The Use of Phytoplankton Communities to Determine the Ecological Status of the Büyükçekmece Reservoir and Investigate Water Quality Problems

Objective: This study aimed to assess the ecological status of Büyükçekmece Reservoir, a major drinking water resource for Istanbul, by integrating phytoplankton community structure with trophic indices, nutrient dynamics, and multivariate analyses. Temporal patterns were also evaluated to identify potential eutrophication risks and pollution pressures.Materials and Methods: We conducted seasonal sampling at six stations between February and November 2019. Phytoplankton species were identified and quantified microscopically, and biovolume was calculated. The physicochemical parameters and nutrient concentrations were measured using standard methods. Ecological quality was evaluated using functional groups, Shannon diversity, Bray–Curtis similarity, Nygaard and Stockner indices, Carlson’s Trophic State Index, OECD criteria, and national water quality standards. Relationships between biotic and abiotic variables were analysed using Spearman’s correlation and redundancy analysis (RDA).Results: A total of 177 taxa were identified, including six new records for Turkish inland waters and 75 new records for the reservoir. Bacillariophyta dominated species richness, whereas Cryptophyta dominated abundance during winter and spring. Cyanophyta and Euglenophyta remained at low densities. Functional group analysis indicated meso-eutrophic and nutrient-rich conditions. While the basic physicochemical parameters mainly corresponded to Class I-II quality, the total phosphorus frequently reflected Class III conditions. The Carlson index classified the reservoir as predominantly oligotrophic, whereas the OECD and national criteria indicated mesotrophic conditions. RDA revealed that conductivity, nitrite+nitrate nitrogen, silicate, and temperature are the principal drivers of phytoplankton distribution.Conclusion: The Büyükçekmece Reservoir exhibits predominantly mesotrophic characteristics but shows increasing phosphorus pressure, particularly in the Karasu inflow. Continuous monitoring and phosphorus control are essential for preventing future eutrophication and safeguarding this critical drinking water resource.