Deep-sea coral amino acid archives of marine ecosystem function in east and southeast Australian waters
Date
2015
Authors
Strzepek, Kelly Michelle
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The research presented here uses deep-sea bamboo corals (Family Isididae) to reconstruct ecosystem dynamics in southeast Australian waters over the past 50-100 years. The work is divided into two complimentary research themes: methodological considerations and case studies that apply amino acid compound specific isotope analysis (AA-CSIA) to specimens from three regions: the subtropical Lord Howe Rise, and temperate to subantarctic Tasmanian waters. These case studies test the central thesis that Australian marine ecosystems are sensitive to documented oceanographic change. First, improvements to the graphite preparation for radiocarbon analysis are presented. These improvements minimized sampling requirements and enabled routine analysis of less than 100 micrograms of carbon. Optimized protocols reduced sample requirements from 2.5 milligrams to 100 micrograms of coral protein, significantly improving the resolving power of the reconstructions. To improve the spatial resolution of the study, it was necessary to test for artefacts of preservation method from different coral collections. Preservation method had no effect on the composition of radiocarbon, AA stable carbon and nitrogen isotopes or elemental ratios in either the organic or carbonate fractions. However, minimal alteration to organic bulk carbon isotopes, and a significant offset in carbonate lattice Ba:Ca after storage in ethanol were observed. The differing distribution of AA between coral families has significant implications for how deep-sea coral isotope proxies are applied to decipher paleoclimate. Multivariate analyses demonstrated first, the need to consider tissue-specific fractionation before applying pre-existing trophodynamic parameters to coral protein, and second, that the carbon and nitrogen isotopic composition of several amino acids show promise as diagnostic biomarkers of dominant algal clades in surface waters. Second, the isotopic composition of deep-sea corals was used to track or trace the provenance and fate of organic matter produced in surface waters. On the Lord Howe Rise, reconstructions indicate that the input of nitrogen fixation to the 'new' nitrogen budget varies significantly (approximately 60 per cent) on timescales comparable to variability in the East Australian Current's (EAC) flow regime. It is hypothesized that a surface community shift toward unicellular diazatrophs occurs when flow is favoured along the Tasman Front. South of the Tasman Front, three distinct surface community structures emerge. Specimens collected in the northeast are influenced by the EAC, while an east-west dichotomy in particulate provenance is apparent on opposing sides of the South Tasman Saddle. In the east there is evidence of eukaryotic production with particulate delivery originating from the Subantarctic Zone. By contrast in the west there is a distinct prokaryotic isotope profile that shares properties with the Great Australian Bight. Non-essential amino acids distributions provide evidence that the delivery of high protein organic matter supports the region's recently discovered deep biomass maximum. This body of work demonstrates that there is distinct variability in ecosystem structure and function in southeast Australian waters. But moreover, it demonstrates that only AA-CSIA can provide the level of detail that can describe how these ecosystems respond to their changing environment.
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