Abstract
The objective of this research was to characterize the distribution of dissolved nutrients at six sampling stations in the estuary of the Anil River (ARE) and characterize the bottom sediment. The study area is influenced by a tropical climate and semidiurnal macrotidal regime. Sampling of the surface water and sediment was performed in the main channel at six sampling stations distributed along the estuary in the rainy period (May 9th) and dry period (October 5th) during the spring. Physical variables were collected with the aid of a HANNA HI-8424 multi-parameter probe (water temperature, salinity, pH, dissolved oxygen, and turbidity) and YSI CTD (conductivity, temperature, and depth) probe. A van Veen dredge was used for the collection of bottom sediment from the main channel. The sedimentological results were heterogeneous. Grain size ranged from very fine to medium sand. Dissolved nutrients in the ARE had different sources depending on the season. Continental and fluvial inputs, evidenced by salinity values, governed the distribution of nutrients in the rainy period (January to June), whereas nutrient concentrations were related to the recycling of organic material in the estuary in the dry season (July to December). The considerable presence of sand in the main channel indicates that the sediments available in ARE are exposed to intense hydrodynamics. The distribution of nutrients and the grain size revealed the occurrence of different sources in the estuary according to the rainfall regime and the intense hydrodynamics of tidal currents (semidiurnal macrotidal).
 Keywords: dissolved nutrients, estuarine environment, sedimentology.
Highlights
The northern coastal zone of Brazil is home to a combination of high-generation ecosystems of considerable environmental importance, the diversity of which is characterized by the transition between terrestrial and marine environments
Study Area The study area was the Anil River Estuary (ARE), which is located in the northwestern quadrant of São Luís Island in the central portion of the Maranhão Gulf composed of the municipalities of Paço do Luminar, São Luís, São José de Ribamar, and Raposa
The Anil River Estuary was characterized as a shallow environment with low water transparency strongly influenced by the action of the tides, the re-suspension of previously deposited sediments, and the erosion of the banks
Summary
The northern coastal zone of Brazil is home to a combination of high-generation ecosystems of considerable environmental importance, the diversity of which is characterized by the transition between terrestrial and marine environments. The presence of mangroves exerts a direct influence on the dynamics of sediments in estuaries and the coastal zone (Mazda et al, 1995; McLachlan et al, 2020). Estuaries are transition regions between rivers and the ocean characterized by gradients of salinity and density associated with the turbulent mixture of river water and seawater (Pritchard, 1955). Estuaries retain matter and particles originating from the continent to the adjacent marine environment, such as dissolved nutrients, which originate mainly from sedimentary rock. The continent is the main source of these elements in estuarine environments (Barcellos et al 2012; Wolanski and Elliott, 2015)
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