The nitrogen isotopic composition of the organic matrices of coral skeleton : a proxy for historical nitrogen provenance in tropical coastal oceans

Marion, Guy (2007). The nitrogen isotopic composition of the organic matrices of coral skeleton : a proxy for historical nitrogen provenance in tropical coastal oceans PhD Thesis, Centre for Marine Studies, University of Queensland.

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Author Marion, Guy
Thesis Title The nitrogen isotopic composition of the organic matrices of coral skeleton : a proxy for historical nitrogen provenance in tropical coastal oceans
School, Centre or Institute Centre for Marine Studies
Institution University of Queensland
Publication date 2007
Thesis type PhD Thesis
Supervisor Prof Ian Ove Hoegh-Guldberg
Total pages 197
Language eng
Subjects L
770300 Marine Environment
Formatted abstract

This thesis established methodologies for analyzing the stable isotopic composition (δ15N and δ13C) of organic matter (OM) preserved within the skeleton of massive Scleractinian corals (Porites). The technique was applied to Porites coral cores collected from the Great Barrier Reef (GBR), and provides new insights into the controversial issue of how pervasive, European-style land use change has altered water quality of the GBR Lagoon over centennial time scales. It was hypothesized that the isotopic composition of these skeletal organic fractions is primarily controlled by N species incorporated from seawater. A sampling approach was developed that allows preparation and analysis of up to 36 samples day-1, and yields relatively high precision data (1SE = 0.37‰) compared to normal ranges in marine δ 15N values (4-7‰). Validation studies were performed on Porites corals that were previously transplanted into a long-term nutrient enrichment experiment, and results indicated that the δ 15N of the acid insoluble skeletal OM reflects N species incorporated from seawater.       

An integrated catchment to reef study was conducted within a constrained tract of the southern GBR that included a major agricultural river system and coral reef sites ranging from 5 - 50km offshore. In the Pioneer River catchment of Mackay, Queensland, sugarcane farming occupies 71% of the lower alluvial flood plains, which are fertilized at globally high rates of 175 kg N ha-1yr-1. End-member input sources were identified within the catchment by isotopic measurements (δ 15N and δ13C) of particulate organic matter (POM) at representative freshwater and marine sites. Biological transformations of fluvial N species during transport to the river mouth had a greater influence on POM δ 15N values than did the composition of the input sources themselves, which included runoff from forests, sugarcane fields, and urban areas, resulting in a latent pool of isotopically distinctive nitrogen (δ 15N ≥ 9‰) in the lower river. Results suggest that the rapid discharge of enriched, fertilizer-derived nitrogen in river flood plumes would be distinctive from the measured range of inner and midshelf GBR values (4.9 – 5.1‰).       

To determine whether anthropogenic N sources in river discharge is traceable in the skeletons of multi-century old Porites corals, reefs were cored along a transect ranging from 5km to 50 km offshore. Skeletal luminescence measured at 0.25mm increments (ʎ=490nm) showed that freshwater river flood plumes extend at least 33km offshore during non-drought years, with major floods reaching reefs 50km offshore every 3-5 years. Average δ15N values of coral tissues and skeletal organic matter decreased significantly across the GBR shelf, ranging from 7.84 ± 0.24‰ at the inshore reef to 3.82 ± 0.59‰ by 50km offshore. An annually-resolved composite K15N time series was developed by averaging data from the three inshore Porites cores, spanning the years 1945 to 2004. Coral δ15N values ranged from 4.84‰ to 16.87‰ and were positively correlated with Pioneer River discharge. Maximum values coincided with the floods triggered by cyclones “Aivu” (1989), “Joy” (1991), and a period of heavy rainfall in 2000. A highly coherent relationship was obtained (r2 = 0.71) by plotting δ15N values from major flood years against time, which rose by 0.62‰ per decade and correlated strongly with the increase in the amount of N fertilizers applied to Mackay sugarcane paddocks over that period. δ15N systematics indicate that the corals incorporate recycled marine particulate nitrogen and senescent phytoplankton detritus stimulated by flood-associated influxes of terrestrial nutrient sources.

One inshore coral δ15N record was extended back to 1824, prior to the arrival of Europeans to Mackay in 1865. Until the late 1950s, δ15N values consistently ranged between 4.61 and 8.90‰ (av. = 6.90 ± 0.85‰). After 1958, average 15N values increased by 2.4‰ during flood years and by 0.9‰ during ambient flow conditions. Estimates derived from the approximately linear relationship that is obtained between modern coral δ15N flood values and fertilizer application rates in the Pioneer catchment (r2 = 0.67, n = 16, p < 0.001) reveal a 10 to 16-fold increase in accumulation rates of nitrogen delivered in river floods relative to natural baseline conditions. A 4-fold increase occurred even under ambient non-flood conditions, which is consistent with signs of chronic nutrient stress on reefs in this study and in the regional Whitsundays Islands (van Woesik et al. 1999). Elevated N accumulation rates in these nearshore coral reefs occurred over the last 40 – 50 years. Agricultural conservation techniques introduced in the late 1980s (green cane trash blanketing) had no obvious effects on N accumulation rates in the inner reefs of Mackay. The aforementioned coral δ15N records provide an isotopic link between anthropogenic sources of nitrogen, runoff from cleared coastal lands, and degraded nearshore GBR communities, demonstrating the feasibility of using coral δ15N to assess baseline targets for water quality monitoring and remediation programs in Australia and worldwide.

Keyword Corals -- Nitrogen content -- Queensland -- Great Barrier Reef
Animal remains (Archaeology) -- Nitrogen content -- Queensland -- Great Barrier Reef
Corals -- Effect of water pollution on -- Queensland -- Great Barrier Reef

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