The case of Skeletonema potamos (C.I.Weber) Hasle in Serbian lowland rivers: evidence of range expansion, invasion or widespread distribution?

Abstract

Aquatic invasive species are widely recognized as threats to biodiversity, ecosystem services and socio-economic activities. Skeletonema potamos (C.I.Weber) Hasle is small, chain-forming, freshwater diatom broadly distributed across the temperate zone in Europe, but also considered invasive in several regions. This study aimed to assess the current distribution of S. potamos in major lowland rivers in Serbia, evaluate its potential range expansion and assess its invasive status. Phytoplankton and epilithic diatom surveys were conducted in the Danube, Sava, and Tisza rivers between 2016 and 2025. The results confirmed the continuous occurrence of S. potamos in the phytoplankton along the entire Danube course, with maximum biomass contributions remaining within previously reported ranges. In the Sava River phytoplankton, a consistent increase in mean annual biomass was observed, with recent values up to twelvefold higher compared with earlier reports. For the first time, the species was recorded in the phytoplankton of the Tisza River in Serbia, where its mean annual biomass values were the highest among the investigated rivers. S. potamos presence was also confirmed in epilithic diatom samples from all three rivers. Water temperature showed a positive association with its development, although its ecological performance is shaped by multiple interacting environmental factors. Despite its increasing abundance and new records from previously undocumented localities, the results suggest that its current distribution pattern is more likely driven by environmental changes and improved detection rather than representing a true biological invasion.