Trophic state assessment based on summer phytoplankton community structure and trophic indices: a small tectonic lake in Turkey

Lake Gokgol is a small lake with a tectonic origin located in Ordu, Turkey. Algal diversity and distribution as well as the trophic structure of the lake in the summer period were examined. It is a lake with poor acidic character, low dissolved oxygen content, high electrical conductivity, with very hard water characteristics, having nutritive elements particularly phosphorus, and high mineral concentration. In the phytoplankton, the highest diversity of species belongs to Bacillariophyta (33 taxa), followed by Cyanobacteria (6 taxa), Charophyta (5 taxa), Euglenozoa (3 taxa), Chlorophyta (3 taxa), Cryptophyta (1 taxa), and Miozoa (1 taxa), respectively. During the summer months, the biggest contribution to phytoplankton abundance is from Cyanobacteria (96%) and Bacillariophyta (4%). Of blue-green algae, Chroococcus dispersus and Dolicospermum affine, of the diatoms Cyclotella radiosa, Achnanthidium exile, Ampohora ovalis, of euglenoids Euglena gracilis are the taxa making a significant contribution to the phytoplankton abundance. The phytoplankton community was applied with various diversity, species richness, and evenness indices (Shannon, Pileou, Simpson, and Margaleff), and the environmental factors were applied with cluster analysis and correlation analysis. According to the correlation analysis results, the highest positive correlation is between NH4-N, TP, and Cyanobacteria (p < 0.01). To determine the trophic level, phytoplankton compound quotient (PCQ), Palmer's algal genus pollution index, dominant genus scores, Carlson's trophic state index, and Burns's trophic level index were used. In conclusion, when evaluated according to the average of all parameters during the summer months, the Gokgol Lake shows eutrophic character and clean to moderate water quality. Toxic Cyanobacteria bloom was not detected in the lake, which could pose a risk to human health. Gokgol Lake, which does not carry the risk of eutrophication, can be evaluated for health tourism because it contains high mineral salts.