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Biological and physical influences on marine snowfall at the equator

Abstract

High primary productivity in the equatorial Atlantic and Pacific oceans is one of the key features of tropical ocean biogeochemistry and fuels a substantial flux of particulate matter towards the abyssal ocean. How biological processes and equatorial current dynamics shape the particle size distribution and flux, however, is poorly understood. Here we use high-resolution size-resolved particle imaging and Acoustic Doppler Current Profiler data to assess these influences in equatorial oceans. We find an increase in particle abundance and flux at depths of 300 to 600 m at the Atlantic and Pacific equator, a depth range to which zooplankton and nekton migrate vertically in a daily cycle. We attribute this particle maximum to faecal pellet production by these organisms. At depths of 1,000 to 4,000 m, we find that the particulate organic carbon flux is up to three times greater in the equatorial belt (1° S–1° N) than in off-equatorial regions. At 3,000 m, the flux is dominated by small particles less than 0.53 mm in diameter. The dominance of small particles seems to be caused by enhanced active and passive particle export in this region, as well as by the focusing of particles by deep eastward jets found at 2° N and 2° S. We thus suggest that zooplankton movements and ocean currents modulate the transfer of particulate carbon from the surface to the deep ocean.

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Figure 1: Primary production is enhanced at the Atlantic and Pacific equator and fuels marine equatorial snowfall.
Figure 2: Zonal current velocity and calculated POC content transects across the equatorial Atlantic at 23° W.
Figure 3: Zonal current velocity and calculated POC content transects across the equatorial Pacific at different longitudes.
Figure 4: POC flux increase coincides with diel vertical migration depth at the equatorial Atlantic and Pacific.
Figure 5: POC flux at bathypelagic depth is dominated by micrometric particles and enhanced at the equator.

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Acknowledgements

This study was supported by the German Science Foundation through the Collaborative Research Center 754 ‘Climate-Biogeochemistry Interactions in the Tropical Ocean’ and by the German Federal Ministry of Education and Research through the cooperative project ‘RACE’. The enthusiastic and continued support of UVP5 operations and maintenance by Jerome Coindat and Sylvain Fevre (Hydroptic) is gratefully acknowledged. For the Tara Oceans expedition we thank the commitment of the CNRS (in particular Groupement de Recherche GDR3280), European Molecular Biology Laboratory (EMBL), Genoscope/CEA, VIB, Stazione Zoologica Anton Dohrn, UNIMIB, Fund for Scientific Research—Flanders, Rega Institute, KU Leuven, The French Ministry of Research and the French Government ‘Investissements d’Avenir’ programmes OCEANOMICS (ANR-11-BTBR-0008). We are also grateful for the support and commitment of Agnès b. and Etienne Bourgois, the Veolia Environment Foundation, Région Bretagne, Lorient Agglomération, World Courier, Illumina, the EDF Foundation, FRB and the Prince Albert II de Monaco Foundation. This article is contribution number 59 from Tara Oceans. The contributions of M.P. and L.S. were supported by the Chair VISION of the CNRS and UPMC. A.M. acknowledges support from the National Science Foundation, the US Global Ocean Carbon and Repeat Hydrography Program, and the University of Alaska Fairbanks. US GO_SHIP funding through NSF OCE-1437015 is gratefully acknowledged for the acquisition of the P16N L-ADCP data. S.C. and F.M. acknowledge support by the French national programme LEFE/INSU (ZEBRE) and IRD, which also supported the CASSIOPEE cruise. The backward tracking experiments and the underlying ocean model integrations were performed at the North-German Supercomputing Alliance (HLRN) and the computing centre at Kiel University. We would like to thank captains and crews of RV Meteor, RV Maria S. Merian, RV Ron Brown, RV L’Atalante and the Tara schooner for their support, P. Vandromme for conducting UVP5 deployments during RV Meteor cruise M119, DT-INSU and US-IMAGO for the acquisition and processing of CTD, L-ADCP and UVP5 measurements and G. Eldin for the shipboard ADCP data from the CASSIOPEE cruise. Furthermore, we would like to thank G. Krahmann for processing of L-ADCP and CTD data from RV Meteor cruises M106 and M119 and RV Maria S Merian cruise MSM22, as well as H. Mehrtens for help with data management. F. Melzner and C. Bowler provided very valuable comments on the article.

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R.K. and L.S. led the project and designed the study. A.M.P.M., H.H., M.P., L.S., I.K. and R.K. processed and analysed UVP5 data. F.U.S. and A.B. conducted particle backtracking simulations. P.B., R.H., A.M.T., S.C. and F.M. provided CTD and ADCP data. R.K. compiled all data and led the drafting of the manuscript. All authors contributed to the interpretation of the results and provided substantial input to the manuscript.

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Correspondence to R. Kiko.

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Kiko, R., Biastoch, A., Brandt, P. et al. Biological and physical influences on marine snowfall at the equator. Nature Geosci 10, 852–858 (2017). https://doi.org/10.1038/ngeo3042

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