Sivakiruthika Natchimuthu

Spatio‐temporal variability of lake CH4 fluxes and its influence on annual whole lake emission estimates

Lakes are major sources of methane (CH4) to the atmosphere that contribute significantly to the global budget. Recent studies have shown that diffusive fluxes, ebullition and surface water CH4 conc ...

Spatio-temporal patterns of stream methane and carbon dioxide emissions in a hemiboreal catchment in Southwest Sweden

Global stream and river greenhouse gas emissions seem to be as large as the oceanic C uptake. However, stream and river emissions are uncertain until both spatial and temporal variability have been quantified. Here we investigated in detail the stream CH4 and CO2 emissions within a hemiboreal catchment in Southwest Sweden primarily covered by coniferous forest. Gas transfer velocities (k600), CH4 and CO2 concentrations were measured with multiple methods. Our data supported modelling approaches accounting for various stream slopes, water velocities and discharge. The results revealed large but partially predictable spatio-temporal variabilities in k600, dissolved gas concentrations, and emissions. The variability in CO2 emission was best explained by the variability in k, while dissolved CH4 concentrations explained most of the variability in CH4 emission, having implications for future measurements. There were disproportionately large emissions from high slope stream reaches including waterfalls, and from high discharge events. In the catchment, stream reaches with low slope and time periods of moderate discharge dominated (90% of area and 69% of time). Measurements in these stream areas and time periods only accounted for <36% of the total estimated emissions. Hence, not accounting for local or episodic high emissions can lead to substantially underestimated emissions.

Spatiotemporal variability of lake pCO2 and CO2 fluxes in a hemiboreal catchment

Globally, lakes are frequently supersaturated with carbon dioxide (CO2) and are major emitters of carbon to the atmosphere. Recent studies have generated awareness of the high variability in pCO2aq (the partial pressure corresponding to the concentration in water) and CO2 fluxes to the atmosphere and the need for better accounting for this variability. However, studies simultaneously accounting for both spatial and temporal variability of pCO2aq and CO2 fluxes in lakes are rare. We measured pCO2aq (by both manual sampling and mini loggers) and CO2 fluxes, covering spatial variability in open water areas of three lakes of different character in a Swedish catchment for two years. Spatial pCO2aq variability within lakes was linked to distance from shore, proximity to stream inlets, and deep water upwelling events. Temporally, pCO2aq variability was linked with variability in DOC, total nitrogen and dissolved oxygen. While previous studies over short time periods (1 to 6 hours) observed gas transfer velocity (k) to be more variable than pCO2aq, our work shows that over longer time (days to weeks) pCO2aq variability was greater and affected CO2 fluxes much more than k. We demonstrate that ≥ 8 measurement days distributed over multiple seasons in combination with sufficient spatial coverage (≥8 locations during stratification periods and 5 or less in spring and autumn) are a key for representative yearly whole lake flux estimates. This study illustrates the importance of considering spatio-temporal variability in pCO2aq and CO2 fluxes to generate representative whole lake estimates.

Response: Inland water greenhouse gas emissions: when to model and when to measure?

The commentary by Li and Bush (2014) to our article (Panneer Selvam et al., 2014) raises a fundamental question: to what extent can inland water greenhouse gas emission estimates be improved without additional substantial measurement efforts? We estimated open water fluxes from India considering only the type of inland waters for which we had representative measurements and information about within-system spatial variability (Panneer Selvam et al., 2014). These were strict and careful conditions for our estimate, and we clearly state that we likely underestimate fluxes. Therefore, the suggestion that we underestimated the total emissions is not controversial. The question is if this attempt by the commentary to add the missing fluxes really results in improved emission estimates. It is difficult to properly evaluate the revised numbers in the commentary given the limited available information. However, the commentary and its revised emissions illustrate several general and very important challenges including: