|Review of Sims et al. BG “Tidal mixing of estuarine and coastal waters”|
Sims et al. compared the performance of two underway pCO2 systems in the Western English Channel and concluded that tidal mixing is the most important if not the only process controlling pCO2 in estuarine and coastal waters in this system. The data quality is high and the data analysis is generally sound (and much improved during revision). I also agree with the main conclusion that water mass mixing played a main role in controlling the estuarine pCO2 distribution and the air-sea CO2 flux in this system, though I do not like the complete discount of the role of biology. I felt and I feel a complete discount of biological control won’t serve our understanding of this nice dataset well. At a minimum, the authors should explain why the very rapid and nearly vertical changes of pCO2 near L4 and away in some low salinity locations (Fig. 8) are not because of biological activities. I think this can be easily amended with one paragraph in the Discussion (I just don’t understand why the authors chose to ignore this).
Lastly, I think the Introduction could be updated with a more recent status of coastal CO2 research (for example, it appears we are in the time of early 2000s; and not citing Burke Hales paper on a similar membrane equilibrator system). This won’t diminish what you and your nice system have achieved.
Below are few detail notes as I read through it.
After 20 years of efforts in coastal waters, the third sentence is only correct in the sense of the high spatial and seasonal heterogeneity in coastal waters. (Estuarine and coastal water carbon dioxide (CO2) observations are relatively few compared to observations in the open ocean.) So modify it a bit or delete it.
I also suggest you modify at the end of line 21-22, adding something like “…fCO2 except a few locations(?at L4 as shown by Fig. 8 and within Cawsand Bay?) where influences of biological production are also clear”. A complete discount of biological control won’t serve our understanding of the dataset well.
Please use italic font for p in pCO2 throughout the paper. The community uses italic p for “partial pressure” and p for -log unit (as in pH). As for f (fugacity), you are free to write either way as there is nothing to be confused of.
L68, I am not qualified to question if there is any language issue here as the authors are all native English speakers (and I am not), but “Transects underway CO2 measurements” is odd to me. Should it be “Underway CO2 measurements along transects”?
For the membrane equilibrator, if I remember correctly, Burke Hales was the first one using it. Don’t remember when he published it. Please search and cite him (if my memory is correct).
L198: A comparison of two showerhead systems in another coastal system also showed about 3 ppm difference (Jiang et al. 2008).
Jiang, L.-Q., W.-J. Cai, R. Wanninkhof, Y. Wang, and H. Lu¨ger (2008), Air-sea CO2 fluxes on the U.S. South Atlantic Bight: Spatial and seasonal variability, J. Geophys. Res., 113, C07019, doi:10.1029/2007JC004366.
L265, change “ in large rivers” to “in large river plumes” or “on large river influenced shelves”.
L278, It is not clear what does the author mean by “micro-environment”. A semi-isolated small bay? Clarify a bit.
I enjoyed reading it. Thanks.