Historic and recent patterns of dissolved oxygen in the Yaquina Estuary (Oregon, USA): Importance of anthropogenic activities and oceanic conditions

Spatial and temporal patterns of dissolved oxygen (DO) in Yaquina Estuary, Oregon (USA) are examined using historic and recent data. There was a significant increasing trend in DO in the upstream portion of the estuary during the years 1960–1985. Historically, minimum dry season (May–October) DO levels occurred about 20 km from the estuary mouth at warm water temperatures, near both point source discharges and where extensive log rafting occurred. Presently, there is a trend of increasing dry season DO with increasing distance from the mouth of the estuary. Minimum DO levels occur at cool water temperatures (∼8 deg C), suggesting that recently upwelled oceanic water from the shelf is the source of the low DO water. Recent time-series data (2001–2007) demonstrates that the water advected into the estuary from the coastal ocean has DO levels < 5 mg l−1 about 13% of the time during the dry season and that water temperature is a good indicator of the oceanic source. Similarity in the flood-tide DO and water temperature relationship between recent time-series data in Yaquina and in historic data from other estuaries suggests that the hypoxic conditions observed off the Oregon coast since 2002 may have occurred previously, especially during the 1950’s–1960’s. It is important to characterize the natural background DO levels for estuaries in the region to be able to separate anthropogenic from natural influences on DO.