Frontiers in Marine Science (Jul 2014)
Greenhouse gas (N<sub>2</sub>O) emission from Portuguese estuaries
Abstract
Abstract: The contribution of nitrous oxide (N2O) emissions to the estuarine nitrogen (N) balance are investigated for Tagus (May 2006), Minho and Lima estuaries (September 2006). N load to Tagus (3.48 Mg N yr-1) and Minho (3.25 Mg N yr-1) are similar in both estuaries but higher then Lima (0.34 Mg N yr-1), being well related to freshwater inflow. Estuaries act as sink of N. In Tagus and Minho most of the N is removed within the system (67% to 70%), while in Lima this value falls to 38.6 %. Also, emissions of N2O to the atmosphere from Tagus and Minho are similar, each estuary accounting to ~13% of the total N loss. Concerning Lima, the N removed through N2O emissions account to 110% of total N loss, which indicate higher sources of this biogas within the estuary. Additionally, balance of dissolved NO2-, NO3-, and NH4+, reveal simultaneous loss in nitrate (~30%) and a large increase in nitrite (200%) and ammonium (400%), suggesting that point sources and/or denitrification process may have an important role on N2O production/emissions. Further study is still needed in order to better understand the nitrogen cycle dynamics in N2O production and emission and ultimately to assess its impact on global warming. 1. INTRODUCTION In the last few decades the study of nitrous oxide (N2O) has acquired greater importance due to its contribution to the Global Climate Change. N2O is an important greenhouse gas (GHG), with a global warming potential per molecule more than 200 times that of carbon dioxide (IPCC, 2007). N2O is often found in high concentrations of coastal waters, particularly, in estuaries, which are known to contribute with a significant amount of this biogas to the atmosphere. The production of N2O is linked to the microbial turnover of inorganic nitrogen by nitrifying and denitrifying organisms. As N2O production strongly depends on environmental conditions (Naqvi et al., 2000) any natural or anthropogenic-induced shifts on the nitrogen availability in aquatic systems will affect its formation and subsequent release to the atmosphere. Estuaries receive significant anthropogenic inputs from both point and non-point upstream sources and from metropolitan areas, tourism and industries located along the estuarine edges and so, at the present time, pollution emerges as one of the biggest problems potentially affecting estuaries. Environmental concerns on management of the coastal environment have stimulated many investigators to examine nitrogen cycle dynamics. However, despite the substantial advances in scientific research, due to a great diversity of hydrological and geomorphological conditions and anthropogenic pressures on estuaries, uncertainty still remains concerning N2O emissions. The quantification of N2O fluxes is, therefore, of highly importance in order to improve scientific knowledge on the estuarine nitrogen budgets and ultimately to assess its impact on global warming. In order to characterise the role of Tagus, Minho and Lima estuaries, in terms of nitrogen impacts on adjacent coastal waters, studies on nitrogen balance was carried out, during the productive period, in Tagus, Minho and Lima estuaries. 2. ESTUARIES Tagus, Minho and Lima estuaries, well mixed mesotidal estuaries located in the Western Iberian margin (Fig. 1), exhibit quite different hydrodynamic and morphologic characteristics (Table 1). These systems have a great socio-economic importance, providing goods and services to a great number of inhabitants and high ecological value, mainly due to the large diversity of habitats and biodiversity. Fig.1. Map showing location of a) Minho, b) Lima and c) Tagus estuaries. Table1. Characteristics of Tagus, Minho and Lima watershed (APA, 2014). 3. METHODOLOGY 3.1. Data Total nitrogen (TN) data was used to estimate nitrogen balance. Data was reliably extrapolated to 3 months (productive period) and presented on an annual scale (based on the concordance between median temperature measured in each estuary and data available on IPMA, I.P. internal usage physical-chemical database). 