Trophic relationships in an interlinked mangrove-seagrass ecosystem as traced by delta C-13 and delta N-15

The food web structure of a mangrove forest and adjacent seagrass beds in Gazi Bay, Kenya, was examined with stable carbon and nitrogen isotope ratio techniques. A carbon isotopic ratio gradient was found from mangroves with mean (+/-SD) delta(13)C value of -26.75 +/- 1.64 parts per thousand to seagrass beds with -16.23 +/- 4.35 parts per thousand. Seagrasses close to the mangroves were more depleted in C-13 than seagrasses close to the major coral reef. Macroinvertebrates collected along this mangrove seagrass bed transect showed a similar delta(13)C gradient. Fishes collected near the mangroves were depleted in C-13 compared to fishes collected in the seagrass meadows. The fish community was differentiated on the basis of its carbon isotopic ratios and the site where individuals were collected. Three groups were identified: (1) species occurring in seagrass meadows in the close vicinity of the mangrove swamps; (2) species migrating between mangroves and the seagrass meadows, together with species occurring throughout the entire seagrass area, from close to the mangroves to the outer bay; and (3) species that use the seagrass meadows proper as a lifetime habitat. The results show that seagrass stands are the main feeding grounds providing food for all fish species studied. delta(15)N signatures allowed to the identification of the following trophic levels: (1) fish species feeding on seagrasses and macroalgae (herbivores); (2) fish feeding on zoo/benthos-plankton (zoobenthiplanktivores); and (3) other fish and/or macro-crustacea (piscivores/benthivores). A N-15 isotepe enrichment of <2 parts per thousand was found between successive trophic levels suggesting significant omnivory.