MEPS prepress abstract  -  doi: 10.3354/meps07362

ABSTRACT: Herbivory by the sea urchin Tripneustes gratilla on a meadow consisting of the co-occurring seagrass species Thalassia hemprichii (climax species), Halodule uninervis and Cymodocea rotundata (colonizing species) was studied to determine the influence of sea urchin grazing on seagrass biomass and species composition, to quantify herbivory rate and to determine their role at the nitrogen (N) cycling. In a two month enclosure experiment in Sulawesi, Indonesia, we increased T. gratilla density and measured seagrass shoot density and aboveground biomass. Belowground biomass was determined at the start and end of the experiment and leaf production in the end. Carbon (C) and N concentrations were measured for plant material. Increased T. gratilla grazing did not influence seagrass shoot density or areal leaf production. The herbivory had a negative impact on the biomass per shoot for all species, but only for H. uninervis and C. rotundata aboveground biomass declined significantly. The aboveground biomass of T. hemprichii relative to the other seagrasses increased significantly from 37 to 68% of the total biomass, while C. rotundata biomass declined significantly from 25 to 11% of the total biomass. No significant decline was measured for H. uninervis (from 38 to 21%). Over the experiment T. gratilla grazing significantly reduced the aboveground biomass with 74% but had no effect on belowground biomass. Grazing caused an increase in the leaf N concentration for H. uninervis and C. rotundata. The calculated N intake rate was 15.9 ± 1.1 mg N urchin^-1 d^-1. Total herbivory by T. gratilla in the meadow at an average urchin density of 1.55 ± 0.07 m^-2 was calculated to be 1.28 g DW m^-2 d^-1, corresponding to 26% of the net aboveground seagrass production. We conclude that T. gratilla consumes a considerable amount of the leaf production and even influences species composition in tropical seagrass meadows. T. gratilla grazing may play a role in preserving N for the meadow and create short-circuits in the N cycling of seagrass leaf material.