Anomalous thermal and winds condition drove low surface chlorophyll-a in La Paz Bay during the 2015-2016 El Nin∼o event

La Paz Bay (LPB) considered one of the most highly productive bays in the southwestern Gulf of California, Mexico, recently established as Priority Marine Region, supports many species of ecological importance. In this context, daily composites of surface chlorophyll a (Chl-aSAT), sea surface temperature (SST) satellite images derived from the MODIS-Aqua sensor, and a wind intensity index (WI) calculated with data recorded simulta-neously at four points along the coast, were used to describe the strong 2015-2016 El Nin & SIM;o impact over the surface chlorophyll pigments in LPB, relative to its average for 2003-2013. Significant relationships between SST, WI (used as environmental factors) and Chl-aSAT (RSST =-0.74, RWI =-0.40, P < 0.01) were found using exponential models for the entire study period, with moderate correlation during El Nin & SIM;o winter condition (December 2015 to March 2016) between SST and Chl-aSAT (RSST = 0.45, P < 0.02). The Chl-aSAT showed un-expectedly low values (-0.5-1.2 mg m-3) with one month-lag of the winter bloom period jointly with the presence of warmer-than-usual water (-1.0 C) and episodes of strong northwesterly winds (speed 8.0-10.0 m s- 1) that began in autumn that drove a-2 month-lag in its winter peak. The Chl-aSAT and SST winter spatial patterns, showed low Chl-aSAT values (--0.5 to-1.0 mg m-3) along the bay and higher-than-usual SST by -0.5-1.0 C relative to a year under the La Nin & SIM;a winter condition (2017-2018). This triggered the possible thermocline deepening and the lowest levels of nutrient supply to the photic layer during the El Nin & SIM;o winter episode. Hovmoller diagrams of Chl-aSAT and SST anomalies along the coast revealed a sharp drop to negative anomaly values (--0.7 to-1.2 mg m-3), in-phase but negatively correlated with the SST coastal anomalies (R = -0.88, P < 0.01). This biophysical coupling played a crucial role in the exceptional drop to the lowest winter Chl-aSAT levels seen in LPB since the MODIS satellite sensor began taking measurements in 2002 (Chl-aSAT values were lower than usual by -0.9-1.2 mg m-3). Besides the lowest winter levels observed, this research reflected the El Nin & SIM;o signal-driven changes over the basic seasonal average of the LPB climate system The non-significant negative trend observed in the winter average Chl-aSAT values, indicates that LPB may be less resilient to future environmental disturbances.

Automated summary

La Paz Bay is considered a highly productive bay in the Gulf of California, Mexico, and has recently been designated as a Priority Marine Region. This study found that the surface chlorophyll pigments in the bay were significantly affected by the El Niño event in 2015-2016, with lower values compared to the average from 2003-2013. This suggests that La Paz Bay may be less resilient to future environmental disturbances.