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 isot ope 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 sea grasses close to the major coral reef. Macroinvertebrates collected al ong this mangrove seagrass bed transect showed a similar delta(13)C gr adient. Fishes collected near the mangroves were depleted in C-13 comp ared to fishes collected in the seagrass meadows. The fish community w as differentiated on the basis of its carbon isotopic ratios and the s ite where individuals were collected. Three groups were identified: (1 ) species occurring in seagrass meadows in the close vicinity of the m angrove swamps; (2) species migrating between mangroves and the seagra ss meadows, together with species occurring throughout the entire seag rass area, from close to the mangroves to the outer bay; and (3) speci es that use the seagrass meadows proper as a lifetime habitat. The res ults show that seagrass stands are the main feeding grounds providing food for all fish species studied. delta(15)N signatures allowed to th e identification of the following trophic levels: (1) fish species fee ding on seagrasses and macroalgae (herbivores); (2) fish feeding on zo o/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 sug gesting significant omnivory.