Sedimentary record of Polysphaeridium zoharyi (cyst of Pyrodinium bahamense) in El Colorado Lagoon, Gulf of California

The temporal variations in abundances (cyst g⁻¹ and percentages) and fluxes (cyst cm⁻² yr⁻¹) of dinoflagellate cysts, including Polysphaeridium zoharyi from the toxic species Pyrodinium bahamense, were analyzed in a ²¹⁰Pb-dated sediment core from El Colorado Lagoon (Mexico) to investigate potential natural and/or human-driven factors, influencing the proliferation of P. bahamense, by examining the relationship between P. zoharyi abundance and geochemical indicators of sediment provenance and hydrodynamic conditions. P. zoharyi was the most abundant and frequently observed species along the core. The dominance of P. zoharyi was linked to the prevalence of terrigenous influence in most core sections, implying enhanced nutrient delivery. In recent years (∼1970s), the decrease in P. zoharyi percentages was associated with a local shift in hydrodynamic conditions, as revealed by an increase in sand content, changes in the δ¹³C and δ¹⁵N composition, and higher concentrations of marine indicator elements (Br, Ca, Na, and Sr). The change in hydrodynamic conditions suggests enhanced connectivity with the sea, likely triggered by a meteorological event. We highlight the importance of integrating multiple geochemical proxies with cyst analysis to assess changes in species assemblages and how local conditions, such as shifts in hydrodynamic conditions, that can influence species composition. This study contributes to paleoecology and phytoplankton ecology by documenting species responses to natural and anthropogenic disturbances. It confirms the recurrence and dominance of P. bahamense cysts, a toxin-producing species associated with harmful algal blooms. The findings underscore the need for continued monitoring, contingency planning, and water quality protection in coastal ecosystems.