Exploring uncharted waters: insights into groundwater zooplankton of the Brazilian semiarid region

The subterranean environments of the Caatinga drylands (Brazilian biome) host a diverse array of aquatic systems and diversity, comprising both lentic and lotic ecosystems. Species diversity in these environmnts has been overlooked and remains poorly understood, especially regarding zooplankton. We studied the species richness and composition of zooplankton living in groundwater of the Caatinga drylands and explored how they vary in relation to habitat type (lentic and lotic) by testing two hypotheses: (1) species richness, gamma diversity, and zooplankton density are higher in lentic environments and (2) zooplankton species composition differs between habitat types. We sampled 12 lentic and lotic groundwater environments quarterly for 2 years, and identified 100 species of zooplankton in groundwater environments, including testate amoebae (50 species), Rotifera (25 species), Copepoda (16 species), and Cladocera (9 species). Rotifer species richness and copepod density differed between lentic and lotic habitats, as did zooplankton composition. Additionally, each habitat was found to harbor distinct indicator species based on their ecology and morphological characteristics. These findings contribute to current understanding of groundwater biodiversity and ecology, providing support for freshwater and speleological management and conservation programs.