Implications of turbulence in a macrotidal estuary in northeastern Brazil — The São Marcos Estuarine Complex

Abstract

Pollution, hypoxia, and salinization are frequent occurrences in estuarine environments. The management of these issues requires the understanding of estuarine dynamics, which are governed by both freshwater and tidal inputs. The aim of the present study was to investigate the dynamics of the São Marcos Estuarine Complex (SMEC) in northeastern Brazil under different rainfall regimes. Two moorings were deployed in the SMEC in 2017 (one downstream [M-I] and one upstream [M-II]) for the collection of data during a complete tidal cycle in three sampling campaigns representative of the rainy, transition, and dry periods. Circulation had an asymmetrical, bidirectional flow, with ebb currents (V > 0) predominating over flood currents (V < 0). The thermohaline indices indicated the presence of two water masses in the SMEC: coastal water in the rainy and dry seasons and river water in the transition period. High concentrations of suspended particulate matter (SPM) (>1,000 mg l−1) were found in the SMEC, with mean values close to 230 mg l−1. The highest concentrations were found near the bottom in the dry season. High concentrations of dissolved oxygen (>190μmol kg −1) were also found, despite the high concentrations of SPM. The Richardson number indicated vertical instability throughout the SMEC, which explains the occurrence of a potential energy anomaly in the range of 7.0 × 10−6 to 1.1 × 10−5 J m−3 s−1. The SMEC was well mixed and poorly stratified most of the time, except at M-I in the dry season, when it was partially mixed with weak vertical stratification. River discharge and the Stokes drift were the main salt exporting and importing mechanisms in the SMEC, respectively. The present results broaden knowledge on macrotidal estuarine dynamics in the Amazon/semiarid interface that can assist in water quality management.