Affiliation: College of Arts and Sciences, Department of Marine Sciences
Coastal upwelling currents such as the California Current System (CCS) provide for some of the most productive biological systems in the world. The seasonality and topography of these regions dictate the intensity of upwelling and entrainment of nutrients, providing for photosynthetic growth by autotrophic plankton that not only cascades up the marine food chain, but also initiates carbon export and sequestration. Diatoms, a distinct taxon of the very phytoplankton functional groups that lie at the base of this food chain, dominate these upwelling events using a “shift-up” response to nitrate entrainment, where they constitutively maintain an elevated pool of nitrogen transport and assimilation genes even before upwelling events occur. And despite the abundance of macronutrients found within the CCS, phytoplankton are often limited by the micronutrient iron, an important trace element involved in photosynthesis and nitrogen assimilation. The mechanisms behind how these phytoplankton – the basis of the marine food web – acclimate to the different stages of the upwelling conveyor belt cycle (UCBC) with respect to iron limitation remains largely uncharacterized. Adding to this complexity is the knowledge that iron bioavailability heavily influences phytoplankton growth dynamics and elemental stoichiometry in this region, and climate change is projected to alter the availability of iron in many parts of the CCS. This project explores the phytoplankton response to the iron limitation mosaic within the CCS, where varying degrees of iron limitation along the California coast may have significant influences on phytoplankton community structure with respect to the UCBC. Samples were collected in situ during a 2019 field incubation experiment conducted on the research vessel R/V Oceanus off the coasts of Oregon and California. Physiological measurements and meta-transcriptomic analysis indicate that iron limitation was not initially evident at either incubation site, but induced iron limitation during the incubations caused noticeable physiological and differential gene expression changes within the phytoplankton taxa, especially in diatoms. This may have significant implications for upwelling events in future oceans, where ocean acidification is projected to decrease the bioavailability of iron in coastal upwelling regions. The onset of climate change and its many complexities could drastically alter phytoplankton community composition in future coastal oceans, and likewise play a significant role in shaping the seed populations that are so vital to the marine ecosystem processes in the CCS.