Impact of Pacific Ocean Heatwaves on Phytoplankton Community Composition

Authors: Lionel A. Arteaga, and Cecile S. Rousseaux

Since 2013, marine heatwaves have become recurrent throughout the equatorial and northeastern Pacific Ocean and are expected to increase in intensity relative to historic norms (Figure 1). Among the ecological ramifications associated with these high temperature anomalies are increased mortality of higher trophic organisms such as marine mammals and sea-birds, which are likely triggered by changes in the composition of phytoplankton, the base of the marine trophic food web. We assimilated satellite ocean color data into an ocean biogeochemical model to describe changes in the abundance of phytoplankton functional types (PFTs) during the last decade’s (2010s) warm anomalies in the equatorial and northeastern Pacific Ocean. We find important changes associated with the “Blob” warm anomaly in the Gulf of Alaska, where reduced silica supply led to a switch in community composition from diatoms to dinoflagellates, resulting in an increase in surface ocean chlorophyll during the Summer–Fall of 2014. A more dramatic change was observed in the equatorial Pacific, where the extreme warm conditions of the 2016 El Niño resulted in a major decline of about 40% in surface chlorophyll, which was associated with a nearly total collapse in diatoms.

slide graphic from Mean modeled sea surface temperature (SST) and surface chlorophyll (Chl) anomaly over the anomalously warm period
Figure 1. Mean modeled sea surface temperature (SST) and surface chlorophyll (Chl) anomaly over the anomalously warm period between March of 2013 and December 2020, inferred by the NASA Ocean Biogeochemical Model.

Hints for a warmer future?

In the ENSO 3.4 region within the tropical Pacific, the most extreme levels of compound high-temperature and low-Chl were achieved in 2016 during the El Niño event (Figure 2a). An abrupt change in phytoplankton community composition is observed in this region, where typically large phytoplankton cells (diatoms) dominate the regime of low-SST and high-Chl anomalies, while smaller cells, better acclimated to low-nutrient environments (chlorophytes), dominate the opposite extreme of compound high-SST and low-Chl anomalies (Figure 2b). As extreme El Niño events become more frequent and potentially more acute, the phytoplankton community composition could shift more permanently towards groups associated with less efficient productivity, transfer, and export of organic carbon to depth.

slide graphic from pacific ocean heatwaves
Figure 2. Relationship between (a) compound chlorophyll (Chl) and sea surface temperature (SST) anomalies and (b) phytoplankton group dominance. The color scale in panel a) indicates the corresponding year of each monthly data grid point between 2002 and 2020. The color scale on panel b) indicates the dominant phytoplankton functional type (PFT) (i.e., group that occupies the largest fraction of total Chl biomass) in a given monthly grid-cell output within the specified region (diatoms-blue, chlorophytes-light-green).

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