Flux variability of phyto- and zooplankton communities in the Mauritanian coastal upwelling between 2003 and 2008
Authors: Romero Oscar E. , Baumann Karl-Heinz , Zonneveld Karin A. F. , Donner Barbara , Hefter Jens , Hamady Bambaye , Pospelova Vera , Fischer Gerhard 
Published: January 2020

Abstract. Continuous multiyear records of sediment-trap-gained microorganism fluxes
are scarce. Such studies are important to identify and to understand the
main forcings behind seasonal and multiannual evolution of microorganism
flux dynamics. Here, we assess the long-term flux variations and population
dynamics of diatoms, coccolithophores, calcareous and organic dinoflagellate
cysts, foraminifera and pteropods in the eastern boundary upwelling
ecosystem of the Canary Current. A multiannual, continuous sediment trap
experiment was conducted at the mooring site CBeu (Cap Blanc eutrophic; ∼20∘ N, 18∘ W; trap depth is ca. 1300 m) off
Mauritania (northwest Africa), between June 2003 and March 2008. Throughout
the study, the reasonably consistent good match of fluxes of microorganisms
and bulk mass reflects the seasonal occurrence of the main upwelling season
and relaxation and the contribution of microorganisms to mass flux off
Mauritania. A clear successional pattern of microorganisms, i.e., primary
producers followed by secondary producers, is not observed. High fluxes of
diatoms, coccolithophores, organic dinoflagellate cysts, and planktonic
foraminifera occur simultaneously. Peaks of calcareous dinoflagellate cysts
and pteropods mostly occurred during intervals of upwelling relaxation. A
striking feature of the temporal variability of population occurrences is
the persistent pattern of seasonal groups contributions. Species of
planktonic foraminifera, diatoms, and organic dinoflagellate cysts typical
of coastal upwelling, as well as cooler-water planktonic foraminifera and the
coccolithophore Gephyrocapsa oceanica, are abundant at times of intense upwelling (late winter
through early summer). Planktonic foraminifera and calcareous dinoflagellate
cysts are dominant in warm pelagic surface waters, and all pteropod taxa are
more abundant in fall and winter when the water column stratifies.
Similarly, coccolithophores of the upper and lower photic zones, together
with Emiliania huxleyi, and organic dinoflagellate cysts dominate the assemblage during phases
of upwelling relaxation and deeper layer mixing. A significant shift in the “regular” seasonal pattern of taxa relative
contribution is observed between 2004 and 2006. Benthic diatoms strongly
increased after fall 2005 and dominated the diatom assemblage during the main
upwelling season. Additional evidence for a change in population dynamics
is the short dominance of the coccolithophore Umbilicosphaera annulus, the occurrence of the
pteropod Limacina bulimoides and the strong increase in the flux of calcareous dinoflagellate
cysts, abundant in warm tropical oligotrophic waters south of the study
area after fall 2005. Altogether, this suggests that pulses of southern
waters were transported to the sampling site via the northward Mauritania
Current. Our multiannual trap experiment provides a unique opportunity to
characterize temporal patterns of variability that can be extrapolated to
other eastern boundary upwelling ecosystems (EBUEs), which are experiencing or might experience similar future
changes in their plankton community.