Interactive comment on “ Vertically migrating phytoplankton drive seasonal formation of subsurface negative preformed nitrate anomalies in the subtropical North Pacific and North Atlantic ” by Robert T . Letscher and Tracy A . Villareal

Abstract. Summertime drawdown of dissolved inorganic carbon in the absence of measurable nutrients from the mixed layer and subsurface negative preformed nitrate (preNO 3 ) anomalies observed for the ocean's subtropical gyres are two biogeochemical phenomena that have thus far eluded complete description. Many processes are thought to contribute including biological nitrogen fixation, lateral nutrient transport, carbon overconsumption or non-Redfield C : N : P organic matter cycling, heterotrophic nutrient uptake, and the actions of vertically migrating phytoplankton. Here we investigate the seasonal formation rates and potential contributing mechanisms for negative preformed nitrate anomalies (oxygen consumption without stoichiometric nitrate release) in the subsurface and positive preformed nitrate anomalies (oxygen production without stoichiometric nitrate drawdown) in the euphotic zone at the subtropical ocean time series stations ALOHA in the North Pacific and BATS in the North Atlantic. Non-Redfield −O 2 : N stoichiometry for dissolved organic matter (DOM) remineralization is found to account for up to ~ 15 mmol N m −2 yr −1 of negative preNO 3 anomaly formation at both stations. Residual negative preNO 3 anomalies in excess of that which can be accounted for by non-Redfield DOM cycling are found to accumulate at a rate of ~ 32–46 mmol N m −2 yr −1 at station ALOHA and ~ 46–87 mmol N m −2 yr −1 at the BATS station. These negative anomaly formation rates are in approximate balance with positive preNO 3 anomaly formation rates from the euphotic zone located immediately above the nutricline in the water column. Cycling of transparent exopolymer particles (TEP) and heterotrophic nitrate uptake can contribute to the formation of these preNO 3 anomalies, however a significant fraction, estimated at ~ 50–95 %, is unexplained by the sum of these processes. Vertically migrating phytoplankton possess the necessary nutrient acquisition strategy and biogeochemical signature to quantitatively explain both the residual negative and positive preNO 3 anomalies as well as the mixed layer dissolved inorganic carbon drawdown at stations ALOHA and BATS. TEP production by the model Rhizosolenia mat system could provide accelerated vertical transport of TEP as well as link the three processes together. Phytoplankton vertical migrators, although rare and easily overlooked, may play a large role in subtropical ocean nutrient cycling and the biological pump.

