Flood pulses reshape phytoplankton richness through aggregation, density, and species abundance distribution across...

The flood pulse is a key driver of species distribution and richness in floodplains, yet the underlying components of its effect on species richness remain incompletely understood. We examined how three key components, namely species spatial aggregation, density, and species abundance distribution (SAD), explain seasonal variation in phytoplankton richness across multiple spatial scales. Our study encompassed 66 lakes from four Brazilian floodplains spanning approximately 2300 km across a subcontinental scale, comparing high- and low-water seasons in 2011-2012, including one dammed floodplain. Species richness declined during floods in most floodplains, coinciding with reduced aggregation, except in the dammed floodplain. However, density and SAD were the primary drivers of richness decline, except in the Dammed floodplain. Overall, the influence of these components varied by scale: aggregation and density were more influential at small scale, while SAD played a dominant role at the regional scale. However, the relative contribution of each component differed among floodplains, likely due to distinct hydrological and environmental conditions, such as damming effect or low seasonal variation in water level. For instance, the dammed floodplain showed distinct species gains during high water, likely due to altered flood pulses from dam regulation, which alters natural flood pulses. These results provide insight into the components shaping biodiversity in floodplains and suggest that changes in flood pulse characteristics, whether from climate variability or human activity, may alter richness patterns by influencing species distribution across space and scales.