the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Ecosystem impacts of marine heat waves in the Northeast Pacific
- 1Department of Geosciences, Princeton University, Princeton, NJ, USA
- 2High Meadow Environmental Institute, Princeton University, Princeton, NJ, USA
- 3Earth, Marine and Environmental Sciences, University of North Carolina, Chapel Hill, NC, USA
- 1Department of Geosciences, Princeton University, Princeton, NJ, USA
- 2High Meadow Environmental Institute, Princeton University, Princeton, NJ, USA
- 3Earth, Marine and Environmental Sciences, University of North Carolina, Chapel Hill, NC, USA
Abstract. Marine heatwaves (MHWs) are a recurrent phenomenon in the Northeast Pacific that impact regional ecosystems and are expected to intensify in the future. These events, including the 2014–2015 “warm blob,” are associated with widespread surface nutrient declines across the subpolar Alaskan Gyre (AG) extending south into the North Pacific Transition Zone (NPTZ) with reduced chlorophyll concentrations confined to the NPTZ only. Here we explain the contrast between these two regions using a coupled global ocean-biogeochemical model (MOM6-COBALT) with Argo float and ship-based observations to investigate how the MHWs influence the productivity of the two primary phytoplankton size classes (large > 10 μm, small < 10 μm) and the subsequent ecosystem response. Differences in seasonal iron and nitrate limitations between the AG and NPTZ explain the differences in ecosystem response to MHWs between the two biomes. The reduced nutrient supply during MHWs most strongly influences large phytoplankton in the NPTZ (-13 % annually), whereas it has a limited impact on the climatologically iron-limited large phytoplankton population in the AG (-2 %). Contrastingly, we find that MHWs yield a springtime increase in small phytoplankton population in both regions due to shallow mixed layers and lower light limitation. These primary production anomalies modify the allometric phytoplankton distribution, resulting in a 2 % decrease in the ratio of large to small phytoplankton in both regions. This shift in the assemblage towards small phytoplankton production is associated with reduced secondary and export production especially in the NPTZ.
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Abigale Wyatt et al.
Status: final response (author comments only)
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RC1: 'Comment on egusphere-2022-17', Anonymous Referee #1, 31 Mar 2022
This paper focuses on quantifying and explaining ecosystem impacts of marine heat waves in the NE Pacific, using a modelling approach supported by observational comparisons. The focus is on anomalies in chlorophyll and phytoplankton and their drivers. The topic is of high interest to the community. Several recent papers and theses look at the impacts of the 2014-15 Blob on productivity rates, ecosystem assemblages, and biogeochemical measurements like trace metals. This study broadens that work to consider a wider spatial area, assess drivers quantitatively, and provide some context for the results of previous work like decreasing nitrate observed by BGC-Argo floats. However, in many places in the paper, I was left wondering about whether the small changes detected were significant. Including statistical tests for significance would strengthen the paper, making the overall message more convincing. In general, the paper is well written, but I do have a few additional comments that I think could further improve it.
This paper reports many anomalies for marine heat waves based mostly on model results. Some of the anomalies are very small relative to the absolute concentrations or rates. Which ones are significantly different from zero? The paper would be strengthened by including a statistical test for significance in each case where a change associated with marine heat waves is reported, providing clarity for which changes are significant and which should be reported as no change within error. Examples include (but are not limited to):
- 2% lower large phytoplankton population in the AG
- 2% decrease in large:small phytoplankton in both regions
- 05 mg m-3 decline in chlorophyll in the NPTZ and 0.02 mg m-3 increase in chlorophyll in the AG – The AG value is especially small compared to mean chlorophyll in this location.Is the mean for MHW years significantly different from the mean for other years, given the fairly high variability in this region?
- The location of the 2 uM nitrate contour in Figure 5b.Given the variability in the location of this contour in the 9 MHW events, is the mean significantly different from the all-year average?
- Mean-MHW values throughout sections 3.3 and 3.4.With 9 MHW years and many non-MHW years in the model, it should be straightforward to calculate whether the MHW years are significantly different (perhaps a Mann-Whitney test or similar) in different months.
- 13% lower large phytoplankton production
Boundaries of nitrate and iron limitation are discussed, but I’m unsure of how these are defined. Nitrate limitation may be defined by the 2 uM contour line, though this should be explicitly stated. I’m not sure what iron value would be considered limiting here. In general, the iron concentrations in the model (Figure S2) seem very high with modelled values around 100-150 nM iron in winter, whereas typical values for dissolved Fe in the region appear to be < 1 nM (see https://doi.org/10.1016/j.marchem.2015.04.004 for example). Is the model iron limiting anywhere? I could not discern the hatching in Fig. 6 discussed in Line 222 or the gray and purple lines discussed in Lines 225-226. How are the limitation factors shown in Figure 7h defined? This section, along with that around Line 58, also caused me to wonder about the role of light limitation. Is the NPTZ actually iron limited before the spring bloom or is it light limited then? Is there grazing limitation that is important to controlling the size of the spring bloom in the NPTZ?
