Characterization of the organic matter produced by Atlantic Rainforest plants and its influence in the surface sediments deposited in a protected subtropical Estuarine–Lagoon system

The aim of the present study was to characterize the distribution and sources of terrigenous organic matter (OM) in surface sediments of a subtropical estuarine–lagoon​ system surrounded by the Atlantic Rainforest. n-Alkanes and their isotopic composition were evaluated in 20 surface sediment samples, as well as in leaves from nine representative plant species collected from the Cananéia–Iguape Estuarine–Lagoon​ System (CIELS; Brazil). According to the calculated n-alkanes indices, the analyzed plants could be grouped as monocotyledonous (aquatic productivity index [Paq] > 0.1; carbon preference index [CPI] < 6), dicotyledonous (Paq < 0.1; CPI > 10), mangrove (average chain length [ACL23−33] < 29; ratio of n-C31 over n-C29 concentration [Norm31] < 0.1) and riparian (ACL23−33 > 29; Norm31 > 0.2) vegetation. δ13C n-alkane values were higher (> −26‰) in the C4 aquatic macrophyte analyzed (Spartina alterniflora), with intermediate values (−30 to −27‰) for the pteridophyte (Pteridium aquilinum), and lower values for the C3 species (< −31‰). In sediment, total n-alkanes, total organic carbon, and total nitrogen concentrations ranged from 510 to 10,000 ng g−1 in dry sediment, 0.15 to 6.37% and not detected to 0.74%, respectively, being governed by the mud composition. ACL23−33 and the Norm31 values ranged from 29.0 to 30.4 and 0.39 to 0.57, respectively. Paq and CPI values ranged from 0.06 to 0.25 and 4.2 to 6.6, respectively. These indexes showed that the main terrigenous OM source to CIELS sediment was the riparian vegetation, coming from the rivers that discharge into the system. The n-alkanes δ13C composition, ranging from −36.0 ± 0.0 to −24.8 ± 0.3‰, also showed contributions of macrophytes input to the deposited OM. The influence of mangrove trees is not dominant but increases in the CIELS eastern region, where this kind of vegetation is more developed.