Seasonal behavior of dissolved cadmium and Cd/PO4 ratio in Todos Santos Bay: A retention site of upwelled waters in the Baja California peninsula, Mexico

• The dissolved Cd/PO4 ratio varied on a seasonal scale due to an intensification of biological activity.
• There is a decoupling of Cd and PO4 cycles within the bay during the springtime.
• The dissolved Cd/PO4 ratio increases due to preferential uptake of PO4 by phytoplankton.
• Cd/Ca proxy should be used with caution to reconstruct past PO4 concentrations in surface waters of upwelling regions.

This work analyzes the spatial and seasonal variability of dissolved cadmium (Cdd) and the cadmium to phosphate ratio (Cd/PO4) in surface waters of Todos Santos Bay (TSB), a semi-enclosed bay within the upwelling zone of the Baja California peninsula (Mexico). The waters inside the bay (17.74 ± 0.42 °C) were on average warmer than the adjacent Pacific waters (14.67 ± 0.52 °C) indicating the retentive nature of circulation within TSB. Cdd and PO4 concentrations and Cd/PO4 ratios for this region are comparable to those reported for the California Current System. Cdd (0.30 ± 0.036 nM) and PO4 (0.97 ± 0.061 μM) concentrations outside of the bay presented a seasonal signal which was positively correlated with the local coastal upwelling index (CUI), suggesting that the magnitude of temporal variability of Cdd is partially controlled by the intensity of upwelling. In contrast, average Cdd (0.15 ± 0.018 nM) and PO4 (0.51 ± 0.034 μM) concentrations within the bay were approximately 50% less than those of the oceanic region, indicating that both elements are removed inside TSB. The dissolved Cd/PO4 ratio was not constant on a seasonal scale between the inside and outside of the bay, with higher values in the spring associated with upwelling conditions and greater phytoplankton biomass. Our findings show that there is a decoupling of Cd and PO4 cycles within TSB during the upwelling season, which is caused by an intensification of biological activity associated with the retention of water inside the bay. To explain the decoupling between Cd and PO4, we speculate that during the upwelling season this coastal system is temporarily relieved from micronutrient Zn limitation, causing an increase in the dissolved Cd/PO4 ratio due to preferential uptake of PO4 by phytoplankton. The seasonal variability of the dissolved Cd/PO4 ratios suggests that the Cd/Ca proxy must be used with caution to reconstruct past PO4 concentrations in surface waters of upwelling regions.