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 (K enya). Analysis of the stable isotope signature of sediment carbon in the seagrass zone and data on the sediment carbon content indicate tha t outwelling of particulate organic matter (POM) from the mangrove for est occurs, but that deposition of this POM rapidly decreases away fro m the forest. No evidence for any input of mangrove POM in the seagras s zone was found at a distance of 3 km from the mangrove creek, near t he reefs. The gradient in sediment deltaC-13 values in the seagrass zo ne was paralleled by a similar gradient of deltaC-13 values in Thalass odendron ciliatum, the dominant subtidal seagrass. This gradient proba bly reflects the availability of respiratory CO2 derived from mangrove POM as a carbon source for the seagrass. Analysis of C:N ratios of pa rticulate material (< 1 mm) collected with sediment traps in the seagr ass zone yielded values ranging from 8.5 to 11.2. This range is remark ably low compared to C:N ratios of plant material produced in the mang rove forest, and suggests that some of the mangrove-derived organic pa rticles deposited in the seagrass zone have gone through a phase of in tensive processing. During flood tides conspicuous decreases were foun d in deltaC-13 values of seston flowing over the seagrass zone, coinci ding with significant increases in the carbon content of the seston. T hese findings point to a reversed flux of organic particles from the s eagrass zone to the mangrove forest. Our data indicate that, as far as POM fluxes are concerned, the mangrove forest and adjacent seagrass m eadows are tightly coupled, but that the nearby coral reefs may exist in relative isolation.