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What’s Marine Debris Got to Do with a Tsunami Anyway?

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I was on an interesting phone call with a bunch of people who work on corals today. They seemed to have a very different conception of the situation out here than we do. So I’m figuring maybe the casual reader of this blog may also be in the dark about what motivated this project, what we’ve seen, and what we’re thinking.

For the scientists reading along at home, feel free to correct my oversimplifications. I’m not a coral reef biologist, and I’m trying to understand and then translate what I’ve heard and seen.

If we could take human beings out of the picture, a tsunami would be an extreme event, but one that happens periodically and to which coral reef ecosystems have adapted. Corals get toppled or broken or downright creamed, but they generally recover naturally.

An overturned table coral.

Then there’s marine debris. On its own, that’s hard on corals too, because it can cause physical damage like shading, abrading, or breaking coral heads. It can also create a secondary effect—for example, scientists have seen that metal debris is associated with blooms of algae, bacteria, or other species that can shift the ecosystem.

Fabric draped across these corals blocks sunlight, which
is essential to the corals’ survival. It can also abrade the coral
as it moves with waves or tides.

Marine debris or a tsunami can be hard on corals even in an area distant from human habitation. However, when humans enter the picture they may do things that stress the corals to the extent that the damage done by a tsunami is just too much to recover from—things like creating land-based sources of pollution (runoff of sediment, say, that smothers the corals, or nutrients like nitrogen or phosphorus that promote excessive algae growth), overfishing, or knocking over corals with boat anchors or swim fins.

What can happen in an area like American Samoa is the trifecta of stress to the corals. You have that underlying stress level of land-based pollution, overfishing, and a growing population. Then you have the direct injury to corals by the tsunami. I’m not out in the water every day, but I’ve been in two areas with no debris to speak of, and there was plenty of coral damage that was apparently caused by the tsunami itself. On top of that, in areas adjacent to villages that were inundated by the tsunami, we’ve seen sparse to extreme amounts of debris out on the coral reefs. It’s a devastating combination.

This field of marine debris caused damage as it moved to its
current location, blocks sunlight now, and can expand the
footprint of damage as it continues to move around.

The marine debris can harm coral over three timescales: one during the tsunami as it moved with tremendous energy, breaking corals; one right now as it lies atop corals, shading them from sunlight and causing bleaching and death; and one more if it moves around due to the action of tides, waves, or storms and the cycle starts over again. This last threat is imminent since we’re heading into hurricane season.

Kyle was telling us this evening that he saw some fabric draped across a coral head and just knew it had killed the corals it was covering, so he took a photo before and after they removed the cloth. (I’ll post those photos in a day or two.) If the fabric is good and stuck, the only way that coral can recover is for the fabric to gradually rot away or be removed. If the debris in question is a piece of corrugated metal roofing, the rotting away will be gradual indeed. It’s the feeling of the scientific team that the best restoration is to remove the marine debris generated by the tsunami so recovery can start and to prevent additional damage.

I can’t say for sure that we’ll be able to do that removal, but the more that can be removed and the quicker it can happen, the better. Not for the sake of removing marine debris, but for the sake of allowing the coral reef to recover.

Swimming a piece of corrugated
metal roofing to the surface.

Author: NOAA Marine Debris Program

The NOAA Marine Debris Program envisions the global ocean and its coasts, users, and inhabitants free from the impacts of marine debris. Our mission is to investigate and solve the problems that stem from marine debris, in order to protect and conserve our nation's marine environment, natural resources, industries, economy, and people.

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