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Science commentary: The low-oxygen ocean lifestyle

Under CC license from Flickr user Matt McGee

When we talk about climate change, it’s easy to get stuck in our terrestrial mammal mindset. Let’s face it: most of us are total dry land chauvinists. The only time we even notice something’s happening to the ocean is when it’s gnawing away at our coastline. But something else is going on just beneath the surface. Certain sections of the ocean are losing oxygen – and that’s just as bad for sea creatures as it would be for us.

Slow suffocation can be a real drag on productivity. If you’ve ever been continually strangled for a whole workday, you know what I’m talking about. Your boss is all, “Can you give me a progress update?” And you’re all, gurgle, gurgle, frantic hand gesture.

It’s easy to forget, but this is true for marine organisms as well. Even fish need to breathe.

If a fish that’s used to swimming in an oxygen-rich environment is engulfed by a wave of low-oxygen water, it had better escape fast. Otherwise, it might end up – forgive me – sleeping with the fishes.

This situation isn’t hypothetical. Right now, layers of low-oxygen water are expanding upwards all along the California coast.

See, we tend to think of the ocean as a big basin of water, but to understand what’s going on here, it might be more helpful to look at it as an enormous parfait. It’s actually made up of three distinct layers that behave in very different ways.

At the surface, oxygen abounds. The water’s choppy and turbulent, and air gets continuously whipped in. The surface is also well lit, so free-floating algae can add more oxygen to the mix through photosynthesis. That surface layer is by far the most productive and densely populated neighborhood in the open ocean.

Counter-intuitively, there’s also a lot of oxygen at the very bottom of the sea. When ocean currents carry surface water from the tropic up to the poles, that water gets cold – which means it gets dense. It drops to the seafloor and oozes back out across the bottom of the ocean.

But in between, in the mid-water, things get more complicated. Vast populations of marine bacteria survive on organic matter sinking from the surface. They use up a lot of oxygen just by breathing. But because there’s little or no light, it’s hard for them to photosynthesize, which means there’s no way to replace that oxygen. This middle layer is called the oxygen minimum zone, or OMZ.

As it happens, we have the world’s largest OMZ right on our doorstep, along the eastern edge of the Pacific. And it’s getting bigger. Here on the West Coast, the upper edge is about 100 meters closer to the surface than it was fifty years ago.

Climate change models predict this. As the ocean’s surface layer heats up, it becomes less dense, meaning it mixes even less with the very cold, very dense water underneath it. To make matters worse, hot water already holds less dissolved oxygen than colder water.  This one-two punch means much less oxygen makes it into the ocean depths, and the OMZ gets beefed up accordingly. Then, on top of that, less available oxygen means marine bacteria eat nitrate instead and produce nitrous oxide, a major greenhouse gas.  This means things heat up a little more. You see where this is going.

Obviously, none of this is great news for animals that, like us, prefer their oxygen in heaping portions. That includes most of the fish that you probably think of when I say the word “fish,” that is, the ones we eat: tuna, sardines, hake – none of them can last long in the OMZ, so they simply avoid it. As the OMZ gets bigger, they get compressed into the thinning surface layer. It’s a form of invisible habitat loss.

Then again, animals that can handle the punishing conditions inside the zone are totally enthused. Jellyfish, comb jellies, the demonic umbrella we call the vampire squid – this is the moment that this much-maligned group of low metabolic rate, detritus-eating weirdos has been waiting for. With its surface-dwelling overlords in a chokehold, the ocean’s seedy underbelly is realizing that its time has come. Or rather, is gradually coming It’s happening, but don’t hold your breath.

Learn more about the ocean at San Francisco's Ocean Film Festival, opening March 7, 2013. Find details here