Origem da matéria orgânica sedimentar no delta-estuarino do Rio São Francisco, AL/SE - Brasil

Abstract

The São Francisco River (RSF) has a cascade of dams, constructed from 1975 to 1994, along its middle and middle-lower basin, which reduced the discharge, the input of dissolved and the particulate matter to the coastal zone. This study characterized the relative contributions of terrestrial and marine sources of organic matter for the total sediments of the RSF estuary, in 15 surface samples. The spatial grain size variability, in which fine-grained sediments are less than 70% of the total, indicates highly dynamic characteristic of the region. Deposi- tional areas were identified only in the enclosed part of the mangrove and in a fluvial embayment. The amount of organic matter increased as the fine-grained sediment fraction (<63 μm) increases, the instead of which highest values were observed in enclosed regions. The low values of organic carbon (0.02–6.64%), total nitrogen (0.00–0.33%), total phosphorus (18–539 μg/g) and C/N molar ratio (11–45) in the sediments reflect an oligotrophic environment. The δ13C and δ15N in the sediments vary, respectively, from -28.24 and 2.20‰ in the mangrove area to -20.30 and 8.21‰ in the small fluvial embayment. These values reflect strong influence of isotopic signature of the three important plant species in the region: Elodea sp. (-15.77 and 4.10‰), Montrichadia linifera Schott (-26.93 and 5.48‰) and Rhizophora mangle (-28.21 and 3.05‰). The isotopic signature of some samples also indicates a contribution of a fourth significant source of organic matter to the region, possibly the phytoplankton. Lignin confirmed the importance of the three plant species analyzed, indicating the main contribution of riverine-terrestrial sources and evidencing that the sedimentary organic matter was little degraded.