Nitrous oxide and methane in two tropical estuaries in a peat-dominated region of northwestern Borneo

Abstract

Abstract. Estuaries are sources of nitrous oxide (N 2 O) and methane (CH 4 ) to the atmosphere. However, our present knowledge of N 2 O and CH 4 emissions from estuaries in the tropics is very limited because data are scarce. In this study, we present first measurements of dissolved N 2 O and CH 4 from two estuaries in a peat-dominated region of northwestern Borneo. Two campaigns (during the dry season in June 2013 and during the wet season in March 2014) were conducted in the estuaries of the Lupar and Saribas rivers. Median N 2 O concentrations ranged between 7.2 and 12.3 nmol L −1 and were higher in the marine end-member (13.0 ± 7.0 nmol L −1 ). CH 4 concentrations were low in the coastal ocean (3.6 ± 0.2 nmol L −1 ) and higher in the estuaries (medians between 10.6 and 64.0 nmol L −1 ). The respiration of abundant organic matter and presumably anthropogenic input caused slight eutrophication, which did not lead to hypoxia or enhanced N 2 O concentrations, however. Generally, N 2 O concentrations were not related to dissolved inorganic nitrogen concentrations. Thus, the use of an emission factor for the calculation of N 2 O emissions from the inorganic nitrogen load leads to an overestimation of the flux from the Lupar and Saribas estuaries. N 2 O was negatively correlated with salinity during the dry season, which suggests a riverine source. In contrast, N 2 O concentrations during the wet season were not correlated with salinity but locally enhanced within the estuaries, implying that there were additional estuarine sources during the wet (i.e., monsoon) season. Estuarine CH 4 distributions were not driven by freshwater input but rather by tidal variations. Both N 2 O and CH 4 concentrations were more variable during the wet season. We infer that the wet season dominates the variability of the N 2 O and CH 4 concentrations and subsequent emissions from tropical estuaries. Thus, we speculate that any changes in the Southeast Asian monsoon system will lead to changes in the N 2 O and CH 4 emissions from these systems. We also suggest that the ongoing cultivation of peat soil in Borneo is likely to increase N 2 O emissions from these estuaries, while the effect on CH 4 remains uncertain.

Mangrove Methane Biogeochemistry in the Indian Sundarbans: A Proposed Budget

“…The concentrations measured for this tropical mangrovedominated estuary (Figure 10) were within the range of that reported for other tropical estuaries located at west (113 ± 40 nM) and east (27 ± 6 nM) coast of India, such as Luper (5.2-59 nM) and Saribus (3.7-135 nM), as well as estuaries located in northwestern Borneo (Muller et al, 2016;Rao and Sarma, 2016). Seasonally, estuarine surface water CH 4 concentrations during post-monsoon (90.91 ± 21.20 nM) were significantly higher [p (= 0.01) < 0.05] compared to pre-monsoon (54.20 ± 5.06 nM) conditions.…”

Seasonal and spatial controls on N2O concentrations and emissions in low-nitrogen estuaries: Evidence from three tropical systems

“…In other words the potential N 2 O ventilation is higher than the observed N 2 O water-air flux, and any in situ production of N 2 O is overwhelmed by the river load. the spatial transect (Mueller et al, 2016). In the dry season however, Mueller et al (2016) found that N 2 O concentrations were less variable and most of the N 2 O came from upstream, freshwater inputs.…”

“…the spatial transect (Mueller et al, 2016). In the dry season however, Mueller et al (2016) found that N 2 O concentrations were less variable and most of the N 2 O came from upstream, freshwater inputs.…”

“…40 There can be significant variability in water-air N 2 O fluxes across larger estuaries due to N 2 O exchange, groundwater input, and benthic/water-column microbial activity. Large spatial N 2 O concentration gradients have been observed at 'hot spots' of N 2 O influx (Mueller et al, 2016;Wong et al, 2013). However, most estuarine N 2 O surveys employ discrete sampling at coarse reso-45 lution, which may not capture the full range of variation.…”

“…However, most estuarine N 2 O surveys employ discrete sampling at coarse reso-45 lution, which may not capture the full range of variation. Continuous sampling, employing real-time N 2 O concentration monitoring, is emerging as a way of investigating spatial variability in estuaries at a very fine scale (Bange et al, 1998;Brase et al, 2016;Mueller et al, 2016;Wells et al, 2018), however there has only been one such sampling campaign in a tropical estuary (Mueller et al, 50 2016).…”