Leaf production and nutrient contents of the seagrass Thalassodendron ciliatum in the proximity of a mangrove forest (Gazi Bay, Kenya)

Abstract

Mangrove forests, seagrass meadows and coral reefs may occur as adjacent ecosystems in tropical coastal zones, where tide-mediated chemical fluxes allow one system to influence another. Previously, stable carbon isotope ( 13 C 12 C ) analyses have been used to show that outwelling of carbon from the mangrove forest of Gazi Bay (Kenya) was followed by trapping of this element in the adjacent seagrass zone. In the present study it was investigated whether the input of mangrove carbon coincides with shifts in functional and chemical characteristics of Thalassodendron ciliatum (Forsk.) den Hartog, the dominant subtidal seagrass. It appeared that the input of mangrove carbon did not coincide with enhanced leaf production of T. ciliatum, nor with consistent shifts in its nitrogen and phosphorus contents. Presumably, carbon outwelling from the mangrove coincides with an only limited export of nitrogen and phosphorus, and the restricted effects of these nutrients on the seagrasses (if any) are masked by other, local factors. The outwelling of mangrove carbon probably includes, in addition to particulate organic matter, dissolved inorganic compounds without nitrogen and phosphorus constituents, such as carbon dioxide and bicarbonate. Although in Gazi Bay seagrass beds are directly adjacent to a mangrove forest and connected with this forest via the shuttle movement of the tidal water, the influence of abiotic fluxes from the mangrove forest on the functioning of the seagrass beds appears to be inconspicuous.