Individual and population responses of the caribbean sponge, Aplysina cauliformis, to natural and anthropogenic stressors.

摘要

Coral reefs are under increasing threat from a diversity of stressors. These reefs have undergone a phase shift from coral-dominated to algal-dominated ecosystems, and in many cases other functional groups such as sponges are now the predominant organisms on many reefs. This dissertation investigated the responses of a common branching sponge, Aplysina cauliformis, to algal contact, eutrophication, hurricanes, and disease.;The effects of algal competition, anthropogenic nutrients and a combination of these stressors on A. cauliformis were examined using factorial designed field experiments on Bahamian reefs. These experiments demonstrated a complex interaction between sponge and alga, in which the green alga, Microdictyon marinum, elicited a competitive effect on the sponge, through shading of its photosymbionts, but contact with the sponge facilitated increased algal productivity. Elevated nutrient concentrations had a positive effect on M. marinum by increasing algal productivity, but showed mixed effects on A. cauliformis, by increasing sponge symbiont abundance, while decreasing overall holobiont health. A further investigation of the sponge-algal interaction using stable isotopes of carbon and nitrogen showed that algal facilitation was likely due to nitrogen transfer from the sponge. In addition, these experiments showed that algal contact did not have acute effects on internal sponge regulation and partitioning of carbon and nitrogen resources.;The fate and dynamics of the sponge disease Aplysina Red Band Syndrome (ARBS) in A. cauliformis were investigated in situ. This study showed that ARBS infection decreased an individual's chance of survival on the reef. Dynamics of ARBS were investigated using spatial pattern analysis of A. cauliformis populations and revealed that direct physical contact was the transmission mechanism for ARBS within a population. In the three year period of this study, hurricane effects on sponge population an disease dynamics were also investigated, and showed a dramatic loss in sponge population biomass, increased breakage, especially in diseased individuals, and randomization of ARBS distribution within the population. With current environmental conditions often favoring alternative states in which organisms such as sponges and/or algae are the dominant organisms, it is important to understand how these organisms respond to multiple environmental stressors.