|Review of paper submitted to Biogeosciences|
Title: Fate of peat-derived carbon and associated CO2 and CO emissions from two Southeast Asian estuaries
Authors: Müller et al. [bg-2015-199]
The authors present a revised manuscript which focuses on DOC, CO and CO2 observations in two Tropical estuaries in Malaysia. The authors have addressed my previous comments. The second reviewer’s main concerns centred on a) the absence of a discussion of upland DOC sources, b) the exclusion of CH4 data and c) neglecting the role of sediments in estuaries. These are valid points and have largely been addressed in the revised manuscript. I have a few comments to add to this discussion which the authors may want to address. I am satisfied that the revised manuscript has improved and recommend its publication in BG.
a) Page 16, line 27: Regarding estuarine retention of C, I agree with the authors, but sediments may also be a source of DOC. For example, DOC may be released into the water during the respiration of POC which has previously settled out of the water column. This may explain the ‘apparent’ estuarine source of DOC.
b) Section 4.1.3: The diurnal pattern of CO clearly suggests photochemical production which has been shown before. The interpretation and discussion is sound, but could perhaps be a little more explicit. The reason why this decreases so dramatically with depth is related to the spectral attenuation of light in natural waters (highest in the UV) and the photochemical efficiency of the reaction (also highest in the UV) (e.g. Kitidis et al., 2011; Stubbins et al., 2011; Zafiriou et al., 2003). I would also advise the authors to be cautious with regards to the increase of DOM bioavailability following photochemical reactions - The evidence is really mixed for this. Photochemical reactions consume O2 and produce a number of highly reactive oxygen species (hydroxyl-, superoxide- and singlet-O2-radicals) (e.g. Kitidis et al., 2014). These are extremely toxic, but also short-lived, so that microbial incubations in irradiated water often show an ‘apparent’ decrease in bioavailability if inoculated straight away. On the other hand, if the irradiated water is allowed to stand for a few hours/days before inoculation, increased bioavailability may be observed. The result seems to depend on how long you let an irradiated sample stand before you add microbes (!), but this isn’t realistic in nature. I would advise the authors to offer this alternative to ‘enhanced bioavailability’.
c) Regarding CO2/CO unit discrepancy (reviewer 2 comment) – I agree with the authors decision to use different units for these gases, precisely in order to maintain consistency with the literature for each gas. Perhaps the authors could give one common unit in brackets where the other unit is used.
d) Regarding the weak correlation between O2/CO2 (reviewer 2 comment) – As the reviewer correctly suggests, one would expect an anti-correlation between these gases from respiration/photosynthesis. The authors correctly point out that this anti-correlation breaks down due to other processes’ involvement in CO2 (chemical reactions and advection). Whilst I agree with this response, it is important to also point out that CO2, as part of the carbonate system, is buffered by DIC (Dissolved Inorganic Carbon). CO2(g) is only 1-2% of DIC in seawater (depending on pH, Temp., salinity). Photosynthesis/respiration should give an anti-correlation, but any change in CO2 is buffered by the vast DIC reservoir. Unsurprisingly therefore, the correlation is weak at times. This “buffering” probably also contributes to the absence of diurnal CO2 cycles and the lack of an obvious relationship between temperature and fluxes. Off course if the pH is low, that the carbonate equilibrium will shift towards CO2(g) and the anti-correlation with O2 may be more obvious.
1. Kitidis et al., 2011, Carbon monoxide emission from a Mauritanian upwelling filament, Marine Chemistry 127: 123–133
2. Kitidis et al., 2014, Oxygen photolysis in the Mauritanian upwelling: Implications for net community production, Limnol. Oceanogr., 59(2), 299–310
3. Stubbins et al., 2011. Carbon monoxide apparent quantum yields and photoproduction in the estuary Tyne. Biogeosciences 8, 703–713.
4. Zafiriou et al., 2003. Concordant estimates of oceanic carbon monoxide source and sink processes in the Pacific yield a balanced global “bluewater” CO budget. Global Biogeochemical Cycles 17 (1).