Fading Underwater Forests: Long-Term Decline of Canopy-Forming Macroalgae on the Slovenian Coast of the Northern Adriatic

Canopy-forming macroalgae, particularly Cystoseira sensu lato species, are key habitat-forming organisms and important indicators of ecological status (ES) in Mediterranean coastal ecosystems. We assessed long-term changes in infralittoral macroalgal assemblages along the Slovenian coastline (northern Adriatic) using data from the national monitoring programme. Spring and autumn surveys conducted between 2007 and 2025 at 13 stations across four coastal water bodies were analysed using the Ecological Evaluation Index continuous formula (EEI-c) and complementary structural and functional indicators. Temporal trends were tested with Spearman rank correlations. Total macrophyte coverage, the relative coverage and taxonomic proportion of canopy-forming taxa, and the relative coverage of Ecological State Group I (ESG I) taxa declined significantly in both seasons. In contrast, the proportion of ESG I taxa in total taxonomic richness showed no significant temporal trend. Autumn assemblages generally had higher total coverage and relative ESG I coverage, whereas the relative canopy contribution was generally higher in spring. Seasonal differences therefore accompanied a decline detectable in both seasonal series. Stable ESG I taxonomic ratios did not imply stable coverage or habitat structure. These results support seasonally replicated monitoring and conservation of remaining macroalgal forests. Identifying the drivers of decline requires environmental measurements alongside biological surveys.