Nutrient dynamics in coastal ecosystems of the Bahamas
Allgeier, Jacob Edward
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This dissertation explores the relative importance of two macronutrients, nitrogen (N) and phosphorus (P), for primary production in coastal ecosystems of The Bahamas and contextualizes these findings with respect to nutrient supply from a widely underappreciated source of nutrients, fish excretion. I used observational and experimental approaches to examine the spatial variation of nutrient limitation of seagrasses and benthic algae within and across mangrove estuaries representing a gradient of habitat degradation, i.e., ecosystem fragmentation. In Bahamian mangrove ecosystems, primary production is strongly nutrient limited and that while production is always fundamentally limited by both nutrients, the degree of limitation is typically decreased due to ecosystem fragmentation. To further explore the relative degree of nutrient limitation across sites I developed the Interaction Effect Index (IEI) to quantify the degree to which benthic algae across these mangrove ecosystems respond to dual versus single nutrient enrichment. These results demonstrated that Bahamian mangrove estuaries are among the most nutrient poor marine environments. I then applied this metric to 653 multiple nutrient enrichment studies across terrestrial, freshwater and marine ecosystems using a meta-analytic approach. Findings from this study suggest that the vast majority of ecosystem types were characterized by antagonistic responses to dual enrichment, suggesting weak colimitation and that one nutrient is likely more limiting that the other. However, when ecosystems tended to be strongly colimited, the magnitude of limitation was often great. Finally, I quantified nutrient supply rates from 144 Caribbean fish species to assess the relative importance of a widely underappreciated nutrient source in coastal ecosystems. This research explored which community characteristics explain the supply and storage of nutrients by fish across different ecosystem types (i.e., seagrass beds, mangroves, coral reefs). Findings suggest a strong relationship between species richness and nutrient supply and storage, but that dominant species tend to have disproportionate effects on these processes. Simulation modes supported these findings but indicated that the relative importance of dominant species decreased in communities with greater species richness. Collectively, these findings provide a thorough assessment of how nutrients limit primary production, and the role of fishes in provisioning these nutrients in tropical coastal ecosystems.