Influence of environmental gradients on C and N stable isotope ratios in coral reef biota of the Red Sea, Saudi Arabia

The Red Sea features a natural environmental gradient characterized by increasing water temperature, nutrient and chlorophyll a concentrations from North to South. The aim of this study was to assess the relationships between ecohydrography, particulate organic matter (POM) and coral reef biota that are poorly understood by means of carbon (delta C-13) and nitrogen (delta N-15) stable isotopes. Herbivorous, planktivorous and carnivorous fishes, zooplankton, soft corals (Alcyonidae), and bivalves (Tridacna squamosa) were a priori defined as biota guilds. Environmental samples (nutrients, chlorophyll a), oceanographic data (salinity, temperature), POM and biota were collected at eight coral reefs between 28 degrees 31'N and 16 degrees 31'N. Isotopic niches of guilds separated in delta C-13 and delta N-15 isotopic niche spaces and were significantly correlated with environmental factors at latitudinal scale. Dietary end member contributions were estimated using the Bayesian isotope mixing model SIAR. POM and zooplankton N-15 enrichment suggested influences by urban run-off in the industrialized central region of the Red Sea. Both delta N-15 and their relative trophic positions (RTPs) tend to increase southwards, but urban runoff offsets the natural environmental gradient in the central region of the Red Sea toward higher delta N-15.and RTPs. The present study reveals that consumer delta C-13 and delta N-15 in Red Sea coral reefs are influenced primarily by the latitudinal environmental gradient and localized urban runoff. This study illustrates the importance of ecohydrography when interpreting trophic relationships from stable isotopes in Red Sea coral reefs. (C) 2013 Elsevier B.V. All rights reserved.