Response of phytoplankton biomass to sea surface temperature fluctuations in the Bay of La Paz, Southern Gulf of California: a 17-year satellite study

Phytoplankton biomass is a vital indicator of ecosystem health, yet its dynamics are increasingly sensitive to thermal variability and large-scale climatic events. Utilizing a 17-year satellite-derived dataset (2002 to 2018) from the Moderate Resolution Imaging Spectroradiometer (MODIS), this study investigates the relationship between chlorophyll-a (Chl-a) concentration and sea surface temperature (SST) in the Bay of La Paz, Gulf of California, with a focus on the influence of the El Niño-Southern Oscillation (ENSO). Our findings reveal a pronounced seasonal cycle. Chl-a peaked at 1.5 mg m-3 during winter cooling (<22°C) and dropped to minimum values of 0.2 mg m-3 during summer warming (>30°C). Interannual variability was closely coupled with ENSO phases. Notably, the intense 2015-2016 El Niño drove record-high SSTs that suppressed phytoplankton biomass, whereas the 2011-2012 La Niña fostered cooler conditions and elevated Chl-a levels. Wavelet analysis confirms that while seasonal cycles remain the dominant temporal component, spectral distortions occurred during major thermal anomalies. These results identify ENSO-driven thermal shifts as primary determinants of localized SST and Chl-a variability, underscoring the critical role of long-term remote sensing in monitoring marine ecosystems where traditional in-situ observations are logistically constrained.