|Characterising the surface microlayer in the Mediterranean Sea: trace metal concentrations and microbial plankton abundance of Antonio Tovar-Sánchez and co authors.|
It is avery interesting subject, executed in a very interesting place, the Mediterranean under the Sahara dust plume, although during the cruise transport of aerosols was often from the north. The authors analysed a lot of parameters, but unluckily they could not harvest many conclusions from their efforts, which makes the manuscript quite descriptive.
I think the manuscript deserves to be published, it will stimulate further research but I have some serious comments.
I think a table should be added with more details than only the recovery on the chemical analyses with ICPMS. A table with the CASS values for example related to the defined concentration in the CASS sample.
However, my main comments are on unclear reasoning in the discussion and on the discrepancy between the results-discussion section and the conclusion, sometimes also the abstract section. In the last sections I have read statements that are not supported by the former mentioned sections.
In the abstract the authors write about particulate metals, these were not determined. Unfiltered water was analysed, so call these values unfiltered concentrations. Particulate metals were not sampled and were as far as I understand also not calculated by subtracting dissolved from total concentrations (which I do not recommend, let that be clear).
In the conclusion it reads:
“While some metals entering the SML (e.g. Cd, Co, Ni and V) show efficient diffusive mixing from the SML to the SSW, more reactive metals such as Cu, Fe, Pb and Zn seem to exhibit a slower diffusion.”
Diffusion and diffusive mixing was not discussed, residence times were. And these residence times show that the residence time of Cu is one of the largest whereas the one of Fe is the shortest, as is also stated in the abstract. Still Fe and Cu both seem to exhibit a slower diffusion. This has to be explained better or if not supported by the results changed into another conclusion
“ 3.2.1. Trace metals in the SML
Dissolved concentrations of Co, Zn, Pb, Cu and Ni showed a decreasing trend from the SML to
the SSW, with average concentrations (± SD) 10.4 ± 0.7, 9.3 ± 5.5, 4.2 ± 1.8, 3.1 ± 1.5, and 1.2 ± 0.1 times higher in the SML than in the SSW, respectively. The SML to SSW concentration ratio for V (1.2 ± 0.42) and Fe (1.3 ± 1.5) indicated only slight enrichment in the SML over the underlying water, while the ratio for Mo (1.0 ± 0.1) indicated no difference between layers (Table 1). Only Cd concentrations were consistently lower in the SML compared to the underlying water (0.8 ± 0.2 times lower). Such depletion of dissolved metals in the SML compared to the underlying water has been previously observed in areas without significant aerosol inputs (Ebling and Landing, 2015, 2017). Although not fully understood, some mechanisms such as the dominance of removal mechanisms versus diffusion, or the higher influence of underlying metal sources have been suggested previously to explain this metal depletion (Ebling and Landing, 2017; Hunter, 1980).”
First: V and Fe have a slight enrichment but Ni has not, whereas Ni enrichment is lower than Fe and equal to V
Second: Such depletion: Only Cd has a lower ratio. So is it Cd the authors write about or is the enrichment factor so low compared to ??? that the word depletion is used. This needs explanation or a different reasoning.
Third : Apart from my confusion about this, the actual word depletion in my book has a relation to not enough for biology, blooms stop. Perhaps the word depleted must be changed by low ratio, low concentration. (but the above first and second comments, remain)
Pae 11 lines 11 onwards
Quote: “During the cruise, Al and Fe atmospheric concentrations were correlated at all the stations and the ratio Fe/Al is typical of a crustal source (Fu et al., in prep.). It is known that the atmospheric iron deposition fluxes are associated to mineral dust particles even during the period when the Saharan dust inputs are very low (Desboeufs et al., 2018; Guieu et al., 2010). On the contrary, no correlation with Al is observed for the other metals, except during FAST1-3. Thus, we used a velocity of mineral dust deposition for Fe 1 cm.s- 1 and an average velocity of fine anthropogenic particles for the other metals, i.e. 0.1 cm.s-1 (Baker et al.,2010; Duce et al., 1991).”
