Limnological characterization and phytoplankton structure in a stone tank located on an inselberg in the semiarid region of Paraíba, Brazil

Introduction: Inselbergs, also known as rocky outcrops, represent unique ecological formations within the Brazilian semiarid region, characterized by intense solar radiation, limited water availability, and shallow, nutrient-poor soils. These environments often contain natural rock pools that accumulate rainwater and serve as temporary microhabitats for aquatic communities. This study aimed to characterize the limnological conditions and phytoplankton structure of a rock pool located on Pedra do Touro, in the municipality of Queimadas, Paraíba, Brazil. Methods: Physicochemical parameters of the water (pH, electrical conductivity, temperature, and dissolved oxygen) were measured using a multiparameter probe, while water transparency was determined with a Secchi disk. Phytoplankton and zooplankton samples were collected using 20 µm and 50 µm plankton nets, respectively, and preserved with Lugol’s solution and formaldehyde. Results: A total of 16 phytoplankton species were identified, distributed across five classes, with a marked dominance of Euglenophyceae (48.6%), followed by Chlorophyceae (27.4%) and Zygnematophyceae (16.4%). The most abundant species, Trachelomonas ovata (212.10 cells/mL), commonly associated with environments enriched in organic matter, suggests initial stages of organic accumulation within the reservoir. Conclusions: Overall, the findings indicate that rock pools function as ecologically relevant freshwater microecosystems that support diverse phytoplankton communities. These systems warrant greater attention regarding biodiversity conservation, ecological monitoring, and the management of water resources in semiarid landscapes.