Phytoplankton dynamics in the Gulf of Mexico: A satellite perspective

Remotely-sensed optical, thermal, and altimetry data have significantly advanced our understanding of biological and physical processes in the Gulf of Mexico. Of particular note synoptic satellite data have established the importance of mesoscale fronts and eddies on governing phytoplankton dynamics, and have also provided a metric to examine long term trends. This research provides a comprehensive analysis of the Gulf’s satellite record (1998-present) by exploiting the most recent improvements in satellite algorithms including phytoplankton size class membership (Muow et al. 2019), absorption-based net primary productivity (NPP, Silsbe et al., 2016), phytoplankton carbon biomass (Graff et al., 2015), and changes in phytoplankton biomass (Behrenfeld, 2010; Behrenfeld and Boss 2014). Amongst the most prominent long term trends, this analysis shows a statistically significant decline in NPP in the offshore waters (maximum depth > 200 m) of the Gulf beginning around 2010. This decline is (surprisingly) independent of long term changes in chlorophyll a, and instead is largely driven by diminishing phytoplankton absorption per unit chlorophyll. We then extend our analysis to investigate and contrast phytoplankton growth and loss rates in cyclonic and anti-cyclonic eddies, and briefly discuss our findings within the context of food web models.