Flux variability of phyto- and zooplankton communities in the Mauritanian coastal upwelling between 2003 and 2008

Abstract

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.

A 2-decade (1988–2009) record of diatom fluxes in the Mauritanian coastal upwelling: impact of low-frequency forcing and a two-step shift in the species composition

“…For this study, a total of 282 sediment trap samples were processed. Each sample was chemically treated with potassium permanganate, hydrogen peroxide (33 %), and concentrated hydrochloric acid (32 %) following previously used methodology (Romero and Fischer, 2017;Romero et al, 1999aRomero et al, , b, 2002Romero et al, , 2009aRomero et al, , b, 2020. Qualitative and quantitative analyses of the diatom community were carried out on permanent slides of acid-cleaned material (Mountex ® mounting medium) at ×1000 magnification by using a Zeiss ® Axioscope with phase-contrast illumination (MARUM, University of Bremen).…”

“…We discuss the new results in view of the high frequency of climate indices, which are proxies for atmospheric and hydrographic dynamics along the CC EBUE, and low-frequency climate variability in the North Atlantic, and we compare our new data set at the site CBmeso with previous diatom (Romero and Fischer, 2017;Lange et al, 1998;Romero et al, 1999aRomero et al, , b, 2002Romero et al, , 2020 and bulk flux results off Mauritania (Helmke et al, 2005;Fischer et al, 2016Fischer et al, , 2019. We also discuss our new results with recent results from the nearby coastal site CBeu (Cape Blanc eutrophic) (Romero and Fischer, 2017;Romero et al, 2020). We used deep-moored (> 700 m water depth), largeaperture time-series sediment traps of the Kiel and Honjo types with 20 cups and 0.5 m 2 openings, equipped with a honeycomb baffle (Kremling et al, 1996).…”

“…We compare our data with those previously published at the mooring location CBeu (ca. 20 • 45 N, 18 • 45 W), also deployed off Mauritania (Romero and Fischer, 2017;Romero et al, 2020). It locates ca.…”

“…2750 m water depth. For site CBeu, only the upper trap fluxes are shown (Romero and Fischer, 2017;Fischer et al, 2019;Romero et al, 2020).…”

Long-Term Changes of Particle Flux in the Canary Basin Between 1991 and 2009 and Comparison to Sediment Trap Records Off Mauritania

“…The year 2005 was discussed by Fischer et al (2016Fischer et al ( , 2019 as an exceptional year with high fluxes despite a low NAO. Romero and Fischer (2017) and Romero et al (2020) found a change in diatom fluxes and composition at the coastal Cape Blanc site (CB eu) starting in 2005/2006. Unusually high particle fluxes in 2005 in the bathypelagic were also observed far offshore at site Kiel 276 (ca.…”