δ 13 C, δ 15 N and TOC/TN as indicators of the origin of organic matter in sediment samples from the estuary of a tropical river.

The present study aimed to determine the total organic carbon (TOC), total nitrogen (TN), the carbon‑nitrogen ratio (TOC/TN), carbon isotope (δ ¹³ C), and nitrogen isotope (δ ¹⁵ N) in five sediment cores collected from upstream to downstream of the Rio Serinhaem estuary, State of Bahia, Northeast Brazil, in order to investigate the origin of the deposited organic matter (OM).Significant positive correlation was found between TOC and NT (r _s  = 0.75); TOC/TN and TOC (r _s  = 0.64); δ ¹⁵ N and TOC (r _s  = 0.72); and δ ¹⁵ N and TOC/TN (r _s  = 0.63). The values of δ ¹³ C and δ ¹⁵ N found are characteristic of terrestrial sources. The TOC/TN ratio confirmed the data found for δ ¹³ C and δ ¹⁵ N, which recorded the origin of organic matter from terrestrial C3 type plants. Upstream of the estuary, the highest means of TOC/TN were found (T1 = 36.9 and T2 = 24.4), as reflected by the increase in TOC content or reduction in TN. The OM along the estuary is predominantly from plants with a C3 photosynthetic pattern, indicating that the Serinhaem River estuary is considered a relatively well-preserved environment.
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