Inland waters are an important source of CO2 to the atmosphere and a known but poorly quantified source of CH4 to the atmosphere (Barros et al., 2011; Bastviken et al., 2004; St Louis et al., 2000). In SOCCR-1, rivers and lakes were considered a net carbon sink. A key message for this chapter is that much of the carbon that moves from terrestrial ecosystems into aquatic systems through lateral transfer is emitted to the atmosphere as CO2. A fraction is either buried in lakes and reservoirs or transported to the coast. Significant biogeochemical processing of carbon happens in inland waters; there are significant data gaps that impede a more complete understanding of transformations under different physical, chemical, and biological conditions as well as a result of anthropogenic disturbance, both direct and indirect.
There are additional key messages about the potential impact of impoundments on carbon biogeochemistry, which are mainly to change the flow paths of carbon as well as the rates of CO2 and CH4 cycling. A final key message (which is not highlighted as such) is the statement that “changes in aquatic carbon fluxes are directly linked to the residence time of water in both terrestrial and aquatic environments...” and “the half-life of organic carbon in inland waters is about 2.5 years, much shorter than the decades to millennia required for soil systems to completely turn over.” This has huge implications for carbon movement and processing in the face of changes in the frequency, timing, and intensity of precipitation events, for example.
Overall this chapter is mostly well written and clear. However, it could use a simple figure that illustrates the points made in section 14.1.2 about the many forms of carbon and their sources.
One general concern is how carbon fluxes are framed in this chapter. For instance, the chapter should make clear that Equation 14.1 (p.567, line 20) applies to the total (background + anthropogenic) carbon cycle—i.e., in the absence of anthropogenic perturbations, there would still be a net flux to (or from) the atmosphere, and the net flux would be balanced by lateral transports. Also, Key Finding1 suggests that the forest carbon sink is countered by CO2 outgassing from inland waters. Indeed, this is reflected in Figure ES5. Taken at face value, Figure ES5 suggests that there is a very small net source/sink associated with non-fossil component of the North American carbon budget. One could easily come to the (wrong) conclusion that the North American sink is close to zero, contradicting top-down estimates that do not separate out inland waters. The authors need to figure out how the findings from this chapter could be integrated into the North American carbon budget, which is commonly thought of as that related to anthropogenic perturbations.
Statement of Task questions
- Does the report accurately reflect the scientific literature? Are there any critical content areas missing from the report?
One of the major new pieces of information for SOCCR2 is the emission contribution from inland waters.
Subsection 14.5 appears to need additional sub-subsection(s) on “North American and Regional Context”. Needs reorganization.
- Are the findings documented in a consistent, transparent and credible way?
Yes although the report should be checked for consistency in reporting fluxes and pools within the chapter, and between text and figures presented in this chapter as well as in the first chapter.
- Are the report’s key messages and graphics clear and appropriate? Specifically, do they reflect supporting evidence, include an assessment of likelihood, and communicate effectively?
Mostly, but see comments above about consistency. Also, the addition of a simple figure to illustrate section 14.1.2 would be helpful.
- Are the research needs identified in the report appropriate?
Yes, although it’s a familiar tune: we need (high frequency) data on all aspects of the C cycle from a diversity of inland waters. It would be better for the authors to provide a more clearly prioritized list that could help advance the science for the next round of SOCCR assessment.
The authors could also discuss here methodological difficulties in understanding the carbon system in freshwater environments. For instance, Golub et al. (2017) illustrate that 30 years of data (DIC, alkalinity, pH) at the North Temperate Lakes LTER site are unable to yield consistent pCO2 estimates, nor are these estimates consistent with co-located direct pCO2 observations. The freshwater community needs to follow the approach of marine scientists in developing best practice laboratory techniques and inter-calibrated standards. The SOCCR2 authors could highlight these needed methodological improvements as an important part of the research agenda. Better spatial resolution is certainly important, but will be of no use if the data are unreliable.
- Are the document’s presentation, level of technicality, and organization effective? Are the questions outlined in the prospectus addressed and communicated in a manner that is appropriate and accessible for the intended audience?
Yes, this is a well written chapter.
- Are the key findings in your chapter well stated and supported by the detail provided in the chapter?
The first key finding’s statement that “This quantity is nearly identical to the estimated 223 Tg C….” is confusing in its comparison as well as the language (“almost identical?”).
P565, Line 15
Perhaps change “The total flux” to “The total emission”?
P567, Line 12
Change “…suggest inland…” to “…suggest that inland ….”
P571, Line 18
Change to “… suggests that…”
Change to “assuming that 25%...”
P574, Line 10
It is not clear what “regional” refers to, in addition to “North America”. There are no other sub-subsections, except 14.5.1 on global perspective.
P592, Figure 14.1.
This is a nice figure.
P565, Line 15-25
“flux … from inland waters across the coterminous U.S. …” could be interpreted as lateral flux. Also, it is not clear (until later) whether flux refers to a source or a sink here. Please restate the first sentence of both Key Findings 1 and 2.
P565, Line 28
Per meter2 of what? Inland water? Or of continental area?
P572, Line 40
Regarding the increase in discharge: how about reduced precipitation or droughts?
P573, Line 16-18
“The rate of change” refers to the pCO2 increases in 6 lakes or decreases in 3 lakes? Also, recovery from acid rain should increase pH, and hence decrease pCO2?
Page 577, Line 16
Page 577, Line 33
Could add a reference to Baehr and DeGrandpre (2002) to illustrate that probes have been around for a while.
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