Resolving carbon and nitrogen transformations along subterranean estuary- sediment water/water interface continuums in carbonate sands
Funding: ARC Linkage(2010 to 2013), AusAID (partners AusAid CI MOIP)
Investigators: Bradley Eyre (SCU), Isaac Santos (SCU), Dirk Erler (SCU), Li Ling (UQ), Caroline Slomp (Utrecht University, Netherlands)
Humans are modifying global nitrogen (N) and phosphorus (P) cycles at an alarming rate. Anthropogenic rates of N2 fixation now exceed natural (pre-industrial) rates (Galloway et al. 2008) and the mining of P is rapidly accelerating. The release of this fixed N and mined P into the environment drives eutrophication (production of excess organic matter), which is one of the greatest threats to coastal ecosystems worldwide (Howarth 2008).
Significant N and P contamination of coastal aquifers may arise from agricultural runoff, septic tank discharge and landfill leachate (Burnett et al. 2006; Valiela et al. 1990). The transport of contaminated groundwater via submarine groundwater discharge (SGD) is a major source of N and P to the coastal zone and an overlooked driver of eutrophication (Burnett et al. 2006). However research into the biogeochemical processes in the subterranean estuary (STE), the fresh - saline groundwater mixing zone (Moore 1999), indicates that it may also be a site of appreciable N and P attenuation (Kroeger et al. 2008). Furthermore, in permeable sandy sediments biogeochemical processes at the sediment-water interface (SWI) may also modify N and P forms and fluxes (Huettel et al. 2000).
Previous research into the biogeochemical processes within the STE or SWI is scarce, focuses on temperate West European and North American quartz sand systems (Charette et al. 2006; Kroeger et al. 2008; Spiteri et al. 2006) and has dealt with the SWI and the STE separately. However we suspect that biogeochemical processes occurring along the fresh groundwater - STE - SWI continuum (Fig. 1) will be interdependent. We anticipate that the quantity and forms of groundwater N and P released to the coastal ocean via SGD will be determined by the biogeochemical interactions along this continuum.
While eutrophication in temperate coastal systems, which are mostly located in developed countries (i.e. Western Europe, North America), is being reduced (Nixon 2008), there is a rapid increase in nutrient over-enrichment in developing countries, most of which are located in the tropics. Tropical coastal systems mostly consist of carbonate sands. SGD is likely to be equally important in tropical systems as temperate systems, but there have been no studies of the STE or the STW-SWI continuum in carbonate sands. Also lacking is an understanding of how biogeochemical processes along the STE - SWI continuum interact and respond to increased waste loading.
This project will combine cutting edge techniques (such as radioisotope tracing of groundwater, resistivity imaging, stable isotope amendment experiments and advanced numerical models) to resolve the biogeochemical transformations and fluxes of nutrients along a fresh groundwater - STE-SWI continuum. The project will address three main hypotheses: (1) Biogeochemical processes along carbonate sand STE-SWI continuums will dramatically alter the flux and form of N and P in fresh groundwater discharging into the ocean via SGD; (2) N:P ratios in carbonate sand systems will be significantly higher than quartz sand systems; and (3) Reduced compounds exert a greater influence on N and P transformations within the STE - SWI continuum as carbonate sands become waste loaded.
We will conduct comprehensive groundwater surveys and a number of field and lab based experiments during the wet and dry seasons at three carbonate sand groundwater sites, one human waste loaded (Cook Islands), one bird waste loaded (Heron Island, GBR, Australia) and one pristine (Ningaloo Reef, Australia). Concurrently we will develop 2D reactive transfer models to describe idealised carbonate sand STE- - SWI continuum. We will then simulate N and P biogeochemistry under different waste loading regimes using the collected field data to refine the calibrate the models.
Santos, I. R., Tait, D., Erler, D. and Eyre, B. D. The breathing of a coal cay: tracing tidally-driven seawater recirculation in permeable coral reef sediments. Journal of Geophysical Research - Oceans (accepted August 2010).
Updated: 21 December 2010