Getting turfed: The population and habitat impacts of Lophocladia lallemandii invasions on endemic Posidonia oceanica meadows

• We assessed the impact of the invasive Lophocladia lallemandii in seagrasses.
• Seagrass mortality was 2.5–5 times higher in invaded than in turf-free sites.
• Seagrass mortality increased when the algal turf biomass exceeded 200–300 g DW m−2.
• L. lallemandii could represent an important treat to P. oceanica resilience.

We examined the extent and mechanism of impact of the invasive alga Lophocladia lallemandii in two otherwise healthy shallow seagrass meadows (Es Caló de s’Oli and Cala Torreta) in a Marine Natural Reserve on Formentera Island (Balearic Islands, Spain). The algae epiphytically invaded healthy seagrass leaves and rhizomes, and occasionally coalesced to form dense turf patches that trapped dead leaves, sediment and debris. Turf density was 0.21 turfs m−2 and 0.43 turfs m−2, and median individual turf size was 0.14 m2 and 0.12 m2 at Es Caló de s’Oli and Cala Torreta, respectively. These turfs occupied 3.5–10.9% of the area of sampled meadows. We examined the ramet- and population-level impacts of these turfs on P. oceanica along a gradient of turf size. Under these turfs, annual seagrass shoot mortality was, on average, 2.5 to 5 times higher than turf-free sites, with mortality rates that ranged from 8% per year to 10% per year. Seagrass shoot mortality rates rapidly increased when the biomass of L. lallemandii turf and accumulated debris exceeded 200–300 g DW m−2. In addition, the size of P. oceanica shoots, aboveground biomass and rhizome growth significantly decreased with increasing turf area (Least squares regression analyses, p < 0.05). Our results indicate that the turf-forming nature of L. lallemandii directly modifies vital demographic parameters of P. oceanica and may have significant consequences for the meadow. L. lallemandii could represent an important yet largely unrecognized threat to the resilience of this endemic ecosystem that is already subject to a plethora of other anthropogenic stresses.