TIDE-SALINITY PATTERNS CHARACTERIZE SEAWATER-FRESHWATER MIXING DYNAMICS IN A TROPICAL MANGROVE ESTUARY

We conducted a time series investigation of water level (tides), salinity and temperature at the mouth of a river and in a tidal creek in a mangrove forest at mid-estuary and another in the estuary head. Our objective was to investigate the processes that controlled salinity and temperature over multiple tidal cycles. Because of the differences in tide levels at each location, we normalized for tides and developed tide-salinity and tide-temperature graphical patterns. The tide-salinity patterns at the river mouth revealed seawater mixing at >0.4 normalized tide and complete seawater removal during tidal ebbing by ~0.2 normalized tide. The salinity during tidal ebbing was higher than during flooding because of saline water drainage from the mangrove forest. Whereas, the tide-salinity patterns for the tidal creek at mid-estuary showed saline water mixing between 0.1 to ~0.2 normalized tide and seawater mixing between ~0.2 to 1.0 normalized tide. The salinity decreased rapidly at the onset of tidal ebbing, followed by a slow decrease to background values at <0.2 normalized tide. Furthermore, the salinity during tidal ebbing was lower than during tidal flooding, indicating freshwater dilution from the mangrove forest. Meanwhile, the tide-salinity patterns at the tidal creek in the estuary head showed salinity decrease at >0.8 normalized tide. During tidal ebbing, the salinity which was low, increased at <0.8 normalized tide and either continuously decreased or stayed nearly constant to background levels. The tide-salinity patterns indicate freshwater mixing during flood tide. The tide-temperature patterns at the river mouth and the tidal creek at mid-estuary and at the estuary head were characteristically different and not diagnostic, due to multiple factors that affect temperature. We conclude that tide-salinity patterns over multiple tidal cycles are characteristic and can serve as an important tool for assessing the spatio-temporal mixing and in characterizing mixing associated with riverine tidal freshwater mixing zones in tropical mangrove estuaries.