Foraminiferal and testate amoeba diversity, distribution and ecology in transitional environments of the Tramandaí Basin (Rio Grande do Sul, South Brazil)

In order to document benthic foraminiferal and testate amoeba diversity, a total of 115 sediment samples were collected from nine different transitional environments along a 100-km long coastal area located in Rio Grande do Sul (Brazil). This area is directly affected by both the input of freshwater from the Tramandai River and a marine influence due to its proximity to the Atlantic Ocean. In particular, 14 and 101 species are recognized within the benthic foraminiferal and testate amoeba assemblages, respectively. Testate amoebae diversity is significantly higher than that documented in previous investigations carried out in other parts of the Brazilian coast. The two dominant testate amoeba families are Difflugidae and Hyalospheniidae, which are represented by 74 and 9 species, respectively. On the other hand, the benthic foraminiferal assemblages are poorly diversified and mainly dominated by agglutinated forms that are typical of transitional environments under the direct influence of a freshwater input. The distribution of these two groups characterizes both the freshwater environments, where testate amoebae are the only representatives, and the more marine conditions, where benthic foraminifera tend to dominate the benthic community, and allows the recognition of a marine influence gradient. The benthic foraminiferal assemblages are mainly observed in front of the direct opening to the sea, but are completely absent in the innermost environments, whereas testate amoebae are more widely distributed, albeit in variable abundances, but are completely missing close to the Tramandai Basin’s outlet. Although the diversity of the benthic community in South Atlantic coastal regions is difficult to determine, this study represents, to our knowledge, the first investigation of benthic foraminifera and testate amoebae along the 100-km long coastal area, characterized by lagoons and lakes interconnected by canals and small rivers.