Lower levels of oxygen in the Earth's oceans, particularly off the United States' Pacific Northwest coast, could be another sign of fundamental changes linked to global climate change, scientists say.

They warn that the oceans' complex undersea ecosystems and fragile food chains could be disrupted.

In some spots off Washington state and Oregon, the almost complete absence of oxygen has left piles of Dungeness crab carcasses littering the ocean floor, killed off 25-year-old sea stars, crippled colonies of sea anemones and produced mats of potentially noxious bacteria that thrive in such conditions.