Hydrology of a Tropical Estuarine System: Itamaracá, Brazil

Abstract

The Itamaracá estuarine system, located in northeastern Brazil is formed by an elongated U-shaped channel, Canal de Santa Cruz, which connects to the South Atlantic Ocean at the two ends and receives runoff from six small rivers. The estuary is surrounded by mangrove wetlands, while its off-shore area is shielded by extensive shore-parallel sandstone reefs and sand banks, which reduce the water exchange. Tides at both entrances are semidiurnal, in phase, and have mean and spring ranges of 1·8 m and 2·2 m, respectively. Typically, flood tides last 6·9 h while ebb tides extend for 5·5 h. Tidal currents lag water elevation by 86-94° and there is a phase lag of 15-20 min. in water elevation between the inner channel and the estuarine mouth. Monthly rainfall during peak rainy season (PRS) is 0·34 m per month and during peak dry season (PDS) only 0·06 m per month. Considering evaporation, evapotranspiration, runoff and river discharge, the fresh-water input into the estuary is 57·7 m3 s-1 during PRS, but only 0·2 m3 s-1 during PRS, a 300-fold difference. This difference in water supply governs, to a large extent, the hydrodynamics of the system. During PDS, the channel is well mixed, and slightly hypersaline. Mean salinity ranges from 31 to 36%, reaching 38·6% near the bottom. This condition results from a combination of low river discharge, high evapotranspiration, east-southeast winds and from the presence of saline water masses at the estuarine entrances. Large concentrations of TSS inside the channel during PDS suggest that these water masses act as a dynamic harrier, inhibiting flushing. During PRS, this situation no longer exists, and exchange of material between the estuary and off-shore waters occurs more readily. The system is partially mixed, gravitational circulation prevails, and mean salinity ranges from 20 to 32%.