Baía de Guanabara, Rio De Janeiro, Brazil

Abstract

Baia de Guanabara (approx. 23°45′S 44°45′W) is a eutrophic and polluted coastal bay in Brazil, impacted by the discharge from the Rio de Janeiro metropolitan area. The shallow bay measures 28 km from west to east and 30 km from south to north, has a 131-km perimeter, a mean water volume of 1.87 x 10⁹m³, and 384 km² surface area. The entrance to Baia de Guanabara is only 1.6 km wide (Fig. 8.1). The bay is structurally controlled and has a central channel with a depth of 30 m and a sandy bottom near the entrance, reflecting wave and tidal forcing. Bottom sediments are mostly muds as a result of the Holocene transgression and rapid fluvial sedimentation, accelerated by channelization of rivers and deforestation in the drainage basin. An extensive sandbank is located seaward of the bay entrance and a flood-oriented sand wave system indicates sand transport into the bay. The mean freshwater runoff measures 125 m3 s⁻¹ and peaks in January. Tides are mixed and mainly semidiurnal with a range of 0.7 m, and peak spring tidal currents reach 0.5 m s⁻¹ inside the bay and 1.6 m s⁻¹ in the bay entrance. The passage of northward propagating polar fronts occasionally results in strong winds from the southwest and high-energy, long-period waves from the south. The salinity of the bay varies from 21.0 to 34.5 with a weak vertical salinity stratification. An estuarine circulation is characterized by a 900 m³ s⁻¹ net surface outflow. Fifty percent of the water volume in Baia de Guanabara flushes every 11.4 days. Largely untreated sewage enters the bay from the west, resulting locally in very poor water quality. The near-bottom mean dissolved oxygen concentration at the inner-most station measures 3.1 mg l⁻¹ and results in anoxic bottom muds. The worst water quality is indicated by average fecal coliform counts higher than 1,000 ml⁻¹. The average chlorophyll concentration in the inner bay, in response to high nutrient loading, exceeds 130 µg l⁻¹.