((N2-N / (DIN + N2-N) * 100%)
Denitrification is one of the few processes capable of counteracting eutrophication because it can permanently remove nitrogen from the system as dinitrogen gas (Figure 1) [1,2,3,4]. When denitrification efficiencies are lowered, more nitrogen is recycled to the water column as ammonium (Figure 1). Nitrogen that is recycled to the water column (and not denitrified) stimulates further cycles of primary production, and therefore can continue the supply of organic matter to the sediment, leading to more decomposition and more dissolved oxygen consumption, and potentially to anoxic and hypoxic events [1,2,3].
Figure 1. Schematic diagram of N cycling in sediments (Modified after Heggie et al., 1999 ).
Carbon loading (a.k.a. trophic status) is perhaps the main control on the denitrification efficiencies of sediment [1-4]. Carbon loadings are reflected in the benthic carbon dioxide flux. There is evidence that denitrification efficiencies of >~68%, ~40%, 18% and 8% are indicative of oligotrophic, mesotrophic, eutrophic and hypertrophic conditions respectively, in shallow coastal waterways in southeastern Australia .The denitrification efficiency optimum occurs between carbon loadings of about 50 and 100 g C m-2 y-1.  There is likely an overlap of aerobic and anaerobic respiration zones at the denitrification efficiency optimum . This is caused by the existence of anaerobic micro-niches within the oxic zone and also by oxidised burrow structures penetrating into the anaerobic zone . These enhance denitrification by improving both the NO3 and organic carbon suppy to the denitrifying bacteria.,Sediments tend to lose this 3-dimensional complexity at higher carbon loads.
The lowering of denitrification efficiencies with increased carbon loadings results from either:
Denitrification efficiency is a good indicator of sustainable carbon loading rates in bays [2,3,4], shallow coastal lagoons [1,4] and wave-dominated estuaries [4,9], where it is almost always linked to nitrification occurring in the sediment (Figure 2a).
There is some evidence that the relationship between denitrification efficiency and trophic status described above may not hold in coastal waterways where denitrification is linked to water column nitrate [1,14] (Figure 2b). High water column nitrate concentrations are most common in waterways with large amounts of suspended sediment that limits light penetration and therefore plant and algal biomass production. This would include most tide-dominated coastal waterways (e.g. deltas, estuaries and tidal creeks) because strong tidal currents resuspend fine sediment .
Figure 2. A comparison of denitrification in wave- (a) and tide-dominated waterways (b). Algal biomass is naturally lower in tide-dominated systems (b) than in wave-dominated waterways (a) because tidal mixing reduces the residence time of algae (green disks = diatoms) in the photic zone, and because sediment resuspended by tides increases turbidity, which reduces light available for photosynthesis . As a consequence, the amount of nutrients that can be taken up by plants is also lower in tide-dominated waterways in comparison to wave-dominated waterways. As such, dissolved nutrient concentrations build up in the water column in tide-dominated systems, and denitrification can be coupled to nitrate in the water column rather than to nitrification occurring in the sediment. In wave-dominated systems, denitrification is more likely to be coupled to nitrification occurring in the sediment.
The denitrification efficiency of sediments in estuaries is determined using benthic chambers. Benthic chambers are open-bottom containers (usually constructed out of perspex) that enclose an area of sediment and overlying water. The chambers are deployed on the sediments/substratum, to capture solute movement (including nitrogen) between the sediment and the water column. The denitrification efficiency of sediments in an estuary is determined by comparing the amount of nitrogen released as a gas to the total amount of nitrogen released.
The released nitrogen gas is quantified using a Membrane Inlet Mass Spectrometry (MIMS) technique . This involves stripping the gases from a water sample using a membrane under high vacuum. Carbon dioxide and water are removed from the gas-stream via a cold trap. The dry gas is then introduced to the mass spectrometer to measure N2/Ar ratios. The N2 concentrations are then calculated from the measured ratios using the N2/Ar calibration curves and theoretically determined argon concentrations for the sampled waters. The N2/Ar technique has several assumptions that must be met to achieve accurate rate calculations . Different tests can be applied to ensure these assumptions are being met .
Photo 1. A benthic chamber being deployed
In the absence of dinitrogen measurements, denitrification rates (DR) and efficiencies (%DE) can be estimated from the following equations:
DR = TDINp - DINm
DE% = =[TDINp - DINm ] * 100 / TDINp
in which [DIN]m = the measured dissolved inorganic nitrogen liberated from the sediment during the degradation of organic matter and [TDIN]p = total dissolved inorganic nitrogen flux predicted on the basis of stoichiometric relationship with the benthic carbon dioxide flux . However, the stoichiometric approach requires that the molar proportions of carbon and nitrogen in the organic matter being degraded are Redfield, or are at least known. Problems with the stoichiometric approach can occur if one assumes that nitrogen missing from the DIN flux has been denitrified, when it may have been utilised by bacteria in sediment. This is most likely to occur in sediments with high TOC:TN ratios (>20) because microbial decomposition can be nitrogen limited under such conditions .
Major research institutions (GA, MAFRI, AIMS), universities and government (local and State) agencies gather denitrification-related data for specific research studies. Denitrification rate and efficiency data (mainly stoichiometric) for thirteen coastal waterways were compiled during the National Land and Water Resources Audit. This information is presented in summary form (e.g. medians and 25 and 75th percentiles) in the OzEstuaries database. Relationships between nutrient loads and denitrification efficiencies in different types of coastal waterways can be explored in the Simple Estuarine Response Models developed by CSIRO.
More information on nutrients (changed from natural).