Seasonal and synodical variability in the distribution of physical water properties in São José de Ribamar Bay, Brazil

Abstract

São José de Ribamar Bay, located in Maranhão State, is one of the largest estuarine systems in Brazil. This study analyzes tidal and riverine regimes, as well as the spatial and seasonal distribution of physical water properties (salinity, temperature, and density) along the bay. Water-level observations were examined using harmonic analysis, and the annual modulation of tidal amplitude was obtained using the Hilbert transform. Historical discharge data from the Itapecurú and Munim rivers - the main tributaries supplying the bay - were analyzed to determine the climatological pattern of river flow. Salinity and temperature data were collected during four oceanographic surveys conducted along the bay: two during the wet season and two during the dry season. In each season, one survey was performed during spring tides and the other during neap tides. Water-column stratification was assessed based on these measurements. The tidal regime is purely semidiurnal and dominated by the M2 harmonic constituent. Tidal range varies from approximately 6 m during spring tides to 2.5 m during neap tides. Mean river discharge reaches approximately 1,000 m3 s-1 in the wet season and decreases to around 100 m3 s-1 in the dry season. Expressed as a flow ratio, river discharge represents only about 1% of the volume periodically exchanged during the wet season and about 0.1% during the dry season. Increased freshwater input in the wet season promotes a buoyancy flux that strengthens stratification. Consequently, the bay behaves as a well-mixed estuary during the dry season and as a strain-induced, periodically stratified estuary during the wet season.