The Combined Effect of El Niño Events and Climate Change on Growth and Photophysiology of the Coccolithophore E. huxleyi in the Santa Barbara Channel

Phytoplankton growth and productivity is heavily influenced by temperature, CO₂, light, salinity, and nutrients in their environment. A change to their environment can have unknown and unpredicted influence on the health of the organism, and in turn the health of the ecosystem. Historically, phytoplankton have been affected by the irregular warming event in the equatorial Pacific region known as El Niño. The Santa Barbara Basin has the highest phytoplankton biomass and production levels in the Southern California Bight, but the response of local phytoplankton to El Niño events, future climate change conditions, and El Niño events in a changed climate remain unclear. In these experiments, we investigated the growth and photophysiology of a coccolithophore, E. huxleyi that was isolated from off the Santa Barbara coast during a bloom that accompanied the 2015 El Niño event. We measured growth and fluorescence-derived photosynthesis in E. huxleyi under four different temperature and CO₂ environments - (1) present-day conditions, (2) future climate change conditions (3) present-day El Niño conditions, and (4) future climate change impacted El Niño conditions. Elevated CO₂ resulted in a decreased growth at all temperatures. A combination of high CO₂ and high temperature - conditions expected during a El Niño event in the future – resulted in a decreased growth as well as decreased photosynthetic efficiency of photosystem II (PSII) in E. huxleyi. These results suggest a potential decrease in E. huxleyi productivity in the Santa Barbara Basin during a El Niño event in the future, and highlights the need to investigate these processes further.