CARBON OUTWELLING FROM A MANGROVE FOREST WITH ADJACENT SEAGRASS BEDS AND CORAL-REEFS (GAZI BAY, KENYA)

Carbon fluxes from a mangrove creek with adjacent seagrass meadows and coral reefs (at 4 km from the creek) were investigated in Gazi Bay (Kenya). Analysis of the stable isotope signature of sediment carbon in the seagrass zone and data on the sediment carbon content indicate that outwelling of particulate organic matter (POM) from the mangrove forest occurs, but that deposition of this POM rapidly decreases away from the forest. No evidence for any input of mangrove POM in the seagrass zone was found at a distance of 3 km from the mangrove creek, near the reefs. The gradient in sediment deltaC-13 values in the seagrass zone was paralleled by a similar gradient of deltaC-13 values in Thalassodendron ciliatum, the dominant subtidal seagrass. This gradient probably reflects the availability of respiratory CO2 derived from mangrove POM as a carbon source for the seagrass. Analysis of C:N ratios of particulate material (< 1 mm) collected with sediment traps in the seagrass zone yielded values ranging from 8.5 to 11.2. This range is remarkably low compared to C:N ratios of plant material produced in the mangrove forest, and suggests that some of the mangrove-derived organic particles deposited in the seagrass zone have gone through a phase of intensive processing. During flood tides conspicuous decreases were found in deltaC-13 values of seston flowing over the seagrass zone, coinciding with significant increases in the carbon content of the seston. These findings point to a reversed flux of organic particles from the seagrass zone to the mangrove forest. Our data indicate that, as far as POM fluxes are concerned, the mangrove forest and adjacent seagrass meadows are tightly coupled, but that the nearby coral reefs may exist in relative isolation.