Publication Date

2008-01-03

Availability

Open access

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PHD)

Department

Marine Geology and Geophysics (Marine)

Date of Defense

2006-12-08

First Committee Member

Peter K. Swart - Committee Chair

Second Committee Member

Ralph Mead - Committee Member

Third Committee Member

Larry Peterson - Committee Member

Fourth Committee Member

Eugene Rankey - Committee Member

Fifth Committee Member

William Anderson - Outside Committee Member

Abstract

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 delta 15 N 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 delta 15 N 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 delta 15 N of POM (3.64 per mil), aquatic vegetation (Dictyota = 2.39 per mil; Thalassia = 1.91 per mil; Rhizophora mangle = 1.46 per mil; Halimeda = 1.62 per mil; sponges = 4.13 per mil; turf algae = 2.67 per mil), herbivorous fish (4.92 to 8.47 per mil), as well as the delta 15 N and delta 18 O of nitrate (4.40 and 20.36 per mil, respectively) suggest that the primary nutrient sources directly impacting the reef are from natural sources, principally nitrogen 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 per mil enrichments over their food source. The presence of anthropogenic wastes was detected in the delta 15 N and delta 18 O of nitrate from Key Largo canal waters (10.09 per mil), 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 nitrogen fixation was the largest contributor of nitrogen to the ecosystem, delivering approximately 43 percent of all nitrogen. Anthropogenic wastes, however, contributed only about 8 percent to the total nitrogen budget, far less than biotic (non-human) wastes (13 percent), upwelling (10 percent), and gyre waters from the Gulf of Mexico (9 percent). For the first time, a long term, spatially diverse investigation has presented a more complete depiction of delta 15 N 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.

Keywords

Anthropogenic Waste; Sewage; Herbiverous Fish; Seagrass; Mangrove; Algae; DIN; DON; POM; Delta 13 C; Delta 15 N; Nitrogen Fixation

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