Estimating an estuarine mixing and exchange ratio from boundary data with application to Mt. Hope Bay (Massachusetts/Rhode Island)

Methods are presented to evaluate an estuarine residence time using volume, salt, and temperature budgets from system boundaries. Changes in the distribution of flux with respect to salinity and/or temperature are used to evaluate a replacement ratio, M, for the system, indicating how much incoming tidal flux volume is replaced with ambient estuarine water prior to discharge on the ebb. Estimates of M can also be generated using total flood and ebb apportioned fluxes, and salinity–temperature relationships of water sampled during the tidal cycle. The generic methods are applied to the Mt. Hope Bay estuary (Massachusetts/Rhode Island, USA), with relevant estimates of M ranging from 10 to 15%, implying a tidal exchange driven estuarine residence time of an order of 2 weeks to a month. The proposed estimation techniques are valuable for deriving simple estimates of residence time, which are often sufficient for many ecological applications, and will also enable more sophisticated calibration of numerical models of estuaries.