Primary production of seagrass beds in South Sulawesi (Indonesia):A comparison of habitats, methods and species

Primary production of tropical seagrass meadows was studied between April and August 1990 in South Sulawesi, Indonesia. Oxygen evolution studies in enclosures over seagrass vegetation revealed gross community production values between 900 and 4400 mg C m-2 day-1. Assumed community respiration ranged from 800 to 1800 mg C m-2 day-1 in non-vegetated areas and from 1400 to 5100 mg C m-2 day-1 for areas with varying amounts of seagrass. Benthic community respiration varied considerably in response to diurnal fluctuations in the dissolved oxygen concentration in the water column. Net production was small (less than 500 mg C m-2 day-1) and slightly negative on seven of 12 occasions. Light compensation points ranged from 50 to 340 muE m-2 s-1. Bell jar measurements revealed no significant differences in seagrass production between coastal and reef island habitats. Leaf marking experiments with Enhalus acoroides (L.f.) Royle revealed significantly higher leaf growth in a coastal muddy area (3.1+/-0.8 cm per shoot day-1) than at an offshore sandy reef site (1.6+/-0.5 cm per shoot day-1), but relative growth rates were comparable between the two habitats (0.019+/-0.005 g g-1 leaf biomass (AFDW) day-1 and 0.012+/-0.003 g g-1 AFDW day-1, respectively). Leaf marking and plastochrone interval methods revealed similar results for net leaf production in a monospecific stand of Thalassia hemprichii (Ehrenb.) Aschers. at a reef site which averaged 1.6 g C m-2 day-1. Rhizome tagging experiments revealed net rhizome production values of 0.1-0.3 g C m-2 day-1 for Thalassia hemprichii at this reef site. Relative growth rate of leaves of Thalassia hemprichii (0.039+/-0.010 g g-1 AFDW day-1) at the reef site was significantly higher than that of E. acoroides (0.012+/-0.003 g g-1 AFDW day-1). The pioneering seagrass species Halodule uninervis (Forssk.) Aschers. and Cymodocea rotundata Ehrenb. & Hempr. ex Aschers. had considerably higher horizontal apical rhizome growth rates (0.7-0.8 cm per apex day-1) than the constant species Thalassia hemprichii (0.1 cm per apex day-1). The suitability of different methods to measure seagrass productivity is evaluated.