Benthic foraminifera as indicators of river discharge in the Western South Atlantic continental shelf

Abstract

The present work focuses on freshwater signatures at the sediment-water interface (1 cm) identified using foraminifera. Samples were collected during both the Southern Hemisphere winter and summer along eleven longitudinal transects on the Western South Atlantic continental margin between 27° and 37°S, at water depths of 11.7 to 250 m. Here we show that depth, salinity, temperature, oxygen, grain size (mud and sand percentages), suspended particulate matter, organic matter, SiO4, NO2, and NO3, in this order of importance, are responsible for the distribution of foraminiferal species and thecamoebians. The presence of these microfossils, which indicate freshwater influx in four sectors over the continental shelf: Itajaí-Açu River, Laguna estuarine system, Patos Lagoon and Rio de la Plata (RdlP), are explored further in detail. A freshwater signature in the continental shelf sediments indicates deposition sites wherein large amounts of pollutants and contaminants transported by river outflows on the shelf eventually accumulate in the ocean. Foraminiferal microhabitat occupation within these zones is discussed in detail. We conclude that together with the fauna, abiotic parameters are important in highlighting the occurrence and degree of marine impact induced by inputs from polluted river waters, including possible hypoxic environments on the shelf. Our findings of freshwater signatures on the continental shelf sediments using benthic species as bioindicators are comparable to reports from other continental shelves worldwide, and a paleo-record study could be useful for three South American countries (Brazil, Argentina and Uruguay).