The Yaquina Estuary is a small drowned-river valley estuary along the central Oregon coast. Because of strong tidal forcing, the estuary and coastal ocean are closely coupled. Approximately 70 percent of the estuary volume is exchanged during each tidal cycle. Spatially, the Yaquina Estuary can be divided into two zones: a lower marine-dominated zone (zone 1) and an upper riverine-dominated zone (zone 2). November through April is the wet season at which time the estuary is river-dominated. In the dryer months, it is ocean-dominated. These seasonal and spatial differences combined with strong tidal forcing greatly influences the nutrient, DO, chlorophyll a, and other water quality conditions in the estuary.
Several assessment endpoints were reviewed in EPA’s case study of the Yaquina Estuary presented in the EPA’s Nutrients in Estuaries report. The case study examines long-term trends in the estuary. The authors discussed spatial and temporal conditions and trends for the endpoints: DO; turbidity and light; chlorophyll, primary production, algal blooms, and species composition; and benthic primary producers such as seagrass. All the assessment endpoints are affected either directly or indirectly by nitrogen and phosphorus loading and ambient nutrient concentrations in Yaquina Estuary.
Zone 1 receives low-oxygen water from hypoxic shelf waters during flood tides. Long-term data show that DO is increasing in zone 2 during both the wet and dry seasons. Chlorophyll a also is imported from the ocean, especially during the dry season with peak levels occurring from June to August. Algal blooms occur in the upper estuary in the summer. Most of the nutrient input to the Yaquina Estuary comes from the ocean in the dry season and from red alders growing in the watershed in the wet season. The estuary is well-mixed as the result of strong tidal forcing, but hypoxic ocean water is sometimes imported into the lower estuary during flood tides. The water is relatively clear throughout the year and supports the growth of two species of seagrass. Macroalgae blooms occur during the dry season with peak biomasses typically occurring in September.
Trend analysis indicates that there have not been any major changes in water column nutrients or chlorophyll a levels in the estuary or any changes in frequency or intensity of macroalgal blooms or spatial distribution of seagrasses. Most of the nutrient input to the Yaquina Estuary comes from natural sources and anthropogenic eutrophication is not a problem.
Water quality stations established by various research groups are located throughout the estuary and are used to collect nutrient, Secchi disk, temperature, DO, and other data. Those data are used to examine spatial and temporal trends and relationships between parameters (National Estuarine Experts Workgroup 2010).
Reference:
National Estuarine Experts Workgroup. 2010. Nutrients in Estuaries: A Summary Report of the National Estuarine Experts Workgroup 2005–2007. U.S. Environmental Protection Agency. Accessed October 2016. https://www.epa.gov/sites/production/files/documents/nutrients-in-estuaries-november-2010.pdf.