanography). The Wirewalker is capable of continuous profiling with a repeat profile every few minutes over the course of weeks, with vertical resolution on scales <1m. The Wirewalker moorings carried high resolution physical and optical instrumentation, and were deployed in association with moorings maintained by OCSD and the Monterey Bay Aquarium Research Institute. The rapid, continuous profiling resulted in a unique view of the distribution and evolution of the effluent plume and receiving waters over the course of the 2012 diversion. We collected a total of 39,759 profiles of temperature, salinity, denisty, pressure, and chlorophyll fluorescence our the course of the deployment. This corresponds to 1,940 km of profiles at the 3 moorings over the deployment (roughly the distance from Los Angeles to US/Canada border). These data gave us an unprecendented view of the evolution of the wastewater plume in the shallow, coastal waters of the Southern California Bight. These dynamics are critical to understanding and predicting the influence of wastewater discharge on the heavily utlized coastlines of the United States. We observed the fine-scale vertical variability of a wastewater plume discharged into shallow, stratified coastal waters. The effluent plume occupied a variable portion of the water column, but was typically near the surface. The plume was characterized by small vertical scales away from the surface, while complicated patterns of vertical temperature and salinity compensation were found in the plume above the pycnocline. Far from the outfall, the plume acted as a passive tracer. At times, we apparently observed the final stages of near-field mixing at the profiling mooring closest to the outfall. Complicated vertical patterns in plume water temperature and salinity compensation and near-surface mixing dynamics demonstrated the complexity of the plume dilution process even in a surfacing plume. The co-occurence of elevated chlorophyll fluorescence and plume waters was evident in the later part of the diversion period, but the overall response of the phytoplankton to the effluent diversion was weak. There were no apparent physical barriers between the plume and ambient waters, and, given the elevated nutrients available in the plume, processes other than the physics of plume evolution were likely inhibiting the phytoplankton response to the wastewater diversion.
