SEDIMENT-NUTRIENT INTERACTIONS IN TROPICAL SEAGRASS BEDS - A COMPARISON BETWEEN A TERRIGENOUS AND A CARBONATE SEDIMENTARY ENVIRONMENT IN SOUTH SULAWESI (INDONESIA)

The relationship between porewater nutrient concentrations and sediment characteristics was studied in seagrass beds on 2 sediment types in South Sulawesi, Indonesia. Porewater nutrient concentration gradients with sediment depth and ratios between ammonium and phosphate porewater concentrations in a terrigenous muddy sedimentary environment could be explained by modelling based on stoichiometric decomposition of organic material and molecular diffusion. Measured porewater phosphate concentrations in a carbonate sedimentary environment, however, were significantly higher (10 muM excess) in the upper few cm of the sediment than would be expected based on stoichiometry. This apparent phosphate enrichment is attributed to rapid regeneration of both N and P in the rhizosphere and subsequent rapid removal of ammonium by nitrification. Sampling artefacts and additional geochemical sources of dissolved P (reduction of hydrous ferric oxides, calcium carbonate dissolution) could be excluded as the cause of the enrichment. The capacity of carbonate sediments to adsorb phosphate was directly related to their grain-size composition. The coarse-grained carbonate sediment in the area maintained relatively high porewater phosphate concentrations as a result of its limited adsorption capacity, in contrast to extremely low porewater phosphate concentrations reported from fine-grained carbonate sediments in the Caribbean, where strong evidence for P-limitation of seagrass growth has been found.