SUFB 056: Warming Oceans Can Cause Dead Zones

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Oxygen, much like water and access to Facebook, are necessary requirements for almost all life on earth. If our earth was running out of oxygen…I can’t even complete that sentence because it’s such a outlandish and horrifying scenario that really no comparison can be made with anything that has ever happened before. Which makes it pretty worrisome that the ocean may be facing a similar situation in the near future. A study published just last week in Nature showed that warming oceans most likely were a major driver of hypoxia in oceans during the end of the last ice age.

Hypoxia is the scientific term for areas of low oxygen concentration. You may have heard to these areas in the ocean referred to as “dead zones.” This term is quite fitting because even though we may not think of marine taxa as requiring oxygen the same way that land animals do, many ocean critters would be dead without it. And while natural levels of dissolved oxygen do fluctuate depending on the depth, habitat, and geographic location of a marine ecosystem, certain conditions can lead to an across the board drop in dissolved oxygen. Dr. Summer Praetorius, a postdoctoral researcher at Carnegie Mellon University and the lead author of the Nature study, concluded that rising ocean temperatures were one of these conditions. In fact, during the last ice age between 10,000-15,000 years ago, warming sea surface temperatures contributed to hypoxic conditions that lasted for about 1,000 years.

The link between rising sea surface temperatures and low oxygen levels is complex, but it begins with marine phytoplankton called diatoms. Warm waters are one of the conditions that diatoms love, so as temperatures increase you typically see a similar increase in diatom abundance on the sea surface. And although this population boom has its own threats to the ecosystem, the problem doesn’t really begin until the diatoms die off. Dead phytoplankton sink to the ocean floor, where they are decomposed by bacteria. This decomposition process uses up oxygen, and a lot of decomposition uses up a lot of oxygen. Therefore, these phytoplankton “blooms” are typically followed by a decrease in dissolved oxygen within the surrounding habitat. And while scientists have understood this connection between diatom blooms and low oxygen levels for quite some time, Praetorius’ article was the first to find evidence for rapid ocean warming events during the last ice age and tie those directly to significant hypoxic conditions within the same time frame. Additionally, her team found evidence of a positive feedback loop within these hypoxic areas, suggesting that once a habitat experiences hypoxia, it is difficult for restore oxygen levels back to normal.

Now you may be thinking, “well that’s all fine and depressing, but what does that have to do with me?” Well, we’re seeing the early stages of similar ocean warming events right now, specifically in the Pacific. As I mentioned earlier, other factors also have to be in play before phytoplankton blooms occur. Nutrient levels, specifically nitrate, phosphate, and iron, have to be high enough to support a rapid increase in diatom abundance. Well, the Pacific has two out of three.

Less dissolved oxygen in the water means less oxygen available for fish, sharks, corals, crabs, shrimp, and snails. As these animals die, we’ll start to see a steep decline in global fisheries output and a drastic shift in marine habitats. Fewer people will want to go fishing, whale watching, or scuba diving if there’s less to see, so ecotourism industries will take a hit as well. Overall, the ocean will just become a more desolate place. However, there are steps you can personally take to prevent this situation from happening in your area. Putting on a jacket instead of turning up your thermostat, or biking around town instead of driving are two ways you can cut down on the amount of carbon dioxide being put out into the atmosphere. By reducing carbon emissions, you’ll be working towards slowing down the rate at which sea surface temperatures are increasing. And with everyone doing their own part to prevent rising sea surface temperatures, we can make sure the ocean remains a healthy and habitable ecosystem.

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