DIFFERENCES IN NUTRIENT CONCENTRATIONS AND RESOURCES BETWEEN SEAGRASSCOMMUNITIES ON CARBONATE AND TERRIGENOUS SEDIMENTS IN SOUTH SULAWESI,INDONESIA

Water column, sediment and plant parameters were studied in six tropic al seagrass beds in South Sulawesi, Indonesia, to evaluate the relatio n between seagrass bed nutrient concentrations and sediment type. Coas tal seagrass beds on terrigenous sediments had considerably higher bio mass of phytoplankton, epiphytic algae and macroalgae, if compared to seagrass beds growing on carbonate sediments in oligotrophic reef flat environments. The size of leaves of seagrass plants of the same speci es was considerably larger at terrigenous sites than at carbonate site s. Seagrasses in carbonate-rich environments had invested considerably more in below-ground biomass relative to above-ground biomass than th ose growing on terrigenous sediments. Elementary composition of plant material indicated a richer nutrient supply of both N and P at the ter rigenous sites (C:N:P = 340:19:1) than at carbonate sites (C:N:P = 565 :18:1). Concentrations of dissolved reactive phosphate, ammonium, and nitrate + nitrite were low (<2 mum) in the water column at all sites, often below detectable limits, but considerably higher in sediment por ewaters. Porewater phosphate concentrations (3-13 mum) were comparable between the two sediment types, but exchangeable phosphorus contents were 2 to 5 times higher in carbonate sediments (18.6-23.6 mg P2O5.100 g-1 versus 4.4-10.9 mg P2O5.100 g-1 in terrigenous sediments). Porewa ter phosphate concentration decreased with increasing sediment depth i n carbonate sediments. The relatively coarse composition of these sand s might have limited the adsorption of phosphate onto the carbonate mi neral surfaces. Carbonate sediments were extremely low in organic matt er, compared to terrigenous sediments. Both biogeochemical properties of the sediment type and the degree of influence from terrigenous run- off were found to be important factors affecting the availability of n utrients to seagrass growth and determining the response in morphology , biomass and chemical composition of the seagrass material.