Assessment of the Cape Blanc (northwest Africa) upwelling ecosystem response to recent climate change, using wavelet analyses on dinoflagellate cyst export

Abstract

Accelerated rates of change in recent climate have urged comprehensive investigations of its impact on marine ecosystems, notably those with high bio-, socio-, and economic importance, such as the upwelling ecosystem off Cape Blanc, Northwest Africa. This paper discusses how phyto- and microzooplankton export flux, represented by dinoflagellate cysts (dinocysts), in this ecosystem can be affected by variable climatic conditions prevailing between 2003 and 2020. The study area is characterised by annual permanent upwelling with cyclic intensity and strong inter-annual variability. Thus, we employed Morlet wavelet analyses to detect periodicities and interannual variations on an 18 year high-resolution sediment trap record of organic-walled dinocyst export flux and local environmental steering factors (e.g., wind direction, wind speed, Saharan dust input and sea-surface temperature). A dinocyst is a fossilisable structure produced by dinoflagellates, a plankton group containing both primary and secondary producers. Significant half-year and annual cycles in the time series of dinocyst export fluxes, upwelling winds, and the dust input time series were detected. Those cycles presented variations that were divided into three distinct phases: Phase I (2003–2008), Phase II (2009–2012), and Phase III (2013–2020). We also observed changes in the taxonomic composition of dinocyst assemblages in every phase, demonstrating their potential as bioindicators for environmental changes. The significant variations within each phase were mostly explained by changes in upwelling intensity and dust input into the area. Our results suggest that there is a strong interaction between these two factors (which depend on surface wind dynamics) and the export flux of dinocysts off Cape Blanc, highlighting the ecosystem's sensitivity to local climate variability.