Award: OCE-1538525

The ocean is rapidly becoming warmer and more acidic, with possibly large consequences for marine biological communities and the food chains that support important fisheries. In the future, not only will temperature continue rising and acidification keep increasing as more carbon dioxide enters the ocean, but these properties are also predicted to become much more variable due to global change. Projections for many parts of the ocean include more frequent extreme heatwave events, and larger daily and seasonal shifts in carbon dioxide levels. This project used a combination of culture experiments and computer modeling to understand how increased variability in these two key environmental factors may affect the marine phytoplankton that support almost all ocean food webs. Experiments with California coastal plankton communities suggested that the maximum temperature observed over the last 10 years acts as a "tipping point". If fluctuating temperatures cross this upper threshold, even briefly, a major re-shuffling of the dominant phytoplankton species in the community occurs. Experiments with nitrogen-fixing cyanobacteria, which provide a crucial source of this essential nutrient to the whole biological community, suggest that these organisms from relatively constant open ocean environments are poorly adapted to deal with large temperature fluctuations. Work with coccolithophores, which have cells covered in calcium carbonate plates and can form large blooms, shows that warming and thermal variability decreased growth and calcification. Shorter variation cycles (1 day) were less inhibitory than longer variations (2 days) under high temperature. In general, these experiments demonstrate that growing phytoplankton under constant conditions in the laboratory may not allow us to accurately predict how they will respond as the ocean becomes more variable and unpredictable in the future. A series of modeling studies were conducted to understand the impact of the experimental results on marine ecosystems and carbon cycling. By analyzing field data, we show that phytoplankton in the Southern Ocean typically experience temperature fluctuations of +/- 1oC in a 7 day period. Coupling these observed temperature fluctuations with a numerical model, we showed that temperature fluctuations significantly depress phytoplankton growth rates and induce a "memory effect", which creates large differences between actual community growth rates and predicted growth rates. Temperature variability due to long-distance transport by currents was also shown to significantly impact the rate of phytoplankton adaptation to warming. Finally, explicitly including relevant environmental fluctuations into marine ecosystem models alters both the phytoplankton community composition and the rate of carbon cycling. Educational impacts include training three Ph.D. students; two of these have completed their degrees so far. Four undergraduate students also received valuable hands-on research experience. This project also supported a series of efforts aimed at increasing the representation of under-represented minorities in oceanography, including the development of pipelines and mentoring programs for high school and undergraduate students. Scientific impacts include showing that new extremes of ocean environmental variability may be just as important as changes in average ocean temperatures and acidity in determining how ocean biology will react to global change. Last Modified: 02/08/2021 Submitted by: David A Hutchins