Environmental Drivers of Phytoplankton Community Composition in the Galápagos Archipelago

Phytoplankton are important for ocean primary production and global carbon cycling. Understanding the environmental drivers of phytoplankton community composition is crucial for assessing marine ecosystem productivity under the threat of climate change. The Galápagos Archipelago, with its unique and complex oceanographic setting, serves as an ideal natural laboratory. This study investigates how nutrient availability and temperature influence the spatial distribution and size structure of phytoplankton communities across the archipelago. Two distinct regions were compared: Isla San Cristobal and Isla Isabela (the upwelling region). Phytoplankton biomass and composition data were obtained from samples collected during the Galápagos 2025 Cruise. The biomass data were approximated by size-fractioned chlorophyll-a concentrations and cell biovolume, and the composition data were obtained from sample cell counts. The results indicate that nutrient availability is the primary driver of phytoplankton community composition (PC1 = 95.2%). Temperature shapes the distribution of phytoplankton commuties among stations. Specifically, centric diatoms (e.g., Corethron hystrix, Chaetoceros sp.) exhibited a strong preference for cool upwelling waters rather than warm oligotrophic conditions, where the community also shifted toward larger cell size (≥ 5µm).