3.2. Sampling and analytical procedure Surface water samples were collected for analysis of dissolved N2O and TN, along salinity gradient of Tagus (May 2006), Minho and Lima (September 2006) estuaries. Additionally physical, chemical and biological parameters were also determined. Meteorological param¬eters were measured using a portable meteorological station (Campbell Scientific CR 510). N2O concen¬tration was determined using a gas chromatograph (GC-3800, Varian) equipped with an electron capture detector (63Ni-ECD) and a headspace CombiPAL autosampler. The N2O equilibrium concentrations were calculated assuming an atmospheric N2O mixing ratio of 319±0.12 ppb (IPCC, 2007). Precision of the method was 2.6%. The N2O air-sea flux, FN2O was estimated as FN2O = kN2O.ΔN2O, where ΔN2O is the difference between the measured and the equilibrium concentration with the atmosphere; kN2O is the N2O transfer velocity, function of the wind speed and the Schmidt number calculated according to the equation of Wanninkhof (1992). We used the k-wind parameterization proposed by Carini et al. (1996). Total nitrogen (TN) determination was based on oxidative digestion with peroxodisulfate method (ISO 11905-1:1997), which converts ammonia, nitrite and many organic nitrogen-containing compounds to nitrate. Dissolved inorganic nitrogen (DIN) (nitrite NO2-, nitrate NO3-, ammonium NH4+) analyses were carried out using a TRAACS auto-ana¬lyzer following colorimetric techniques outlined by the manufacturer. Precision of the method was 0.8% (NO2- and NO3-) and 2.0% (NH4+). Accuracy of measurements was maintained by using daily CKS standards (Wako, Japan). 4. RESULTS Total N loads to the estuaries are well related to freshwater inflows. Also, anthropogenic N load fraction presents good correlation with inhabitants in each watershed (Table 1). Load of N to Tagus (3.48 Mg N yr-1) and Minho (3.25 Mg N yr-1) estuaries are similar but about 10 times higher than Lima (0.34 Mg N yr-1) (Fig.2). Estuaries are acting as sink of N, although in different ratios. In Tagus and Minho most of the N (67% to 70%) is removed within the system, being only a minor fraction exported to the ocean. Also, emissions of N2O to the atmosphere were similar, accounting each for ~13% of total N loss. Lima estuary revealed an opposite trend, with major N content (~60%) being exported to ocean. However, N2O emission accounts with a much higher fraction (110%) of total N loss, suggesting either its production through biological processes (nitrification/denitrification) or the existence of point sources. Biological processes could not be deduced in this study, however, despite no available information on the nitrifying bacterial community of Lima estuary, nitrification does not seem a relevant process inN2Oproduction in the water column during the study period, particularly, as the substrate concentration, NH4+, (values varying only between 0.5-1.8 µmol L-1) is not adequate for this process to occur (Koops and Pommerening-Röser, 2001). Additionally, balance of dissolved nutrients revealed simultaneous loss of NO3- (~30%) and large increase of NO2- (200%) and a NH4+ (400%), which suggests an anthropogenic source. In fact, taking in account the combined effect of Lima lower estuarine area and river flow, it is likely that the effect of anthropogenic pressure plays a more important role on nitrogen cycle within this estuary, and ultimately in N2O production process and emission to the atmosphere. However, clarification on this subject can only be done on the basis of further studies. Fig.2. Annual nitrogen inputs and outputs in Tagus, Minho and Lima estuaries. DIN is present in brackets. Values in Mg N yr-1. 5. CONCLUSION Tagus, Minho and Lima estuaries are source of N2O to the atmosphere. Particularly, in Lima estuary anthropogenic N input seems to play an important role on N2O emission. However, in a global perspective N2O attained emissions represent a reduced fraction (2O yr-1, Barnes and Upstill-Goddard, 2011). Values are comparable with those registered in some Portuguese estuaries and other European less eutrophic estuaries. However, it is known that higher N2O emissions in estuaries may occur during winter and spring (Sun et al., 2014). Thus, these systems may represent on an annual basis a larger source of N2O, which can only be clarified in future studies. Only a full comprehension of the global estuarine nitrogen cycle will provide an efficient basis of scientific knowledge for sustainably management of such ecosystems and ultimately reduce N2O emissions.
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