Natural aliphatic lipids and sterols in sediments from Obhur Lagoon, Red Sea coast of Saudi Arabia: Concentrations, spatial distributions, and sources

Abstract

Samples from the upper surface sediments of Obhur Lagoon - north Jeddah were collected to determine the concentrations, spatial distribution, and sources of natural lipids. The lagoon was divided into three zones based on their immediate ecosystems: Z I (adjoining inland), Z II (the region between Z I and the adjacent coastal Z III), and Z III (coastal region). The major natural biogenic lipid compounds of the total extractable organic matter (TEOM) were n-alkanes (partial), fatty acids, fatty alcohols, and steroids. The n-alkanes of biogenic sources were mainly from terrestrial higher plant wax and decreased from about 38% to 12% from Z I to Z III. Their aquatic algal and diatom sources increased from ~ 1% to 8% for Z I to Z III and microbial inputs decreased from ~ 3% to 0.5% for Z I to Z III. Relative concentrations of fatty acid inputs from higher plants varied from ~ 6% in Z I, 4% in Z II, and 5% in Z III; from aquatic algae sources ~ 80% in all regions; and from microbes ~14-12% with a slight decrease from Z I to Z III. The terrestrial input of fatty n-alcohols decreased from ~ 32% to 11% for Z I to Z III, from ~ 62% to 45% in Z I to Z III from aquatic algae and diatom sources, whereas microbial inputs varied around 10%. Steroid inputs from terrestrial plants were in decreasing order from Z I (37%) to Z III (16%), whilst from the aquatic biota, they increased from Z I (58%) to Z III (76%). The microbial inputs of steroids were in the order of Z III (11.5%) > Z II (9.9%) > Z I (9.4%). The contributions of the total natural lipids from terrestrial sources decreased from Z I (42.8%) to Z III (19.2%), whereas the aquatic source component increased from Z I (53.0%) to Z III (77.4%). The results indicate that the lagoon biogeochemistry is influenced by the immediate ecosystems, hydrodynamic of the lagoon, and human and social activities in the area.