Several dead crabs and some seaweed are found on a sandy beach.
Photo: NOAA

Hypoxia, which occurs when dissolved oxygen concentrations are low enough to cause stress to aquatic animals, has increased globally in recent decades. Off the coasts of Washington and Oregon, unusually severe and prolonged hypoxic events in recent years have been associated with widespread mortality of fish, crabs, and other marine life. Climate change projections indicate that hypoxic regions are likely to expand due to both sea surface warming and changing circulation patterns. As ocean waters warm, they are naturally able to hold less dissolved oxygen. Ocean warming can also increase stratification in the water column and reduce vertical mixing throughout the water column, exacerbating the impacts of hypoxia.

Over the last decade, the Olympic Coast has experienced progressively worsening seasonal hypoxia, predominantly in the southern portion of the sanctuary. During the summer upwelling season, low concentrations of dissolved oxygen are a common feature in the subsurface waters of the sanctuary. Mass mortality events caused by hypoxia are becoming more frequent, as evidenced by the diversity of organisms that tend to wash up dead on local beaches. Analysis of sanctuary mooring data suggests a concerning long term trend for hypoxia along the sanctuary’s 135-mile coastline, and forecasts of future conditions predict further worsening, although actual impacts may be variable across the geography of the Olympic Coast.