the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 09 Oct 2024
Phytoplankton responses to iron and macronutrient fluxes from subsurface waters in the western North Pacific in summer
Abstract. Iron (Fe) and macronutrient supplies and their ratios are major factors determining phytoplankton abundance and community composition in the North Pacific. Previous studies have indicated that Okhotsk Sea Intermediate Water and North Pacific Intermediate Water (NPIW) transport sedimentary Fe to the western subarctic Pacific. Although the supply of Fe and macronutrients from subsurface waters is critical for surface phytoplankton productivity, return paths from NPIW to the subsurface and their impact on the abundance and community composition of the organisms have not been fully understood. In this study, Fe and macronutrient turbulent fluxes, as well as the flux ratios from NPIW to surface waters, were calculated based on a chemical dataset, which included Fe and macronutrient concentrations, with turbulent mixing parameters obtained from the same cruise and same station along the 155° E transect in summer. Additionally, vertical flux divergence was calculated from the estimated vertical fluxes. Surface and subsurface phytoplankton community composition was evaluated in the CHEMTAX program based on algal pigment measurements. The results show that diatom abundance is significantly correlated with the vertical fluxes of Fe and macronutrients, especially with Fe and silicate (Si) fluxes, and with the Fe/N flux ratio along the section line. These results suggest that diatom abundance was controlled by Fe supply from subsurface waters in summer. The computed turbulent flux divergence in the subarctic and Kuroshio-Oyashio Transition Area suggests that enhanced concentrations of Fe and Si in the subsurface layer were supplied from NPIW.
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RC1: 'Comment on egusphere-2024-3064', Anonymous Referee #1, 11 Nov 2024
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General comments
This manuscript investigates the phytoplankton community species composition response to iron and macronutrient in the western North Pacific in summer by looking at physically driven inputs. Results show that the response of the diatom community was driven by the vertical fluxes of Fe and macronutrients (especially silicate) being supplied from the North Pacific Intermediate Water. Overall, the results are relevant to the field, and this paper should be published; however, some revisions are required. Currently, the figures would need a bit of work to increase the quality. The discussion on phytoplankton genus-specific response lacks a bit of literature and is not contextualised enough.
Specific and Technical comments
line 52: It is unclear how Si* was derived in Sarmiento et al. 2004 method the way it is written now. In the paper, they use the following: Si*= [Si(OH)4] -[NO3 2].
Line 63: verb missing ‘processes ARE controlling’
Line 66 and 67: repetition of ‘investigated’Paragraph 3.5 – both parts could be better connected (previous work from Kaneko et al. 2012 and this one), and how this is surprising could be discussed.
Line 262-263: which previous studies? Authors could give examples to contextualise the results. After discussing genetic and remote sensing studies, authors could first compare with in situ data (even from other locations) and then contextualise with remote sensing data.
Line 282-285: Martin’s work was the first to report that, but since then, numerous Fe-addition bottle incubation experiments were performed- As well as bioavailable Fe assays were performed in the SO; this could be added to contextualise this part of the discussion better.
Line 285: Here, the genus-specific requirements towards trace metals and macronutrients could be added; why do diatoms may have higher Fe requirements compared to the other members of the phytoplankton community in the area?
288: As it ends with the seasonality aspect, the authors could hint at other controls that could happen during winter, for instance.
It is a bit confusing to have the part 3.7 at the end as the genus-specific response is discussed and the physical drivers were treated before. Maybe authors could consider having in part 1: the physical drivers (incl. fluxes and flux divergence) and then the phytoplankton responses.
Fig. 6, 7 – the quality is not so good, especially when zooming. Would need more space between the 3 panels to see the titles better.
Citation: https://doi.org/10.5194/egusphere-2024-3064-RC1 -
RC2: 'Comment on egusphere-2024-3064', Kiefer Forsch, 17 Nov 2024
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In this manuscript, Deng et al. take advantage of existing datasets: discrete bottle measurements and vertical microscale velocity fluxuations, to derive diffusive flux of the essential nutrients, iron, nitrate, phosphate, and silicate across an oceanographic transect which extends from the subarctic gyre to the subtropical gyre. The authors relate the derived diffusive fluxes with phytoplankton abundance, separated by taxa using pigment HPLC and CHEMTAX analysis. This work stands to contribute to our understanding of how nutrients and water mass mixing select for specific taxa during the productive summer season. It is important to relate rates of nutrient supply and their ratios to phytoplankton as this would be expected to relate directly to biological metrics, such as growth.
