Protistan Plankton Responses to Variable Light and Upwelling in the Peruvian Humboldt Current System: Insights Into...

The Humboldt Current System (HCS) sustains one of the most productive fisheries globally, driven by nutrient-rich upwelling. Understanding the impact of climate change, including variations in upwelling intensities and light availability, on protistan plankton communities is critical due to their pivotal role in food web dynamics and biogeochemical cycles. During the CUSCO campaign, eight off-shore mesocosms were deployed to simulate varying upwelling intensities (0%, 15%, 30%, and 45% deep-water addition) under high-light (HL) and low-light (LL) scenarios representing austral summer and winter conditions. DNA metabarcoding targeting the V9 18S rRNA gene revealed significant changes in diversity, community structure, and functional roles. Increased upwelling enhanced alpha diversity, likely reflecting a combination of nutrient enrichment and dispersal of deep-water protists, with diatoms and chlorophytes dominating under nutrient-enriched and depleted conditions, respectively. In contrast, dinoflagellates thrived under high-light, low-upwelling conditions, showing a functional shift toward parasitism and osmotrophic lifestyle under nutrient deficiency. These findings underscore the potential impacts of climate-induced changes on protistan plankton diversity, food web structures, and ecosystem services in upwelling systems.