Distribution of total suspended solids and dynamics of the estuarine turbidity maximum in the Ipojuca River estuary

This study focused on the distribution of total suspended solids (TSS) concentration along the Ipojuca River estuary and on identifying the average location of the estuarine turbidity maximum (ETM) zone considering the seasonality of its rainfall and tidal regime. This research was carried out along a 13.6 km stretch of the chosen river, with 21 sampling stations during the rainy (Jun-Jul/17) and dry periods (Dec/17). Temperature and salinity data were obtained at each station using CTD profiling. Furthermore, current intensity and direction were measured using a current meter, and water samples were collected to determine TSS concentrations. The water column showed vertically homogeneous temperature (~27° C). Salinity distribution varied seasonally and along the tidal cycle throughout the system. The estuary shows a weakly to moderately stratified water column that intensifies itself upstream. At just 2.28 km from the river mouth (second station), salinity varies by 0.2-32.9. This stratification primarily stems from the constriction of its river mouth, trapping freshwater in its interior. Currents showed higher values during the rainy period (-5 to 95.8 cms-1). TSS concentrations were higher during the rainy season, ranging from 8.6 to 241.2 mg L-1 during spring tides and from 6.5 to 223.0 mg L-1 during neap tides. The ETM was located at 1.8-2.2 km from the river mouth during the rainy season and at 2-8 km during the dry season. The ETM coincided with the boundaries of salt propagation in the estuary, corresponding to salinities of 0.2-10, and longitudinal currents ranging from 60 to -10 cm s-1, indicating the mixing zone of water masses. Since the ETM is also associated with the area of highest sediment deposition in the estuary, it is likely that most materials transported to the estuary that are associated with sediment transport remain trapped in this zone, making it an important area for further studies.