Impact of altitudinal variation on phytoplankton community dynamics and ecological status across 70 lentic systems in Türkiye

In 70 lentic systems across Türkiye, phytoplankton distribution, composition, biovolume, species richness (both alpha and gamma diversity), and Shannon diversity index were determined and environmental variables were measured using samples collected in the spring, summer, and fall of 2017–2019 to determine the impact of altitude (ALT) on phytoplankton assemblages. The lentic systems were categorized into three ALT groups: group I (< 800 m a.s.l.), group II (800–1600 m a.s.l.), and group III (> 1600 m a.s.l.). Changes in species composition and dominant species were observed with increasing ALT, with certain species showing a preference for higher ALT and exhibiting higher ALT optima values. The species composition of group III was markedly different from the other groups. Phytoplankton biovolume, species richness (alpha diversity), and the Shannon diversity index did not show a correlation with ALT. However, group III lentic systems exhibited the lowest biovolume values, while group I systems had the lowest species richness. The total species richness (gamma diversity) of Cryptista (Cryptophytes) and the dominance of flagellate species increased with ALT. While phytoplankton trophic index (PTI) indicated all the systems had only moderate and poor ecological qualities, trophic state index (TSI) did not detect any high-quality lentic systems in group III. In contrast, the PHYTO-TR index demonstrated that group III exhibited a higher proportion of lentic systems classified as good and high quality, providing a more accurate evaluation of the ecological quality of lentic systems. ALT indirectly influenced habitat diversity and the degree of human impact, thereby affecting some parameters, such as temperature, electrical conductivity, alkalinity, pH, total nitrogen, and total phosphorus. Consequently, these parameters were significant determinants of phytoplankton distribution, composition, biovolume, species richness (alpha and gamma diversity), and the ecological quality of these lentic systems.