# The Role of Habitat, Climate, and Space in the Species and Traits Variation of Phytoplankton in Large Cascade Reservoirs

## TL;DR

This study shows how the age and location of reservoirs affect the diversity of phytoplankton in the Tocantins River, with younger and more distant reservoirs showing higher diversity.

## Contribution

The study reveals that spatial variables more strongly influence phytoplankton beta diversity than local environmental conditions in cascade reservoirs.

## Key findings

- Younger and more distant reservoirs have higher taxonomic and functional phytoplankton beta diversity.
- Intermediate reservoirs show phytoplankton homogenization due to environmental similarity.
- Spatial variables explain more variation in phytoplankton communities than local environmental factors.

## Abstract

The construction of cascade dams on large rivers reduces river connectivity, increases environmental homogenization by altering the longitudinal processes responsible for high biodiversity, and may restrict the establishment and survival of species without functional traits in these locations. Our study evaluated phytoplankton beta diversity regarding taxonomic and functional facets in the seven cascade reservoirs of the River Tocantins (Brazil), along 1500 km in length, between 2006 and 2014. We tested whether (1) younger reservoir age leads to higher values of taxonomic and functional phytoplankton beta diversity than the others; (2) whether there is a positive correlation between functional and taxonomic phytoplankton beta diversity in cascade reservoirs; (3) and the taxonomic and functional beta diversity of phytoplankton is more strongly determined by spatial mechanisms than by local hydroclimatic and environmental conditions, with a stronger explanation expected for functional beta diversity. We demonstrated that younger and more distant reservoirs present higher values of taxonomic and functional phytoplankton beta diversity, and that reservoirs in the intermediate regions of the cascade, close to each other and with greater environmental similarity, presented taxonomic and functional homogenization of phytoplankton. Finally, we found that spatial variables were more explanatory of the variation in species and functional characteristics of phytoplankton communities compared to local environmental and hydroclimatic variables.

Phytoplankton beta diversity along cascade reservoirs of the Tocantins River is shaped by reservoir age, spatial arrangement, and environmental conditions. Younger, distant reservoirs show higher diversity, while intermediate ones tend toward homogenization. Environmental and spatial drivers differentially influence taxonomic and functional composition.