Physical Controls on Estuarine Gas Exchange: The Estuarine Gas Exchange Maximum (GEM)

Results from a suite of numerical simulations are used to demonstrate that high outgassing rates of CO₂ commonly observed in the upper regions of estuaries are strongly controlled by the underlying estuarine circulation. The basic conceptual model for this process is analogous to the convergence in physical transport that results in the estuarine turbidity maximum (ETM). Like the commonly observed ETM, these simulations suggest that an estuarine gas exchange maximum (GEM) is likely a common feature in many estuarine systems. The presence of vertical density stratification effectively isolates the respiratory demand of sub-pycnocline waters from surface exchange. As a result, bottom waters become depleted in O₂and enriched in pCO₂ as they advect landward in the residual estuarine circulation. Convergence in the bottom residual circulation results in entrainment and mixing of high pCO₂ into the surface waters. The greatest upwelling/mixing occurs near the limit of salt where bottom pCO₂ values are maximal (and pH and O₂ are minimal) and vertical density stratification is weak, resulting in a region of enhanced evasion of pCO₂ (and invasion of O₂). In the numerical simulations, the along-estuary distribution of net ecosystem metabolism is held constant, so the GEM is solely the result of convergence in physical transport. However, the overall pattern of enhanced CO₂e vasion near the limit of salt persists even when the upper-estuary is autotrophic and the lower estuary is heterotrophic, highlighting the first order control that estuarine physics play in controlling the magnitude and location of gas exchange.