Enough air in the room: Monitoring hypoxia in Cordell Bank National Marine Sanctuary
By Elizabeth Weinberg
Suck all the oxygen out of a room and it's impossible to breathe. In the ocean, reduction of oxygen can also be a major problem -- so scientists in Cordell Bank National Marine Sanctuary are working to track and understand hypoxic conditions.
In extreme cases, hypoxia, or low-oxygen conditions, can cause die-offs of fish, shellfish, corals, and other organisms, which, like us, require oxygen to survive. More commonly, animals may experience physiological stress in low oxygen conditions. Animals may also avoid areas that are prone to hypoxia, reducing biodiversity.
Around the world, hypoxic areas have been increasing in severity and occurring more often, because hypoxic conditions are linked to the effects of climate change. As we burn fossil fuels, we release excess carbon dioxide into the atmosphere. That carbon dioxide builds up and acts as a heat-trapping blanket, warming the planet, and, in turn, the ocean. This warming acts on the ocean in several ways. First, warm water simply holds less oxygen. Second, oxygen exchange between the atmosphere and the ocean occurs at the surface and the oxygenated surface water later mixes into deeper waters. But as surface water warms, it's less likely to mix with colder, deeper water, leaving that deep water depleted in oxygen.
In addition to warming, climate change causes other large scale changes in weather and the ocean. For example, climate change may shift seasonal upwelling patterns, which bring nutrient-rich water to the surface and cause mixing of the oxygen-rich surface water with deeper water. Upwelled nutrient-rich water fuels algae blooms, which then decompose in deeper water where they consume oxygen. Upwelling can also bring oxygen-poor water to the surface.
In recent years, seasonal and episodic hypoxia has been observed on continental margins, shelves, and estuaries that were previously well-oxygenated. Hypoxic conditions have been observed frequently in the Pacific Northwest since 2002, and less severe events have been observed along the north-central California coast -- meaning national marine sanctuaries like Cordell Bank and Greater Farallones are at risk.
Danielle Lipski, research coordinator at Cordell Bank National Marine Sanctuary explains that "If long-term, large-scale hypoxic conditions were to occur in the sanctuary, it could be devastating to the sanctuary's abundant marine life." The deep-water reef of Cordell Bank is inhabited by a vibrant invertebrate and rockfish community, which could be vulnerable to hypoxic conditions.
With that in mind, since 2013, researchers from Cordell Bank National Marine Sanctuary and UC Davis Bodega Marine Lab have recorded hypoxic conditions at moorings in these sanctuaries. "Cordell Bank is a special place, and many of the organisms inhabiting this place are susceptible to low levels of oxygen," says Dr. John Largier, professor of oceanography and associate director for research at Bodega Marine Lab. Particularly because Cordell Bank is offshore, he explains, the sanctuary is far from human influences and coastal runoff; this makes it a particularly good location to study the impact of climate-change-driven hypoxia. "These deep data from Cordell Bank will help us gain a more profound understanding of hypoxia and future ocean conditions in our region."
Researchers deploy instruments for several months at a time each year, during the upwelling season from spring until fall. These instruments record dissolved oxygen concentration and temperature in both shallow and deep waters in Cordell Bank, and more recently have been fitted to track salinity.
Lipski explains that in the last three years, hypoxia conditions in Cordell Bank have varied based on weather, and by season and year. In some cases, hypoxic conditions have been observed for weeks at a time. Researchers are still evaluating what is driving the hypoxia, but it appears to be influenced by both local conditions like upwelling, and larger phenomena like overall sea surface warming.
Hypoxia is just one factor influencing Cordell Bank's marine communities. And as a global phenomenon spurred on by climate change, hypoxia can be a difficult issue to manage on the local level. Lipski says the first step is for sanctuary scientists to best understand the conditions the sanctuary experiences and the factors influencing hypoxia. Then, she says, "our best approach as stewards of Cordell Bank National Marine Sanctuary is to protect the sanctuary from other stressors that may happen at the same time but that can be managed locally." That includes making sure that intact habitats are maintained, that water quality doesn't suffer from impacts like pollutants, and that biological communities remain strong.
By protecting ocean ecosystems, Lipski says, "we maintain their resilience and give them the best chance to thrive in the face of changing global conditions."
Funding for this project is provided by the Cordell Marine Sanctuary Foundation.
Elizabeth Weinberg is the social media coordinator and writer/editor for NOAA's Office of National Marine Sanctuaries.