Climatic variability, hydrologic anomaly, and methane emission can turn productive freshwater marshes into net carbon sources

Abstract

Freshwater marshes are well-known for their ecological functions in carbon sequestration, but complete carbon budgets that include both methane (CH4 ) and lateral carbon fluxes for these ecosystems are rarely available. To the best of our knowledge, this is the first full carbon balance for a freshwater marsh where vertical gaseous [carbon dioxide (CO2 ) and CH4 ] and lateral hydrologic fluxes (dissolved and particulate organic carbon) have been simultaneously measured for multiple years (2011-2013). Carbon accumulation in the sediments suggested that the marsh was a long-term carbon sink and accumulated ~96.9 ± 10.3 (±95% CI) g C m(-2) yr(-1) during the last ~50 years. However, abnormal climate conditions in the last 3 years turned the marsh to a source of carbon (42.7 ± 23.4 g C m(-2) yr(-1) ). Gross ecosystem production and ecosystem respiration were the two largest fluxes in the annual carbon budget. Yet, these two fluxes compensated each other to a large extent and led to the marsh being a CO2 sink in 2011 (-78.8 ± 33.6 g C m(-2) yr(-1) ), near CO2 -neutral in 2012 (29.7 ± 37.2 g C m(-2) yr(-1) ), and a CO2 source in 2013 (92.9 ± 28.0 g C m(-2) yr(-1) ). The CH4 emission was consistently high with a three-year average of 50.8 ± 1.0 g C m(-2) yr(-1) . Considerable hydrologic carbon flowed laterally both into and out of the marsh (108.3 ± 5.4 and 86.2 ± 10.5 g C m(-2) yr(-1) , respectively). In total, hydrologic carbon fluxes contributed ~23 ± 13 g C m(-2) yr(-1) to the three-year carbon budget. Our findings highlight the importance of lateral hydrologic inflows/outflows in wetland carbon budgets, especially in those characterized by a flow-through hydrologic regime. In addition, different carbon fluxes responded unequally to climate variability/anomalies and, thus, the total carbon budgets may vary drastically among years.

Water level, vegetation composition, and plant productivity explain greenhouse gas fluxes in temperate cutover fens after inundation

“…Methane emissions from GK Typha-Hydrocharis and GK Carex-Lysimachia were higher compared to a pristine, water saturated sedge fen (dominated by Carex aquatilis) in the southern Rocky Mountains (30 to 34 g CH 4 −C m −2 yr −1 ; Table 6; Wickland et al, 2001) or to Carex acutiformis and Typha latifolia sites during the wet year in the above mentioned rewetted fen grassland (47 and 10 g CH 4 −C m −2 yr −1 , respectively; Günther et al, 2014). They were comparable to temperate Typha latifolia (82 g CH 4 −C m −2 yr −1 ; Whiting and Chanton, 2001) and T. angustifolia marshes (51 g CH 4 −C m −2 yr −1 , Chu et al, 2015; 127 g CH 4 −C m −2 yr −1 , Strachan et al, 2015). The constantly high water levels made us expect a net CO 2 uptake at GK Typha-Hydrocharis and GK CarexLysimachia, as was found for Typha latifolia and T. angustifolia marshes (Whiting and Chanton, 2001;Strachan et al, 2015), for a water saturated temperate sedge fen in the Czech Republic (Dušek et al, 2012), and in the wet year for Carex acutiformis and Typha latifolia (Günther et al, 2014).…”

“…However, in contrast to our first hypothesis the sites GK Typha-Hydrocharis and GK Carex-Lysimachia were net CO 2 sources. Similar, a wet sedge fen in the southern Rocky Mountains (Wickland et al, 2001) and a water saturated Typha angustifolia marsh (Chu et al, 2015) were found to be CO 2 sources (Table 6). Chu et al (2015) explain their findings by abnormal climatic conditions.…”

“…of species that are potentially strong pathways of methane (Kim et al, 1998;Brix et al, 2001;Whiting and Chanton, 2001;Kankaala et al, 2004;Hendriks et al, 2007;Chu et al, 2015;Knox et al, 2015;Strachan et al, 2015). Whereas earlier studies indicate that the radiative forcing of such methane emissions may be compensated for by the simultaneous very strong net CO 2 uptake (Brix et al, 2001;Whiting and Chanton, 2001), recent observations described Typha dominated wetlands as often only weak CO 2 sinks Chu et al, 2015;Strachan et al, 2015; but cf. Knox et al, 2015).…”

High net CO<sub>2</sub> and CH<sub>4</sub> release at a eutrophic shallow lake on a formerly drained fen

“…We could not observe a considerable decrease of the spatial extent of the open water body as emergent vegetation mainly covers the shallower edges of the water body. The effect of water table lowering at Typha sites due to dry conditions is also shown by Günther et al (2015) and Chu et al (2015): relative increase of R eco rates, resulting in net CO 2 release. This might be of special interest in terms of climate change, as a temperature increase and significantly less precipitation in summer are expected for NE Germany and meteorological conditions are more frequently characterised as "unusually" warm and dry.…”

“…Allochthonous organic matter import into the NE bay due to lateral transport, as discussed for CH 4 , might have further enhanced decomposition (e.g. Chu et al, 2015). Longer data gaps in summer 2013 (see Fig.…”