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, as well as seasonal wind-driven coastal upwelling which supplies nutrient-rich waters to the surface and drives high coastal 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 into the more oligotrophic California Current waters. The interaction between waters within and outside of the eddies is limited, and so the eddies act as natural mesocosms, where the resident phytoplankton population undergo ecological succession as the eddy ages. Diatoms, a unicellular and eukaryotic subgroup of phytoplankton, have high sensitivities to changes in their environment, particularly temperature and nutrient distributions. In this study, I examine how diatom communities trapped within mesoscale eddies in the CCS evolve in response to environmental shifts as they travel offshore. In a transect that bisected two cyclonic eddies off the coast of northern California near Cape Mendocino, diatom samples were collected and later sequenced using high throughput sequencing. Although the eddies both originated in broadly the same location, they had formed 2 and 10 months previous to sampling, respectively. Because of this difference in the age of the eddies, I 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, the last result largely driven by the relative abundance of a single Rhizosolenia species. Rhizosolenia accounted for over 50% of the diatom community in 5 out of 8 offshore eddy stations, with one of these stations characterized by nearly 75% Rhizosolenia. Our results suggest that the Rhizosolenia species present in the offshore eddy is one that bypasses nitrate limitation by forming vertically migrating mats. I also found elevated relative abundances of Pseudo-nitzschia cf. sp. and Thalassiothrix sp. in the offshore eddy. The younger, nearshore eddy was higher in macronutrients, Fe-limited, and higher in diversity. Top abundances for this eddy include Pseudo-nitzschia sp., Fragilariopsis kerguelensis, F. cf. kerguelensis, Thalassiosira ritscheri, Asteromphalus sp., and T. oestrupii. Our results show that the biogeochemistry and diatom community structure within cyclonic eddies evolve as the eddies move offshore from the coast. The high-nutrient coastal waters are initially dominated by coastal diatoms known to have higher nutrient requirements. As the nutrients within the eddy are drawn down over time, species equipped with low-nutrient adaptations can become dominant. 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