Tracking the record of sewage discharge off Jeddah, Saudi Arabia, since 1950, using stable isotope records from antipatharians

In 2007, a study was undertaken of the long-term records of the ratios of stable isotopes of carbon (δ13C) and nitrogen (δ15N) in the skeletons of antipatharians (black corals) growing on reefs near the city of Jeddah, Saudi Arabia. Antipatharians were collected from more than 100 sites, and detailed records obtained from 31 individuals from 16 sites. Annual growth bands were identified and counted. Precision of age estimates was verified by bomb radiocarbon analyses. Some of the specimens were >70 yr old. Samples from a comparison reef, 15 km offshore in the Red Sea, showed δ15N values of 4‰ and δ13C values of about –18‰. The early years of growth of antipatharians from affected areas also showed δ15N values of 4‰ and δ13C values of about –18‰, but then in subsequent years these values changed, presumably as water quality deteriorated. Records from specimens in the harbour showed a steady increase in δ15N that paralleled population growth in the city — values from 2007 frequently exceeded 8.0‰. The carbon record also showed progressive change: since 1950, values of δ13C in affected areas gradually moved from about –18 to –17‰. We postulate that δ15N values track the input of sewage-dominated wastewater. By using correlations with species of algae that co-occur with antipatharians on outerand mid-shelf reefs, we were able to build up a spatial and temporal picture of 60 yr of sewage input off this coastline. The advantage of the use of bioindicators is the ability to provide baseline water quality data in places where no such data exist.