the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
The influence of zooplankton and oxygen on the particulate organic carbon flux in the Benguela Upwelling System
Abstract. We conducted extensive sediment trap experiments in the Benguela Upwelling System (BUS) in the south-eastern Atlantic Ocean to study the influence of zooplankton on the flux of particulate organic carbon (POC) through the water column and its sedimentation. Two long term moored and sixteen short term free-floating sediment trap systems were deployed. The mooring experiments were conducted for several years and the sixteen drifters were deployed on three different research cruises between 2019 and 2021. Zooplankton was separated from the trapped material and divided into 8 different zooplankton groups. In contrast to zooplankton which actively carries POC into the traps in the form of biomass (active POC flux), the remaining fraction of the trapped material was assumed to fall passively into the traps along with sinking particles (passive POC flux). The results show, in line with other studies, that copepods dominate the active POC flux, with the active POC flux in the southern BUS (sBUS) being about three times higher than in the northern BUS (nBUS). In contrast, the differences between the passive POC fluxes in the nBUS and sBUS were small. Despite large variations, which reflected the variability within the two subsystems, the mean passive POC fluxes from the drifters and the moored traps could be described using a common POC flux attenuation equation. However, the almost equal passive POC flux, on the one hand, and large variations in the POC concentration in the surface sediments between the nBUS and sBUS, on the other hand, imply that factors others than the POC supply exert the main control on POC sedimentation in the BUS. The varying intensity of the near-bottom oxygen minimum zone (OMZ), which is more pronounced in the nBUS than in the sBUS, could in turn explain the differences in the sediments, as the lack of oxygen reduces the POC degradation. Hence, globally expanding OMZs might favour POC sedimentation in regions formerly exposed to oxygenated bottom water but bear the risk of increasing the frequency of anoxic events in the oxygen-poor upwelling systems. Apart from associated release of CH4, which is a much more potent greenhouse gas than CO2, such events pose a major threat to the pelagic ecosystem and fisheries.
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Status: open (until 22 May 2024)
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RC1: 'Comment on egusphere-2024-700', Anonymous Referee #1, 24 Apr 2024
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This paper presents organic carbon and biomass fluxes sampled with bottom tethered and free-drifting sediment traps in the Benguela Upwelling System. A main finding is that in the southern BUS the flux carried by identifiable zooplankton organisms (called 'active flux') is larger than in the northern BUS while the small particle POC flux ('passive flux') is quite similar between both systems which do differ in terms of sedimentary POC accumulation. Authors discuss possible mechanisms explaining these observations, including suboxic conditions in the north preserving POC and enhanced zooplankton-related active fluxes in the south.
Although the paper presents useful information, It is in my opinion not ready for publication in its present form.
I have some concerns regarding the methodology. The > 1 mm particle fraction is analysed for zooplankton group identifcation an deduction of biomass flux. To what extent are these composed of sinking zooplankton and swimmers who got caught accidentally on the traps? Authors just briefly mention this possibility but do not enter into a detailed discussion about this issue.
When determining biomass weight and elemental composition, carbonate content is not taken into account (as mentioned by the authors), although pteropods appear to be present. What impact would pteropod carbonate have?
What is the time lag between sampling and laboratory analysis? Is the used protocol safe for ensuring biological material remains unaltered till analysis?
Explain how the filtered material is recovered from the polycarbonate filters after drying.
Vertical trends of <1mm POX fluxes are investigated by applying Martin curve fits to avergae profiles. To what extent is information lost due to this averaging of profiles? What sense does it have to fix the MLD at 10m ? In the Martin approach the MLD is set at 100m depth. Is there any reason to choose a shallower MLD depth? No information is given on observed MLD and its variability. Martin curves do not provide useful information when extrapolated to shallow depths (<100m). In that sense the discussion at page 15 (lines 320-330) needs to be reconsidered.
Discussion about delivery of POC to the sediments (pp14-15) is focussed on the passive <1mm POC flux. But what about the larger stuff ? Unclear how it is taken into account.
Specific comments:
Figures and legends are incomplete. Fig 1 does not show Hondeklip, Cape Columbine, nor mentions units for POC concentration. Specify these these are sediment POC contents; nBUS and sBUS delimitation should be indicated. Fig 2 legend should provide a reference for the O2 data of cruise SO285. Fig 3 legend lacks information about the considered surface areas, and time period used for averaging, red and black dots are not identified; the graphs should have markers indicating the timing of the 3 cruises; there is no reference for the used data set. Fig 6 specify that % of zooplankton is biomass % . Fig 9 is not necessary in my opinion and if shown should indicate error bars on the measures POC fluxes.
Line 183: the value of 1.8 to convert POC to OM comes out of the blue
Line 210: analysis of the main components of the dried material 'as described before'.. I don't think this was described previously in the text.
Lines 245 tyo 252: Not clear why passive, active POC fluxes are averaged over the water column when comparing sites. Onlmy comparing fluxes at given horizons between sites would make sense.
Line 286: Zooplankton abundance is mentioned here. Nothing has been set before in the methiods section about his.
Line 297: Possible occurrence of active swimmers. This important issue should have been tackled before in the method section.
Line Line 332: Assuming a mean water depth of 150m ... ? Unclear what the purpose is. Holds for the moored traps only ?
Citation: https://doi.org/10.5194/egusphere-2024-700-RC1
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