Measuring Estuarine Total Exchange Flow From Discrete Observations.

The exchange between estuaries and the coastal ocean is a key dynamical driver impacting nutrient and phytoplankton concentrations and regulating estuarine residence time, hypoxia, and acidification. Estuarine exchange flows can be particularly challenging to monitor because many systems have strong vertical and lateral velocity shear and sharp gradients in water properties that vary over space and time, requiring high‐resolution measurements in order to accurately constrain the flux. The total exchange flow (TEF) method provides detailed information about the salinity structure of the exchange, but requires observations (or model resolution) that resolve the time and spatial co‐variability of salinity and currents. The goal of this analysis is to provide recommendations for measuring TEF with the most efficient spatial sampling resolution. Results from three realistic hydrodynamic models were investigated. These model domains included three estuary types: a bay (San Diego Bay), a salt‐wedge (Columbia River), and a fjord (Salish Sea). Model fields were sampled using three different mooring strategies, varying the number of mooring locations (lateral resolution) and sample depths (vertical resolution) with each method. The exchange volume transport was more sensitive than salinity to the sampling resolution. Most (>90%) of the exchange flow magnitude was captured by three to four moorings evenly distributed across the estuarine channel with a minimum threshold of 1–5 sample depths, which varied depending on the vertical stratification. These results can improve our ability to observe and monitor the exchange and transport of water masses efficiently with limited resources. Plain Language Summary: The two‐way exchange of water and properties such as heat and salinity as well as other suspended material between estuaries and the coastal ocean is important to regulating these marine habitats. This exchange can be challenging to measure. The total exchange flow (TEF) method provides a way to organize the complexity of this exchange into distinct layers based on a given water property. This method has primarily been applied in numerical models that provide high resolution output in space and time. The goal here is to identify the minimum horizontal and vertical sampling resolutions needed to measure TEF depending on estuary type. Results from three realistic hydrodynamic models were investigated. These models included three estuary types: bay (San Diego Bay), salt‐wedge (Columbia River), and fjord (Salish Sea). The models were sampled using three different mooring strategies, varying the number of mooring locations and sample depths with each method. Most of the exchange magnitude was captured by three to four moorings evenly distributed across the estuarine channel with a minimum threshold of 1–5 sample depths, which varied by estuary. These results can improve our ability to observe and monitor the exchange and transport of water masses efficiently with limited resources. Key Points: A threshold of 1–5 sample depths was needed to measure the exchange flowMost (>90%) of the exchange flow magnitude was captured by 3–4 moorings spread across the channelThe exchange volume transport was more sensitive than salinity to the sampling resolution [ABSTRACT FROM AUTHOR]