THE LIMITED EFFECT OF IN-SITU PHOSPHORUS AND NITROGEN ADDITIONS TO SEAGRASS BEDS ON CARBONATE AND TERRIGENOUS SEDIMENTS IN SOUTH SULAWESI, INDONESIA

Seagrass response to in situ sediment nutrient enrichment with slow-release fertilizers was studied (4 to 5 months after fertilization) in three tropical seagrass beds in South Sulawesi, Indonesia. Nitrogen-addition and P-addition had no significant effects on seagrass biomass, shoot density, and leaf production in mixed seagrass beds in a terrigenous sandy bay and a carbonate sedimentary reef Aat environment, simultaneously fertilized in 1990. An additional experiment using a slightly adapted methodology (1991) at another carbonate site, characterized by a more homogeneously vegetated, and largely monospecific seagrass meadow of Thalassia hemprichii, again showed no significant response of seagrass growth, biomass, and shoot density to fertilization. C and N contents of plant tissue, however, had increased significantly (p < 0.05) to 12-25% above the controls in response to N-addition at this locality. The relatively high ambient porewater nutrient concentrations (about 10 mu M PO4, about 100 mu M NH4) and rather low C:N:P ratios (565:18:1) in seagrass tissue, indicate that nutrient supply meets seagrass demand and provide an explanation for the lack of response to the manipulations. The relatively high porewater phosphate concentrations in carbonate sediments are attributed to the relatively coarse particle-size distribution of these sediments in the study area, which limits their adsorption capacity and prevents P-limitation of seagrass growth, which is generally considered to be characteristic of tropical carbonate-rich environments.