Concentrations, spatial distributions, and origins of natural aliphatic lipids and sterols in sediments of Obhur Lagoon, Red Sea Coast, Saudi Arabia

Samples from the upper surface sediments of Obhur Lagoon, located north of Jeddah, were collected to analyze the concentrations, spatial distribution, and origins of natural lipids. The lagoon was segmented into three zones based on their surrounding ecosystems: Z I (adjacent to inland areas), Z II (the transitional region between Z I and the coastal Z III), and Z III (the coastal region). The primary natural biogenic lipid compounds found in the total extractable organic matter (TEOM) included n-alkanes (partial), fatty acids, fatty alcohols, and steroids. The n-alkanes of biogenic origin were primarily derived from terrestrial higher plant wax, decreasing from approximately 38% in Z I to 12% in Z III. Conversely, contributions from aquatic algae and diatoms increased from about 1% to 8% from Z I to Z III, while microbial inputs declined from roughly 3%–0.5% over the same zones. The relative concentrations of fatty acids from higher plants varied, accounting for about 6% in Z I, 4% in Z II, and 5% in Z III. Aquatic algae sources contributed approximately 80% in all regions, and microbial inputs slightly decreased from 14% to 12% from Z I to Z III. Terrestrial inputs of fatty n-alcohols declined from around 32% in Z I to 11% in Z III, while contributions from aquatic algae and diatoms decreased from about 62% to 45% from Z I to Z III, with microbial inputs remaining around 10%. Steroid inputs from terrestrial plants decreased from 37% in Z I to 16% in Z III, while those from aquatic biota increased from 58% in Z I to 76% in Z III. The microbial inputs of steroids were highest in Z III (11.5%), followed by Z II (9.9%) and Z I (9.4%). The total natural lipid contributions from terrestrial sources decreased from 42.8% in Z I to 19.2% in Z III, while the aquatic source component increased from 53.0% in Z I to 77.4% in Z III. These results suggest that the lagoon's biogeochemistry is influenced by the surrounding ecosystems, lagoon hydrodynamics, and local human and social activities.