Aquatic nitrous oxide dynamics from rivers to reefs

Michael John Reading

doi:10.25918/thesis.197

Nitrous oxide (N2O) is a powerful greenhouse gas and ozone-depleting substance generated mainly from microbial nitrogen cycling. Coastal waters are important and dynamic environments for nitrogen cycling and N2O due to nutrient inputs from terrestrial catchments and strong gradients in oxygen and redox conditions. However, substantial uncertainties presently remain in constraining global N2O budgets due to the multitude of drivers in different environments and wide spatial and temporal variability in N2O emissions. This thesis provides critical data and novel insights from understudied southern hemisphere coastal waters and reveals links between multiple-scale anthropogenic activity, submarine groundwater discharge, and N2O dynamics along the river to reef continuum from terrestrial nutrient sources to coastal waters. This thesis investigates several key knowledge gaps in coastal N2O dynamics, including the influence of regional and global scale anthropogenic land use, the role of coastal submarine groundwater discharge on N2O dynamics, the impacts of high-density urbanization on waterway N2O dynamics, and N2O dynamics in understudied coral reef lagoons. I found that increasing catchment modification (agricultural and urban area coverage) can increase N2O emissions from estuaries at the regional scale, using spatial surveys in four estuaries sharing similar climatic, geologic, and hydrologic properties. In applying a global literature meta analysis, I found a positive relationship between global estuarine N2O concentrations and dissolved inorganic nitrogen, and a negative relationship with dissolved oxygen saturation. In a major metropolitan estuary (Sydney Harbour, Australia), I found N2O emissions were much lower than most previously published studies of urbanised estuaries, presumably due to the low surface water dissolved inorganic nitrogen (range 2.5 to 5.4 μmol L-1) and high dissolved oxygen saturation (range 75 to 109 %sat) observed during the study. The N2O dynamics in the metropolitan estuary were influenced by recirculating submarine groundwater discharge, which led to a sink of N2O in some upstream embayments through the transport of dissolved N2O into sediments that were conducive for microbial N2O consumption. While N2O uptake and undersaturation in upper estuary sub-embayments are occasionally reported within urbanised waters when compared to pristine systems (e.g., Rajkumar et al., 2008), the embayment N2O uptake was not high enough to offset central estuary and downstream sub-embayment groundwater derived N2O sources. In a large coral reef fringing lagoon (Great Barrier Reef, Australia), I found the lagoon was a sink of N2O, with N2O CO2 equivalent (eq) uptake almost completely offsetting the regional methane eq emissions based on 20 year warming potentials of 86 and 268 for N2O and CH4 respectively. However, the Great Barrier Reef lagoon was an overall greenhouse gas source due to CO2 emissions that were an order of magnitude larger than CO2 equivalent units of CH4 and N2O fluxes combined. N2O uptake appeared to be driven by carbonate sediment denitrification along the coastline and central lagoon. There appeared to be no influence on the lagoon N2O dynamics from agriculturally dominated catchments along the coast, which contrasts with previous shelf studies.

Aquatic nitrous oxide dynamics from rivers to reefs

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Aquatic nitrous oxide dynamics from rivers to reefs

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