Seasonality of hydrographic conditions and foraminiferal species in Guanabara Bay (Rio de Janeiro, Brazil)

Abstract

Guanabara Bay was characterized through benthic foraminiferal assemblages and its correlations with changes resulted from physical processes in semidiurnal and seasonal time scales. Biological and physical sampling were made in 26 oceanographic stations distributed in the bay and four anchored stations sampled during the semidiurnal tidal cycle and in the neap and spring tidal phases; the experiments were performed in July (2000) and February (2001). The top 1–2cm of undisturbed sediments were taken, stained and preserved in alcohol solution for biological analysis. Physical properties were sampled with a CTD/Current temperature depth (Falmouth model 2AD) and vertical profiles of salinity, temperature, intensity and current velocity were measured in the water column in the anchored stations. The distribution of foraminifera presented a similar biodiversity pattern, both in winter and summer seasons. The low diversity and high dominance of a few opportunistic-tolerant species demonstrate that GB is highly impacted. During winter and summer, the bay entrance presented high bottom salinities (35,0 to 35,5UPS), higher currents in spring tide (0,85 to –0,81 ms−1, during ebbing and flooding, respectively), being classified as a partially mixed (type 2a) system, with S=0,99 and S=0,86, an indication that the tidal diffusion was the main process responsible for the inward salt transport. Foraminiferal species that can be correlated to this more dynamic and saltier environment are Cassidulina subglobosa, Discorbis williamsoni, Pseudononion atlanticum. The water mass close to the Governador Island is a chocked part of the system where velocities are lower (0,2 to -0,2 ms−1, maximum speeds of flooding and ebbing, respectively) indicating weak tidal influence. The residual circulation, driven by the river discharge and density stratification, indicates lower velocities (-0,04 to 0,03 m s−1). East of the Governador Island, the environmental conditions during the spring tide of the summer favors the transport of anthropogenic substances towards the coast. Stations located in the inner parts of the bay presented similar characteristics, related to low water renovation state, revealing high anthropic impact in the region. Foraminiferal species linked to these low renewable water environments and dynamics are Ammonia tepida, Buliminella elegantissima, Bolivina striatula, Bulimmina elongata, Fursenkoina pontoni.