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

Abstract

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.

Expansion and fragment settlement of the non-native seagrass Halophila stipulacea in a Caribbean bay

“…Randomly collected fragments of different sizes and lifespans all appeared viable and after tethering the fragments started to grow new roots and leaves within 10 days. Apart from our study, clonal growth rates of H. stipulacea have been reported only once from a mesocosm study in the Mediterranean (Georgiou et al 2016). Reported maximum growth rates are one shoot every two days at the maximum studied temperature of 30°C (Georgiou et al 2016).…”

“…However, this species can potentially have a large ecological impact at introduced locations due to its extensive range expansion and high tolerance to broad salinity, irradiance and temperature ranges and substrate types (Lipkin 1975;Georgiou et al 2016). These qualities can favour the species in competition with native species and enable it to spread quickly.…”

Sulfate Reduction and Sulfur Cycles at Two Seagrass Beds Inhabited by Cold Affinity Zostera marina and Warm Affinity Halophila nipponica in Temperate Coastal Waters

“…(Hillman et al 1995;Bianchi 2007), H. nipponica remained alive by maintaining a high ratio of BGB to AGB when the water temperature was below 15°C along the southern coast of Korea (Kim et al 2012). Similarly, Georgiou et al (2016) demonstrated that Halophila stipulacea, an invasive species in the Mediterranean Sea, ceases clonal growth during winter (at 10°C) but continues to undergo photosynthesis and produces leaf biomass. The average water temperature in the winter (February) increased by approximately 2°C (from 8.4 to 10.1°C) during the past 70 years in southern coastal waters of the Korean peninsula ).…”

“…These results imply that sulfate reduction in seagrass sediment is not directly influenced by temperature, but is substantially affected by the amount of DOC exudation from plant roots (Holmer and Nielsen 1997;Kristensen and Alongi 2006), which co-varies with temperature. The clear differences in the optimal growth temperature for the two species further indicated that warming might deteriorate Z. marina growth and that this dominant species might eventually be replaced by H. nipponica in the study area (Pergent et al 2014;Georgiou et al 2016).…”

“…Temperature is a critical factor regulating the growth and survival of seagrass, and thus, global warming could ultimately induce a distributional range shift, resulting in a decline of temperate seagrass species and the expansion of subtropical/ tropical seagrass species (Moore and Jarvis 2008;ISAC 2011;Georgiou et al 2016). For example, the shoot density of Z. marina decreased by 48% at elevated temperatures to 4°C above ambient temperature (Touchette et al 2003).…”