Changes in the nature of organic matter in a large tropical urban bay: Temporal variations and environmental implications

Abstract

We used information on the elemental (C/N ratio), isotopic (δ13C and δ15N) and molecular (aliphatic hydrocarbons) composition of organic matter in sediments from two cores collected in the Todos os Santos Bay to investigate the sources, distribution and fate of organic matter (OM). TOC (0.66–2.23%) and TN (0.12–0.26%) concentrations increased over time but still indicate an oligotrophic system. A simple isotopic mixing model shows a mixture of sources, with autochthonous OM dominating (47.1–93.2% of TOC). ALCTOT concentrations in the sediments varied from 0.25 to 5.84 µg g−1. The ACL (27.79–29.89) and AI (0.02–0.57) indices show the dominance of the terrestrial contribution coming from C3 vascular plants, such as the mangrove vegetation present on the BTS tidal flats. In core T2, collected at a more offshore site, the parameters analyzed indicate better environmental conditions with natural sources of hydrocarbons. At the T4 core collection site, which was close to the shoreline, a gradual and persistent increase in terrestrial input from 1950 onward was detected, probably related to changes in land use. From 1980 onward, a clear petrogenic signature is seen with the presence of a complex unresolved mixture (nd to 349.94 µg g−1) and increased concentrations of total resolved aliphatic hydrocarbons (0.35–380.34 µg g−1) and normal alkanes (0.25–5.84 µg g−1). Three decades of moderate chronic contamination are revealed in the T4 core, reflecting the history of economic development of the region, including extraction, refining, transportation and burning of fossil fuels and changes in land use. The effects of implementing environmental policies to improve the Bay's water quality began to be reflected in the sediment quality at the beginning of the 21st century.