Functional roles of sea cucumbers in a coral reef-seagrass ecosystem

Animals can mediate energy and material movement within a system, which can drive ecosystem function. Through excretion and egestion, animals can supply ammonium to nutrient-poor tropical marine systems, potentially altering primary productivity. My thesis explores how sea cucumber identity influences rates of ammonium excretion and bioturbation, and how this affects seagrass growth in The Bahamas. I quantified ammonium excretion and bioturbation rates in two species, Holothuria mexicana and Actinopyga agassizii, and found that H. mexicana excretes ammonium and bioturbates sediment at higher rates per-individual. I conducted a manipulative field experiment to test whether the differences I found between species translate to differences in seagrass productivity when one sea cucumber species is replaced by the other. I found that fish biomass was more important in predicting seagrass responses than sea cucumber identity or density, and that reef proximity and sampling day best predicted all seagrass responses. My research suggests that species-specific differences in bioturbation and ammonium excretion rates by sea cucumbers do not extrapolate at the ecosystem level. Thus, H. mexicana and A. agassizii may be functionally redundant, at least in terms of nutrient provisioning, in the patch reef-seagrass system I investigated in The Bahamas.