Interactive comment on “ Export fluxes in a naturally fertilized area of the Southern Ocean , the Kerguelen Plateau : ecological vectors of carbon and biogenic silica to depth ( Part 2 ) ” by M .

This discussion paper is/has been under review for the journal Biogeosciences (BG). Please refer to the corresponding final paper in BG if available. Abstract The chemical (particulate organic carbon and nitrogen, biogenic silica) and biological (diatoms and faecal pellets) composition of the material exported to a moored sediment trap located under the winter mixed layer of the naturally-fertilized Kerguelen Plateau in the Southern Ocean was studied over an annual cycle. Despite iron availability in 5 spring, the annual particulate organic carbon (POC) export (98.2 mmol m −2) at 289 m was low but annual biogenic silica export was significant (114 mmol m −2). This feature was related to the abundance of empty diatom frustules and the ratio of full : empty cell exerted a first order control in BSi : POC export stoichiometry of biological pump. Chaetoceros Hyalochaete spp. and Thalassiosira antarctica resting spores were found 10 to be responsible for more than 60 % of the annual POC that occurred during two very short export events (< 14 days in spring-summer) representing the majority of captured export. Low diatom fluxes were observed over the remainder of the year. Faecal pellet contribution to annual carbon flux was low (34 %) and reached it's seasonal maximum in autumn and winter (> 80 %). The seasonal progression of faecal pellet types re-15 vealed a clear transition from small spherical shapes (small copepods) in spring, larger cylindrical and ellipsoid shapes in summer (euphausiids and large copepods) and finally large tabular shapes (salps) in autumn and winter. We propose that in this High Biomass, Low Export (HBLE) environment, small, highly silicified, fast-sinking resting spores are able to bypass the high grazing pressure and efficient carbon transfer to 20 higher trophic levels that are responsible for the low fluxes observed the during the remainder of the year. Our study also provides a statistical framework linking the ecological succession of diatom and zooplankton communities to the seasonality of carbon and silicon export within an iron-fertilized bloom region in the Southern Ocean.


Introduction
Is there a particular reason why the authors present the fluxes in mol m-2?Many sediment trap studies in the Southern Ocean (e.g.Honjo et al. 2000;Fischer et al., 2002;Trull et al. 2000) show the data in g or mg.I would suggest to at least include the data in these other units in Table 1 to facilitate comparisons with previous studies." Highest diatom fluxes (> 109 cellsm−2 d−1) are observed in the Seasonal Ice Zone (SIZ) near Prydz Bay and Adélie Land and are dominated by Fragilariopsis kerguelensis and small species of Fragilariopsis curta and Fragilariopsis cylindrus (Suzuki et al., 2001;Pilskaln et al., 2004)."Are these values annual averages or the highest values registered by the traps?Please clarify.
Instead of "and small species of Fragilariopsis curta and Fragilariopsis cylindrus" it C7897 would be better to say: "and smaller Fragilariopsis species such as Fragilariopsis curta and Fragilariopsis cylindrus"."These high fluxes occur in spring and are associated with the melting of sea ice.Changes in light availability and melt water input appear to establish favorable conditions for the production and export of phytoplankton cells (Romero and Armand, 2010).In the Permanently Open Ocean Zone 5 (POOZ), diatom fluxes are two orders of magnitude lower _ 107 cellm−2 d−1 (Abelmann and Gersonde, 1991;Salter et al., 2012;Grigorov et al., 2014) and typically represented by F. kerguelensis and Thalassionema nitzschioides, except in the naturally fertilized waters downstream of the Crozet Plateau where resting spores of Eucampia antarctica var.antarctica dominate the diatom export assemblage (Salter et al., 2012)."Same comment as above.Are the diatom valve fluxes provided here annual averages or highest values recorded?Please clarify."These high fluxes occur in spring and are associated with the melting of sea ice." These studies reported maximum fluxes during summer not spring.

