The coast of Oregon is known for it’s 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 lead to harmful conditions of hypoxia (low oxygen) in bottom waters that can have devasting impacts on local fisheries.
Coastal areas area uniquely susceptible to low oxygen (hypoxia) events near the seafloor. In the case of the Oregon coast, these events are linked to shifts in upwelling dynamics and the decay of organic matter from 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.
Though these data focus on physical changes (upwelling/downwelling) that can lead to hypoxic conditions, it is important to note the biological drivers in the system as well. Namely, high summer productivity in the surface waters leads to more organic matter sinking to the seafloor, the decomposition of which removes oxygen from the water, potentially leading to hypoxic conditions.