Strong tidal currents and labile organic matter stimulate benthic decomposition and carbonate fluxes on the southern Great Barrier Reef shelf

Abstract

The southern Great Barrier Reef (GBR) shelf is characterized by a sharp across-shelf gradient from terrigenous to marine-derived organic matter, and by the presence on the outer shelf of the Pompey Reef Complex (PRC). The PRC runs parallel to the shelf edge and consists of many narrow, turbid channels where strong tidal currents and eddies foster high suspended loads and phytoplankton production that sustain lush gardens of suspension-feeding, benthic communities. Rates and pathways of benthic carbon decomposition and carbonate kinetics in relation to these characteristics were measured across the shelf. Flux rates of DIC, O 2, Mn, and dissolved inorganic nutrients across the sediment–water interface were rapid, increasing from inshore and peaking at the channels within the PRC. Rates of DIC (mean: 39.5 mmol m −2 d −1; range: 14.5–103.2) and NH 4 + production (mean=5.4 mmol m −2 d −1; range=1.6–23.7) from incubated sediments were rapid compared with other shelves. Sulfate reduction (mean:1.2 mmol S m −2 d −1; range: 0.1–6.1) and iron reduction (mean: 2.7 mmol Fe m −2 d −1; range: 0.6–4.6) were minor diagenetic pathways, measured only in inshore and mid-shelf deposits. Manganese reduction (mean: 12.5 mmol Mn m −2 d −1; range: 0.5–55.9) was the second most important pathway, as sites seaward of the inner shelf were dominated by aerobic respiration (63–99% of total C oxidation). There was no detectable production of either CH 4 or N 2O. Rates of O 2 consumption were rapid (mean: 44.6 mmol m −2 d −1; range: 10.2–121.9) with the percentage of O 2 involved in chemical oxidation declining from 90% to 92% inshore to <10% at the shelf edge. From inshore to the mid-shelf reefs, ≈20% of remineralized DIC was involved in carbonate dissolution whereas ≈10% was involved in authigenic mineral formation on the outer shelf and at the shelf edge. N 2 production was rapid and much greater than nitrogen fixation but neither showed across-shelf patterns. High tidal energy within the PRC leads to increased exposure of high-quality organic matter to microbes and oxygen, fostering rapid rates of aerobic respiration, manganese reduction, and carbonate dissolution. Our data supports the notion that tropical shelves act as efficient ‘incinerators’ of organic matter.