The residual circulation profile of the Bons Sinais Estuary in central Mozambique - potential implications for larval dispersal and fisheries

Abstract

The residual circulation in estuaries determines the net exchange of water, heat, salt, fine sediments and drifting biological organisms between estuarine and nearshore marine waters. The Bons Sinais Estuary in central Mozambique is ~ 30 km long with the city of Quelimane and an industrial port on the northern bank of its upper reaches. To investigate residual circulation in the estuary, seasonal (wet, dry and transition season) CTD profiling data were collected at 11 fixed stations between the upper estuary and mouth, and vertical current profiles were measured over a full tidal cycle at a fixed mid-estuary station. Strong longitudinal gradients in salinity and density indicated that the estuary was river-dominated during the wet season and tide-dominated during the dry season, but the water column remained partially mixed. Tidally averaged vertical profiles from the mid-estuary station revealed: uniform vertical temperatures, warmest during the wet season; depth-stratified salinity during the wet season, but uniform profiles during the dry and transition seasons with highest salinity during the dry season when the density was also highest. The density was uniform and somewhat lower in the transition season, and in the wet season the density was even lower, but stratified. The vertical velocity profile showed a classical two-layer circulation model, with downstream flow intensifying at the surface, and upstream flow at the bottom, during the wet and transitory seasons, when freshwater discharges into the estuary. The flow velocities obtained from a calibrated Hansen and Rattray model fitted the observed data well, confirming that a simplified modelling approach is adequate to describe the residual flow of the Bons Sinais Estuary. The residual circulation model provides insights useful for fisheries research and predicting the spread of water-borne pollutants in the estuary.

The Diversity of Fish Larvae in the Bons Sinais Estuary (Mozambique) and Its Role as a Nursery to Marine Fish Resources

“…On the other hand, in the upper estuary, salinity averages of less than 5 were still recorded in the first months of the dry season, namely, May to June. A previous study found similar results [54] in the BSE by noting strong spatial gradients and seasonal variations in salinity, and concluded that the estuary is river-dominated during the wet season and sea-tide-dominated during the dry season.…”

“…According to the critical swimming speed (CSS) of some larval fish families, currents in estuaries are somewhat stronger and stifle the ingress and retention of larvae. For instance, the CSS of 18.7 cm/s for Clupeidae [108], CSS of 18.4 cm/s for Blennidae [108], and CSS of 22.1 cm/s Scianidae [109] are lower than the maximum currents in the estuary of around 100 cm/s determined by [54] at spring tides. Under such severe hydrodynamic conditions, some species may use different strategies to be retained in the estuary, including short-range directional horizontal swimming against the axial currents, vertical migration, and the use of residual upstream flows, especially over shallower areas [29].…”

“…There is a reduction in river flow from the early dry season (May to August) and a lowering of water temperature, which can explain the lower larval density found in this period. In the BSE, the current velocities are higher in the dry season (up to 100 m/s at spring tides) compared to the wet season, which are around 60 cm/s for the spring tides [54], a condition that may favor a major ingress of larvae in the later season.…”

“…Measurements in the upper zone were likely taken in locations that, in the dry season, were at the interface between freshwater and seawater, with the lowest water transparency, but the interface moved downstream in the wet season. On the other hand, erosion and the accumulation of sediments seaward are features of estuaries in which ebb currents are stronger than flood currents [70]-as in the BSE-where ebb currents peak in the wet season [54].…”