Study of the origin and fate of organic matter in a tropical bay using elemental, isotopic and fatty acid biomarkers.

Abstract

Organic matter (OM) exchanges between adjacent coastal ecosystems affect the role of the different primary producers as energy and nutrient sources in food webs. Elemental and isotope parameters (C/N ratios, δ13C) and fatty acid (FA) biomarkers were used to assess the origin and distribution of OM in both surface sediment and water along a strongly tidally-influenced tropical area, Gazi Bay (Kenya), in two climatic seasons (dry and rainy). Dominant primary producers (i.e. mangroves, macroalgae and seagrasses) were also characterized through the same combined approach. Export of the mangrove-derived OM, highly depleted, to the adjacent bay was evident from a gradual 13C-enrichment of primary producers and sediment along the land-sea transect, with more depleted values close to the mangroves and more enriched ones in the coral reef. Contextually, a step-by-step decrease of both C/N ratio and mangrove FA biomarker concentration was also evident in the surface sediment. This export was higher in the rainy season but reduced from the wide seagrass bed area. This result is consistent with the buffer action of seagrasses, which efficiently act in trapping the suspended OM. Isotopic and FA signatures of the particulate OM gave indication of a mixed contribute of both allochthonous and autochthonous OM sources, but, unlike sediment, were overall homogeneous within the bay because of the mixing effect of tides. FA biomarkers revealed that brown algae, seagrasses and diatoms were also important OM sources in the whole bay. These results suggest an high ecological connectivity in Gazi Bay, which may have important consequences on the functioning of the whole ecosystem and on the local food webs.