It is all very well that Fe correlates with Al, but there is hardly transport from the south. And this Fe/Al ratio, is that not related to the mineral lattice? So not really Fe that will dissolve so easily in such a short residence time. Moreover, this factor 10 difference in velocity decreases the residence time with a factor 10. If the authors had used the same deposition for Fe, the residence time ± the error would overlap the residence time of Co and Zn. And then only Ni would have a residence time that is extremely short, which would fit the lack of reactivity of this element in contrast to an element as Fe. This might interfere with the role of Ni in the discussion though.
Quote: “ Since, such a quick transfer of these metal particles to the underlying water
(in the order minutes) is unlikely (mainly due to their affinities to organic ligands), and the dissolution is not immediately reflected in an increase in the concentration in the dissolved fraction (i.e. D-SML), other variables (linked to physical processes, photochemistry or biological activity) probably affected the residence time of this and the other metals in the SML.”
Gerringa et al, 2017 ((mar. chem. 194, 100-113) concluded that ligands are saturated in the surface Mediterranean so that might form one explanation that a quick transfer as particles to the underlying water is possible.
One of the co-authors of this manuscript, Wagener (in Wagener et al. 2010), might have some opinions here, that could explain this quick transfer also very well!
Quote: “On average, while the highest residence times obtained for Cu and Pb are in agreement with their strong affinity to particles and therefore with a high probability of retention in the SML, other reactive elements such as Fe presented the shortest residence times. Since, such a quick transfer of these metal particles to the underlying water (in the order minutes) is unlikely (mainly due to their affinities to organic ligands), and the dissolution is not immediately reflected in an increase in the concentration in the dissolved fraction (i.e. D-SML), other variables (linked to physical processes, photochemistry or biological activity) probably affected the
residence time of this and the other metals in the SML” .
I cannot follow the reasoning: the long residence times of Cu and Pb are explained by a strong affinity to particles. However, one of the possible explanations for a short residence time is also binding by particles (a quick transfer to particles). Although according to the authorsthis is not the case for Fe, it still served as a possible explanation for a short residence.
Quote: “Since surface salinity showed the same eastward increase and was closely correlated with those metals (rs ranged from 0.51 p<0.05 for Mo to 0.97 p<0.01 for Ni; Table 3), the exchange with the surface Atlantic Ocean waters seems to be the main cause of this gradient of concentrations in our study, although higher aerosol inputs in the western MS could also contribute to this gradient.”
How can more aerosol in the west decrease dissolved concentrations? I think dissolution would increase the conc, if you mean ballasting and adsorption processes are connected with the decrease in dissolved concentrations than this must be better explained.
Abstract, Line 33: Ni concentration: do you mean total or dissolved ? Since both are discussed in the lines just above.
Page 2: lines 17-19
Copied from the Conclusion in Sarthou and Jeandel 2001: “The very low surface concentrations may limit primary production or, at least control phytoplankton species composition. Such an iron depletion was not expected in the Mediterranean Sea, characterised by important continental inputs.”
For me “may limit” is not equal to saying that “ Fe has been considered an important factor controlling phytoplankton growth (Sarthou and Jeandel, 2001).”
So tone down here please.
Line 25, were the metals in the FAST station comparable then with Tovar-Sanchez et al., 2014?
Line 17 however, the strong dependence of redox seawater chemistry and complexation of
elements such as Cu and Fe on solar radiation is well known (Croot and Heller, 2012; Moffett and Zika,1988).
I would not say that this is really well known, some studies have been done, true but in my opinion nothing on the combination of complexation and redox chemistry is well known.
Please change “well known” in “has been studied” or some knowledge is present or another rewording.
Page 12, lines12-14
Our results indicate that Fast 1-4 stations were affected by the dusty rain events,
which increased the concentration of some metals in the T-SML and consequently the residence time (Table 4). However, the reasons for the increase at stations 3-4 are not evident.
For me this is unclear, 1-4 means 1,2,3,4. So if 1-4 can be explained then why 3 and 4 not?