Bio-optical characteristics of a phytoplankton bloom event off Baja California Peninsula (30-31°N)

A phytoplankton bloom was detected in the Southern California Current System, off the Baja California Peninsula (Mexico) on June 2003 with chlorophyll-a concentration (TChla) of 10.13 mg m−3. Two stations (D1 and D2) were sampled on June 24, and D2 was resampled 6 days later; chlorophyll-a concentration had decreased by about one half. LAC MODIS-Chla images were obtained and showed the spread of the bloom on the day after sampling. The phytoplankton community consisted primarily of dinoflagellate temporary cysts, mainly at the surface and at 5 m in station D1. Two Pseudo-nitzschia species (P. australis, P. seriata) were also very abundant. Samples from the bloom had a specific phytoplankton absorption coefficient (aph*(λ)) lower than the rest of the samples. Values varied from 0.0186 to 0.0455 m2 mg−1 for aph*(440) and from 0.0092 to 0.0294 m2 mg−1 for aph*(675), with ratios aph*(440): aph*(675) ranging from 0.99 to 2.20. These low ratios were associated with the combined effect of packaging, and with the relatively high ratios of fucoxanthin, peridinin, diadinoxanthin and chlorophyll-c2 to TChla. Samples from the surface and 5 m depth at station D1 had higher ratios of Perid:TChla (0.12-0.32) than the rest of the samples, suggesting that cysts have similar Perid:TChla as free-living dinoflagellates. An unusual absorption spectrum with a broad maximum around 480-500 nm was associated with the high proportion of cysts and diatoms. The slope of the spectra between 443 and 488 nm was a good index to differentiate bloom samples containing high proportions of dinoflagellate temporary cysts. Further investigation of the absorption properties of dinoflagellate cysts is needed in order to detect these waters by remote sensing. Although much work is still necessary to understand and explain the bio-optical properties of a bloom, the present study is the first assessment off the Baja California coast to simultaneously consider aspects such as absorption properties, pigment composition and to include a spatial evaluation of the extension of a bloom with satellite images.