Minor suggestions:
Line 92: I was very surprised to read that chlorophyll data was not available for 2008-2010 from Line P. I contacted chief scientist, Marie Robert, to ask. She looked into it and has found that the data exists but that there is a problem with some of the summary .csv files. Some individual casts seem to contain the data but the whole cruise files do not. She is working on updating the files. I suggest you contact her directly for updates: Marie.Robert@dfo-mpo.gc.ca
Section 2.5: Suggest BGC-Argo rather than bioArgo. Suggest referencing Appendix A here.
Line 175: Fig. X
Lines 203-214: Suggest mentioning in this section that the high nutrient regime near the coast is temporally variable and mainly controlled by the timing of upwelling events.
Figure 1: I’m unclear why the bounding box for the average anomalies shown in panels a and b is different from either box shown on the maps. The targeted area appears to be mainly in the NPTZ. I suggest that it would be more illuminating to show the time series for the black NPTZ box in panels a and b rather than a different region that is not used in further analysis. Panel b: The region for this anomaly is probably the same as for panel a, but it would be good to state that. I think the location of OSP should be 50oN 145oW, not 50.1oN 149.9oW.
Figure 2: the colours of the float trajectories in the upper panels should match those in the lower time period panel. In particular, the brown colour in the upper panels is orange in the lower panel. Colour bar label is cutoff for panel b.
Figure 3: Suggest adding property labels to the colour bars, i.e. not just units. Also for the y-axis of Figure 10.
Figure 4: Colour bar labels are cut-off. Figure 5: Colour bar labels and legend are cut-off.
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RC2: 'Comment on egusphere-2022-17', Anonymous Referee #2, 11 Apr 2022
I think this is a good paper that is publishable with more or less minor revisions. Some aspects of the methodology are insufficiently explained. The terminology is confusing in some places, and some unnecessary jargon is used (see details below).
Major points:
1) There are some important details missing from the description of the methods and the data. Most importantly, a marine heatwave is defined as "anomalies that exceed 1 standard deviation for 5 months or more". But standard deviation of what and anomalies relative to what? The obvious answer is relative to a climatology calculated over the period of the ERSSTv4 data product, but that needs to be stated explicitly, and which years of this data product were used does not appear to be stated anywhere. And is there an area threshold? Is the criterion applied point-by-point, or only to the regional mean? Would it be a heat wave if only 1 grid point exceeded the threshold? (And why does the "MOM6-COBALT climatology" in Figure 10 appear to have interannual variability?)
"The model was spun-up using three repetitions of ... 1958 to 1985" (112). But spun up from what? From rest? 81 years doesn't seem very long to spin up a global ocean model. And why go to the trouble of initializing short-lived (i.e., insensitive to initial conditions) biological tracers from an ESM piControl (116-117), but not the physical ocean? I think it would make more sense to use the ESM piControl data to initialize the physical ocean, or 1958 of the historical run.
I find it difficult to believe that there are no Line P chlorophyll data before 2011 (175-176). Line P is one of the longest-running ocean time series programs, and the basic methodology for chlorophyll concentration has not changed in half a century. Satellite chlorophyll data should be available back to 1996 or 1997. "GlobColour" is referred to several times in the figure captions but never in the main text.
Are these really all of the Argo floats available in this region? Or is there some other selection criterion being applied that is not spelled out here (e.g., availability of nitrate data or data within a certain area)? I find it hard to believe that these are the only Argo floats deployed in this region over an 11 year period.
2) The Abstract ends by saying that "primary production anomalies modify the allometric phytoplankton distribution, resulting in a 2 % decrease in the ratio of large to small phytoplankton in both regions". Firstly, this seems like a very small change to emphasize as a key point in the Abstract: I am wondering if it is a mistake and it should be 2X or 20%. Secondly, it isn't easy to tell whether this passage is talking about production or biomass, and seems to shift arbitrarily between the two. Finally, where exactly in the main text is this assertion substantiated? Figures 7 and 8 illustrate the seasonal decoupling of large and small phytoplankton production, but can not be used to directly infer the Large/Small ratio of either biomass or production. Figure 9 shows only summer data. (BTW "allometric phytoplankton distribution" here is a good example of unnecessary jargon: "phytoplankton size distribution" would suffice. And if one wishes to get dogmatic, the anomalies do not "modify" the size distribution. This sort of quasi-teleological confusion of subject and object is characteristic of inexperienced authors receiving inadequate guidance (see also 208, "Salinity maintains ...")).
3) The interaction of the N and Fe cycles is sometime characterized in superficial terms, although I think the overall conclusions are mostly sound. It might help to spend a few sentences in the Introduction sketching out a conceptual model of how the authors think the overall system works.
