Trophic relationships in an interlinked mangrove-seagrass ecosystem as traced by delta13C and delta15N

Abstract

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) S13C value of -26.75 + 1.64% to seagrass beds with -16.23 + 4.35Y~. Seagrasses close to the mangroves were more depleted in ''C than seagrasses close to the major coral reef. Macroinvertebrates collected along this mangrove seagrass bed transect showed a similar 6C gradient. Fishes collected near the mangroves were depleted in I3C compared to fishes collected in the seagrass meadows. The fish community was dlfferentiated on the basis of its carbon isotopic ratlos and the site where individuals were collected. Three groups were identified: (1) species occurring in seagrass meadows in the close vicinlty of the mangrove swamps; (2) species mlgrating between mangroves and the seagrass meadows, together with species occurring throughout the entlre seagrass area, from close to the mangroves to the outer hay, 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 F1'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 isotope enrichment of <2%. was found between successive trophic levels suggesting significant omnivory.