Nitrogen cycling on coral reefs: A stable isotopic investigation of nutrient dynamics within the Florida Keys coral reef tract

Date of Award




Degree Name

Doctor of Philosophy (Ph.D.)


Marine Geology and Geophysics (Marine)

First Committee Member

Peter K. Swart - Committee Chair


This dissertation serves as a comprehensive, natural-abundance analysis of the present-day spatial and temporal dynamics and trophic linkages of nitrogen from within the Florida Keys National Marine Sanctuary (FKNMS). This work was accomplished by documenting the delta15N of particulate organic matter (POM), several genera of aquatic vegetation and herbivorous fish, as well as dissolved water column nitrogen. Seasonal and spatial variations in delta15N were assessed, trophic level variations among reef constituents were quantified, and relative contributions of both natural and anthropogenic nitrogen into the reef tract were determined.The measured mean delta15N of POM (+3.64‰), aquatic vegetation (Dictyota = +2.39‰; Thalassia = +1.91‰; Rhizophora mangle = +1.46‰; Halimeda = +1.62‰; sponges = +4.13‰; turf algae = +2.67‰), herbivorous fish (+4.92‰ to +8.47‰), as well as the delta 15N and delta18O of nitrate (+4.40‰ and +20.36‰, respectively) suggest that the primary nutrient sources directly impacting the reef are from natural sources, principally N2 fixation, and not anthropogenic wastes. Clear trophic linkages, without evidence of disturbances from anthropogenic wastes, are apparent in this study; herbivorous fishes show distinct +3 to +4‰ enrichments over their food source. The presence of anthropogenic wastes was detected in the delta15N and delta 18O of nitrate from Key Largo canal waters (+10.09‰, however, sewage derived nutrients did not exist in any measurable or detectable amount outside the canals. Additionally, an assessment of the total yearly nitrogen contributions to the FKNMS was conducted, demonstrating that N2 fixation was the largest contributor of nitrogen to the ecosystem, delivering approximately 43% of all nitrogen. Anthropogenic wastes, however, contributed only ∼8% to the total nitrogen budget, far less than biotic (non-human) wastes (13%), upwelling (10%), and gyre waters from the Gulf of Mexico (9%).For the first time, a long term, spatially diverse investigation has presented a more complete depiction of delta15N composition of various reef components found in the FKNMS, critical and imperative for accurately assessing nutrient pressures on coral reefs. As such, the data presented in this study do not support the theory that continuous anthropogenic nutrient loading from nearshore populations is the sole cause of reef decline in the FKNMS.



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