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Dissolved oxygen in two Oregon estuaries: The importance of the ocean-estuary connection
Citation:
BROWN, C., J. McPherson, T. MOCHON COLLURA, AND H. A. STECHER, III. Dissolved oxygen in two Oregon estuaries: The importance of the ocean-estuary connection. Presented at MidCoast Watershed Council, Newport, OR, February 06, 2010.
Impact/Purpose:
We examined the role of the ocean –estuary connection in influencing periodic reductions in dissolved oxygen (DO) levels in Yaquina and Yachats estuaries, Oregon, USA.
Description:
We examined the role of the ocean –estuary connection in influencing periodic reductions in dissolved oxygen (DO) levels in Yaquina and Yachats estuaries, Oregon, USA. In the Yaquina Estuary, there is close coupling between the coastal ocean and the estuary. As a result, low DO water is periodically advected from the shelf into the estuary during flood tides. The low DO water has a distinctive oceanic temperature and salinity signature. Temporal variability in DO concentrations of water entering the estuary is significantly correlated with wind stress and water temperature. Similarity in the flood-tide DO and water temperature relationship between recent time-series data in Yaquina and historic data from other estuaries suggests that the hypoxic conditions recently observed (since 2002) off the Oregon coast may have occurred previously, especially in the 1950’s-1960’s. In contrast, for the Yachats Estuary, the ocean does not appear to be the source of hypoxic conditions. Instead the physical connection between the estuary and the ocean is the determining factor, due to the extremely shallow depths at the estuary mouth. The Yachats Estuary is connected to the ocean when the water level exceeds about 2.6 m above MLLW. When connected, the DO levels within the estuary rapidly increase and water temperatures decrease. When disconnected from the ocean, strong stratification develops and the DO levels in the bottom layer decrease to the level of severe hypoxia, with a concurrent increase in water temperature. While both systems have periodic, naturally occurring hypoxia, the causes are completely different. Thus, it is critical to characterize the factors determining the natural background DO levels for estuaries in the Pacific Northwest region in order to be able to separate anthropogenic from natural variations.