Deep-sea sponge grounds as nutrient sinks: High denitrification rates in boreo-arctic sponges

Department of Biological Sciences, University of Bergen, Postboks 7803, 5020, Bergen, Norway. Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Hong Kong. Department of Earth Sciences, University of Bergen, Postboks 7803, 5020, Bergen, Norway. 10 K.G. Jebsen Centre for Deep Sea Research, University of Bergen, Postboks 7803, 5020, Bergen, Norway. NORCE, Norwegian Research Centre, NORCE Environment, Nygårdsgaten 112, 5008 Bergen, Norway CIIMAR – Interdisciplinary Centre of Marine and Environmental Research of the University of 15 Porto, 4450-208 Matosinhos, Portugal.


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Printer-friendly version Discussion paper Upscaling of the rates indicates high nitrogen removal rates in sponge grounds.
Denitrification was previously demonstrated in a few sponge species, but this survey represents a substantial expansion of the small database, particularly for colder waters. It further contributes to the growing literature on nitrogen transformations in "exotic" environments such as marine snow, animal microbiomes, etc. Thus, it is an original and relevant study and well-suited for Biogeosciences. The study was generally well designed and the results are of good quality. The writing and presentation of results are generally clear. While some conclusions are justified others require further discussion and likely need to be moderated.
Major issues 1) Experiments were conducted with nitrate and ammonium added at at least 10 fold higher concentrations than in situ values (100 µM vs. 10 µM and 10 µM vs. ≤ 1 µM, respectively, i.e., 1000% above ambient, and not 90% as stated in the text p. 11 l. 6). This means that the measured rates must be treated as potential rates unless the authors can establish an argument for 0th-order kinetics for both denitrification and nitrification. In turn, this implies that the estimated sponge-ground rates may be vastly (10-fold) overestimated. This issue should be discussed and the conclusions modified accordingly. In the oxic experiments, denitrification rates could, in principle, be calculated using the classic isotope pairing calculations for sediment cores (D14 sensu Nielsen 1992), but then the incubations should have been performed without addition of unlabelled ammonium and with maintenance of steady state.
2) Nitrification-based denitrification rates are calculated from the accumulation of single labelled 29N2. Firstly, it is not entirely clear how these rates and relative contributions were calculated, and I suggest to include the essential equations in Methods. Secondly, the concept of water-based and nitrification-based denitrification was developed by Nielsen for sediment cores with steady state distributions of oxygen and nitrate (and it was challenged by Middelburg in L&O 41:1839). In the present study, oxygen was clearly not at steady state during the oxic incubations, and it also seems likely that new formed nitrate may have leaked from the sponge tissue thus gradually decreasing the C2

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Printer-friendly version Discussion paper labelling of the ambient nitrate pool, and increasing 29N2 production from the ambient water. Moreover, the data presented in Fig. 1, for one of the six sponges, suggests that there is an issue with the mass balance of unlabelled N in the incubations. Thus, at the end of the anoxic incubations, excess 29N2 dominated over 30N2 in two of three incubations despite the stated ∼90% labelling of the nitrate pool, and the accumulated 29N2, reaching up to ∼23 µM, exceeds the amount of unlabelled nitrate initially available (10 µM in situ + 1 µM from the 99% 15N tracer). Also during the first 24 h, 29N2 production in the anoxic incubations seems higher than predicted by nitrate labelling in the absence of nitrification. Altogether, these uncertainties and discrepancies undermine the conclusion concerning the role of nitrification. Plots of excess 29N2 vs. excess 30N2 could potentially help the authors to evaluate and constrain some of these issues.
Specific comments 3, 8-12: The final statement is highly speculative and does not belong in an abstract. 4, 16-7: The statement about nif genes seems out of context. 6, 14: Science should never aim to show specific results but rather test hypotheses! 7, 11 9, 4-5: "Upper few centimetres" is vague -considering the negative result, the question is whether only the oxic surface layer was sampled. 9, 20: There was no "atmosphere" in the vials? However, incubation with a helium/oxygen headspace would have kept the incubations oxic throughout. 10, 7-8: This seems a very shaky assumption. Respiration rates must vary with species, temperature, and trophic state.
10, 18-9: Some oxygen is likely introduced during transfer -did you test the water in the Exetainers?

C3
Printer-friendly version Discussion paper 23, 9-10: With 6 orders of magnitude variation, this is not very telling.
23, 19 on: The calculations of sponge ground rates need explanation, but see Major issue #1. Furthermore, it seems that results of population density surveys are presented here for the first time. If this is the case, the methods and results should be specified i the appropriate sections. Otherwise, a reference should be included. 24, 24: What was the frequency of non-pumping? 25, 11-2: Is this a short-term or permanent effect? Would reduced pumping rates/increased anoxia not result in reduced growth, reduced biomass, and thereby reduced nitrogen removal in a longer perspective? The system effect of the stressors seems speculative.