The manuscript is well-written and constructed in a manner that is easy to follow. I especially like all the figures, they do look almost publication-ready, though could be made slightly larger. I found that while the correlations are the most important contribution (along with derivation of fluxes for this region), the interpretation could be elaborated upon (see listed comments below). I do not think the focus was on phytoplankton responses, but rather relating distributions of specific taxa with vertical fluxes, and the title should reflect this.
Specific comments:
Line 28: rephrase. I think the main goal is to understand how the environment selects for specific phytoplankton groups.
Line 30: “need to be studied, simultaneously.”
Line 32: I think you could add a sentence of why it is important to understand Fe supply in this region. For example, distribution of zooplankton, fisheries, higher trophic organisms?
Line 38: “A more complete view of Fe biogeochemistry requires oceanic sources also be considered…” I think in this paragraph, it would be good to elaborate a little about why dust deposition doesn’t stimulate blooms. Timescale for biological response versus episodic dust deposition events? Bioavailability of this Fe? Not Fe/P limited (N-limited?).
Line 42: I think for non-trace metal chemists, you might want to indicate that sedimentary Fe is derived from the continental shelves. (see also line 45)
Line 49: “winter surface mixing” do you mean deep convective mixing?
Line 53: please put the equation for Si* or be more specific about how silicate and nitrate concentrations are treated.
Line 57: I am not a physicist. By internal tides, do you mean internal tide breaking or isopycnal deformation for the mechanism of mixing?
Line 62: “…North Pacific varies seasonally…”
Line 64: “…processes control the seasonal…” What do you mean by variability of the biology? Variability occurs through changing abundance and taxonomy.
Line 66: What do phytoplankton pigment data indicate? A short explanation could help here.
Line 135: some typos exist in the supplement (line 41)
Line 167: instead of “are mainly contributed by” you could write “the main components derive from”
Figure 2: I think it would be great if you indicated on these hydrographic sections where the main water masses are located with depth.
Line 187: How is the “surface layer” defined?
Line 209: Is this a known behavior for nitrate? Nitrate will act conservatively along a subsurface flow-path only if there are no biological processes occurring, or regeneration is balanced by uptake.
Line 210: What is a “high concentration”? I would include specific values for these concentrations, or concentration ranges. Similarly, how are the high cores defined as both nutrients have red colors in Figure 4 extending to STG at sigma 27.0. Only station 18 looks exceptionally high.
Line 231: It can not be discerned from the current color scheme that a “nitrate flux is downward” in Figure 5. It looks to be around zero in the STG below sigma 27.0.
Figure 7: It would be nice to see a line which indicates the depth of maximum diatom-derived chlorophyll-a on the section plot. This could be drawn between profiles for where diatoms are present.
Line 276: “…dFe flux…”
Line 279: What is meant by “…influence the diatom-derived Chl a…”? You could describe the trend in the plot.
Line 283: In fact, I think these were diatoms which responded to the Fe additions (silicate was drawn down). A feature that is often seen in HNLC and HNLC-like waters.
Line 285: “…diatoms form silicified frustrules…”
Line 314: Please indicate what is meant by “nitrate plus nitrite”.
Line 338: It is well-known that diatoms occupy high nutrient regimes due to their competitive advantage over slow-growing small cells. Diatoms have high affinity uptake of nitrate and have many strategies to store nutrients when available. I wonder if you could include some discussion of how pro occupies oligotrophic conditions (where negative flux can occur) due to their ability to deal with lower nutrients through small cell size, slow growth rates, and lower nutrient requirements. Some discussion of these well-known features which drive phytoplankton distributions would provide more context of these great findings. Few studies connect supply ratios with phytoplankton, which is more indicative of the conditions phytoplankton experience (compared to static concentration measurements).
Citation: https://doi.org/10.5194/egusphere-2024-3064-RC2
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