Nutrients from human waste boost ocean acidification & hypoxia in CA―wastewater treatment agencies say regulation is premature, environmentalists say it’s overdue & researchers say it’s “fair to ask hard questions” about the science.
By Robin Meadows
The State Water Resources Control Board is exploring regulating nutrients emitted from Southern California wastewater treatment plants into the ocean. The controversial move is prompted by concerns that these discharges may accelerate acidification and oxygen loss in the region’s coastal waters, harming nearshore marine life.
The wastewater treatment industry says this nutrient regulation is premature. Environmentalists say it’s overdue. To help inform decision-making, scientists are investigating this issue further.
“This is a hotly debated topic right now,” says Martha Sutula, who leads biogeochemical studies at the Southern California Coastal Water Research Project, a research institute dedicated to improving management of aquatic systems. The institute’s members include agencies that treat wastewater, manage stormwater, regulate water quality, and set ocean policy.
Wastewater effluent from 23 million people is piped offshore in Southern California. The resulting acidity boost could be enough to start dissolving the shells of crabs and small snails called pteropods, which swim near the ocean surface and are a favorite food of many fish and whales. And the resulting oxygen depletion could deprive anchovies, which many commercial fish eat, of their habitat.
OCEAN ACIDIFICATION AND OXYGEN LOSS
Climate change drives most of the ocean’s increasing acidity and decreasing oxygen (hypoxia). When carbon dioxide emitted into the air dissolves in the ocean, some of it reacts with seawater to produce carbonic acid. In turn, carbonic acid reacts with and so depletes carbonate, a compound that many marine organisms need to make shells. Warming causes hypoxia directly: as ocean temperatures rise, oxygen levels fall because warmer water holds less of this gas.
Ocean acidification and hypoxia can be exacerbated by nutrients from human sources, notably those discharged from wastewater treatment plants. These excess nutrients, which come primarily from nitrogen-containing compounds in human waste, can trigger chronic algal blooms. As the algae die and decompose, the bacteria that break them down both consume oxygen and release carbon dioxide that further acidifies the water.
NUTRIENTS IN THE SOUTHERN CALIFORNIA BIGHT
Sutula is part of a team studying the effects of nutrients on the Southern California Bight, an embayment that stretches hundreds of miles from Santa Barbara to San Diego. “We were curious about the long-held assumption by scientists that nutrients from humans are small compared to those from natural ocean sources like upwelling,” she says. Upwelling occurs along the coast when winds push surface waters away, bringing nutrient-rich waters up from the ocean’s depths.
Based on an ocean model, the team predicted that the assumption that most nutrients in the Southern California Bight came from upwelling was false in nearshore waters. A 2014 study by Sutula and colleagues revealed that about half of the nutrients near major metropolitan areas came from human sources, which are primarily wastewater effluent but also include agricultural runoff, industrial discharges, and fossil fuel combustion.
Next the researchers developed the model further to tease apart the various drivers of acidification and oxygen loss in the Southern California Bight, including nutrients from upwelling and climate change versus those from human sources. The model predicted that the nutrients from human sources were exacerbating acidification and oxygen loss in coastal waters, according to a 2021 study.
BIOLOGICAL EFFECTS OF EXCESS NUTRIENTS
Wastewater treatment managers then requested follow-up work, which led to a trio of studies that included delving deeper into the biological effects of nutrients from wastewater outfalls. One of these studies was published last year, and the second and third have been submitted for publication but have not yet been peer-reviewed.
These studies predicted that the extra acidity and oxygen loss due to nutrients from human sources was enough to seasonally shrink the habitat available to marine life in the top 200 meters of the ocean. These surface waters are home to most fish that are harvested commercially as well as many protected marine mammals and sea turtles. Importantly, surface waters are also rich in these species’ prey, such as the sea snails and anchovies that are sensitive to acidification and oxygen loss.
Specifically, nutrients from human sources make the habitable part of the surface waters considerably shallower for sea snails, anchovies and other sensitive species in an area near Santa Catalina Island about three months per year, peaking during the late summer and early fall. The affected area measures thousands of square kilometers, and the vertical loss of habitat in this region averages about 25% but can be as much as 60% of the depth that would otherwise be available to species sensitive to low pH, a measure of acidification, and oxygen levels.
“The predicted levels are not low enough to kill the animals,” Sutula says. “But it might make them swim away if they can or spend more energy coping with those conditions if they can’t.”
NEXT STEPS
Now an independent expert panel is reviewing the model, drilling down on sources of uncertainty in its predictions. “We’re confident in the science we’ve published but there are lots of nuances,” Sutula says. “It’s fair for people to ask hard questions about whether these model findings can be trusted.”
Uncertainties include how well the model reflects the real world, which can be ascertained by comparing its predictions to observations. The team did one round of model validation in a 2021 study, and is currently doing a second round.
Another question is how sensitive predicted acidity and oxygen levels are to each of the various factors accounted for by the model, such as the growth rates of phytoplankton, tiny plants collectively called algae, as well as the grazing rates of zooplankton, tiny creatures that eat algae.
There are also nuances beyond these uncertainties. For example, raw sewage spills from Mexico make their way into Southern California waters, raising questions about whether regulating nutrients from San Diego’s wastewater treatment plants would do any good.
A key consideration is that the model has run only all-or-nothing scenarios, where either every wastewater treatment plant in the region discharges all its nutrients into the ocean or none of them discharge any nutrients at all. Next steps will include running more realistic scenarios―such as cutting nutrients by 50 percent from many of the treatment plants versus by 90 percent from the four largest treatment plants―to help determine what can be done and how well it will work.
Moreover, reducing nutrients in wastewater effluent may not be the only option for protecting marine life from acidification and oxygen loss. Sutula and colleagues are using the model to investigate alternatives, such as using the nutrients in outfalls to grow kelp or shellfish, which “are powerful filter feeders that hoover up algal blooms.” Another possibility is using methods such as electrolysis to remove the excess carbon dioxide that acidifies the ocean.
TO REGULATE OR NOT TO REGULATE
“There are lingering doubts with a lot of this,” says Jared Voskuhl, who manages regulatory affairs for the California Association of Sanitation Agencies, which represents more than 130 wastewater treatment agencies.
To Voskuhl, resolving these issues before regulating the nitrogen-containing nutrients in wastewater effluent is prudent. “Upgrading to denitrify is incredibly expensive―we need some certainty if the model is going to be used to require this,” he says, adding that the cost of upgrades could run into the tens of billions of dollars.
But Sean Bothwell, who directs the environmental nonprofit California Coastkeeper Alliance, has no doubts. “The science is alarming,” he says. “In my opinion, it’s clear that wastewater treatment discharges are creating ocean acidification and hypoxia hotspots off the coast.”
“Ocean acidification is pretty devastating to the coast, and land-based inputs are just making it worse,” he continues, explaining that acidification is already dissolving the shells of plankton. These tiny organisms are at the bottom of the food web, so plankton losses could eventually affect the many species higher up the web.
Both Voskuhl and Bothwell serve on the steering committee that chose expert panelists for the independent review of the model, and Sutula welcomes the opportunity to improve confidence in the model. The implications go far beyond ocean acidification and oxygen loss. “Nutrients are not the only challenging issue confronting us,” she says. “The model also can help us grapple with issues like climate change, microplastics, and kelp bed loss.”
