Searching for the competitive ability of the alien seagrass Halophila stipulacea with the autochthonous species Cymodocea nodosa.

The tropical seagrass Halophila stipulacea (Forsskål) Ascherson, 1867 entered in the Mediterranean Sea through the Suez Canal more than 100 years ago. In coastal-marine ecosystems the spatial niche of H. stipulacea is often overlapped with that of native Mediterranean Sea seagrasses and therefore it might out-compete them. Aiming to better understand its invasiveness potential, we monitored a Southern Mediterranean shallow coastal-marine water habitat from August 2010 to August 2011, where H. stipulacea co-occurred with the native seagrass Cymodocea nodosa (Ucria) Ascherson, 1870. Besides, the year-round dynamics of H. stipulacea was also monitored in four periods. To test the hypothesis that the presence/absence of H. stipulacea may have an effect on C. nodosa density, we analyzed the shoot density of C. nodosa in 8 sites, 4 sites where H. stipulacea was present (impacted sites) and 4 where H. stipulacea was absent (control sites). The results showed significant differences in C. nodosa shoot density according to the presence/absence of H. stipulacea, with the lowest values observed in sites where it co-occurred with H. stipulacea. We hypothesize that the dense rhizome-sediment net created by H. stipulacea can interfere with C. nodosa density, pushing down its rhizomes in the anoxic layer. The leaf features of H. stipulacea were generally comparable to those of other Mediterranean populations. In January 2011 a significant decline of H. stipulacea was observed, maybe related to changes in the environmental conditions that have become unfavorable (e.g. hydrodynamics, turbidity) and, unexpectedly, the seagrass disappeared in April 2011. In January, we also observed the occurrence of the green alien alga Caulerpa cylindracea Sonder, 1945 which rapidly invaded the bare substrate left by H. stipulacea.