A two-decades (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

Abstract. Eastern Boundary Upwelling Ecosystems (EBUEs) are among the most productive marine regions in the world's oceans. Understanding the degree of interannual to decadal variability in the Mauritania upwelling system is key for the prediction of future changes of primary productivity and carbon sequestration in the Canary Current EBUE as well as in similar environments. A multiannual sediment trap experiment was conducted at the mooring site CBmeso (= 'Cape Blanc mesotrophic', ca. 20° N, ca. 20°40' W) in the high-productive Mauritanian coastal waters. Here, we present results on fluxes and the species-specific composition of the diatom assemblage for the time interval between March 1988 and May 2009. The temporal dynamics of diatom populations allows to propose three main diatom productivity/flux intervals: (i) early 1988–late 1996; (ii) 1997–1999, and (iii) early 2002–mid 2009. The impact of the Atlantic Multidecadal Oscillation appears to be an important forcing of the long-term dynamics of diatom population. The impact of cold (1988–1996) and warm AMO phases (2001–2009) is reflected by the outstanding shifts in species-specific composition. This AMO-impacted, long-term trend is interrupted by the occurrence of the strong 1997 ENSO. The extraordinary shift in the relative abundance of benthic diatoms in May 2002 suggests the strengthening of offshore advective transport within the uppermost layer of filament waters, and in the subsurface and in deeper and bottom‐near layers. It is hypothesized that the dominance of benthic diatoms was the response of the diatom community to the intensification of the slope and shelf poleward undercurrents. This dominance followed the intensification of the warm phase of AMO and the associated changes of the Atlantic Meridional Overturning Circulation. Transported valves (siliceous remains) from shallow coastal waters into the deeper bathypelagial should be considered for the calculation and model experiments of bathy- and pelagial nutrients budgets (especially Si), the burial of diatoms and the paleoenvironmental signal preserved in downcore sediments. Our 1988–2009 data set contributes to the distinction between climate-forced and intrinsic variability of populations of diatoms and will be especially helpful for establishing the scientific basis for forecasting and modelling future states of this ecosystem and its decadal changes.