Influence of the Amazon River on the composition of particulate organic carbon in the western tropical Atlantic Ocean

Particulate organic carbon (POC) from surface and multiple depths up to 2000 m, and from sediment traps (placed at 150 and 250 m deep) were collected in the tropical Atlantic Ocean under high and low Amazon River discharges. Sources and levels of POC samples were assessed through bulk elemental and stable carbon isotopic composition, as well as molecular biomarkers. Carbon contents of POC in mesohaline plume waters were up to 10 times higher than those in the non-plume sites with higher salinity. Overall, stable carbon isotopic values of POC samples were enriched, averaging -21.0‰ (± 3.0) in the plume sites. Major contributions to surface POC included low molecular weight fatty acids (saturated and unsaturated) and mono- and disaccharides, i.e., short-lived compounds and indicators of fresh inputs. These biomarkers are presumably derived from algal blooms (primarily diatoms) observed along the core of the plume during both high and low discharges, and were vertically transported and recorded at lower levels in the sediment traps. Terrestrial biomarker inputs, including plant wax homologous series, triterpenoids, and levoglucosan were minor in all samples, including those influenced by the Amazon plume, even at high discharge. Zooplankton (e.g., cholesterol, occelasterol) and bacteria (e.g., branched alkanoic acids) biomarkers were relatively higher in the sediment traps compared to the surface POC. Our preliminary results showed that while sediment traps recorded predominantly inputs from bacteria/zooplankton activity, the overlying POC samples showed a marked autochthonous input, likely from algae.