Linkages between coral assemblages and coral proxies of terrestrial exposure along a cross-shelf gradient on the southern Great Barrier Reef |
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Authors: | S Jupiter G Marion M Henderson V Schrameyer M McCulloch O Hoegh-Guldberg |
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Institution: | (1) ARC Centre of Excellence for Coral Reef Studies, The Australian National University, Canberra, ACT, 0200, Australia;(2) Australian Institute of Marine Science, PMB #3, Townsville MC, QLD, 4810, Australia;(3) ARC Centre of Excellence for Coral Reef Studies, Centre for Marine Studies, The University of Queensland, Brisbane, QLD, 4072, Australia;(4) WCS South Pacific Country Program, 11 Ma’afu Street, Suva, Fiji Islands |
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Abstract: | Coral core records, combined with measurements of coral community structure, were used to assess the long-term impact of multiple
environmental stressors on reef assemblages along an environmental gradient. Multiple proxies (luminescent lines, Ba/Ca, δ15N) that reflect different environmental conditions (freshwater discharge, sediment delivery to the nearshore, nutrient availability
and transformations) were measured in Porites coral cores collected from nearshore reefs at increasing distance from the intensively agricultural region of Mackay (Queensland,
Australia). The corals provide a record (1968–2002) of the frequency and intensity of exposure to terrestrial runoff and fertilizer-derived
nitrogen and were used to assess how the present-day coral community composition may have been influenced by flood-related
disturbance. Reefs closest to the mainland (5–32 km offshore) were characterized by low hard coral cover (≤10%), with no significant
differences among locations. Distinct annual luminescent lines and elevated Ba/Ca values (4.98 ± 0.63 μmol mol−1; mean ± SD) in the most inshore corals (Round Top Island; 5 km offshore) indicated chronic, sub-annual exposure to freshwater
and resuspended terrestrial sediment that may have historically prevented reef formation. By contrast, corals from Keswick
Island (32 km offshore) indicated episodic, high-magnitude exposure to Pioneer River discharge during extreme flood events
(e.g., 1974, 1991), with strongly luminescent lines and substantially enriched coral skeletal δ15N (12–14‰). The reef assemblages at Keswick and St. Bees islands were categorically different from all other locations, with
high fleshy macroalgal cover (80.1 ± 7.2% and 62.7 ± 7.1%, respective mean ± SE) overgrowing dead reef matrix. Coral records
from Scawfell Island (51 km offshore) indicated little exposure to Pioneer catchment influence: all locations from Scawfell
and further offshore had total hard and soft coral cover comparable to largely undisturbed nearshore to middle shelf reefs
of the southern Great Barrier Reef. |
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Keywords: | Porites Luminescence Barium Nitrogen isotope Coral assemblage Disturbance frequency |
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