Along coastlines around the world, streams of fresh water pour into the sea. Those meeting points often contain lower-than-normal dissolved oxygen levels—a condition known as hypoxia.
Hypoxia can arise naturally, but scientists have noticed that human activity is causing these low-oxygen zones to grow and intensify. The worst hypoxic zones are called dead zones because the lack of oxygen drives away most life. Last year, the world’s largest dead zone—covering an area as large as the state of Florida—was confirmed in the Arabian Sea.
As our planet changes, scientists must both assess the “new normal” and predict how conditions will continue to shift. In a new study, Foster and Fulweiler investigated how estuary sediments interact with oxygen levels in water to affect nutrient levels and greenhouse gases. They used a review of previous research, practical experiments, and predictive models to gain a fuller understanding of the dynamics of hypoxia.
Over 3 years, the research team collected cores from a shallow, temperate estuary on Massachusetts’s Cape Cod. They carefully siphoned the water off the cores and replaced it with water from the same source, filtered to remove biota and adjusted to normal or mildly hypoxic oxygen levels.
The scientists found that hypoxic conditions had mixed effects. When oxygen levels were low, nitrogen and other nutrient fluxes did not change, but more phosphorus was released into the water column, a finding that is consistent with previous studies. The higher phosphorus levels and thus altered nutrient ratios could fuel phytoplankton or algae blooms and potentially extend the growing season of the estuary’s flora. Such blooms are responsible for the world’s largest dead zones.
The researchers also discovered that sediment was unable to take up the same levels of nitrous oxide, a potent greenhouse gas and ozone-depleting substance. In hypoxic conditions, sediment removed only half as much nitrous oxide as it did at normal oxygen levels.
Surprisingly, sediment released less methane—another powerful greenhouse gas—in mildly hypoxic conditions. The authors propose that the lower oxygen levels in the experiment boosted anaerobic, methane-consuming reactions. However, in prolonged hypoxia, they anticipate that anaerobic, methane-producing reactions will outpace the methane-consuming ones. In this scenario, there will be a net increase in methane released from the sediment as hypoxia increases.
The scientists combined their results with those of numerous other studies to predict the sediment trends they studied not only for short, mild hypoxic episodes but also for longer and more intense stretches. They noted that hypoxia’s effects were dynamic; more intense hypoxia did not have the same trends as milder conditions. As a result, they warn that there could be unknown thresholds in hypoxia-sediment interactions, and the effects of hypoxia on estuaries worldwide will likely be dramatic and varied. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1029/2018JG004663, 2019)
—Elizabeth Thompson, Freelance Writer