Cadmium in the waters off South Morocco: Nature of particles hosting Cd and insights into the mechanisms fractionating Cd from phosphate

Abstract

In this study, we report the distributions of total dissolvable cadmium and particulate cadmium from 27 stations in southern Moroccan coastal waters (22°N–30°N), which is part of the North-West African upwelling system. These distributions were predominantly controlled by upwelling of the North Atlantic Central Waters (NACWs) and uptake by primary production. Atmospheric inputs and phosphogypsum slurry inputs from the phosphate industry at Jorf Lasfar (33°N), recently estimated as an important source of dissolved cadmium (240 t Cd yr−1), are at best of minor importance for the studied waters. Our study provides new insights into the mechanisms fractionating cadmium from phosphate. In the upper 30 m, the anomalies observed in terms of Cd:P ratios in both the particulate and total dissolvable fractions were related to an overall preferential uptake of phosphate. We show that the type of phytoplanktonic assemblage (diatoms versus dinoflagellates) is also a determinant of the fractionation intensity. In subsurface waters (30–60 m), a clear preferential release of P (versus Cd) was observed indicating that remineralization in Oxygen Minimum Zones is a key process in sequestering Cd.