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 ºC 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 year-1. Despite being a tropical native species, the species was able to survive, photosynthesise and grow within a broad range of temperatures (10 to 30 ºC). At 10 ºC, 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 30º C but the optimal leaf growth rate was within the range of temperatures for temperate seagrass species (11.5-26 ºC). Based on the present spread rate and on the non-limiting effect of temperature we estimate that in the next 100 years H. stipulacea will be present throughout the whole Mediterranean Sea (but perhaps the north Adriatic), potentially spreading into the Atlantic.

The tropical seagrass Halophila stipulacea is dominant in most regions of the Indo-Pacific and the Red Sea and was introduced into the Mediterranean Sea after the opening of the Suez canal. The species is considered invasive in the Mediterranean Sea and has been progressively colonizing new areas westward. Growth and photosynthetic responses of H. stipulacea have been described but no information is yet available on the nitrogen nutrition of the species. Here we simultaneously investigated the uptake kinetics of ammonium and nitrate and the internal translocation of incorporated nitrogen in H. stipulacea using 15 N-labelled substrates across a range of N i levels (5, 25, 50 and 100 lM). The ammonium uptake rates exceeded the nitrate uptake rates 100-fold, revealing a limited capacity of H. stipulacea to use nitrate as an alternative nitrogen source. The uptake rates of ammonium by leaves and roots were comparable up to 100 lM 15 NH 4 Cl. At this concentration, the leaf uptake rate was 1.4-fold higher (6.22 AE 0.70 lmolÁg À1 DW h À1 ) than the root uptake rate (4.54 AE 0.28 lmolÁg À1 DW h À1 ). The uptake of ammonium followed Michaelis-Menten kinetics, whereas nitrate uptake rates were relatively constant at all nutrient concentrations. The maximum ammonium uptake rate (V max ) and the half-saturation constant (K m ) of leaves (9.79 lmolÁg À1 DW h À1 and 57.95 lM, respectively) were slightly higher than that of roots (6.09 lmolÁg À1 DW h À1 and 30.85 lM, respectively), whereas the affinity coefficients (a = V max /K m ) for ammonium of leaves (0.17) and roots (0.20) were comparable, a characteristic that is unique among seagrass species. No substantial translocation (<2.5%) of 15 N incorporated as ammonium was detected between plant parts, whereas the translocation of 15 N incorporated as nitrate was higher (40-100%). We conclude that the N i acquisition strategy of H. stipulacea, characterized by a similar uptake capacity and efficiency of leaves and roots, favors the geographical expansion potential of the species into areas with variable water-sediment N levels throughout the Mediterranean.Marine Ecology. ISSN 0173-9565