Material and methods
The depth of the water column should be mentioned in this section.Sediment traps deployed at shallow depths are subject to several hydrodynamic biases.Despite the fact that the efficiency of this sed trap is assessed in the companion paper a general statement about the efficiency of the trap should be given here as well.The companion manuscript (Rembauville et al. 2014) has already been accepted or just submitted?It probably would be better to cite it as (Rembauville et al., this issue).Please check with the journal guidelines and correct if necessary.

Chemical measurements
"POC and PON analyses have been previously described in Rembauville et al. (2014)."POC and PON data have already been presented in the companion paper and there-C7898 fore shouldn t be presented as part of the results of this manuscript.The abstract should also be corrected accordingly.
"Diatom enumeration and identification was made from one quarter to one half of the counting chamber (depending on cell abundance) following the taxonomic description in Hasle and Syvertsen (1997)." How many diatoms per sample were counted?300? 400?Why was this number chosen?With the magnification used in this study (especially with x200) it can be quite hard to differentiate an empty complete frustule (i.e. two valves together) from a single valve.This can be particularly difficult for some species such as those of the genus Pseudo-nitzschia or small Fragilariopsis species.Were the authors able to make such a difference?And if so, most of the empty frustules were found as separate valves or forming a complete frustule?.Please provide the raw frustule and cell counts as supplementary material.Are all the species identified in this study described in Hasle and Syversten (1997)?Authorships for all the species should be provided in Table 4 or in a separate appendix.
"We directly compared the micropaleontological and biological counting techniques in our sediment trap samples and noted the loss of several species (Chaetoceros decipiens/dichaeta, Corethron pennatum/inerme, Guinardia cylindrus and Rhizosolenia chunii) with the micropaleontological technique.We attribute this to the aggressive chemical oxidation techniques used to "clean" the samples which appears to selectively destroy/dissolve certain frustules.For the species that were commonly observed by both techniques, total valve flux was in good agreement (Spearman 10 rank correlation, n = 12, _ = 0.91, p < 0.001, data not shown) although consistently lower with the C7899 micropaleontolgical technique, probably due to the loss of certain frustules described above.Full details of this method comparison are in preparation for a separate submission." This is an interesting observation and something worth looking further into.However the authors should be cautious when referring to the "micropaleontological technique" as different authors use different versions of this technique.For example, after the acid cleaning treatment some researchers centrifuge the samples several times to wash and buffer them to a neutral pH.In contrast, other authors wait 24 h between washings allowing the diatom to settle without the use of the centrifuge.The use of the centrifuge could also be a responsible, at least partially, for the fragmentation of weakly silicified diatoms.Something similar occurs with the acid treatment, various concentrations, temperatures, and times of treatment are used in the literature and thus results may vary between authors.
"Biomass calculations for both Chaetoceros Hayalochaete resting spores..." Why did the authors decide to estimate the biomass of Chaetoceros RS and Thalassiosira antarctica?At the beginning of this paragraph the authors should briefly explain why these particular species were chosen.Perhaps it would be preferable to start the paragraph with the last sentence.

Statistical analyses
"Chi2 distance is very sensitive to rare events.Consequently, only full-and empty-cells fluxes > 10% of the total mean flux of all sample cups were retained in the correspondence analysis."Do the authors mean that only the species representing more than 10% of the annual assemblage were used in the statistical analysis?Please clarify.A list with the annual relative contribution of each species should be provided in Table 4 or in a separate figure or table.Results

Chemical composition of the settling material C7900
Satellite chlorophyll-a estimates belong to material and methods not to this section.
POC fluxes have already been presented in the companion paper and therefore shouldn t be presented in the results section of this paper.This should also be clarified in the abstract, as it written now it seems that these results belong to this paper.
Was the total mass flux estimated?What was the relative contribution of the biogenic silica fraction to the total mass flux?Did the authors estimate the contribution of the carbonate fraction?I would like to see the annual relative contribution of each bulk compound in Table 1 or in a new table or figure.
In order to facilitate comparisons with other sites, fluxes of all the bulk compounds should be also provided in mg m-2 d-1 and annual estimates in g m-2 yr-1 in Table 1.

