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

Abstract. The constant changing of the recent climate has 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 the relationship between changes in this ecosystem and climate-induced changes of major environmental steering factors between 2003 and 2020. The study area is characterised by annual permanent upwelling, indicating a cyclic character, with a strong interannual variability. Thus, we employed Morlet wavelet analysis to detect periodicities and interannual variations on an 18-year high-resolution sediment trap record of dinoflagellate cyst (dinocyst) export flux and the local environmental steering factors (e.g., wind direction, wind speed, Saharan dust input and sea surface temperature). Dinocyst is a fossilisable structure produced by dinoflagellates, which is a major plankton group that contains both primary and secondary producers. Significant half-year and annual cycles in the time series of dinocyst, the upwelling winds, and the dust input time series were detected. Those cycles demonstrated 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 the dinocyst assemblages in every phase, demonstrating dinocysts as a bioindicator 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 suggested that there is a strong interaction between these two factors (which depend on the surface wind dynamics) and the export flux of dinocysts off Cape Blanc, representing the ecosystem's sensitivity to local climate variability.