Christmas tree worms (Spirobranchus giganteus) and their role as bioindicators of environmental stress on coral reefs of Bonaire, N.A.
The use of biological entities as indicators of environmental stress can provide links between changes in ecological conditions and ecosystem productivity. Historically, bioindicators have been used as a rapid-assessment tool of areas declining in sustainability for the inhabiting organisms. This study investigated the utility of sessile, filter feeding Christmas tree worms (Spirobranchus giganteus) as bioindicators of the presence of potential coral reef stressors. Christmas tree worm density was compared at low impact sites (> 200 m from a commercial establishment) and high impact sites (< 200 m from a commercial establishment). For each site, four quadrats were randomly placed along a 10 m transect at 6, 12 and 18 m depths to assess percent live coral cover and Christmas tree worm density. These data were compared with potential environmental stressors such as excess nutrients (nitrite, nitrate, ammonia and phosphate), human gut (Enterococcus) bacteria, sedimentation rates, and sediment particle size distributions between high and low impacted sites and among depths. Approximately 97% of the worms inhabited live coral. Live coral cover was similar for 12 and 18 m at both high and low impacted sites (~ 17 % - 20 %) but significantly lower at 6 m depth (~ 2 % - 8 %). Despite the similarity in live coral cover at depth, there were significantly more Christmas tree worms at 12 m of high impact sites. At all other sites and depths, the worms never exceeded ~ 1.5 worms m-2. At 12 m, water chemistry analyses did not show any differences between site impact except for phosphate, with significantly greater concentrations at high impact sites. Bacterial loads, sedimentation rate and particle size distributions did not show any differences between site impact although there were finer sediments at high impact sites and coarser sediments at low impact sites. S. giganteus may be found at high densities at high impact sites due to a greater availability of food acquired through filter-feeding biota. Therefore, they may be used as novel indicators of the presence of environmental stressors, such as excess nutrients and finer sediments, in Caribbean coral reef systems.