Modeling shows nutrient discharges can influence coastal acidification, hypoxia

A computer modeling initiative to understand how Southern California’s coastal ocean will be affected by ocean acidification and hypoxia has shown that land-based sources of nutrients can have a measurable effect on seawater chemistry in nearshore waters, although more investigation is needed to interpret whether such changes could be significant enough to warrant management action.

The modeling initiative – which is being conducted by a consortium of West Coast researchers that includes the University of California, Los Angeles, University of Washington, National Oceanic and
Atmospheric Administration and SCCWRP – used historical data from the late 1990s to explain how land-based nutrient discharges can measurably exacerbate coastal acidification and hypoxia conditions. The modeling work was presented to SCCWRP member agencies and other stakeholders during an all-day meeting at SCCWRP in May.

The model’s initial findings address a decades-long debate about the influence of land-based nutrient discharges on the coastal ocean vs. the influence of natural upwelling events that transport large volumes of nutrient-rich waters to the shallow coastal zone.

The modeling work found that the land-based nutrient discharges accelerate growth of microscopic algae known as phytoplankton. The increased phytoplankton production, in turn, triggers complex biogeochemical cycling processes that raise pH and dissolved oxygen levels in surface waters, while lowering pH and oxygen levels at depth.

The modeling work in the Southern California Bight is part of a multi-year, West Coast-wide initiative to help managers understand which marine habitats are most vulnerable to ocean acidification and to what extent local, land-based sources of nutrients are exacerbating coastal conditions.

The modeling work involves coupling West Coast physical and biogeochemical ocean models together to understand the relative contributions of global carbon dioxide emissions, natural upwelling processes, and nutrients introduced via wastewater effluent, stormwater runoff and atmospheric deposition. In the Southern California Bight, land-based nutrients are being introduced mainly via discharges of treated wastewater effluent through ocean outfalls.

The model findings presented in May represent the first results to be generated by the computer model. Researchers have spent the past few years developing a downscaled, high-resolution Southern California Bight model and validating its performance using locally collected field data.

During the May meeting, modelers presented the results of initial modeling runs that simulate the influence of human sources of nutrients on coastal seawater chemistry. The runs reflect conditions during the years 1997 to 2000; subsequent runs are planned for the years 2013 to 2016 to reflect current-day nutrient loading scenarios.

Meeting stakeholders agreed to form a subcommittee that will participate in ongoing discussions between scientists and managers about how to interpret the model’s findings. In particular, scientists and managers are working toward consensus on if and/or how the changes in coastal seawater chemistry that are predicted by the model are impacting sensitive marine life. State Water Board staff are leading this stakeholder subcommittee.

Meeting stakeholders also agreed to form a second subcommittee to provide additional input on ongoing efforts to validate the computer model using field data, as well as to use the model to run additional, managerially relevant scenarios. The intensive model validation effort is essential for providing confidence that the model’s predictions can be used reliably for management decision-making. The Orange County Sanitation District is leading this subcommittee.

For more information, contact Dr. Martha Sutula and Dr. Faycal Kessouri.