In situ quantification of Tripneustes gratilla grazing and its effects on three co-occurring tropical seagrass species

We studied sea urchin Tripneustes gratilla herbivory in a meadow of co-occurring seagrass species Thalassia hemprichii (climax species), Halodule uninervis and Cymodocea rotundata (colonizing species) to determine effects of sea urchins on seagrass biomass and species composition, to quantify herbivory rate and to determine the role of urchins in the nitrogen (N) cycle. In a 2 mo 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 at the end. Carbon (C) and N concentrations were measured in plant material. Increased T gratilla grazing did not influence seagrass shoot density or leaf production. Herbivory had a negative impact on the biomass per shoot for all species, but only H, uninervis and C. rotundata aboveground biomass declined significantly. The proportion of aboveground biomass made up by T hemprichii increased significantly from 37 to 68% of the total seagrass biomass, while C. rotundata biomass declined significantly from 25 to 11% of the total biomass. No significant decline occurred in H. uninervis (from 38 to 21%). Over the experiment, T gratilla grazing significantly reduced the aboveground biomass by 74%, but had no effect on belowground biomass. Grazing caused an increase in leaf N concentration for H. uninervis and C. rotundata. The calculated mean +/- SE N intake rate was 15.9 +/- 1.1 mg N urchin(-1) d(-1). Total consumption by T gratilla in the meadow at an average urchin density of 1.55 +/- 0.07 urchins m(-2) Was calculated to be 1.28 g dry weight 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 conserving N within the meadow and create short-circuits in the N cycling of seagrass leaf material.