SEARCH

SEARCH BY CITATION

Keywords:

  • seagrass biomass;
  • canopy height;
  • shoot density;
  • sea urchin abundance;
  • Tanzania

Abstract

  • 1.
    Seagrasses biomass, canopy height, shoot density, percentage cover, and sea urchin abundance were intermittently (between July and November 2007) studied at four littoral sites in the Dar es Salaam area (Mjimwema, Mbweni, Bongoyo Island and Mbudya Island) in order to investigate the seagrass–sea urchin association.
  • 2.
    Seagrass biomass ranged from 126.7±65.62 g dwt m−2 in the upper sub-tidal area at Bongoyo Island to 508.1±133.4 g dwt m−2 in the upper sub-littoral area at Mbudya Island. Canopy height ranged from 6.51±2.76 cm in the mid-littoral zone at Mjimwema to 23.8±8.93 cm in the upper sub-littoral zone at Mbudya Island. Shoot densities ranged from 363.6±268.9 shoots m−2 in the mid-littoral zone at Mjimwema to 744.0±466.9 shoots m−2 in the lower littoral zone at Mbudya Island.
  • 3.
    Seagrass biomass, canopy height and percentage cover differed significantly among study sites (P=0.001, 0.0001, 0.008 respectively). However, there was no significant difference in shoot density among the sites (P=0.376).
  • 4.
    Ten species of sea urchins were recorded, Echinometra mathaei being the most abundant followed by Tripneustes gratilla. Total sea urchin abundance was significantly different among the study sites (P=0.001). Seagrass–sea urchin interaction was depicted by significant negative correlations between sea urchin densities with seagrass biomass, canopy height, shoot density and percentage cover. This suggests that grazing by sea urchins might have contributed to the reduction of above ground seagrass biomass in locations with higher sea urchin densities. However, further studies are required to corroborate the present results and assess effects of other factors (e.g. light, nutrients and currents), which also influence seagrass growth.

Copyright © 2009 John Wiley & Sons, Ltd.