Authors used pre NO3-concept to highlight the importance of phytoplankton vertical migration in nutrient supply and DIC drawdown in the surface ocean primarily focusing on the HOTS and BATS data.Preformed NO3-anomalies are smaller than true concentrations of preformed nutrients, which actually result from transports by mixing and circulation.However, the authors seem to have downplayed contributions of mixing in rNPN and rPPN as they did not consider this physical factor among causative factors in Table 2 (see Comment 9).Vertical migration of phytoplankton assumes significance C1 in upward transport of nutrients through their intracellular accumulation in nutricline layers.However, I wonder whether the authors' present option of using preformed nutrients and their anomalies to highlight the significance of vertical migration is justifiable as presented.This approach forced them make several assumptions (see Comment 16) and where calculations were made the uncertainties were not shown/assessed making the exercise not convincing.It may be possible to evaluate the quantitative significance of phytoplankton vertical migration to upward supply of nutrients and disproportional DIC drawdown from nutrient poor surface ocean by measuring intracellular upward nutrient transport and its possible leakage during vertical migration.
The authors are well aware of limitations associated with their assumptions (e.g.lines 453-465; 533-535).The central theme on showing the importance of phytoplankton vertical migration is appreciable several assumptions made with unknown uncertainties leave the reader wondering 'how much to be convinced'.specific for ALOHA and BATS.I believe this rPOM will be more realistic.Why use constants of 10.6 and 6.9 from literature?One can make a comparison with literature data but when one has an opportunity to use realistic values one should do so.In the present case the authors seem to prefer using literature values than their own results.Also I am not clear whether the values 10.6 and 6.9 are specific to PON or TON.
5. Lines 205 -210: The periods of occurrence of seasonal rNPN and rPPN anomalies and their trends may be shown in x-y plots.), fPOM (50%), rDOM (18.1-18.9)and rPOM of 10.6 or 6.9 in the computations for the entire water column of 200 m (see Table 1).Then the results in Figure 1 are mainly reflective of changes and trends in TDN, Nitrate+Nitrite and Oxygen!I guess results in Figure 1a C3 and b could be different if the authors used their evaluated values of fDOM, fPOM, rDOM and rPOM!!! 9. Line 369: "Having ruled out lateral mixing. ..." This seems to be a very tentative statement since lines 333-335 for BATS and 353-357 ALOHA clearly indicate lateral mixing influence is considerable in deeper layers.Ignoring mixing effects here is not justifiable.
10. Lines 402 and 427: A TEP gradient of 5-10 µg XG eq l-1 was used to assess its contribution to rPPN and rNPN anpmalies.I wonder whether such small gradient is sufficient to assessing its role in view of the semi-quantitative nature of TEP measurements and results.The authors should clearly discuss the uncertainties associated with TEP measurements and justify that the gradients used between surface and deep layers are significantly above the analytical errors.Other constraints associated with TEP are identified by the authors in lines 460-465.
11. Lines 413-414: "We assume TEP is comprised of pure carbohydrate with no N content. .." -This is a simplified statement.The TEP has dominant polysaccharide composition but to assume that no other organic materials (nitrogen containing substances) are attached to TEP is not realistic.
12. Lines 499-502: ". ..it is clear that neither remineralization of N-poor DOM and TEP or heterotrophic bacterial nitrate uptake can quantitatively explain both the. ...." -How justifiable is this statement given several assumptions involved.The authors have to quantify uncertainties to give confidence to readers at some level!13.Lines 547-550: The logic in the estimation of the contribution of vertically migrating phytoplankton to the rNPN and rPPN ignoring the contribution of physical N transports is not justified (see Comment 9).
14. Lines 557-558 and 564-565 are confusing!When vertically migrating phytoplankton can help explain the observed summertime DIC drawdown in the absence of mea-surable nitrate (557-558) why do they state 'the mixed layer DIC drawdown need not be entirely supported by migrator photosynthesis, instead their nitrate leakage could help explain. ..(564-565)'.Is not nitrate leaked through excretion used and included in migrator supported photosynthesis?
16. SEVERAL ASSUMPTIONS: a. Lines 186-187: "our approach assumes rDOM is constant over time within each density horizon investigated at each station" b.Lines 202-205: "For the calculation of the preNO3 tracer within the euphotic zone, we made the assumption that the values of fDOM and rDOM were equivalent to those empirically derived for the upper mesopelagic density layer present immediately below the euphotic zone at each site" c.Lines 413-414: "We assume TEP is comprised of pure carbohydrate with no N content. .." -This is probably highly simplified.We know that TEP has dominant polysaccharide composition but to assume that no other organic materials (say proteins etc.) attached to TEP is not realistic.d.Authors are well versed with the topic and aware of limitations of this study.Therefore, they should justify their logical statements through evaluations of relevant uncertainties wherever possible.Probably a better approach to convince the readers on the significance of nutrient export by phytoplankton vertical migration is by conducting experiments to quantify intracellular accumulation of N and P and the extent of nutrient leakage through excretion by vertically migrating phytoplankton.
Assumption of rDOM constancy over time in each density layer may compromise the examination of its temporal variability in the regions.More over near constancy of rDOM values shown in Table1for each region might oversimplify vertical variability.If such use is necessary why not consider the top 250 m as just one layer?Please see Comment 16 (a) and (b) below that seem to simplify or play down on actual temporal and spatial variabilities.I am afraid such assumptions may undermine understanding of natural variability and compromise the significance of this study.b.Now that authors evaluated rDOM we know the values of rPOM (since DOM and POM remineralization should account for 100% of AOU, see also lines[190][191] 6. Lines 223-225: 'DOM remineralization.....that utilizes the preNO3-tracer' is an appreciable observation and suggestion made in this study.7.Lines 226-230: It appears that 40 to 67% of estimated AOU is explainable by POM oxidation and thus is equally important as that of DOM.Again authors chose to use climatological averages of fDOM than the actual values observed (Comments 2 and 3 are relevant here).
8. Lines 242-245: Are increases in rPPN in the euphotic zone and rNPN in the subsurface waters between May-June and Oct-Nov at ALOHA connected?It should be remembered that we have used near constant values of fDOM (50%

Table 2 :
(a) Lines 133-134 in text: 'Phytoplankton vertical migration.... ..at both stations ALOHA and BATS' is not convincing by the information provided in Table2, (b) It is not clear whether the FOUR sources listed account for 100% of NPN or PPN features listed, (c) what are the total NPN and PPN values computed?What about contributions from lateral and vertical/diapycnal mixing, however small they are?(d) Consider all factors/sources and show they account for 100% of NPN or PPN evaluated, and (e) vertical migration appears more significant at BATS than at ALOHA?This will not be clear unless one shows the contributions of various sources in terms of percentage totaling to 100.