On 164-165, would not a prolonged period of stratification also result in depletion of surface iron concentrations? In the absence of significant aeolian sources I think it would. However, it would also tend to drive the system towards N limitation even in the absence of new aeolian Fe. It also seems to be implied that only large phytoplankton are subject to iron limitation (130-135), which I think is questionable. Iron is potentially limiting for nanophytoplankton even if iron limitation is the main driver of the dominance of diatoms or nanophytoplankton. On 268 it is stated that "small phytoplankton are not simulated with iron limitation" so possibly the lack of Fe limitation is by construction in this model. If this is the case it should be stated up front in the Methods.
The limitation factors are never really explained. I assume this means a number between 0 and 1 where 1 means N or Fe replete and 0 means no growth, but this should be clearly stated in the Methods. (On a terminological note, I think "nitrate limited" and "nitrate limitation" should be changed to "nitrogen" across the board.)
In the last paragraph of section 3.2, the terminology is sometimes vague or confusing, wrt what is meant by a "boundary". On 223, the "2 uM nitrate boundary" could be "2 uM nitrate contour". In the next sentence, "nitrate boundary" occurs without any context. I assume this means the boundary between regions of N and Fe limitation, but it could be spelled out more clearly. This is an example of a place where adding a few more words could increase clarity substantially. The last few sentences (226-229) read like a description of the model solution, and this seems like a missed opportunity to state what the authors think is happening in terms of physical processes (see also 339-343).
Some details:
10 and elsewhere I would change "Alaskan gyre" to "Alaska gyre" across the board
15 change "limitations" to "limitation"
17 delete "climatologically" or change it to e.g., "usually" or "chronically"
18 "Contrastingly, we find that ..." conversely? in contrast? by contrast?
19 maybe change "lower light limitation" to "higher mean irradiance"
20 change "allometric phytoplankton distribution" to "phytoplankton size distribution"
26 not sure "recorded" is the appropriate term here; how many of these were recognized as such when they occurred?
31 " a redistribution of marine biogeography " ???
32 delete "geographical"
35, 37 "Chlrophyll"
36 change "demarks" to "demarcates"
37 delete "Pacific"
38 change "nitrate surface concentrations" to "surface nitrate concentrations"
48-50 this sentence is very awkwardly worded
57 change "Ekman-driven transport" to "Ekman transport" or "Ekman flow driven transport"
60-61 I would consider also citing Glover et al 1994 (10.1029/93JC02144) here (Bograd et al appears to be missing from the ref list)
67, 387 change "contrasted" to "contrasting"
127 delete "re-"
132-133 delete "and are efficient ... Geider et al., 1997)"
158, 160 mmol kg^-1 should be umol
159 add a ' on "floats"
171-172 "nitrate concentrations are near-zero for most stations (P4–P20)" Is this unusual? Don't some of these stations always see drawdown in summer? (e.g., Pena and Varela 2007).
186 add "North" before "American"
191 change "biophysical" to "biogeochemical"
205 change "values" to "concentrations"
207 "> 5 mg m^-3" Is this a mistake? This is an extremely high concentration for an open-ocean environment.
211-214 this assertion seems disconnected from the preceding text; not clear what its relevance is
217 add a "~" before "130 W"?
219 "nitrate becomes more depleted" more than what? (unclear antecedent)
250 not clear what is meant by "in this region of the model"
267 "the limitation factor is significantly lower (-0.06)" significant by what criterion? P<what?
285-287 "Lg Chl" and "Sm Chl" appear only in this one place, as does "chl" (elsewhere Chl)
286-296 "southern-like" and "northern-like" appear only in this paragraph and are not defined or explained
306 specify mmol of C or N
311-313 another very awkwardly worded sentence
333-336 Does this sentence make sense? It reads like it is sort of arbitrarily combining different levels of causation. If there is a clear hypothesis as to "A leads to B leads to C", it would be better to express it that way.
350 add "concentration" after "nitrate"
360 "changes sampled along the floats" along the floats' trajectories?
364 change "this data" to "these data"
398 comma in wrong place
416-417 I'm not sure this sort of editorializing is necessary, and I doubt that it is discussed by Frölicher and Laufkötter. As for the following sentence (418-420), the intended meaning is fairly clear but the wording could be improved.
Figure 7d, 8d unit should be nM?
Figure 9 unit needs a space between mg and m-3
Figure 9 caption: There are a bunch of details about this Figure that are not really explained in the caption: the meaning of the vertical bars (probably mean, but needs to be stated, and panel (b) is different from the other 3), the vertical position of the symbols (arbitrary, but again should be stated), and the meaning of the symbol colours (obvious from the positions, but in this case is having two colours even necessary?) And there appear to be more years than there are symbols.
Abigale Wyatt et al.
Abigale Wyatt et al.
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