While polar bear populations face the challenge of habitat melting beneath their feet, organisms that call water home appear to be grappling with a stranger difficulty: More and more areas of the ocean have oxygen levels too low to sustain them. A report just published in the journal Science asserts that, as tropical oceans warm, regions of low oxygen content are expanding. While researchers make no specific predictions of the ways this will affect individual species, they describe the changes as having potential for wide ecosystem disruptions, with shifts in populations (increased presence of tiny organisms that don’t need much oxygen; fewer predatory fish) affecting animals up and down the food chain.

The scientists analyzed a database of readings taken over the last 50 years, and focused on the most dramatic changes at depths between 300 and 700 meters. According to a statement by team leader Lothar Stramma, of Germany’s Leibniz Institute of Marine Sciences, “whether or not these observed changes in oxygen can be attributed to global warming alone is still unresolved. The reduction in oxygen may also be caused by natural processes on shorter time scales.”

It’s certainly something that has happened cyclically in the past: Stramma points to trends 250 million years ago that caused massive extinctions for Permian-period marine life.

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