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

Abstract

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 360:107-114 (2008) - DOI: https://doi.org/10.3354/meps07362 In situ quantification of Tripneustes gratilla grazing and its effects on three co-occurring tropical seagrass species Jan Arie Vonk1,2,*, Marjolein H. J. Pijnappels1, Johan Stapel1 1Department of Environmental Science, Institute for Wetland and Water Research, Faculty of Science, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands 2Present address: Laboratory for Ecological Risk Assessment, National Institute for Public Health and the Environment (RIVM), PO Box 1, 1370 BA Bilthoven, The Netherlands *Email: javonk@gmail.com ABSTRACT: 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. KEY WORDS: Herbivory · Species composition · Tropical seagrass · Sea urchin · Tripneustes gratilla · Indonesia Full text in pdf format PreviousNextCite this article as: Vonk JA, Pijnappels MHJ, Stapel J (2008) In situ quantification of Tripneustes gratilla grazing and its effects on three co-occurring tropical seagrass species. Mar Ecol Prog Ser 360:107-114. https://doi.org/10.3354/meps07362Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 360. Online publication date: May 22, 2008 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2008 Inter-Research.