Controlling factors on the abundance, diversity and size of living benthic foraminiferal assemblages in the NE sector of Guanabara Bay (Brazil)

The disposal of waste and sewage by anthropic activities in Guanabara Bay (GB), Rio de Janeiro State (Brazil) is negatively affected this coastal ecosystem over the last few decades. This research intends to document how organic matter and metal enrichment affect the benthic organisms. Physicochemical, textural, geochemical and microfaunal data (foraminifera) are analyzed in nine stations in the NE sector of Guanabara Bay. The size and structure of the living foraminiferal assemblages are analyzed on each sedimentary fractions (63-150 mu m, 150-250 mu m, 250-500 mu m and >500 mu m) of every station. In the fraction >500 mu m no foraminifera were found. Results suggest that organic matter contents are high everywhere whereas metals concentrations increase in Sao Goncalo region. The dimension, diversity and equitability of the living foraminiferal assemblages were reduced in the study area. Ammonia tepida was the dominant taxon in all samples. Living specimens identified in the 63-150 mu m sedimentary fraction were found at all stations except in front of Sao Goncalo city (stations GB9 and GB8). Most of species were not found in the 250-500 mu m sediment fraction in most sites except for instance Ammonia tepida. This species dominate in eutrophic areas but where organic matter is in an oxide stage and regardless of whether there is or not gas seep. Foraminiferal density and diversity increased in areas with a prevalence of organic matter from oceanic biological production and declined in areas where organic matter and metals are retained in disoxic/anoxic phases of the sediments.