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Emission of global warming gases from northern Portuguese estuaries: the contribution of CH4 and N2O

Alexandra Cristina D. Santos1, Catarina Teixeira1, 2* and Adriano A. Bordalo1, 2
  • 1 Abel Salazar Institute of Biomedical Sciences, University of Porto, Portugal
  • 2 Interdisciplinary Center for Marine and Environmental Research, Abel Salazar Institute of Biomedical Sciences, University of Porto, Portugal

BACKGROUND The continuous increase of emissions of greenhouse gases to the environment, such as carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and chlorofluorocarbons, leads to the increase of their concentration in the atmosphere with commensurable impact on the climate (Sturm et al., 2017). In the last 100 years, the global average temperature has increased by 0.6 °C, and a larger increase in the next 100 years is forecasted, if no corrective action is taken (Root et al., 2003). Our research focused on two major gases responsible for global warming, CH4 and N2O, due to their high capacity to absorb infrared radiation (Sturm et al., 2017). Estuaries are one of the natural sources of emissions of those gases to the atmosphere (Bange et al., 1996). As such, the objectives of this work were to estimate the potential emissions of CH4 and N2O to the atmosphere from the Douro, Ave, and Cávado estuaries. METHODOLOGY Water and sediment samples were collected seasonally at each estuary. The emissions of CH4 and N2O by sediments were estimated using slurry incubations. In 50 mL serum bottles with 10 mL of incubation water and 5 mL of homogenized sediment sealed with butyl stopper and aluminum crimp and purged 20 min with nitrogen to remove oxygen. A separate set of time zero samples was sacrificed immediately after purge, to quantify CH4 and N2O levels (time 0). All samples were incubated for 4h at constant stirring (100 rpm). CH4 and N2O were determined by Gas Chromatography with Flame Ionization Detector (GC-FID), and by Gas Chromatography with Electron Capture Detector (GC-ECD), respectively. The GC was calibrated using a certified gas standard mixture of N2O and CH4. RESULTS & DISCUSSION Sediments of the three studied estuaries presented higher potential rates of CH4 production in the winter, reaching 26.48 nmol h-1 g-1 wet sed, 1.97 nmol h-1 g-1 wet sed, and 0.12 nmol h-1 g-1 wet sed, for the Douro, Ave, and Cávado estuaries, respectively (Figure 1). The higher production values were measured in fine-grained sediments rich in organic matter in the middle estuarine stretch of Douro River suitable for methanogens. Regarding N2O, sediments from Douro estuary acted as a sink rather than source of this gas during the fall and winter (Figure 2). On the other hand, sediments from the Cávado and lower Ave estuary, were active places of N2O production, especially in the summer, with the maximum value found in Ave estuary (5.18x10-2 nmol h-1 g-1 wet sed). Under anaerobic conditions, N2O is likely to be produced by denitrification (Bange, 2000). In agreement, being temperature a well-known denitrification regulator, higher potential emissions were measured during summer. The obtained results were made comparable with other studies by, estimating the fluxes in square meters. The Douro estuary reached 57.0 µmol h-1 m-2 of CH4 in winter survey, while Ave and Cávado estuaries presented maximum values one to two order of magnitude lower (5.9 µmol h-1 m-2, 0.32 µmol h-1 m-2, respectively). Methane production in the Douro estuary was higher than previously reported in Australian estuaries (23.4 µmol h-1 m-2 (Allen et al., 2011). Emissions of N2O reached 27.8 nmol h-1 m-2, 153.7 nmol h-1 m-2, and 15.5 nmol h-1 m-2 in the Douro, Ave and Cávado estuaries, respectively. The measured N2O values were in the lower range of those reported for other estuaries (Li et al., 2019; Middelburg et al., 1995). FINAL REMARKS The Douro estuary was an active CH4 producer, while N2O presented higher production rates in the Ave estuary. The different range of emissions of both CH4 and N2O from the three estuaries reflected the specific environmental conditions. Future research will explore the environmental controls and dissolved CH4 and N2O in the water column.

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Acknowledgements

Strategic Funding UID/Multi/04423/2019 and Project UNNOWN (PTDC/BTA-BTA/31098/2017), co-financed by COMPETE 2020, Portugal 2020 and the European Union through the ERDF, and by FCT through national funds.

References

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Keywords: Global Warming, Methane, Nitrous Oxide, sediments, Estuaries

Conference: XX Iberian Symposium on Marine Biology Studies (SIEBM XX) , Braga, Portugal, 9 Sep - 12 Sep, 2019.

Presentation Type: Poster Presentation

Topic: Global Change, Invasive Species and Conservation

Citation: Santos AD, Teixeira C and Bordalo AA (2019). Emission of global warming gases from northern Portuguese estuaries: the contribution of CH4 and N2O. Front. Mar. Sci. Conference Abstract: XX Iberian Symposium on Marine Biology Studies (SIEBM XX) . doi: 10.3389/conf.fmars.2019.08.00120

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Received: 08 Jun 2019; Published Online: 27 Sep 2019.

* Correspondence: PhD. Catarina Teixeira, Abel Salazar Institute of Biomedical Sciences, University of Porto, Porto, Portugal, catarina@icbas.up.pt