Seasonal changes in oceanographic processes at an equatorial macrotidal beach in northern Brazil

Coastal waters influenced by the Amazon River are highly dynamic and productive environments, although few studies have focused on the seasonal changes in oceanographic processes, except for the Amazon plume. The aim of this paper is to evaluate the effects of short- (daily) and medium-term (seasonal) variations in meteorological and hydrodynamic conditions on hydrological variables in a sub-tidal zone located to the southeast of the Amazon estuary, in an area influenced by other rivers. The study was undertaken between November 2008 and September 2009. The month with the lowest fluvial discharge (November, dry season) was characterized by strong tidal currents and high significant wave heights, the highest salinity and pH, and the lowest dissolved nutrient concentrations and turbidity. During the equinoctial spring tide (March, rainy season), when discharge was increasing, similar significant wave height and tidal current speeds were recorded, with high tide elevations, and the highest concentrations of chlorophyll a and nitrate. The maximum fluvial discharge (June, rainy season) was characterized by the lowest hydrodynamic energy, salinity and pH, but also the highest silicate concentrations. During the other equinoctial spring tide (September, dry season), when discharge was decreasing, tides and tidal currents were similar to those recorded in March, whereas phosphate and nitrite concentrations were the highest recorded in the study. Overall, coastal circulation and hydrological variables shifted seasonally, responding primarily to atmospheric, hydrographic, and astronomical factors, including winds, waves, fluvial discharge, tides, and tidal currents. The study also confirmed that coastal areas to the southeast of the Amazon plume are highly dynamic and productive.