Intratidal fluctuations in stratification within a mangrove estuary

Abstract

The dependence of salinity stratification on the vertical structure of the longitudinal salinity gradient is investigated by solving the equations of longitudinal momentum and salt conservation. The results are used to interpret measurements made throughout 31 tidal cycles in the Merbok Estuary, Malaysia, as part of an ecological study of a tropical mangrove estuary. The solutions show that a substantial part of the salinity stratification was caused by vertical shear in the currents coupled with advection of the longitudinal salinity gradient. This mechanism led to maximum stratification around low water and minimum stratification around high water. Observed intratidal variations in stratification at neap tides greatly exceeded modelled values when a depth-independent, longitudinal salinity gradient was assumed. Simulations made using a longitudinal salinity gradient of simple depth-dependent, power-law form implied that this difference arose mainly because the near-surface, longitudinal salinity gradient in the field was significantly larger than that near the bed during neap tides and at times of high run-off. The longitudinal, dispersive transport of salt, fresh water and solutes due to vertical shear is shown to be important during periods of stratification at neap tides and during high run-off. The relevance of stratification to mangrove estuaries is discussed.