School of Environmental Sciences
Natural Estuarine Particles and their Uptake of Nitrogen in Estuaries (NEPTUNE)
Funded by the Leverhulme Trust
The quality of natural waters has major consequences for human and environmental health. The global importance of water quality and estuarine-coastal zone management has been recognized through its inclusion in a Key Framework Document presented at the Johannesburg Earth Summit in August 2002. Within Europe, the emphasis on water quality is manifested via the Water Framework Directive (2000/60/EC) which insists that member states "establish good ecological and chemical status of surface waters by 2015". The concentrations of dissolved and particulate nutrients, including nitrogen compounds, are fundamental components of the ecological and chemical status of river-estuary systems.
Inorganic and organic forms of nitrogen play a crucial role in the fertility of natural waters since they are essential for primary production (plankton), which is a key food source sustaining fisheries. However, human activities are significantly altering the natural balance of nitrogen in rivers and estuaries and inputs of nitrogen, in all its forms, are predicted to increase as a result of changes in agriculture, urbanisation, transport and climate. Potential consequences include eutrophication (i.e. excess plant growth) which will become more common and a greater prevalence of nuisance algae, some of which can be highly toxic to fish and humans.
Approximately half of the annual global riverine flux of nitrogen is in the form of particulate nitrogen, of which most is organic, with amino acids making up the largest reservoir. However, we cannot accurately predict how much organic nitrogen is carried by particles in rivers and what will happen to bound organic nitrogen compounds, including amino acids, as they cross the estuarine salinity gradient. In estuaries, suspended particulate matter can be broadly separated into two categories (a) that which is permanently suspended and is transported as though it were a quasi-dissolved constituent and (b) that which undergoes resuspension caused by the tides. Almost all of the studies on particulate nitrogen have focused on the total particle population and the differential transport and reactivity of particles within a population has simply not been tackled.
Because there are missing links in our understanding of the chemical behaviour of organic nitrogen there are major uncertainties in our assessment of the overall flux of nitrogen compounds from estuaries to coastal seas. This is not helpful to environmental managers who are trying to plan for better water quality and improvements in the health of the aquatic environment. This project will provide vital, strategic knowledge for regulators and governments in their quest to meet the objectives of the European Directive and any new proposals arising from the Earth Summit.
This project will test the hypothesis that suspended particles significantly control the reactivity and flux of organic nitrogen in rivers and estuaries. Amino acids have been chosen as surrogate compounds, since they have been shown to represent the behaviour of bulk organic nitrogen. Specific objectives:-
The originality in this proposal focuses on the application of relatively new technology for the fractionation of natural particles, using split-flow, thin cell, lateral transport (SPLITT). There are inherent weaknesses in the use of traditional, size-based separations in aquatic studies but the SPLITT system overcomes these by employing user-defined settling velocities to fractionate suspended particles. This approach was first described in 2000 but has not yet been applied in turbid estuarine environments. A major advantage of this method is that the separation process mimics the hydrodynamic sorting of suspended particles found in river-estuary systems. Importantly, it has two major advantages (a) it can be used in the field and (b) it operates in continuous mode and rapidly delivers significant amounts of material within each fraction for physical and chemical analysis and laboratory experimentation.
This work will provide significant new information on the role of particles in the estuarine cycling of organic nitrogen. With the aid of the end-user community we intend to integrate our new concepts into existing river-estuary-coastal water management practices in order to give better estimates of estuarine nitrogen fluxes and their potential impacts on water quality and biological productivity.
The study will focus on the Tamar Estuary, SW England. We have opted for this site because extensive background water quality information is available to aid the planning of sample collection and the interpretation of results. Water samples will be collected from three critical estuarine zones viz. tidal freshwater, the region of maximum suspended particle concentration and the high salinity region, under contrasting seasonal conditions. In situ measurements and particle fractionation will be undertaken, from a vessel, and the particle fractions returned to the laboratory for characterisation and experimentation.
Water samples will be characterised according to the defined estuarine variables, dissolved and particulate nitrogen concentrations and particle characteristics. Bacterial populations will be counted. The free and combined amino acids will be quantified by chromatography after extraction.
To determine the reactivity of amino acids with the particle fractions, experiments will be conducted under controlled conditions representing those encountered in the tidal freshwater, turbidity maximum and higher salinity regions of the estuary, using the characterised water samples. Incubations with specific amino acids labelled with 14C (enabling sensitive radiochemical monitoring of their behaviour) will be undertaken on fractionated and unmodified suspensions. These will include assessments of the amino acid release from particles as the salt content of the water changes. The role of bacterial processes will be assessed under biotic and abiotic conditions.