Physiological Responses of Cystoseira compressa in Relation to the Presence of the Invasive Macroalga Batophora occidentalis Under Differing Habitat Conditions in a Mediterranean Coastal Lagoon

The spread of the green macroalga Batophora occidentalis into shallow, sheltered Mediterranean systems may alter habitat structure and impose sublethal stress on resident habitat-forming species. We assessed whether the presence of B. occidentalis in the s’Estany des Peix lagoon (Formentera, Balearic Islands) is associated with physiological alterations in the perennial brown alga Cystoseira compressa. Samples of C. compressa were collected from areas with high (n = 8) and low (n = 8) abundance of B. occidentalis, and benthic cover was surveyed using 40 × 40 cm quadrats (n = 8 per area). Thermal monitoring indicated greater short-term variability inside the lagoon compared to the outer mouth. Biochemical assays showed that individuals from invaded patches exhibited significantly elevated antioxidant enzyme activities (superoxide dismutase and catalase), higher glutathione S-transferase activity and increased total polyphenol content, together with greater reactive oxygen species production. In contrast, malondialdehyde levels did not differ between areas, suggesting that enhanced antioxidant and detoxification responses may prevent detectable lipid peroxidation under the conditions studied. Habitat characteristics, notably higher availability of rocky substrate in invaded sectors, likely facilitate B. occidentalis establishment and modulate the interaction outcome. In conclusion, the coexistence with B. occidentalis was associated with a moderate oxidative challenge in C. compressa, within the environmental context of the invaded areas, and we recommend long-term studies to determine whether these sublethal responses translate into demographic effects.