Yaquina Estuary, located along the central Oregon coast, is home to at least 62 species of fish and crustaceans, including breeding populations of five salmonid species and the prized Dungeness crab. Currently the estuary shows no symptoms of cultural eutrophication. The management goal is to protect and maintain healthy aquatic life in Yaquina Estuary. The principal objective in developing nutrient criteria for Yaquina Estuary is to prevent future degradation of estuarine water quality and the accompanying loss of beneficial uses from the system. Yaquina Estuary supports several designated uses, including aquatic life harvesting (shellfish growing and fishing), recreation (water contact recreation), ecological (resident fish and aquatic life, salmonid spawning and rearing, anadromous fish passage), and aesthetics. Currently the Yaquina Estuary is home to almost 100 hectares of seagrass, covering approximately 5 percent of its total area.
In Oregon, estuarine waters have a numeric chlorophyll a criteria of 0.015 milligrams per liter (mg/L), which triggers additional study. Brown et al. (2007) (this study) developed a case study in An Approach to Developing Nutrient Criteria for Pacific Northwest Estuaries: A Case Study of Yaquina Estuary, Oregon, in which they proposed a methodology for developing numeric nutrient criteria in Yaquina Estuary because the chlorophyll a criteria codified in the water quality criteria (0.015 mg/L) might not provide enough protection for this estuary. Because there was not enough data to apply the reference condition approach to the class of estuaries similar to the Yaquina Estuary, Brown et al. (2007) used in situ observations within Yaquina Estuary as a basis for determining an estuarine reference condition.
The authors set eelgrass (Zostera marina), a principal seagrass in Pacific Northwest estuaries, as their endpoint (depth and area) using a seagrass stressor-response model (SRM). They also considered macroalgal cover and biomass as response indicators, but determined that their usefulness as indicators would be limited. The authors found that benthic green macroalgae closer in the zone closer to the outlet to the ocean derive most of their nutrients from tidal influx of nearshore marine waters; and summer green macroalgal blooms appear to be a natural response of the estuarine system. The authors concluded that much of the nutrient loading came from natural sources, especially with the close exchange with the coastal ocean, and that numeric standards should incorporate natural variability associated with ocean conditions.
Approximately 48 percent of the Yaquina Estuary is intertidal, effectively dividing the estuary into two ecologically different zones: a lower segment dominated by ocean influences (zone 1) and an upper segment dominated by freshwater riverine influences, including watershed and point source inputs (zone 2). As a result, the two zones exhibit wide differences in nutrient levels and sources as well as in their response to nutrient inputs. In addition to these spatial differences, there is a distinct dry season (May—October) and wet season, which also affects nutrient loads and sources to the estuary. These spatial and temporal differences resulted in four estuary classes: (1) upper estuary—dry season, (2) upper estuary—wet season, (3) lower estuary—dry season, and (4) lower estuary—wet season. These classes were incorporated into the criteria development process to improve the accuracy of stressor-response relationship estimates.
The authors gathered available historical data (nonconsistent between 1964 and 1992) and recent data (1998–2006) and assembled causal (nutrient) and response variables (e.g., chlorophyll a, dissolved oxygen [DO], water clarity, total suspended solids [TSS], macroalgal biomass, distribution of submerged aquatic vegetation) and physical data (i.e., temperature and salinity) at three spatial scales. Historical data included parameters such as salinity, water temperature, DO, chlorophyll a, nitrogen, and phosphorus. Recent data included DO, conductivity, temperature, depth, turbidity, fluorescence, photosynthetically active radiation, nitrogen, and phosphorus. The authors separated data into wet season (November–April) and dry season (May–October) to address strong seasonal variation in nutrient loads and sources, and used cumulative distribution functions (CDFs) and seagrass SRM to determine median percentile for water clarity.
The relationships between causal variables (nutrients, mostly nitrogen in Yaquina Estuary) and response variables (e.g., seagrass biomass and carbohydrate content, water clarity, macroalgae, chlorophyll a, TSS) were assessed. The authors used the seagrass SRM to determine median percentile for water clarity, using the 25th, median, and 75th percentile results from the CDFs to determine if potential criteria are protective of seagrass distribution and biomass in Yaquina Estuary. Based on that relationship, the authors derived proposed criteria that protect the current population of seagrass in Yaquina Estuary. Multiple water quality parameters were assessed for use as possible criteria variables, including nitrogen, phosphorus, chlorophyll a, DO, TSS, and water column light attenuation. Because there were limited water quality variable data available from estuaries similar to Yaquina, in situ observations within Yaquina Estuary were used to create the database. Previous assessments of water quality conditions were hindered by the limited availability of water quality data for Oregon estuaries. The authors produced CDFs for Yaquina Estuary and compared them to CDFs of other Oregon estuaries using two independent data sets.
The 25th, median, and 75th percentiles of distributions of the water quality parameter data were used as inputs to the seagrass SRM. Model simulation runs statistically determined which quartile values would protect and maintain current eelgrass populations and which values would result in a decline of eelgrass. Results indicated that the median value could be used as criteria for most water quality parameters. As discussed earlier, the U.S. Environmental Protection Agency recommends that criteria should be developed only for the dry season, with priority emphasis on the upper estuary. The table below presents the proposed criteria based on the results of the stressor-response approach.
Field results were used to confirm output from the seagrass SRM. Researchers used in situ observations to determine estuarine reference conditions. Based on weight-of-scientific evidence, the Yaquina Estuary is showing no signs of cultural eutrophication. The seagrass SRM confirmed that the median percentile for water clarity would be protective of eelgrass within the estuary. Researchers recommended setting dry season criteria for two zones for dissolved inorganic nitrogen (DIN), phosphate (PO4), chlorophyll a, water clarity, and DO (Brown et al. 2007):
| Parameter | Zone 1 (lower estuary) | Zone 2 (upper estuary) |
| DIN | 14 µM | 14 µM |
| PO4 | 1.3 µM | 0.6 µM |
| Chlorophyll a | 3 µg/L | 5 µg/L |
| Clarity | 0.8 m | 1.5 m |
| DO | 6.5 mg/L | 6.5 mg/L |
Notes: µM = micromole; µg/L = microgram per liter; m = meter.
Reference:
Brown, C.A., W.G. Nelson, B.L. Boese, T.H. DeWitt, P.M. Eldridge, J.E. Kaldy, H. Lee, J.H. Power, and D.R. Young. 2007. An Approach to Developing Nutrient Criteria for Pacific Northwest Estuaries: A Case Study of Yaquina Estuary, Oregon. EPA 600/R-07/046. U.S. Environmental Protection Agency, Office of Research and Development, National Health and Environmental Effects Laboratory, Western Ecology Division. Accessed October 2016. https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P10077PT.txt.