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

[摘要] We investigated the biogeochemistry of low dissolved oxygen high-nitrate(LDOHN) layers forming against the backdrop of several interleaving regionalwater masses in the eastern Indian Ocean, off northwest Australia adjacentto Ningaloo Reef. These water masses, including the forming Leeuwin Current,have been shown directly to impact the ecological function of Ningaloo Reefand other iconic coastal habitats downstream. Our results indicate thatLDOHN layers are formed from multiple subduction events of the Eastern GyralCurrent beneath the Leeuwin Current (LC); the LC originates from both theIndonesian Throughflow and tropical Indian Ocean. Density differences of upto 0.025 kg m⁻³ between the Eastern Gyral Current and the LeeuwinCurrent produce sharp gradients that can trap high concentrations ofparticles (measured as low transmission) along the density interfaces. Theoxidation of the trapped particulate matter results in local depletion ofdissolved oxygen and regeneration of dissolved nitrate (nitrification). Wedocument an associated increase in total dissolved carbon dioxide, whichlowers the seawater pH by 0.04 units. Based on isotopic measurements(δ¹⁵N and δ¹⁸O) of dissolved nitrate, we determinethat ~ 40–100% of the nitrate found in LDOHN layersis likely to originate from nitrogen fixation, and that, regionally, theimportance of N-fixation in contributing to LDOHN layers is likely to behighest at the surface and offshore.

[学科分类] 地球化学与岩石