Global Warming and Fish Diversity Changes in the Po River (Northern Italy)

In the context of climate change, the current rise in temperature, changes in precipitation, and extreme weather events are exceptional and impact biodiversity. Using the Mann–Kendall trend test, change-point analysis, and linear mixed models, we investigated the long-term trends (1978–2022) of water temperature and flow in the Po River, Italy’s largest river, and examined changes in the fish community over the same period. Our findings indicate that the daily water temperature of the Po River increased by ~4 ◦C from 1978 to 2022, with a significant rise starting in 2005. The river’s daily discharge showed higher variability and decreased from 2003 onwards. The number of days per year with water temperatures above the summer average increased steadily by 1 day per year, resulting in over 40 additional days with above-average temperatures in the last four decades. The number of summer days above the seasonal average water temperature was the most influential factor affecting fish diversity. Total fish species richness and native species richness significantly decreased between 1978 and 2022 with the increasing number of days above the summer average water temperature, while non-native species increased. Our results demonstrate that the Po River is experiencing significant impacts from global warming, affecting freshwater communities.