by Wendy Noreña
The effects of oceanic dead zones and lower dissolved oxygen on marine populations are now generally common knowledge as media reports about fishery devastation and coastal habitat destruction have reached popular media. However, serious scientific inquiries into declining O2 in our oceans have moved beyond the macroscale of events like dead zones and have begun to focus on the day-to-day utilization and depletion of oceanic oxygen in the face of climate change. Deutsch et al. (2015) contribute to future oceanic warming predictions with a metabolic index that puts the combined effects of decreased oxygen and increased temperature into perspective. Using data on four extensively researched marine ectotherms, including an open water fish (Atlantic cod), a benthic crustacean (Atlantic rock crab), a subtropic fish (sharpsnout seabream), and a common eelpout, the researchers calculate a ratio that compares the, “maximum sustainable metabolic rate,” of an oceanic region or depth with the minimum metabolic rate needed for the survival of a defined species. Ultimately, the study finds that we can expect a decrease of 14 to 26% in the habitable ocean regions for the four species outlined in their research and that similar numbers could likely be found for any other species’ data put through their metabolic model.
Deutsch et al. focused on marine organisms that have been widely researched and their individual temperature-dependent hypoxic levels and metabolic requirements are relatively well known, making them ideal subjects for the study model. Deutsch et al. compared the data from these species with data on oceanic temperature and dissolved oxygen, which they took from databases such as the World Ocean Atlas and NOAA, in order to create ratios that compared and demonstrated the relationships between oxygen, temperature increase, species’ tolerance, resilience, and ability to shift up through ocean zones or geographically away from hypoxic areas.
They found that, overall, organisms required an environment that could accommodate 2 to 5 times their resting metabolic rate in order to perform basic survival activities such as hunting, fleeing from predators, and reproducing. Interestingly enough, this correlates with metabolic rates used by terrestrial species, which use 1.5 to 5 times their resting metabolic rate for similar survival activities. They also found that while decreased oxygen levels are a major concern, increased temperature will actually be responsible for 2/3 of future hypoxic marine conditions. Additionally, because higher temperatures increase metabolic rates which in turn increase the amount of oxygen needed, temperature will also be a main reason that native marine habitats will no longer be inhabitable in the future. Ultimately, because conditions around the equator already tend to be hypoxic, it is likely that marine species will start to move towards the poles, which have much higher levels of dissolved oxygen. That being said, even these polar regions will be negatively affected by the decrease in O2, not only because oxygen will generally decrease but because migrations towards the poles from equatorial regions will likely result in increased competition and ecosystemic shifts.
However, Deutsch et. al suggest that some of these negative projections may be offset by other climate change effects such as sea level rise and, “habitat expansions,” or even by species and ecosystem-driven changes like evolutionary adaptations, general resilience, and acclimation. Despite these hopeful suggestions, the overall results seem to be that hypoxic ocean conditions are soon to be standard, especially in the equatorial regions. What this means on a global scale is yet to be known, as the four species sampled were all from the Atlantic. Deutsch et. al offer their metabolic index as a simple way to map potential changes in a species’ distribution and suggest that if the index is used widely, the collective data could be used to generate an overall better understanding of what awaits our oceans in the future.
Deutsch, C., Ferrel, A., Seibel, B., Pörtner, H.-O., Huey, R.B., 2015. Climate change tightens a metabolic constraint on marine habitats. Science (80). 348, 1132–1135