Diatom fluxes
In order to compare the results with data from the sedimentary record, it would be useful that the authors present the annual relative contribution of each diatom species as I already suggested previously.In this section, the authors describe the changes in the relative contribution of the different diatom taxa but this information is not shown in the graphs or in the tables.This information should be included in Figure 2 or in a separate figure.The authors should explain in more detail how the annually integrated empty:full ratio presented in Figure 4 was estimated.Did the authors take into consideration the flux of each species in each cup for this calculation or was it just an average of the empty:full ratio of each species of all the cups?Discussion 4.1 The significance of resting spores for POC flux Please briefly resume in a sentence or two why the authors know that there was a strong attenuation of flux between the WML and 300 m.The reader should be able to understand the paper without reading the companion manuscript.Also, please provide the depth of the WML.C7901 "We did not observe any full cells of the vegetative stage of Chaetoceros Hyalochaete, a feature possibly related to its high susceptibility to grazing pressure in the mixed layer (Smetacek et al., 2004;Quéguiner, 2013;Assmy et al., 2013)".
Silica dissolution in the upper water column should also be considered as an important factor determining the absence of full vegetative cells in the sediment trap.Weakly silicified diatoms such as Chaetoceros may lose one of their valves or the girdle band more easily than more heavily silicified and compact diatoms, which would facilitate the remineralization of its cellular content.
What do these values represent?maximum fluxes or annual average values?
Most of these studies present the diatom fluxes in valves m-2 d-1and some do not provide annual estimates.When the authors estimated the values presented in the text from these publications, did they divided the results of these authors by two (i.e. two valves = one cell)?Please check and correct if necessary.
"Although the resting spore formation strategy is typically associated with 5 neritic areas (Smetacek, 1985;Crosta et al., 1997;Salter et al., 2012), their very high export and transfer efficiency together with advection can explain their contribution to deep open ocean fluxes (e.g.Rynearson et al., 2013)." C7903 "can explain their contribution to deep open ocean fluxes"?This sentence is not clear, please clarify.
"Highest Pseudo-nitzschia spp.full cell fluxes were observed in summer, concomitantly with the second export event (cup #9).Pseudo-nitzschia species are rarely found 15 in deep sediment trap studies and are absent from the sediment diatom assemblages due to their susceptibility to dissolution (Grigorov et al., 2014;Rigual-Hernandez et al., 2014).The genera have been reported to accumulate in summer in deep chlorophyll maximum, benefiting from nutrient diffusion through the pycnocline (Parslow et al., 2001).This ecological characteristic, together with the shallow sediment trap depth (289 m) may explain our observations of peaks in Pseudo-nitzschia spp.fluxes during summer." "Genera" should be replaced by "genus".
Only some species of this genus have been found in association to a DCM, and therefore it is incorrect to state that all the species of this genus have similar ecological affinities.Parslow et al. do not report this taxon, it was Kopczynska et al. ( 2001) who reported the species composition of the DCM.
In regard to the last sentence, why does the shallow depth of the trap explain the enhanced Pseudo-nizschia fluxes during summer?Please explain better.
"Although their fluxes were very low, species of the Rhizosolenia and Proboscia genus were mostly exported as empty cells at the end of summer and during autumn (cups #8 to #11), occurring in parallel with the full cell fluxes of the giant diatom Thalassiothrix antarctica (Table 4).It has been suggested that these species belong to a group of "deep shade flora" that accumulate at the subsurface chlorophyll maxima in summer with their highly silicified, large frustules protecting them from grazing pressure (Kemp and Villareal, 2013)." Many species of the genus Rhizosolenia exhibit weakly silicified frustules.

