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Could low oxygen levels be just as threatening to Florida's coral reefs as overheated seas?

Scientists expect increasing marine heat waves to cause coral bleaching, which can result in reefs dying off.
Scientists expect increasing marine heat waves to cause coral bleaching, which can result in reefs dying off.

Florida's coral reefs have been decimated by record high temperatures, spurring researchers to relocate some species to onshore tanks. But lowered levels of oxygen may be as much of a threat to their existence.

WUSF's Steve Newborn talks about this peril with Andrew Altieri. He's an assistant professor in environmental engineering sciences at the University of Florida, and associate director at the university's Center for Coastal Solutions.

SN: Could you give our audience a general overview of the research you've been working on?

AA: Generally, the reefs that people are familiar with, say in the Florida Keys are pretty well oxygenated. We've been conducting some monitoring there that's essentially verified that the latest sort of heatwave that we're having in coastal waters around the Keys and South Florida are definitely a threat directly to the corals. But the sort of hidden or kind of compounding threat is that the oxygen in the water may be lowering to dangerous levels in association with that heatwave.

 Andrew Altieri
Zoom screen capture
Andrew Altieri

Now, that's something that's in progress. And we're monitoring that with instruments. We have oxygen loggers, as well as temperature loggers and devices that measure other parameters as well, such as pH. But I don't have the data yet to understand exactly just how low the oxygen levels might be going. But typically, when temperatures go up, oxygen goes down in the ocean, and marine life becomes more sensitive to it. Research is ongoing. And so we are conducting experiments, and we are analyzing interpreting data from prior experiments. And we're collecting data from the field in terms of the conditions there. And we'll know even more six months to 12 months from now once we're able to kind of go through our records as well as go through the results of experiments that are ongoing in the laboratory.

But there is a general understanding that I can share with you. And that is that when temperature goes up, oxygen typically goes down. And related to that is that the demand for oxygen by marine life goes up as well, in association with increasing temperatures, I can kind of quickly run through this relationship between temperature and oxygen that I think is important to not just corals, but other marine life as well. And so the first is that just the laws of physics is that as temperature goes up in water, its capacity to hold oxygen decreases. And that's the same principle you see, in action, when you have a warm soda that goes flat much faster, it's not able to hold the carbonation for nearly as long it loses it much more quickly. And so that's essentially what's happening in the oceans as well - when the temperature goes up, the amount of oxygen that it's able to hold on to goes down.

So specifically with the corals right now, how much of a decline in the oxygen levels are we seeing? Can you quantify that?

The oxygen levels are likely decreasing as temperature goes up. And we've seen that because we've got a multi-year record from the Florida Keys at multiple sites. And we see that there's essentially a seasonal cycle, where as temperatures spike in the summer, that typically oxygen dips a little bit. And right now, what appears to be happening is we have some of the warmest temperatures, recorded at least recently in and around the Florida Keys. And as a consequence, the oxygen levels are probably dropping even lower than in recent summers, at least while we've been recording our data. If the trend from the past few years continues, once you get below about 5 milligrams per liter, scientists kind of pay attention. But it's usually a right around 3 milligrams per liter where scientists start to get really concerned, because that's where a broad group of organisms start to show responses.

What is the normal level the optimum level for the coral reef in particular in the wintertime? I'm trying to see how much it is declining.

When we've looked at our records for the past few years, we see that in the winter, the oxygen levels are typically 6, maybe 6 to 7 milligrams per liter at most sites in the summer. It can definitely dip below 5 milligrams per liter, and it can get even lower for short periods of time.

So in essence, what we're seeing here is a halving of the amount of oxygen in the water at this depth.

Yeah, it could be getting to that extreme. What I would say is that in a typical summer, the oxygen levels are dipping down to a level where someone like me starts to pay attention. And so, you know, what I'm worried about is in a year like this where it's even warmer, we basically don't have much of a margin here. And so when we have an especially warm summer, we're having now that the oxygen levels can be really dropping into a danger zone.

Corals can be surprisingly resilient to low oxygen. They can tolerate oxygen levels dropping to a level that we once thought that they would not really be able to tolerate, but that's at relatively lower kind of very moderate summertime temperatures. And what a lot of the research that's come out of my group would indicate is that you need to look at the oxygen levels as well, because it's probably also a factor in that response that's been attributed solely to increase temperatures. Because that's another stress that oftentimes accompanies elevated temperatures. And the responses that you see in an organism can be triggered by low oxygen as well.

And I'll just take as an example, we know that when temperatures exceed some sort of level for many corals, they will bleach, which is a very visible sign that those corals are being stressed. They've expelled their symbiotic algae that they depend on for kind of a healthy life. Well, it turns out that just lowering is enough to also cause a bleaching response. And so it may be that in some instances where, you know, in a in a kind of degraded habitat, where corals exhibit bleaching, in the past that may have been attributed to just warming and it may, in fact, have been due to warming as well as low oxygen or maybe entirely the low oxygen that was associated with those circumstances.

The Florida Keys in particular are home to a lot of hurricanes. Big storms come through, churning the water. Would that maybe ease the lack of oxygen in the depths where the coral reefs are located?

In the very short term, you've basically got a hurricane coming and churning the water. And, there's going to be wind waves and, and lots of energy entering the system. And it has a potential to kind of mix the water, and it could have some re-oxygenation effect, like as the hurricane's happening.

But there's some indications that in even the slightly longer term of like days or maybe weeks, that those hurricanes could actually compound the problems associated with low oxygen for a few reasons. So one is that oftentimes after a hurricane, you get a very calm period and a very warm period, and you got these elevated temperatures and this kind of layering of the water, that's going to contribute it to decreased oxygen. The other thing is that you're kind of mixing maybe some nutrients or organic matter that was kind of locked up or sequestered down in the sediments, suddenly, it becomes much more available. And you can have primary production or growth of algae, based on these resources that are suddenly been freed up by the hurricane, which can lead to depletion of oxygen once that microbial activity ramps up. And then you also have got lots of runoff associated with the precipitation that comes from the hurricane. And that precipitation can further as you get runoff entering into the coastal waters. You get additional layering of the water, you also have that runoff bringing in lots of nutrients, and causing further oxygen depletion.

And based on some preliminary evidence that some colleagues of mine from here at the University of Florida have collected in the wake of Ian, was that there did appear to be some oxygen depletion in coastal waters immediately following the hurricane.

I would say overall, their effect is probably a negative, and I would not look for them as sort of to rescue on the event.

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Steve Newborn
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