Mechanisms structuring phytoplankton richness in floodplains: the role of aggregation, density, and species abundance distribution across spatial and temporal scales.

Abstract

Species richness in floodplains is shaped by multiple ecological processes that vary across space and time, reflecting the complexity of hydrological dynamics. Although progress has been made in understanding biodiversity patterns, gaps remain regarding how the underlying ecological mechanisms structure species richness, particularly across different spatial and temporal scales and in contrasting environmental contexts. In this thesis, a conceptual framework originally developed to explain spatial diversity patterns was innovatively applied to also investigate temporal variation in the taxonomic richness of phytoplankton. The approach considered two complementary analytical dimensions: (1) the spatial variation in phytoplankton richness across Brazilian floodplains, and (2) the temporal variation over a long-term historical time series. Three community structure components were evaluated as direct determinants of observed richness: the spatial or temporal aggregation of individuals, the density of individuals per sample, and the species abundance distribution (SAD). The main objective was to quantify how these components’ structure phytoplankton richness patterns and how their contributions are modulated by environmental impacts, especially hydrological regulation by dams. The first approach, data from lakes across four floodplains spanning 2,300 km were analyzed, comparing high- and low-water periods. Richness tended to decrease during flood periods, primarily due to reduced aggregation, except for the floodplain influenced by dam operation. Density and SAD emerged as the main components driving richness loss, with effects varying by scale: aggregation and density were more influential at the local scale, while SAD dominated at the regional scale. The Paraná River floodplain displayed distinct patterns, likely associated with hydrological regulation influenced by dam operations. In the second approach, a 22-year time series was used to assess the effects of dam construction on the temporal variation of phytoplankton richness in a subtropical floodplain. In areas affected by dam operations, increased aggregation and reduced density contributed to richness loss, while SAD had a positive effect, though insufficient to offset the decline. The relative contribution of each component varied over time and differed between species groups, such as typical and rare species, underscoring the central role of hydrological regimes in structuring richness in aquatic environments. In summary, the richness patterns observed across time and spatial scales reflect the interaction between internal community processes and environmental conditions, particularly both natural and dam-induced hydrological variation. This thesis contributes to our understanding of the processes shaping species richness in floodplains and offers a promising analytical framework for community ecology.