Fish-mediated nutrients in tropical marine systems

Animals can mediate biogeochemical processes and drive nutrient availability and productivity in ecosystems. Fish may be a major source of nutrient supply through metabolic processes such as excretion, but their role in influencing primary productivity in nutrient-poor tropical coastal marine systems is not well understood. Broadly, my thesis explores how variations in fish-mediated nutrient provisioning can influence reef primary productivity, and how invasive species can alter fish-mediated nutrient supply in coral reefs and seagrass ecosystems. I quantified excretion rates for several groups of reef fish in The Bahamas, demonstrating that excretion is primarily governed by body size, and that nutrient supply in an area is influenced by fish movements and diel activity levels. I then examined how variations in fish excretion altered three groups of primary producers on reefs using a large manipulative experiment. I demonstrated that increases in fish excretion can result in increased algal biomass and phytoplankton growth and can also lead to higher nutrient content in seagrass blades. I examined natural patterns of seagrass growth in proximity to coral reef fish communities to try and infer how fish communities shape these habitats. I found that while fish communities may have a minor influence on these primary producers through excreted nutrients, other factors such as sand depth likely play a larger role in shaping seagrass in immediate proximity to reefs. Finally, I examined how nutrient supply from native fish communities is altered by an invasive predator, the Indo-Pacific lionfish (Pterois sp). I found that, through their own excretion, these invasive predators replace the nutrient supply of the native fish they consume. Fish-mediated nutrient provisioning is therefore relatively robust to predation from lionfish on short time scales. Overall, my thesis helps to fill critical gaps in our understanding of the role of fish in shaping coral reef productivity. Bottom-up processes such as nutrient provisioning from reef fish may be a key aspect of reef ecosystem functioning and this specific functional process should be incorporated into future reef management and conservation efforts.