Urban sewage lipids in the suspended particulate matter of a coral reef under river influence in the South West Gulf of Mexico

Nutritional quality of suspended particulate matter (SPM) and the degree of human fecal pollution in the largest coral reef system in the southwest Gulf of Mexico were evaluated using lipid classes, fatty acids (FA) and sterols in the dry and rainy seasons. High proportions of triacylglycerols and saturated and monounsaturated FA were detected in the SPM however it was considered poor quality because it had low proportions of highly unsaturated FA which can be used to determine production of marine biogenic material of dietary value to pelagic and benthic organisms. Urban sewage organic carbon was traced with coprostanol. The reference value of coprostanol from the point source of pollution was set using two samples from a sewage treatment plant processing waste from >140,000 people near the coral reef system, and it was contrasted with one river station and nine marine stations including six coral reefs. The concentration of coprostanol in the SPM was 3621 ± 98 ng L −1 comprising 26% of total sterols. During the dry season, the river was contaminated upstream with human feces as evidenced by coprostanol at 1823 ng L −1, the 5β-coprostanol: cholesterol ratio at 0.5, and 5β-coprostanol: [5α-cholestanol+5β-coprostanol] at 0.7. In contrast, marine stations had concentrations of coprostanol lower than a suggested regulation limit for tropical marine coastal waters (30 ng L −1 ), ranging between 6 and 28 ng L −1. During the rainy season a dilution effect was detected in the river, however significantly higher concentrations of coprostanol in the marine stations were detected ranging between 15 and 215 ng L −1, higher than the tentative tropical regulation range (30–100 ng L −1 ). Among the reefs, the nearshore one, 14.3 km from the treatment plant, was more exposed to human-fecal pollution, and offshore reefs, >17.3 km from the plant, had a lower degree of contamination. Finally, only three stations were clearly uncontaminated during both seasons including two reefs in the south located 21.8 and 35.6 km from the plant, with no presence of coprostanol. Contamination in the rainy season likely comes from a village with untreated sewage located 9.3–32 km from the reefs, and from the second largest Mexican river flowing into the Gulf of Mexico which has a watershed covering three states with lower than average sewage treatment. Inclusion of coprostanol monitoring could be a key factor in the management of this coral reef system.