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-re lease fertilizers was studied (4 to 5 months after fertilization) in t hree tropical seagrass beds in South Sulawesi, Indonesia. Nitrogen-add ition and P-addition had no significant effects on seagrass biomass, s hoot density, and leaf production in mixed seagrass beds in a terrigen ous sandy bay and a carbonate sedimentary reef Aat environment, simult aneously fertilized in 1990. An additional experiment using a slightly adapted methodology (1991) at another carbonate site, characterized b y a more homogeneously vegetated, and largely monospecific seagrass me adow 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 loc ality. The relatively high ambient porewater nutrient concentrations ( about 10 mu M PO4, about 100 mu M NH4) and rather low C:N:P ratios (56 5:18:1) in seagrass tissue, indicate that nutrient supply meets seagra ss demand and provide an explanation for the lack of response to the m anipulations. The relatively high porewater phosphate concentrations i n 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 carbo nate-rich environments.