Portland, Ore. ? Bottom fish and crabs washing up dead on Oregon beaches are being killed by a recurring “dead zone” of low-oxygen water that is larger than in previous years and may be triggered by global warming, scientists said.

There are signs it is spreading north to Washington’s Olympic Peninsula.

Scientists studying the 70-mile-long zone of oxygen-depleted water, along the Continental Shelf between Florence and Lincoln City, conclude that it is being caused by explosive blooms of tiny plants known as phytoplankton, which die and sink to the bottom, then are eaten by bacteria that use up the oxygen in the water.

The recurring phytoplankton blooms are triggered by northerly wind, which generates a process known as upwelling in which nutrient-rich water is brought to the surface from lower depths.

Scientists first noticed a dead zone off Newport in 2002. That one was traced back to a rare influx of cold water rich in nutrients and low in oxygen that had migrated from the Arctic.

Dead zones have returned each summer since then, but these have been marked by intense bursts of upwelling that were followed by calm periods, when the water contains lower nutrient levels.