Doctor of Philosophy
Department of Biological Sciences
Roberts, Daniel E., Structure and dynamics of sponge-dominated assemblages on the temperate reefs: variability associated with anthropogenic disturbance, Doctor of Philosophy thesis, Department of Biological Sciences, University of Wollongong, 2000. http://ro.uow.edu.au/theses/1870
The structure and dynamics of sponge-dominated assemblages living on both shallow and deep coastal and estuarine reefs in temperate NSW, Australia were quantified by examining their spatial and temporal variability at a number of scales. The effect of the discharge of sewage effluent on these assemblages was also examined. Quantifying the natural spatial and temporal variability in these assemblages has received inadequate attention because of the difficulty in sampling these habitats. Following assessment of the efficacy of photoquadrats and appropriate levels of taxonomic resolution, I sought to determine i) spatial and temporal patterns of variability and ii) assess the effects of point source sewage pollution on these assemblages.
A reliable quantitative technique using photo-quadrats was therefore developed and tested for sampling the macrobenthic assemblages living on hard substrata. No loss in the accuracy of the data was detected whilst the increased efficiency of the method enabled these assemblages to be sampled at appropriate spatial and temporal scales. Both univariate and multivariate techniques determined that sufficient taxonomic resolution was attainable at the level of phylum for assemblages in kelp forests, whilst the level of order was necessary for assemblages in crustose habitats. Furthermore, as taxonomic resolution increased towards species level, the detection of differences associated with sewage became less likely.
Patterns and processes in deep-water macrobenthic assemblages have largely been inferred from the study of such assemblages on shaded artificial structures or in relatively shallow water. Patterns in the diversity and abundance of sponges on the coastal reefs off Sydney were examined using photo-quadrats to provide estimates of species richness and percentage cover. Spatial and temporal variability was measured by sampling three nested sites within each of three reefs at each of three depths (20, 30 and 50 m) on three occasions. Sponge richness generally increased with depth, as did the number of erect or massive forms. In contrast, cover decreased with depth particularly for encrusting sponges. Univariate and multivariate analyses revealed considerable small-scale spatial and temporal variation in sponge distribution and abundance. A significant positive relationship between richness and cover was also apparent. In general, there were greater temporal changes in the patterns of abundance for the shallow reef assemblages at 20 m, relative to those at 30 m and 50 m. The structure and dynamics of sponge-dominated assemblages were further examined by contrasting these assemblages on both exposed and sheltered reefs at the same depth. Four sheltered estuarine and four open coastal reef locations were examined where three nested sites were sampled within each location over time. Generally, richness of erect species was greater for the assemblages on sheltered reefs whilst there was greater temporal variability on reefs exposed to wave energy.
The effect of discharging sewage effluent on shallow, nearshore and deep-water macrobenthic assemblages was examined at various spatial and temporal scales. In shallow water kelp forests, the assemblages were sampled according to an experimental design that had two sites nested within each of three outfall and three reference locations. Apart from the richness of sponges, univariate and multivariate analyses determined that there were no obvious patterns associated with the discharge of sewage effluent from these low volume and highly mixed cliff-face outfalls. In general, the assemblages showed natural spatial patchiness that could not be attributed to the effects of sewage.
In contrast, using a 'Beyond BACI' experimental design, the cover and the number of encrusting macrobenthic species inhabiting a nearshore reef in the vicinity of an ocean outfall changed rapidly following the discharge of secondary treated sewage effluent. Within 3 months, significant reductions in the cover of crustose and foliose algae were apparent when compared to reference locations. The cover of several species of sponge, including Cymbastela concentrica, Geodinella sp. and Spongia sp. also underwent marked declines coincident with the commissioning of the outfall. Multivariate analyses revealed marked shifts in the structure of the assemblage at the outfall relative to the reference locations. The overall composition of the community at the outfall changed from one in which algae and sponges were well represented to an assemblage dominated by silt and ascidians.
Finally, the macrobenthic assemblages living on reefs in the vicinity of a newly commissioned deep-water ocean outfall were sampled using an experimental design that had three sites nested within each of three reef locations. After 18 months, significant effects of the outfall were detected for the total number of species, bryozoans, cnidarians and the abundances of two bryozoan species and a non-descript silt matrix. Small-scale interactions among sites through time also occurred for many taxa which represented inherent 'noise' and some evidence for the effects of the outfall at smaller spatial scales
To partition out variation associated with the discharge of sewage, a manipulative field experiment was done to examine some of the processes that may be causing some of the observed patterns within sponge-dominated assemblages. The phototrophic sponge, Cymbastela concentrica was used to test its response to increased shade, silt and nutrients. Furthermore its response to a salinity gradient was also examined. The results indicated that sponge growth and reproductive status were influenced by shading and siltation, and the relationship between the host sponge and its macroalgal symbiont were also affected. Nutrients did not affect the sponge in any way, whereas a decreasing salinity gradient affected the growth, reproductive status and symbiotic algae within the sponge.
In general, the structure and dynamics of sponge-dominated assemblages was found to be highly variable at many scales with natural and anthropogenic disturbance playing a critical role in the dynamism of the assemblages. Water authorities with the responsibility of managing the disposal of sewage effluent to the marine environment commonly use poorly designed "post-commissioning" sampling programs to assess environmental impact. The utility of monitoring programmes that do not sample at appropriate spatial and temporal scales is compromised because of the considerable variability inherent within spongedominated assemblages.