Diatom Community Composition Shifts Driven by Coherent Cyclonic Mesoscale Eddies in the California Current System

The California Current System (CCS) is characterized by an equatorward flowing eastern boundary current and seasonal wind-driven coastal upwelling which supplies nutrient-rich waters to the surface, fueling productivity. Cyclonic mesoscale eddies form off the coast in the CCS where they trap the highly productive upwelled coastal waters, along with their resident planktonic communities, and transport them offshore. As interaction between waters within and outside of the eddies is limited, the eddies act as natural mesocosms where the phytoplankton population will undergo ecological succession. Diatoms have high sensitivities to changes in their environment, particularly nutrient distributions. In this study we examine how diatom communities trapped within CCS mesoscale eddies evolve in response to environmental shifts as they travel offshore. In a transect that bisected two cyclonic eddies off the coast of northern California, samples were collected and later sequenced using high throughput sequencing. Although the eddies originated in broadly the same location, they had formed 2 and 10 months previous to sampling. Because of this difference in the age of the eddies, we can approximate the time evolution of a single CCS eddy by comparing their biogeochemical and ecological characteristics. The older, offshore eddy was low in macronutrients, nitrate-limited, low in Fe, and lower in diversity and evenness largely driven by the relative abundance of a single Rhizosolenia species, capable of bypassing nitrate limitation by forming vertically migrating mats. The younger, nearshore eddy was higher in macronutrients, Fe-limited, and higher in diversity and evenness. Top abundances for this eddy include Pseudo-nitzschia sp., Fragilariopsis kerguelensis, and Thalassiosira ritscheri. Our results show that the biogeochemistry and diatom community structure within cyclonic eddies evolve as the eddies move offshore. The high nutrients in coastal waters are drawn down over time by coastal diatoms with higher nutrient requirements, leaving behind low-nutrient waters suitable for oceanic diatoms and diatoms with low-nutrient adaptations. The combined effect of transport by, and ecological succession within the eddies is likely a key factor in mediating carbon cycling and export across the wider CCS region.