Sponge bioerosion and habitat degradation on Indonesian coral reefs

Abstract

<p>Coral reefs are among the most diverse ecosystems on the planet, yet they are also sensitive to anthropogenic disturbances that can degrade these systems. On many degraded reefs, large increases in bioeroding sponge abundance have occurred. On healthy reefs these sponges contribute to species diversity and habitat complexity, however there is growing concern that their proliferation on degraded reefs could lead to a state of net-erosion. In the Southeast Asian Indo-Pacific, the ecology of bioeroding sponges in relation to coral degradation has been poorly studied compared to other coral reef regions. This thesis aims to increase our understanding of the ecology of these sponges in the Wakatobi region of Indonesia, and their likely trajectory if reefs continue to degrade in the region. My first research chapter aimed to identify the common bioeroding sponge species of the Wakatobi. This was achieved through in-water surveys, and subsequent spicule and phylogenetic analysis. This resulted in the identification of eight commonly occurring Wakatobi bioeroding sponge species, two of which are described for the first time. The assemblage composition was distinctly different from the only other bioeroding sponge study in Indonesian waters (Calcinai et al. 2005), highlighting the need for more clionaid taxonomic information from the region. Having identified the common bioeroding sponge species in the region, my second chapter assessed the major environmental drivers of the abundance and assemblage composition of these sponges. Abundance surveys were conducted at 11 reef sites characterised by different environmental conditions and states of reef health. Bioeroding sponges occupied 8.9% of suitable substrate, and differences in abundance and assemblage composition were primarily attributed to differences in the availability of dead substrate. However, abundance was lowest at a sedimented and turbid reef, despite abundant dead substrate availability. This indicates a limited resilience in some species to conditions associated with terrestrial run-off and that not all forms of reef degradation are beneficial for bioeroding sponges. The capacity to increase spatial occupation of degraded reefs is also dependent upon larval recruitment and my third chapter was a two year recruitment study using in situ experimental calcareous blocks. Recruitment occurred rapidly and consistently with bioeroding sponges recruiting to approximately 70% of experimental blocks and exhibiting a preference for settlement on uncolonised dead calcareous substrates. The importance of substrate settlement cues and extent of larval dispersal appeared to differ between species, indicative of different recruitment mechanisms. Any significant increase in the availability of exposed calcareous substrate (e.g. following a mass coral bleaching event) is therefore likely to result in widespread increases in bioeroding sponge recruitment. Surveys conducted in my second research chapter revealed that two of the three locally abundant zooxanthellate bioeroding species were absent from a highly turbid reef, Sampela. My fourth research chapter investigated whether this was due to light limitation by examining the photoacclimatory capabilities of the Symbiodinium photosymbionts of Cliona aff. viridis n. sp. A. PAM chlorophyll fluorometry was employed in a 25 day shading experiment and Symbiodinium of C. aff. viridis n. sp. A demonstrated an ability to photoacclimate to extreme light reduction and recover quickly when conditions returned to normal. My results demonstrate that the absence of this species at Sampela is not due to light limitation but possibly due to other stressors associated with turbidity, e.g. suspended sediment. My final chapter was an assessment of the environmental drivers of rates of bioerosion in Spheciospongia cf. vagabunda, a common species in the Wakatobi. Erosion rates were determined from changes in dry-weight of calcareous substrates with attached grafts of S. cf. vagabunda after a year deployment across seven reef sites. The average bioerosion rate was 12.0 kg m⁻² sponge tissue yr⁻¹ (± 0.87 SE), but differed between sites and was negatively correlated with settled sediment depth. Bioerosion by this species can play a significant part in the carbonate budget on reefs where it is abundant (up to 20% of available substrate on some reefs in the Wakatobi) but is likely reduced on highly sedimented reefs. In summary, the Wakatobi bioeroding sponge assemblage is diverse and overall, both adult abundance and recruitment are primarily driven by the availability of dead calcareous substrates. Therefore, further coral mortality and a subsequent rise in the availability of dead substrate in the region is likely to result in increased abundance of bioeroding sponges. However, not all forms of reef degradation will benefit these sponges; turbid and sedimented reefs will likely constitute stressful habitats for some bioeroding sponge species and assemblages in these environments will be comprised of fewer more resilient species.</p>