Phenology and ecology of the alien seagrass Halophila stipulacea in its northern range limit in the Mediterranean Sea

The alien seagrass Halophila stipulacea has invaded the Mediterranean Sea from the Red Sea more than 120 y ago, and first recorded in the western Mediterranean Sea in 1995 (Vulcano island, Sicily). The species has further expanded similar to 110 nautical miles northward, reaching the harbour of Palinuro (Tyrrhenian Sea, Italy) in 2006, which currently represents its northern distribution limit. We describe phenology and ecology of H. stipulacea in Palinuro harbour in relation to long-term changes in local sea-surface temperatures (SST). The species occurrence in this area was concomitant to increases in local SSTs, both during winter and summer, especially since the early 2000s. New patches of this species established in the habour in 2018; we monitored two sites from April 2018-November 2019: Patch A (exposed: 1.8-2.0 m depth), and Pier Patches (3-4 m depth; more sheltered conditions). H. stipulacea attained maximal shoot sizes in autumn-winter and minimal sizes in the summer. Shoot density at the Pier was higher (9000 shoots m(-2)) than at Patch A (6000 shoots m(-2)) where plants also presented smaller leaves, and lower Leaf Area Index than those at the Pier, likely an adaptation to higher exposure to wave action, sediment resuspention and tramping. Only male flowers were observed in May-July, with higher frequency at the Pier. Grazing by juveniles of the fish Sarpa salpa was pervasive year-round, but more frequent at Patch A. The species seems well established in this new area showing a high plasticity to differential wave exposure conditions at the study sites.

Automated summary

The alien seagrass Halophila stipulacea invaded the Mediterranean Sea from the Red Sea over 120 years ago, reaching as far north as Palinuro harbour in Italy. The species' occurrence in this area is correlated with increases in local sea-surface temperatures, showing high adaptability to different wave exposure conditions.