Anomalously low diatom fluxes during 2009–2010 at Alfonso Basin, Gulf of California

A long-term information baseline is necessary to identify the seasonal and interannual variability of diatom production driven by environmental dynamics. This motivated our compiling hydrographic data and concurrent diatom fluxes at the Alfonso Basin sub-zone. The diatom data time series spanned from February 2008 to September 2012. These data were analyzed along with primary productivity and chlorophyll-a concentration estimates derived from satellite imagery from 2002 to 2012. Planktonic diatom flux (range: 1.62 × 10⁶–3.55 × 10⁷ valves·m^-2d^-1) was significantly correlated (r² = 0.37) with chlorophyll-a concentration. The bimodal production season (December–July) exhibited a winter mixing period influenced by water from the Gulf of California. The diatom flux was dominated by fast-growing surface species such as Chaetoceros spp. and Thalassiosira spp. The second peak in May occurred under the influence of southwest winds concurrent with the entry of tropical surface water and a cyclonic eddy that injected nutrients into the base of the euphotic zone, eliciting a bloom of Pseudo-nitzschia spp. The summer stratification resulted in the deepening of Subtropical Subsurface Water, and the diatom flux included large species of oceanic affinity such as Chaetoceros (Phaeoceros), Rhizosolenia acuminata, and Rh. imbricata. The environment at Alfonso Basin promotes diatom sinking; thus, material from sediment traps accurately reflects the primary production cycle associated with hydrographic processes. Interannual variability was characterized by anomalously low diatom fluxes between August 2009 and July 2010. A significant negative association between diatom flux anomalies and El Niño Modoki Index values suggests that oceanographic changes are related to this event. Additional reductions in diatom fluxes were coeval with SST positive anomalies recorded from July to February 2008–2009 and 2011–2012, which showed a significant relationship with the tropical Pacific Meridional Mode. The temporal trends like the additive signals of interannual (CP-El Niño) and decadal (Pacific Meridional Mode) time scales indicate that climatic disturbances affected the local ecosystem in the basin, reducing the vertical flux of diatoms initially dominated by Pseudo-nitzschia spp., and boosting the presence of oceanic warm-water species and small highly silicified species such as Thalassionema nitzschioides var. parva.