Land-use impacts on Amazon and Atlantic rainforest coastal ecosystems revealed by multiple biogeochemical markers

Land-use changes were assessed using isotopic and elemental composition of organic matter (OM) and organic pollutants in estuarine sediments from two Brazilian basins: the Caeté River (CR, Amazon Rainforest) and the Paraíba do Sul River (PSR, Atlantic Rainforest). Between 1985 and 2024, anthropogenic land use in CR increased in 39%, while PSR decreased by 2.6%. CR sediments exhibited lower δ¹³C_org (-27.3 ± 0.9‰) and δ¹⁵N_total (2.1 ± 2.2‰) than PSR (-23.6 ± 2.9‰ and 5.6 ± 1.0‰, respectively). Seasonal variability occurred only in CR, with more ¹³C-depleted and ¹⁵N-enriched values in the rainy season, suggesting surface runoff of C₃-derived OM from rainforest or mangroves and upstream anthropogenic inputs. The Bayesian mixing model (MixSIAR) indicated clear contrasts in OM sources between basins. In the CR basin, sedimentary OM was predominantly derived from mangroves (median = 0.76), with a secondary contribution from sewage (0.18). In contrast, in the PSR basin, sewage was the dominant OM source (0.65), followed by mangrove contributions (0.28). Organic pollutants correlated with OC, showing that contaminant distribution is partly governed by OM dynamics. PAH diagnostic ratios revealed the sewage, biomass/coal burning as the main source in PSR, with additional fossil-fuel inputs, while CR reflected mixed combustion sources linked to agricultural fires and boat traffic. Organochlorine patterns in CR suggest historical pesticide use and diffuse atmospheric input, while PSR shows lindane and local/industrial influences. These results highlight contrasting OM dynamics under different land-use intensities and the dual role of estuaries in storing carbon and pollutants.