Abstract
Tropical oceans provide a benchmark for future primary productivity in increasingly warmer, stratified and nutrient-depleted waters. In this context, we assess the export fluxes of calcifying phytoplankton (coccolithophores) across the tropical North Atlantic, from upwelling affected NW Africa, via three ocean sites along 12°N to the Caribbean. Sampling was undertaken by means of a spatial array of four time-series sediment traps collecting particle fluxes in two-week intervals, from October 2012 to February 2014, allowing to track temporal changes along the southern margin of the North Atlantic central gyre. Species composition and seasonal export fluxes of coccolithophores show steep gradients in two groups. Upper photic zone (UPZ) and placolith-bearing species dominated by Emiliania huxleyi and Gephyrocapsa oceanica are most abundant in the mesotrophic surface waters of the Cape Blanc upwelling system off NW Africa. They decline gradually towards the Caribbean, paralleled by increasing surface temperatures and decreasing surface chlorophyll-a. Meanwhile the abundance of lower photic zone (LPZ) species Florisphaera profunda and Gladiolithus flabellatus increase in the same direction, reaching fluxes up to 3–5 times higher in the western end of the transect compared to the UPZ flora in mesotrophic waters. Adapted to low light conditions, the LPZ species follow the geostrophic wind-forced deepening of the thermocline/nutricline westward in ever lower species diversity towards the Caribbean. Temporal changes were marked by weak seasonality in coccolith fluxes at all four sites, modulated by latitudinal migration of the Intertropical Convergence Zone (ITCZ) at the tropical sites M1, M2 and M4, and by spatiotemporal variation in wind-forced upwelling at site CB. A seasonal mismatch was observed between LPZ and UPZ-oligotrophic taxa (i.e. Umbellosphaera spp., Rhabdosphaera spp.) vs. UPZ-opportunistic species (E. huxleyi, G. oceanica) at the western site M4 contrasting with the more similar seasonal patterns amongst all species towards site CB. We interpret this as reflecting the entire photic zone becoming increasingly nutrient enriched towards east whenever wind-forced mixing occurs due to the eastward shoaling of the thermocline/nutricline. Our synoptic observations of seasonally resolved export fluxes at four sites contribute to the spatiotemporal understanding of coccolithophore fluxes across the entire tropical North Atlantic, urging for considering phytoplankton- and carbonate production across the entire photic zone when projecting the effects of ocean warming on future primary production.
| Original language | English |
|---|---|
| Article number | 102140 |
| Pages (from-to) | 1-19 |
| Number of pages | 19 |
| Journal | Progress in Oceanography |
| Volume | 176 |
| Early online date | 24 Jul 2019 |
| DOIs | |
| Publication status | Published - Sept 2019 |
Funding
We thank the crews of Meteor cruise M89, Pelagia cruise 64PE378, RV Poseidon cruises P425, POS445 and POS464 (recovery of CB-24), as well as NIOZ and MARUM technicians for their contributions. Moorings M4, M2 and M1 managed by the Royal NIOZ in the framework of the multidisciplinary projects TRAFFIC funded by NWO (project no. 822.01.008), and DUSTTRAFFIC, funded by ERC (project no. 311152), both awarded to Jan-Berend W. Stuut. Moorings CB-23 and CB-24 managed by MARUM and the University of Bremen. We are deeply thankful to Gerhard Fischer for providing all the samples and total mass flux data from trap CB, and for his great and fruitful contribution during the discussion of the data and the writing of the manuscript. Lab preparation of the 1/5 split of the original sediment trap sample for M4, M2 and M1 was conducted at the NIOZ, whereas the splitting, filtering, and SEM taxonomical analysis were performed at the Geosciences Department of the University of Bremen, Germany. The first author benefited from a Marie Sklodowska-Curie Fellowship supported by the University of Bremen and the European Union FP7 COFUND under grant agreement no. 600411, and currently benefits from a Marie Sklodowska-Curie European Fellowship supported by the European Union H2020-MSCA-IF-2017 under grant agreement no. 796802. This study also had the support of the Portuguese Science Foundation (FCT), through the strategic project UID/MAR/04292/2013 granted to MARE, and the post-doctoral grant awarded to Carolina Sá (SFRH/BPD/118760/2016). Regarding the satellite data used in this study, the authors would like to acknowledge the following entities: the MODIS Atmosphere Science Team and the MODIS Adaptive Processing System (MODAPS); the NASA EOSDIS Physical Oceanography Distributed Active Archive Centre (PO.DAAC) ( http://podaac.jpl.nasa.gov/SeaSurfaceSalinity/Aquarius ); and the Ocean Biology Processing Group (OBPG) and the Atmosphere Archive and Distribution System (LAADS) at the NASA Goddard Space Flight Center. The CCMP version 2.0 vector wind analyses were produced by Remote Sensing Systems ( www.remss.com ) and the CMAP precipitation data were provided by NOAA/OAR/ESRL PSD, Boulder, Colorado, USA ( http://www.esrl.noaa.gov/psd/ ). Modelled MLD data presented in Fig. 6 were obtained from the NASA Ocean Biogeochemical Model ( https://gmao.gsfc.nasa.gov/reanalysis/MERRA-NOBM/ ), whereas the in situ MLD and temperature data presented in Figs. 3 and 6 were collected and made freely available by the International Argo Project, a pilot programme of the Global Ocean Observing System, and by the national programmes that contribute to it ( http://www.argo.net ). We are thankful to Afonso Ferreira for compiling the NAO time-series data downloaded from NOAA’s Climate Prediction Center ( https://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/nao.shtml ). We also gratefully acknowledge the constructive criticism from the editors Nate Mantua and Josep Pelegri, and from two anonymous reviewers. Appendix A
| Funders | Funder number |
|---|---|
| DUSTTRAFFIC | |
| European Union FP7 COFUND | |
| European Union H2020-MSCA-IF-2017 | |
| MARUM | M1 |
| MODIS Atmosphere Science Team | |
| NASA EOSDIS Physical Oceanography Distributed Active Archive Centre | |
| NASA Ocean Biogeochemical Model | |
| NOAA/OAR/ESRL | |
| Portuguese Science Foundation | |
| National Oceanic and Atmospheric Administration | |
| Goddard Space Flight Center | |
| Horizon 2020 Framework Programme | 796802, 311152 |
| European Research Council | |
| Fundação para a Ciência e a Tecnologia | UID/MAR/04292/2013, SFRH/BPD/118760/2016 |
| Nederlandse Organisatie voor Wetenschappelijk Onderzoek | 822.01.008 |
| Seventh Framework Programme | 600411 |
| Total | |
| Koninklijk Nederlands Instituut voor Onderzoek der Zee | |
| Universität Bremen |
