Zooplankton biodiversity assessment and community structure in semi-arid reservoirs of Northwestern Algeria.

Zooplankton communities are essential indicators of aquatic ecosystem health. Understanding their reactions to varying environmental conditions can provide valuable insights into freshwater ecosystems management. This study aims to compare the influence of environmental drivers on zooplankton biodiversity and community structure in two interconnected reservoirs with distinct morphometric and environmental characteristics in Northwestern Algeria: Cheliff, a shallow diversion reservoir supplies Kerrada, a deep storage reservoir. These reservoirs are integral to a large-scale project transferring dam water to provide drinking water for Mostaganem, Arzew, and Oran (M.A.O.) corridor. Monthly sampling over a two-year period (from November 2020 to October 2022) was conducted in these two reservoirs. We assessed the composition, abundance, and diversity of the main zooplankton groups: Rotifera, Copepoda, and Cladocera, to determine their adaptive responses to environmental conditions. Our findings indicate that morphometric, physical, and chemical differences between these reservoirs significantly impact zooplankton dynamics. Cheliff exhibited higher temperatures, alkalinity, salinity, and nutrient concentrations, attributed to its shallow depth (mean depth = 6 m) and anthropogenic pressures. In contrast, Kerrada's greater depth (mean depth = 115 m) contributed to more stable water quality, lower turbidity, and prolonged nutrient retention. Zooplankton diversity patterns highlighted the influence of environmental variables. Cheliff reservoir supported higher species richness (28 species) compared to Kerrada reservoir (20 species), although both reservoirs showed similar Shannon diversity index (H') values. Cheliff's zooplankton community was dominated by opportunistic rotifers such as Keratella valga and Synchaeta pectinata, associated with eutrophic conditions and high turbidity. Conversely, Kerrada exhibited higher species evenness and dominance of cladocerans such as Diaphanosoma brachyurum, indicative of moderate-quality, oxygen-rich waters. Statistical analyses, including redundancy analysis (RDA), revealed strong correlations between environmental factors and zooplankton diversity, particularly microcrustaceans such as copepods and cladocerans. Cheliff was characterized by the high abundance of cyclopoida Acanthocyclops americanus and a lower abundance of cladocerans, while Kerrada was dominated by calanoida Copidodiaptomus numidicus and showed a higher abundance of cladocerans. Key environmental drivers shaping the distribution and abundance of zooplankton groups included temperature, turbidity, salinity, organic matter, and nutrient levels (notably orthophosphates and ammonium) in Cheliff reservoir, while dissolved oxygen, and lower nutrient concentrations played a pivotal role in structuring communities in Kerrada. These results indicate that nutrient availability, water depth, and habitat stability critically shape zooplankton community structure in these semi-arid reservoirs. This study provides new insights into the use of zooplankton as bioindicators, with significant implications for water quality assessment and ecological conservation strategies in similar Mediterranean ecosystems.