The Influence of Ocean Acidification and Warming on DMSP & DMS in New Zealand Coastal Water

Round 1

Reviewer 1 Report

Saint-Macary and team describe the results of a series of four mesocosm studies performed in coastal waters in New Zealand, investigating the effect of future ocean acidification and warming on the concentrations of DMSP and DMS. This study is unique in its location – no previous study reports such results from Southern Hemisphere (temperate) coastal waters. The experiments also addressed the multi-stressor knowledge gap – most previous studies that have considered the response of DMS/P to future conditions have focused on only ocean acidification. Therefore, the data reported in this paper are a useful and valuable addition to the DMS knowledge base. The mesocosm experiments formed part of the CARIM project, a national New Zealand programme focused on the impacts of OA in coastal waters, and were well-designed and appropriately implemented.  In general, the paper is well written and figures/tables are of a good quality. If the authors are able to address my (generally minor) comments, I would be very happy to see this paper accepted for publication.

1. The changes in DMS seen under the future treatments are relatively small, compared to most other studies, and warming seems to have the strongest effect – this is an interesting and important finding. The discussion is limited to the relationships between phytoplankton community composition and DMSP/DMS concentrations – which is certainly interesting and valid. The effects on the phytoplankton communities aren’t huge – in general there are only small differences between treatments, with the exception of the 2150 treatment in ME3. So perhaps it is worth considering whether there may be some resilience to these treatments within these communities. What do we know about the in situ, natural carbonate chemistry conditions of Evans Bay, a fairly enclosed, heavily coastally-influenced body of water? Coastal waters can show greater variability in carbonate chemistry compared to open ocean waters, due to higher nutrients and productivity, and riverine freshwater input. If the communities possess some degree of adaptation to rapid changes in carbonate chemistry, this may explain the fairly muted responses in DMS/P, particularly compared to other studies.
2. GAMMS plots, figure 2 and figure 3: a useful way to present the overall DMSP and DMS trends. However, the text in the legends is a bit confusing and possibly too technical (can the details of the stats be presented elsewhere maybe?). It required flicking back and forth to the methods section to make sense of it. Also, it seems that a presentation of the raw DMSP and DMS data is missing – perhaps a table summarising the DMSP and DMS, as per the other parameters, could be added. Or even plots of the time series (overlaid on the GAMM plots perhaps…if that would work). It would be useful to see how the temporal variability in DMSP/DMS concentrations corresponds to the biological measurements (which are nicely presented).
3. Throughout: Is ‘New-Zealand’ usually hyphenated? I’m not a native but it does seem odd to me.
4. Abstract: it would be helpful if the key findings were communicated more clearly in the abstract. It’s only in the third from last sentence that the actual change in DMSP under OA/warming is mentioned. It’s not really made clear what the DMS response is here. So a clearer take home message/s would be beneficial to the reader.
5. L46: DMSP is not only produced by phytoplankton, many other organisms (bacteria, corals, macroalgae) have the necessary molecular machinery for DMSP synthesis.

Author Response

Please see the attachment

Author Response File: Author Response.docx

Reviewer 2 Report

The authors present the results of a study that tests the effects of future pH and temperature conditions on the production of DMS and DMSP via four mesocosm experiments, typically with two treatments each. The study is well-described and presents relevant research in the field of DMS/P phytoplankton production, is well-written, and deserves publication. My comments mostly concern data presentation and organization.

 

As the paper stands, the various treatments are possible to discern but the methods section is somewhat disorganized. Table 1 is quite clear, but  I would advocate cross-linking the "Treatment target" column from Table 1 with the results shown in figure 1. (eg, instead of b) low-pH, write (ph-2100), for c) ph/T 2100, ph/T 2150, etc. Subplot titles will also help the viewer. Yellow colour on Fig1a) seems different from other yellows-  is this intentional?

 

***Table 3 and Figure 1 should be combined to present some sort of total carbon content for all functional groups present. Total biomass as well as biomass by functional group, for all experiments, should be readily viewable somewhere.

 

***Were nutrients measured? Their effect is discussed on, eg, line 210. Nutrients are assumed to be replete in ME3 and 4 - what is their concentration in ME1 - are they completely drawn down? Multiple factors (time of year, temperature) may account for differences between ME1 and ME3, and should be discussed.

 

*** A brief discussion of the effects of different seasonal conditions in the 4 experiments should be included.

 

*** The discussion of the two spikes in DMS and DMSP production in the control-case in RE3-4 with regards to phytoplankton community composition is very interesting.

>> Again, please consider presenting total biomass combined with comunity composition somewhere (stacked line graph with several shaded areas on top of each other representing different functional groups? If DMS/P production is then overlaid, this will make it more clear for the reader - currently the feeling is that much of the interesting factors are presented, but in a sort of 'scattered' way that makes it hard for the viewer to piece them together despite goodwill.   

 

***Line 20: The abstract mentions four experiments and then lists three; is it possible to list all 4 experiments clearly somehow?

***Line 24: This indicates that future 24 warming may have greater influence on DMS production than ocean acidification. "

>> though this may be true, it does not follow from the preceding sentence.

 

***Line 104 MEs 1, 2 and 4 consisted of 2 different treatments in addition to an ambient control: low pH 104 alone (referred as “low pH”) and low pH & warmer temperature in combination (“pH/T”). ME3 105 simulated the future low pH and warming scenarios predicted for the years 2100 and 2150 and 106 compared these with ambient controls. During ME2, only a low pH treatment was compared to 107 ambient controls. 

>> I believe the authors mean ME 1 and 4 here?

 

 

Author Response

Please see the attachment

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

I have now read Saint-Macary et al's responses to reviewer 2 comments and am satisfied with how they addressed my suggestions. I can recommend publication

Author Response

Dear reviewer,

Thank you again for your comments on this paper and for the time you have dedicated to it.

Kind regards,

The authors