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.
Highlights
A way to obtain insights into the impact of climate variability on marine ecosystems is monitoring multiannual evolution and changes covering key species or groups of species representing different trophic levels
In addition to the previously reported diatom fluxes and reconstructed sea-surface temperature (SST) values, we present here new data of fluxes of coccolithophores, calcareous and organic-walled dinoflagellate cysts, planktonic foraminifera, and pteropods as well as the species-specific composition of the assemblages that have been collected at the mooring site CBeu (Cap Blanc eutrophic), located around 80 nmi west of the Mauritanian coastline below a giant chlorophyll filament (Fig. 1)
Coccolithophores, planktonic foraminifera, calcareous dinoflagellates and pteropods are the main contributors to the CaCO3 flux, while diatoms dominate the siliceous community
Summary
A way to obtain insights into the impact of climate variability on marine ecosystems is monitoring multiannual evolution and changes covering key species or groups of species representing different trophic levels. Despite the fact that they cover only 10 % of the global surface ocean area, they provide about 25 % of the global fish catch (Pauly and Christensen, 1995) and build extraordinary marine hotspots of high primary production and biodiversity (Arístegui et al, 2009). In doing so, they play a key role in the marine biological pump, and, together with other continental margins, they may be responsible for more than 40 % of the CO2 ocean sequestration (Muller-Karger et al, 2005). As EBUEs are highly dynamic with large seasonal and interannual variability (e.g., Chavez and Messié, 2009; Fischer et al, 2016), gaining information on their long-term variability is essential to understand their potential response to current global climate change
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