Source: Journal of Geophysical Research: Oceans
Climate change is driving the oceans to lose oxygen. Marine organisms that need oxygen to survive live in a gradually shoaling, or shallowing, zone of water above a hypoxic, low-oxygen layer. Researchers have studied the long-term deoxygenation trend in marine ecosystems, but investigations on how shorter, transient events can affect ecosystems on weeks- to months-long timescales are lacking.
Now, a new study by Köhn et al. looks at when and where these hypoxic shoaling events occur. These so-called transient habitat reduction extreme events (THREEs) can change biogeochemical processes or alter entire ocean ecosystems. To find THREEs, which are rare because their detection requires data on changes in the hypoxic layer, the researchers used a simulation model to look at data from the eastern Pacific Ocean because it features a vast area of horizontal hypoxic waters that are driven by physical and biogeochemical processes. They detected THREEs by applying a fixed threshold depth for the hypoxic layer. Each event was also characterized in time and space, and drivers were identified.
They found that THREEs compress the oxygenated zone by up to 50%–70% in subtropical and tropical regions. La Niña events appear to precondition the waters for THREEs. As a result, in subtropical regions, THREEs occur primarily during boreal winter (December–February) and spring. In the subtropical eastern Pacific, THREEs appear to be associated with mesoscale eddies, which are known as hot spots for low-oxygen conditions, and occur independently of season. The team also noted that 71% of THREEs go along with cold, low-pH, shoaling waters. These events—low oxygen and low pH—can compound the stressors on fish and other marine organisms.
These findings show how THREEs could be detected in other open-ocean locations to better understand water column biogeochemistry and ocean ecosystems. The authors note that THREEs can also foreshadow long-term changes and shifts in ocean habitats. (Journal of Geophysical Research: Oceans, https://doi.org/10.1029/2022JC018429, 2022)
—Sarah Derouin, Science Writer