Temperature is not a limiting factor for the expansion of Halophila stipulacea throughout the Mediterranean Sea

The seagrass Halophila stipulacea is a Lessepsian migrant that has spread from the Red Sea into the Mediterranean Sea where some authors consider it invasive. It has been suggested that the range of expansion of the species in the Mediterranean will be limited by the 15 degrees C sea surface isotherm. Here we tested the effects of temperature on survival, photosynthesis, leaf growth and clonal growth of H. stipulacea. We analysed the temporal and spatial variation of sea surface temperature (SST) in the Mediterranean Sea and the species' spread rate since its introduction to forecast its future expansion and potential changes in the spread rate due to increased SST. We estimated that the species has been spreading throughout the Mediterranean with a variable rate averaging 12 km yr(-1). Despite being a species native to the tropics, it was able to survive, photosynthesise and grow within a broad range of temperatures (10 to 30 degrees C). At 10 degrees C, a temperature colder than the winter's lowest isotherm for most of the Mediterranean Sea, the clonal growth ceased but plants did not die and continued to photosynthesise and produce new leaf biomass. The maximum photosynthetic rate peaked at 30 degrees C but the optimal leaf growth rate was within the range of temperatures for temperate seagrass species (11.5 to 26 degrees C). Based on the present spread rate and on the non-limiting effect of temperature, we estimate that in the next 100 yr H. stipulacea will be present throughout the whole Mediterranean Sea (but perhaps not the north Adriatic), potentially spreading into the Atlantic