The paper describes the results of microcosm experiments examining the response of DMS and DMSP concentration, synthesis & production rates to acidification in Southern Ocean & Arctic waters, and compares them with previously published results from the NW European shelves. The primary results are the absence of an effect of high CO2 on DMS/DMSP in polar waters, and the contrasting significant effect of high CO2 in decreasing DMSP concentration/DMSP production & increasing DMS production in temperate waters. The authors relate the difference in regional DMS response to variability of the carbonate system of each region; other factors (phytoplankton community, nutrients etc) are rejected due to the absence of significant relationships. The paper makes some interesting points regarding regional variation in response to acidification, which should be considered in models.
I have two “medium” concerns with paper, the first being that the DMSP response to High CO2 is a little overstated; a significant DMSP response to High CO2 is limited to the first 48 hours in the polar experiments, after which there is no significant difference from the control. My other concern is that the interpretation, and proof of their hypothesis, rests on differences in regional variability of pH & the carbonate system, yet the data presented to support this are somewhat limited. I appreciate there are limited pH datasets available, but the data discussed are primarily large-scale spatial variability, and not the temporal variability that the phytoplankton experience.
Title doesn’t scan as written (“IS insensitive”) and is a little confusing with reference to the polar results, & then then the metanalysis of polar AND temperate results. A better title might be:
“DMS sensitivity to ocean acidification as determined in a meta-analysis of temperate to polar microcosm experiments”
Or, to highlight the polar results:
“A metanalysis of microcosm experiments shows that DMS production in polar waters is insensitive to ocean acidification”
Line 26 “resulting in an INCREASE in DMS emissions to the atmosphere…..”
Line 443-445; The text implies there is a significant positive effect on DMSP of high CO2 at the 2nd time point, but this does seem apparent in Fig. 7D
Line 450 “This data suggests that DMSP concentrations in polar waters may be upregulated…” again this is only for the first 48 hours; Fig 7D doesn’t show a significant mean effect at 96 hours for polar waters. This could be a “shock“ response to the dramatic alteration of pH and the carbonate system, before the phytoplankton community acclimate, which should be mentioned.
Also, perhaps more accurate to say the results reflect a downregulation of DMSP in temperate waters (relative to polar waters), as the mean DMSP effect is significant for both experimental time periods temperate waters in Figs 7C&D (unlike in polar waters where there is only a significant difference in the first 24 hours in Fig 7C).
Line 459 “rapid OA on DMSP producers…” more detail required here. Is this an algal physiological response or change in phytoplankton community composition?
Line 460 This paragraph is a little contradictory; starts by saying DMSP production is upregulated in polar relative to temperate, and finishes with “the lesser response seen in polar waters”
Line 466. “In an assessment across...”; this sentence should be rewritten for clarity
Line 533-540. “The polar waters sampled during our study were characterised by pronounced gradients in carbonate chemistry over small spatial scales”. Where is this shown? And what are the “small spatial scales”? The pH range of the whole voyage (“along each cruise track”) is mentioned but theres no information provided on the length of these transects which makes its difficult to compare the pH gradient between regions, or assess how significant the pH spatial gradients are. Phytoplankton won’t experience the total pH range measured on a voyage, as their exposure will be limited by physical constraints such as currents and water masses. Furthermore, the sites sampled within each voyage will experience different pH variability. The information on the drivers of spatial gradients (in paragraph Line 541-554) is interesting, but theres no indication of the spatial scale associated with this to relate spatial pH gradients to phytoplankton. The authors have provided some examples of the temporal pH variation at certain sites (Paragraph Line 555-564), but these do not consider all pH data available (for example, Beare et al (2013) show large variation (7.8-8.5) in the central North Sea). The authors supply supporting evidence from other experiments that polar phytoplankton are relatively insensitive to variation in pH, but to support their hypothesis that the regional variation in pH is the primary factor driving differences in DMSP response, more evidence and analysis of differences in regional pH variability are required.
Beare, D., McQuatters-Gollop, A., van der Hammen, T., Machiels, M., Teoh, S. J., & Hall-Spencer, J. M. (2013). Long-term trends in calcifying plankton and pH in the North Sea. PLoS One, 8(5), e61175.
Line 634 Section 4.4. This section should highly other benefits of mesocosms (inclusion of larger components of the foodweb and physical factors (mixing, stratification, particle export) that are excluded from microcosms, and note that their longer duration & more holistic/inclusive framework makes mesocosm results more relevant for Earth System models. This section should also consider the shock effect of sharply altering pH in microcosms
Line 722. “Our findings contrast with two previous studies…..” this sentence (the reasons for differences in previous polar microcosm responses) should be expanded on in the Discussion section
Technical corrections/Minor comments
Line 52-66; why does this paragraph focus on pack ice & associated climate-related changes?
Line 113 “‘winners vs loser’ dynamic”: this requires some explanation
Line 137 “Polar” not required in this sentence as the paper presents experiments from temperate waters as well as polar
Line 183. For the Drake Passage and Weddell Sea experiments suggest the Fe experiments are not mentioned (as their results aren’t discussed), and the same notation is used as for the other experiments (High CO2, High CO2 +, etc)
Line 215-219. A 48-hour experiment seems very short; however, as pointed out growth rates are faster in temperate waters than polar incubations. The authors may want to use these differential growth rates to justify the comparison of response of shorter duration temperate experiments & longer duration polar experiments.
Line 211. IF the high frequency results are not discussed them exclude this from the Methods
Fig 2. As DMS & DMSP results (G-I) are only presented to depths of 100m, the non-DMS/P parameters (A-F) should be only shown for this depth range, particularly as only the surface values for non-DMS/P variables are discussed in the Results section. Currently details of the depth profiles in A-F are not visible due to the extended vertical axis used.
Fig 2. The depth profiles on DMS/P are not really discussed; the maxima is not always at the surface and these sub-surface maxima appear to be associated chl-a subsurface maxima
Line 344. This description is misleading; both the 48-hr and 96-hr samples were collected within the incubation period
Line 351 Fig 5 is DMSP data, not Fig 4
Line 372 The unpublished temperate data are an important component of this paper and so the Suppl. Table 2 data should instead be a Figure in the main paper (possibly including comparison with the publ. data from Hopkins& Archer (2014)).
Line 388. “effect” missing
Line 487 “For all Arctic stations …”; Drake Passage & the Weddell sea are not in the Arctic
Line 698-702. These two sentences don’t seem to address the topic of Section 4.4, or this paragraph and should perhaps be moved elsewhere in the text