Contribution of global warming to the shift in macroalgal dominance in a Mediterranean eutrophic lagoon ecosystem.

In order to assess the potential consequences of global warming on a Mediterranean eutrophic lagoon, in terms of the communities of opportunistic bloom-forming macroalgae, historical and specifically collected data on macroalgal growth from 1983 to 2024, water temperature (T) from 2013 to 2025, and air T from 1970 to 2025 were studied. In addition, for three plots of the East basin of the lagoon, sediment labile organic matter (LOM), and macroalgal biomass from 2017 to 2024 were also used. Statistical analysis indicated a significant increasing trend in air maximum T from 1970 to 2025 (p < 0.001), and air minimum T in June, July and August (p < 0.05). Water T showed no statistically significant change over the last 12 years (p > 0.05), although a slight upward trend in T can be observed. The Rhodophyceae/Chlorophyceae biomass ratio showed a significant decrease between 1983 and 2024, mainly due to the continued dominance of Chaetomorpha linum from 2008 onwards. However, this decrease was not statistically directly linked to the T rise. In the East basin, the dominance of C. linum increased gradually between 2017 and 2024. A comparison between macroalgal biomass and LOM revealed a significant negative correlation (p = 0.002), indicating that as the dominance of C. linum increased at the expense of Rhodophyceae, LOM decreased. The rise in temperature makes the environment more stressful and anaerobic bacteria are likely to become more aggressive; as a result, some species disappear completely during the summer, but not C. linum. If a rise in T favours the dominance of a species that is tolerant to this phenomenon, LOM would tend to decrease, and consequently the incidence of dystrophies would fall, but at the cost of a further decline in biodiversity, with extreme dominance of a single macroalgal species.