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Formation and maintenance of high-nitrate, low pH layers in the Eastern Indian Ocean and the role of nitrogen fixation


Posted on 13 March 2013

TitleFormation and maintenance of high-nitrate, low pH layers in the Eastern Indian Ocean and the role of nitrogen fixation
Publication TypeJournal Article
Year of Publication2013
AuthorsWaite, AM, Rossi V, Roughan M, Tilbrook B, Akl J, Thompson PA, Feng M, Wyatt ASJ, Raes EJ
JournalBiogeosciences Discussions
Volume10
Issue3
Pagination3951 - 3976
Date Published2013
Keywordsbiogeochemistry, Ningaloo Reef, nirtogen fixation, sea water pH
AbstractWe investigate the biogeochemistry of Low Dissolved Oxygen High Nitrate layers form-ing against the backdrop of several interleaving regional water masses in the EasternIndian Ocean, off northwest Australia adjacent to Ningaloo Reef. These water masses, including the forming Leeuwin Current, have been shown directly to impact the ecological function of Ningaloo Reef and other iconic coastal habitats downstream. Our results indicate that LODHN layers are formed from multiple subduction events of the Eastern Gyral Current beneath the Leeuwin Current (LC); the LC originates from both the Indonesian Throughflow and tropical Indian Ocean. Density differences of up to 0.025kgm between the Eastern Gyral Current and the Leeuwin Current produce sharp gradients that can trap high concentrations of particles (measured as low transmission) along the density interfaces. The oxidation of the trapped particulate matter results in local depletion of dissolved oxygen and regeneration of dissolved nitrate (nitrification). We document an associated increase in total dissolved carbon dioxide, which lowers the seawater pH by 0.04 units. Based on isotopic measurements (δ15N)and (δ18O) of dissolved nitrate, we determine that∼ 40–100% of the nitrate found in LODHN layers is likely to originate from nitrogen fixation, and that regionally, the importance of N fixation in contributing to LODHN layers is likely be highest at the surface and offshore.
URLhttp://biogeosciences-discuss.net/10/3951/2013/bgd-10-3951-2013.pdf
DOI10.5194/bgd-10-3951-2013
Short TitleBiogeosciences Discuss.