Hypoxia off the Oregon Coast

The coast of Oregon is known for its highly productive surface waters fueled by nutrients upwelled to the surface. This high productivity is important to support local fisheries, particularly Dungeness crabs. However, these cycles of upwelling and production can also lead to harmful conditions of hypoxia (low oxygen) in bottom waters that can have devast­ing impacts on local fisheries.

Hypoxia typically occurs near the seafloor in productive coastal areas when high summer productivity leads to the deposition of a large amount of organic matter on the seafloor. As this organic matter decomposes, oxygen is removed from the water via respiration. Under a stratified water column the oxygen-rich surface water cannot mix down to replenish the oxygen poor bottom water and hypoxic conditions set in

In the case of the Oregon coast, hypoxic events are linked to shifts in upwelling dynamics, in addition to the decay of organic matter that sinks out of the productive surface waters. Looking at OOI Coastal Endurance Array data from the seafloor on the Oregon shelf at 25m depth during the of summer 2014, there is a clear period of downwelling captured during the end of June (gray bar). During this period, bottom temperatures and oxygen concentrations are high and salinities are low, indicating that warmer, fresher, oxygen-rich surface waters were mixed throughout the water column. There is then an abrupt shift to a period of upwelling in which cold, salty, low-oxygen water is advected from deeper offshore water onto the shelf.