Research project title
Intertidal sediments and fringing mangroves of Darwin Harbour are hot spots of high denitrification that support oligotrophic condition.
Abstract
Understanding the status of nitrogen (N) processing in Darwin Harbour, a tropical macro-tidal estuary subject to increasing pressures is essential for future management. This study generated a contemporary understanding of nitrogen inputs to the estuary providing a basis for the exploration of denitrification — a critical process which removes nitrogen from estuarine systems. The extent of intertidal sediments and fringing mangroves of Darwin Harbour is significant and highly valued for the ecosystem services they provide. Conceptualised as nutrient hot spots, these depositional habitats persist along the estuary where a gradient of nitrogen processing was expected. Using a combination of both conventional benthic fluxes and molecular approaches, findings suggest that smaller tidal creeks were in fact ‘reactors’ for denitrification rather than the anticipated longitudinal pattern of N removal.
N-cycle qPCR assays were developed that measure the abundance of genes involved in denitrification (nosZ) and ammonia oxidation (amoA) under conditions of high to low nutrient status along intertidal sediments. Outcomes suggest that ammonia oxidation and denitrification are key microbial N transformations in surface sediments. The multiple lines of evidence approach demonstrated that nitrogen metabolism, particularly denitrification is a significant ecosystem function in surface sediments which have the capacity to attenuate human induced inputs and maintain the low nitrogen status of Darwin Harbour.
Drawing on new understanding of process rates critical gaps in budget terms are resolved. A nitrogen budget for Darwin Harbour is developed with an estimated export of approximately 31,031 t N y-1 to the adjacent Beagle Gulf in northern Australia. Denitrification removed ~26% of total nitrogen inputs (~12,918 t y-1) and in conjunction with burial, accounted for up to 36% of N removed from the estuary. Resolution for the process of denitrification highlights the critical role of sediments in tropical estuaries. These hot spots play an integral role in connecting nitrogen processing across the benthic environment relieving N loading.
Research interests
Marine and estuarine water quality, sediment quality, biogeochemistry, science communications, marine assessment, monitoring, management and policy.