Even Brief, Transitory Exposures to Temperatures Exceeding Decadal Maximum Levels Profoundly Reshapes Coastal California Phytoplankton Community Structure

Assemblages of photosynthetic bacteria and protists in the surface ocean play key roles in marine ecosystems. By fixing carbon they support higher trophic levels, and provide long-term carbon removal through the biological pump. However, rising sea surface temperatures will likely change their community composition, biogeochemical rates, and trophic interactions. We studied how episodic warming will impact natural California phytoplankton communities from the San Pedro Ocean Time series (SPOT) during the spring, summer, and fall. Experiments that simulate future warming typically use constant incubator temperatures that do not reflect dynamic thermal conditions such as periodic heat waves in situ. To study impacts of natural climate variability on phytoplankton community structure, we incubated seasonal assemblages under constant and fluctuating conditions at both present-day and warmer future mean temperatures. The resulting seasonal communities were analyzed using 16S and 18S rRNA amplicon sequencing and put into the context of a decade of temperature, nutrient, and sequence data from our collection site. For each season, phytoplankton were able to maintain growth as much as six degrees above the seasonal temperature maximum. Dominant phytoplankton amplicon sequence variants (ASVs) were largely consistent for treatments within a season, but changed dramatically when mean temperatures rose even briefly above the decadal maximum recorded temperature of ~25 degrees. Above ~25 degrees, constant and fluctuating treatments each selected for different dominant phytoplankton ASVs, neither of which were observed at lower temperatures. We hypothesize that multi-year environmental upper temperature limits may represent tipping-points for phytoplankton communities, and that even short-term (days to weeks) excursions above these thresholds will likely cause major shifts in dominant phytoplankton ASVs.