Foraminifera and thecamoebians as indicators of hydrodynamics in a tropical delta – Paraíba do Sul River Delta, Brazil

Abstract Deltas are dynamic transitional environments where complex morphology and hydrodynamics have a great influence on benthic communities. This study investigates the distribution of foraminifera and thecamoebians in the Paraíba do Sul River Delta (Brazil), with emphasis on distinguishing living assemblages (biocenosis) from dead assemblages (thanatocoenosis). A total of 24 sediment samples were analysed for faunal composition, water parameters, and sedimentological characteristics. Results reveal significant discrepancies between living and dead assemblages, reflecting both ecological preferences and taphonomic processes such as dissolution, transport, and differential preservation. Ammonia tepida , Criboelphidium excavatum , Haplophragmoides wilberti and Trochamminita salsa dominated living assemblages. A higher diversity of calcareous species from the inner shelf characterized the dead assemblages, evidencing post-mortem transport from adjacent marine settings. The statistical analysis demonstrates clear sectorization of the delta: the northern sector is dominated by a marine influence zone and another with lagoon characteristics and overwash deposits; the southern sector is characterised by confinement conditions with mangrove fauna and a zone with a strong freshwater influence marked by thecamoebian species particularly Difflugia spp. and Centropyxis spp. These results underline the utility of combined foraminiferal and thecamoebian assemblages as sensitive indicators of hydrodynamic and environmental gradients in tropical deltas. By integrating biological, physical, and geochemical datasets, this study provides new insights into the functioning of a major South American deltaic system and supports the use of these protists as bioindicators in coastal monitoring and management strategies.