MASS-BALANCE CONSTRAINTS ON NUTRIENT CYCLING IN TROPICAL SEAGRASS BEDS

A relatively simple mass balance model is presented to study the cycli ng of nutrients (nitrogen and phosphorus) in tropical seagrass beds. T he model is based on quantitative data on nutrient availability, seagr ass primary production, community oxygen metabolism, seagrass tissue n utrient contents, sediment-water nutrient exchange rates, and seasonal dynamics in seagrass and environmental variables for two contrasting seagrass meadows from a single study area in South Sulawesi (Indonesia ). Possible pathways of nutrient acquisition and nutrient cycling are described and quantified, and implications for leaf/root uptake and au totrophy/heterotrophy are discussed. It is concluded that, on an annua l basis, the tropical seagrass ecosystems in the study area are more o r less equally balanced between net autotrophy and net heterotrophy. R oot uptake may potentially account for 66-98% of total nutrient uptake from external sources. Model simulation based on 50% internal nutrien t resorption (withdrawal of nutrients from old plant parts and translo cation to new growing parts) shifted the ratio between leaf vs. root-u ptake, increasing the importance of leaf uptake. The results of these mass balance calculations indicate that root and leaf uptake contribut e about equally to total plant nutrient uptake in tropical seagrass be ds.