Influence of anomalous subarctic water intrusion on phytoplankton production off Baja California

• An anomalous subarctic water intrusion began off Baja California in 2002.
• In 2005, the negative salinity anomaly was associated with low chlorophyll.
• Primary production estimated by two methods was different in 2002 and 2005.
• The anomalous subarctic water intrusion of 2005 homogenized the ecosystem.
• In 2005, presumably the phytoplankton presents a physiological acclimatization.

The southern region of the California Current (CC) off Baja California represents a transitional environment, with the influence of low temperature and less saline CC water during spring and summer, and warm and salty tropical and subtropical conditions through the end of summer and autumn. From 2002 to 2006, an anomalous subarctic water (SAW) intrusion generated changes in the epipelagic ecosystem dynamics, affecting phytoplankton biomass and primary production. The goal of the present work is to show the effects of the SAW intrusion off Baja California in the phytoplankton production during the two contrasting years 2002 and 2005. Our results show that the minimum water salinity was associated with density levels of 24.6–25.5 kg m−3 in 2002 and 24.3–25.2 kg m−3 in 2005, indicating that the anomalous SAW intrusion was more pronounced in 2005. In 2005, the mean chlorophyll concentration was low (<0.4 mg m−3) and associated with negative salinity anomalies (<−0.02). The mean satellite-derived chlorophyll (CHL) values were 0.25 and 0.26 mg m−3 in 2002 and 2005, although in the coastal zone they were >1.0 mg m−3 in some months. CHL values were significantly different between the north and south zone and among months (p<0.01). The mean values of the photosynthetic parameters (maximum light utilization coefficient (αB) and maximum photosynthesis rate (PBm)) were one order of magnitude higher in 2005 compared to 2002 (p<0.01). The primary production estimated by the Herman and Platt model and the Carr model was very different in 2002 (mean values of 907 and 143 mgC m−2 d−1 respectively), but similar in 2005 (750 and 1006 mgC m−2 d−1 respectively). The differences between models may be due to the diverse chlorophyll data used in the singular models (in situ vs. satellite-derived), and to the weight of the phytoplankton photosynthetic parameters in each algorithm. We conclude that for the present data set the Herman and Platt model is more suitable. To assess the relative importance of physical variables, as well as bio-optical and physiological parameters in primary production, we conducted a principal component analysis (PCA). There was an apparent separation between the two years, while each year experienced a special structuring due to different physical variables (mainly salinity, but also temperature and KPAR in 2002, and temperature and water stability in 2005). No clear structuring is discernible in 2005, suggesting that the anomalous SAW intrusion homogenized the pelagic ecosystem. The decrease in primary production suggests a change in phytoplankton biomass as a result of a phytoplankton physiological acclimatization.