Ecophysiological responses of Fucus virsoides (Phaeophyceae, Fucales) to past and present nutrient conditions in the northern Adriatic

Fucus virsoides is a brown seaweed endemic to the Adriatic and the only species of its genus found in the Mediterranean. Historically widespread from the Venice lagoon (Italy) to Albania, this species has suffered a sharp decline and is currently threatened with extinction. Over the past three decades, the northern Adriatic has seen a shift towards oligotrophy conditions, yet the ecophysiol- ogy of F. virsoides in response to nutrients changes has been poorly studied. Addressing this gap is crucial for understand- ing the extent to which these environmental changes may have contributed to the species’ decline. To test our hypothesis that nutrient changes might be the primary driver of F. virsoides decline, we conducted a two- week experiment exposing germlings and adults to six different nutrient conditions. These included three Redfield ratios recorded in the Gulf of Trieste in 1996, 2007 and 2017, reflecting the shift from eutrophic to oligotrophic conditions experienced by F. virsoides in the wild. Additionally, the adults were exposed to three supplementary eutrophic conditions (naturally and artificially fertilized). Growth and physiological responses (measured via O2 evolution and PAM fluorimetry) of F. virsoides to varying nutri- ent conditions were largely consistent, with only subtle effects observed. Our results highlight the species’ acclimatation potential, suggesting that short-term nutrient changes alone may not fully explain its decline. Understanding F. virsoides resilience to multiple environmental stressors is crucial for developing effective conservation strategies to preserve marine forests in the face of ongoing anthropogenic disturbances.