The impacts of nutrient enrichment and skewed nitrogen: phosphorus (N:P) stoichiometries on the skeletal microstructure and geochemistry of zooxanthellate corals

Abstract

The skeletons of zooxanthellate corals provide the foundations of coral reefs, which in turn are biodiverse and productive marine ecosystems of great importance to human populations. They also provide useful environmental proxies, due to the capacity for changes of climatic conditions or seawater chemistry to be recorded as modifications to the skeletal structure and geochemistry. However, studies on the impacts of nutrient enrichment on coral skeletal growth have provided contradictory results, while the impacts of nutrient stress on skeletal growth, microstructure and geochemistry remain relatively understudied. This thesis aimed to investigate the impacts of nutrient enrichment and stress on the skeletons of zooxanthellate corals, and how this might affect the use of microstructural and geochemical features as environmental proxies. First, the published literature relating to the impacts of nutrient enrichment on coral skeletal growth and microstructure was re-evaluated. By considering coral taxonomy and the resultant seawater nitrogen (N): phosphorus (P) stoichiometry some of the major contradictions were resolved. The impacts of four different nutrient treatments on two species, Acropora polystoma and Euphyllia paradivisa were then assessed using replicate coral colonies cultured in an experimental mesocosm. The nutrient conditions considered were high nitrate: high phosphate (HNHP), low nitrate: low phosphate (LNLP), high nitrate: low phosphate (HNLP) and low nitrate: high phosphate (LNHP). The impacts on zooxanthellae density, photochemical efficiency and skeletal growth were recorded, and the coral skeletons were analysed using micro-computed tomography (µ-CT) and mass spectrometry to determine differences in skeletal micro-structure and geochemistry respectively. HNHP conditions were associated with high zooxanthellae densities in both species, rapid skeletal growth in A. polystoma and a corallite shape and structure indicative of large polyp biomass in E. paradivisa. The HNLP and LNLP treatments caused bleaching in all corals of both species, with the additional effect of P-starvation in the residual symbionts of HNLP corals. In A. polystoma bleaching was associated with reduced skeletal growth and thickened skeletal elements, while in E. paradivisa reduced corallite size and shallow endothecal dissepiments indicated substantial polyp tissue loss, this being most severe in HNLP corals. Bleaching susceptibility differed for the two species under the LNHP treatment, but in both species the effects on coral physiology and skeletal structure indicated a much less severe impact of low N:P ratios than for high ratios (the HNLP treatment). All of the isotopic (δ13C and δ18O) and trace element (Li/Ca, Mg/Ca, Sr/Ca and B/Ca) ratios analysed were impacted by the nutrient treatments such that corals from the HNHP, HNLP and LNLP treatments were identifiable from characteristic geochemical responses common to both species. The differences in δ18O, Li/Ca, Mg/Ca and Sr/Ca values between treatments were equivalent to erroneously large proxy-derived temperature differences between treatment tanks. Patterns of trace element and stable isotope differences between treatments indicated that different forms of nutrient stress likely impact coral skeletal geochemistry through variable impacts on calcifying fluid chemistry, Rayleigh fractionation and the degree of isolation of the calcifying fluid. The skeletons of zooxanthellate corals may record the impacts of nutrient stress in their skeletal structure and geochemistry. Such modifications can impact ecosystem processes and present challenges relating to their proxy use.

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Identifiers

ORCID for Michael Christopher Buckingham: orcid.org/0000-0002-8946-9854

ORCID for Joerg Wiedenmann: orcid.org/0000-0003-2128-2943

ORCID for Bastian Hambach: orcid.org/0000-0003-4546-5672

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