Phytoplankton Community Response to Upwelling Events: Distribution and Abundance Investigated Using Genomic Methods

Upwelling events are known to support blooms of phytoplankton, important primary producers at the base of the oceanic food web. Phytoplankton community structure changes in response to upwelling support higher trophic level growth and increased efficiency of carbon export from the euphotic zone. While these events occur globally, this study examined upwelling in coastal regions, where alongshore winds can drive Ekman transport and upwelling of deeper waters. The two upwelling regimes examined were the California Current System and the Alaskan Beaufort Sea. In the California Current System, the relative diatom community composition was examined using 18S sequencing to determine how diatoms respond to two physically and chemically different upwelling plumes. Results showing that different species of diatoms responded to each upwelling event were discussed in the context of future predictions of diatom proliferation, transport, and carbon export in the California Current System. Upwelling in the Beaufort Sea is becoming more frequent as sea ice melts and the Arctic becomes warmer. The upwelling system in the Beaufort Sea was examined during three upwelling events, specifically focusing on how the phytoplankton community on the shelf responded to upwelling, through sequencing of the 18S v9 region of eukaryotic DNA. Before the first upwelling event Alexandrium tamerense, a dinoflagellate known for its ability to produce the neurotoxin that causes paralytic shellfish poisoning, dominated sequencing reads. Once upwelling began, the relative abundance of Alexandrium sequences disappeared and was replaced by elevated relative sequence abundance of diatoms, picoeukarytoes, and other dinoflagellates. While these results indicate expansion of a possible harmful algae further into the Arctic than previously seen, likely because of climate warming, they also suggest that upwelling events can lead to the proliferation of other phytoplankton, which could in turn diminish the prominence of this harmful algae. The implications of how this dynamic system may change due to increasing strength and number of upwelling events were discussed. Overall, these results underscore the importance of understanding the response of phytoplankton community composition to coastal sub mesoscale dynamics such as upwelling, which is of particular interest given upwelling is predicted to change in a warming climate