Mass balance constraints on nutrient cycling in tropical seagrass beds

Abstract

A relatively simple mass balance model is presented to study the cycling of nutrients (nitrogen and phosphorus) in tropical seagrass beds. The model is based on quantitative data on nutrient availability, seagrass primary production, community oxygen metabolism, seagrass tissue nutrient 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 autotrophy/heterotrophy are discussed. It is concluded that, on an annual basis, the tropical seagrass ecosystems in the study area are more or less equally balanced between net autotrophy and net heterotrophy. Root uptake may potentially account for 66–98% of total nutrient uptake from external sources. Model simulation based on 50% internal nutrient resorption (withdrawal of nutrients from old plant parts and translocation to new growing parts) shifted the ratio between leaf vs. root-uptake, increasing the importance of leaf uptake. The results of these mass balance calculations indicate that root and leaf uptake contribute about equally to total plant nutrient uptake in tropical seagrass beds.