Reservoirs in semi-arid regions are increasingly exposed to eutrophication driven by nutrient enrichment, hydrological alteration and climate variability. In Algeria, water-quality monitoring still relies mainly on physicochemical parameters, whereas biological indicators remain underused. This study evaluates the usefulness of plankton-based indices for assessing trophic state in two connected but also contrasting reservoirs in northwestern Algeria: Cheliff, a shallow diversion reservoir, and Kerrada, a deeper storage reservoir. Water samples were collected monthly from November 2020 to October 2022. The trophic state of reservoirs was compared with different phytoplankton and zooplankton-based metrics. Cheliff was classified as a reservoir with a higher trophy of water (eutrophic to hypereutrophic). In contrast, Kerrada reservoir, which receives water from Cheliff, was classified as a reservoir with lower trophy (mesotrophic). Multivariate analysis indicated that nutrient enrichment, turbidity and particulate matter structured Cheliff plankton communities, while depth and water transparency were the main drivers in Kerrada. Phytoplankton indices reflected the trophic contrast while zooplankton-based metrics showed variable performance due to regional environmental influences such as temperature and predation. Our findings demonstrate the potential of plankton-based metrics to provide complementary information on trophic conditions in contrasting reservoirs. However, differences in absolute trophic-state classification highlight the need for regional calibration of zooplankton-based indices before their application in routine monitoring of semi-arid reservoirs. Nevertheless, this study provides a baseline for Algerian reservoirs.