Seasonal hydrology shapes the taxonomic and functional diversity of fish associated with aquatic macrophytes in a neotropical floodplain lake

We evaluated the influence of hydrological phases and limnological variables on the taxonomic composition and functional diversity of fish associated with aquatic macrophytes in a floodplain lake of Amazon Basin, Brazil. Sampling was conducted using seine nets across the four phases of the local hydrological cycle (rising, high-water, receding, and low-water) during the year 2018 in Lake Maicá. The species were identified and classified into four functional groups on the basis of their ecological traits (migration, life history strategy, feeding habits, and swimming performance/microhabitat use). A total of 6075 individuals were captured, representing nine orders, 26 families, and 104 species. The fish assemblages underwent changes in both taxonomic and functional structure in response to the hydrological phases and limnological variables (pH, temperature, dissolved oxygen, and water transparency), with an increase in functional diversity during the receding water and low-water phase. During the low-water period, there was an increase in the abundance of fish with life history strategies at equilibrium, small periodic species as well as invertivores, piscivores, short-distance migrants, and residents. In contrast, the rising and high-water phases contributed to higher abundances of young fish (larvae and juveniles) and medium-distance migratory fish. The synergy among these factors acts as an environmental filter in the structure of the local ichthyofauna, allowing species with antagonistic ecological traits to coexist, maintain their populations, and reduce competition for space and food under varying environmental conditions during the four phases of the annual hydrological cycle. The information presented is essential for identifying priority areas for the conservation of fish assemblages, which represent the primary source of animal protein and income for the region, and for understanding the mechanisms responsible for changes in the taxonomic and functional structure of fish in Amazonian floodplain lakes.