This study presents the first occurrences of the diatom species Discostella asterocostata in Europe’s second largest river, the Danube River and the earliest occurrence in European freshwater ecosystems. Samples collected between 2016 and 2022 in Croatia and Hungary were analysed using light and scanning electron microscopy (SEM) along with molecular techniques to confirm the presence of the species. Morphometric comparisons with known populations from Asia and the USA revealed no significant morphological differences. Molecular analyses facilitated the detection of D. asterocostata even in samples with low abundance and confirmed the genetic similarity with already sequenced populations. Environmental correlations showed that D. asterocostata had a significant positive correlation with temperature and a negative correlation with total organic carbon and nitrogen forms, suggesting that it favours warmer waters with lower nutrient forms. Given the ongoing increase in water temperatures due to climate change, the occurrence and potential spread of this species in European rivers may be related to changing environmental conditions. Our results emphasise the importance of integrating traditional and molecular approaches for the early detection of non-indigenous diatom species. The establishment of D. asterocostata in Europe raises questions regarding its ecological role and long-term impact on native phytoplankton communities and emphasises the need for continuous monitoring and research on the responses of large river ecosystems to global environmental change.

Phytoplankton is a polyphyletic group of organisms that responds rapidly to environmental conditions and provides a reliable response to changes, making it a good ecological indicator for water quality monitoring. However, a gradient is almost essential for a reliable relationship between pressure and impact. In a low-gradient environment, ingenuity is required to outsmart the limitations of the commonly used linear relationship. Here, we examine changes in biomass and functional biodiversity by analysing larger data sets (2013–2022) in six ecologically diverse, natural, deep Croatian karst lakes with low nutrient gradients using nonlinear correlation coefficients and multivariate analyses in 209 samples. We found that phytoplankton biomass was most strongly influenced by nutrients, salinity and alkalinity, while light availability and total nitrogen strongly influenced phytoplankton functional biodiversity. An additional analysis of the TN:TP ratio revealed that the oligotrophic Lake Vransko is nitrogen-limited, and lakes Kozjak and Prošće are phosphorus-limited. This further clarified the relationship of phytoplankton to nutrients despite the low gradient. The complex analysis in this study provides a new perspective for predicting changes in the structure and succession of phytoplankton in deep karst lakes for successful management under apparent anthropogenic pressure and climate change.