Light absorption properties of dinoflagellate blooms in Todos Santos Bay, Mexico (northeast Pacific Ocean)

Todos Santos Bay (TSB) is located on the northeastern coast of the Baja California peninsula in Mexico. Algal blooms dominated by dinoflagellates in TSB are recurrent and negatively impact the ecosystem of the bay and regional economy. In this study, we analyzed the concentration of chlorophyll a (Chla) and the light absorption coefficients of phytoplankton, colored dissolved organic matter (CDOM), and non-algal particles between 250 and 700 nm with data collected between 2016 and 2023. We calculated the respective spectral slopes of the absorption coefficients (Sphy, SCDOM, and Sdg) using the wavelength range of 370–412 nm. We aimed to define a baseline, identify the light absorption properties associated with blooms, and compare these to the baseline. Eight bloom events were identified, with Chla concentrations ranging between 7.7 and 36.2 mg m−3. Lingulodinium polyedra was dominant in four, while Prorocentrum micans was dominant in two. A mixture of dinoflagellates characterized the other two blooms. Significant changes in the magnitude, shape, and slopes of the spectra due to mycosporine-like amino acids were observed during blooms. However, these changes varied based on the dominant species and Chla concentration. Finally, Sphy was superior to either SCDOM or Sdg when identifying blooms and could potentially be employed for their detection using the new generation of satellites capable of gathering UV-band data. •Eight blooms were identified between 2016 and 2023 in Todos Santos Bay.•MAAs caused magnitude, shape, and slope changes in absorption spectra.•Changes depended on the dominant bloom species and Chla concentration.•Sphy was superior to either SCDOM or Sdg when identifying blooms.•Sphy could be used to detect blooms using UV-band data from new satellites.