# Protistan Plankton Responses to Variable Light and Upwelling in the Peruvian Humboldt Current System: Insights Into Community Dynamics Under Environmental Change

## TL;DR

This study shows how changes in light and upwelling affect plankton in the Humboldt Current, which could impact fisheries due to climate change.

## Contribution

The study experimentally reveals how protistan plankton communities respond to variable upwelling and light conditions in the Humboldt Current System.

## Key findings

- Increased upwelling boosts alpha diversity, with diatoms and chlorophytes dominating under nutrient-rich and poor conditions, respectively.
- Dinoflagellates thrive under high-light, low-upwelling conditions, showing a shift toward parasitism and osmotrophy.
- Climate-induced changes in upwelling and light may alter food web structures and ecosystem services in the Humboldt Current System.

## Abstract

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.

Our study reveals (based on experiments) the shifts in protistan plankton communities, serving as energy and organic matter source for higher trophic levels if the Humboldt Current System is affected by climate change as predicted by current models. This may have profound effects on fisheries in the Humboldt Current System.