In the first line replace genus by genera
Replace "as the fall dump suggests" by "as the fall dump hypothesis suggests" 4.4 Preferential carbon and silica sinkers "The annual BSi :POC ratio of the exported material (1.16) is much higher than the usual ratio proposed for marine diatoms of 0.13 (Brzezinski, 1985).Moreover, the BSi :POC ratio of the exported material in spring (2.1 to 3.4, cups #1 to #3) is significantly higher than the BSi :POC ratio of 0.32±0.06 in the mixed layer of the same station during spring (Lasbleiz et al., 2014).Numerous chemical, physical, biological and ecological factors can impact BSi :POC ratios of marine diatoms (e.g.Ragueneau et al., 2006).However, the ten-fold differences in BSi :POC ratios of exported particles between spring and summer is unlikely to result simply from physiological constraints set during diatoms growth (Hutchins and Bruland, 1998;Takeda, 1998)." Authors should explain the meaning of differences between the BSi:POC ratio in the mixed layer and in the trap during spring.
"These observations are consistent with previous studies of natural (Salter et al., 2012) and artificial (Assmy et al., 2013) iron fertilization that identified C. pennatum, D. antarcticus and F. kerguelensis as major silica sinkers and C. Hyalochaete vegetative cells, CRS and E. antarctica var.antarctica resting spores as major carbon sinkers."Notably, resting spore formation was not observed in the artificial experiment and carbon export was attributed to mass mortality and aggregation of algal cells (Assmy et al., 2013)." Of all the species listed in this paragraph only F. kerguelensis is classified as a silica sinker by Assmy et al. 2013.Moreover, Assmy and colleagues do not report RS in their study (as mentioned here), so it is incorrect to use this reference to support the findings of the authors, (at least as it is written now).This statement should be rewritten C7905 clarifying these points.
The study European Iron Fertilization Experiment (EIFEX) was performed in an open ocean location remote from coastal influence.This probably is the main reason why they did not register RS.
4.5 Seasonal succession of ecological flux vectors over the Kerguelen Plateau "The species succession directly observed in our sediment trap samples differs somewhat to the conceptual model of ecological succession in naturally iron fertilized areas proposed by Quéguiner ( 2013), although the general patterns are similar.The first diatoms exported in spring are indeed small species of F. kerguelensis, T. nitzschioides spp., and small centric species (< 20 µm)." F. kerguelensis and T. nitzschioides are generally considered as relatively large diatoms.Do the authors mean that the size of the specimens of these species found in spring were smaller than the ones found later in the year?
"However we observe that these species are exported almost exclusively as empty cells."Why however?The authors should succinctly describe conceptual scheme proposed by Quéguiner ( 2013) or at least provide some insights of this scheme in order to compare their results with it.
Carlotti et al. 2014 manuscript is in preparation or has been submitted?Please check correct if necessary "The main difference in our observations and the conceptual scheme of Quéguiner ( 2013) is the dominance of Chaetoceros Hyalochaete resting spores to diatom export assemblages and their contribution to carbon fluxes out of the mixed layer in summer, 10 probably triggered by Si(OH)4 limitation.Resting spores appear to efficiently bypass the "carbon trap" represented by grazers and might also physically entrain small faecal pellets in their downward flux." It is generally accepted that zooplankton activity diminishes with depth toward the lower C7906 mesopelagic zone (∼ 1.5 km).Therefore, it is possible that RS are grazed or affected by other processes after 300 m.This should be also addressed in the discussion.

Conclusions
"Despite iron availability,. .." The authors should provide values and references of the iron concentration in the surface layer during the experiment (if available).This information should also be mentioned in the discussion.Table 1.A column with the number of days that each cup was open should be included.The units of the column LSi seems to be erroneous.Please revise all the units on the table and text.Table 2 , 3 and 5.The word bold shouldn t be in bold in the caption figures.Table 4.The heading of the first column "Species-taxa group/Cup number" is confusing, please correct.

Technical corrections
In the penultimate paragraph of the discussion (line 18) change "sport" by "spore".

Figure 1 .
It is not clear what the two curves plotted in figure 1a represent.Please clarify in the figure caption.

Figure 7
Figure 7 is not mentioned in the text.