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Biogeosciences An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/bg-2017-216
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
14 Jun 2017
Review status
This discussion paper is under review for the journal Biogeosciences (BG).
Coccolithophore fluxes in the open tropical North Atlantic: influence of the Amazon river and of Saharan dust deposition
Catarina V. Guerreiro1, Karl-Heinz Baumann1,2, Geert-Jan A. Brummer3,4, Gerhard Fischer1,2, Laura F. Korte3, Ute Merkel1,2, Carolina Sá5, Henko de Stigter3, and Jan-Berend W. Stuut2,3 1University of Bremen, Geosciences Department, Klagenfurter Str., 28359 Bremen, Germany
2University of Bremen, MARUM – Center for Marine and Environmental Sciences, Leobener Str. 8, 28359 Bremen, Germany
3NIOZ Royal Netherlands Institute for Sea Research, Department of Ocean Systems, Den Burg 1790 AB, and Utrecht University, The Netherlands
4VU University, Earth and Climate Cluster, Department of Earth Sciences, Faculty of Earth and Life Sciences, De Boelelaan 1085 1081 HV Amsterdam, The Netherlands
5MARE Marine and Environmental Science Centre, Faculdade de Ciências da Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal
Abstract. Coccolithophores are calcifying phytoplankton and major contributors to both the organic and inorganic oceanic carbon pumps. Their export fluxes, species composition and seasonal patterns were determined in two sediment trap moorings in the open equatorial North Atlantic (M4 at 12° N 49° W and M2 at 14° N 37° W), which collected settling particles synchronously in successive 16-day intervals from October 2012 to November 2013, at 1200 m water depth. The two trap locations show a similar seasonal pattern in total coccolith export fluxes and a predominantly tropical coccolithophore settling assemblage throughout the monitored year. Species fluxes were yearlong dominated by lower photic zone (LPZ) taxa (Florisphaera profunda, Gladiolithus flabellatus), but also included upper photic zone (UPZ) taxa (Umbellosphaera spp., Rhabdosphaera spp., Umbilicosphaera spp., Helicosphaera spp.). The LPZ flora was most abundant during fall 2012, whereas the UPZ flora was more important during summer. In spite of these similarities, the western part of the study area produced persistently higher fluxes, averaging 241 × 107 coccoliths m−2 d−1 (117 × 107 to 423 × 107 coccoliths m−2 d−1) at station M4, compared to only 66 × 107 coccoliths m−2 d−1 (25 × 107 to 153 × 107 coccoliths m−2 d−1) at station M2. Higher fluxes at M4 were mainly produced by the LPZ species, although most UPZ species also contributed higher fluxes, reflecting enhanced productivity in the western equatorial North Atlantic. In addition, we found two marked flux peaks of the more opportunistic species Gephyrocapsa muellerae and Emiliania huxleyi indicating a fast response to nutrient-enrichment of the UPZ, probably by wind-forced mixing, whereas increased fluxes of G. oceanica and E. huxleyi in October/November 2013 coincided with the occurrence of Amazon River affected surface waters. Since the spring and fall events of 2013 were also accompanied by two dust flux peaks we propose a scenario where atmospheric dust also provided fertilizing nutrients to this area. Enhanced surface buoyancy associated to the river plume indicates that the Amazon acted not only as a nutrient source, but also as a surface density retainer for nutrients supplied from the atmosphere. Still, lower total coccolith fluxes during these events compared to the maxima recorded in November 2012 and July 2013 indicate that transient productivity by opportunistic species was less important than background tropical productivity in the equatorial North Atlantic. This study illustrates how two seemingly similar sites in an open-ocean tropical setting actually differ greatly in ecological and oceanographic terms, and provides valuable insights into the processes governing the ecological dynamics and the downward export of coccolithophores in the tropical North Atlantic.

Citation: Guerreiro, C. V., Baumann, K.-H., Brummer, G.-J. A., Fischer, G., Korte, L. F., Merkel, U., Sá, C., de Stigter, H., and Stuut, J.-B. W.: Coccolithophore fluxes in the open tropical North Atlantic: influence of the Amazon river and of Saharan dust deposition, Biogeosciences Discuss., https://doi.org/10.5194/bg-2017-216, in review, 2017.
Catarina V. Guerreiro et al.
Catarina V. Guerreiro et al.
Catarina V. Guerreiro et al.

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Short summary
Our study provides insights into the factors governing the spatiotemporal variability of coccolithophores in the equatorial North Atlantic, and illustrates how this supposedly oligotrophic and stable open-ocean region actually reveals significant ecological variability. We provide evidence for Saharan dust and the Amazon River acting as fertilizers for phytoplankton, and highlight the importance of coccolithophore productivity in the lower photic zone for the global oceanic carbonate budget.
Our study provides insights into the factors governing the spatiotemporal variability of...
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