Abstract
During the last 150 years, the tropical seagrass species Halophila stipulacea has established itself in the southern and eastern parts of the Mediterranean Sea. More recently (2018), Halophila decipiens was observed for the first time in the eastern Mediterranean, and was described as the second non-native seagrass species in the Mediterranean Sea. We implemented a species distribution model (SDM) approach to (1) hindcast the habitat suitability of H. stipulacea over the last 100 years in the Mediterranean basin, and (2) to model the increase in the potential habitat suitability of H. stipulacea and H. decipiens during the current century under two very different climate scenarios, RCP 2.6 (lowest carbon emission scenario) and RCP 8.5 (highest carbon emission scenario). In addition, a principal component analysis (PCA) and k-means cluster based on temperature and salinity drivers were applied to visualize the distance and relatedness between the native and invasive H. stipulacea and H. decipiens populations. Results from this PCA suggest that the H. stipulacea populations of the Mediterranean and Red Sea are likely to be similar. In contrast, H. decipiens from the Mediterranean is more related to the Atlantic populations rather than to the Red Sea populations. The hindcast model suggests that the expansion of H. stipulacea was related to the increases in seawater temperatures in the Mediterranean over the last 100 years. The SDMs predict that more suitable habitat will become available for both tropical species during this century. The habitat suitability for H. stipulacea will keep expanding westward and northward as the Mediterranean continues to become saltier and warmer. In comparison, the SDMs built for H. decipiens forecast a restricted habitat suitability in the south-eastern Mediterranean Sea at the present environmental conditions and predicts a progressive expansion with a potential increase in habitat suitability along 85% of the Mediterranean coastline. The predicted rapid expansion of non-native seagrass species could alter the Mediterranean’s seagrass community and may entail massive impacts on associated ecosystem functions and services, impacts that have severe socio-economic consequences.
Highlights
Ocean warming, the increment of frequency and severity of extreme events such as marine heatwaves or storms, increases in salinity, and biological invasions are emerging as severe threats for marine ecosystems, resulting in a significant loss of biodiversity, functionality and associated ecosystem services (i.e., Rilov and Galil, 2009; Gattuso et al, 2015; Oliver et al, 2019)
Sea surface temperatures (SST) in the Mediterranean Sea have increased at an average rate of +0.04 ± 0.1◦C per decade, while in the eastern Mediterranean basin these rates were specially high with summer increases of +0.12 ± 0.07◦C year−1; surface salinity has increased at an average rate of +0.015 ± 0.002 per decade, and summer increases of +0.008 ± 0.006 year−1 in the eastern Mediterranean basin (i.e., Axaopoulos and Sofianos, 2010; Borghini et al, 2014; Ozer et al, 2017)
The Species Distribution Models (SDMs) built to hindcast the habitat suitability of H. stipulacea over the last 100 years (1920–2019) (Figure 1) had an area under the curve (AUC) of 0.97 and a sensitivity of 92% (Supplementary Table S2)
Summary
The increment of frequency and severity of extreme events such as marine heatwaves or storms, increases in salinity, and biological invasions are emerging as severe threats for marine ecosystems, resulting in a significant loss of biodiversity, functionality and associated ecosystem services (i.e., Rilov and Galil, 2009; Gattuso et al, 2015; Oliver et al, 2019). Seagrasses are key foundation species that create highly productive habitats and nursery grounds for fish and invertebrates, and provide essential functions including CO2 sequestration, production of organic carbon and nutrient cycling (Duarte et al, 2010; Costanza et al, 2014; Nordlund et al, 2018). Seawater warming rates in this enclosed sea are four times faster than the average of the warming rates of coastal waters of the world (Bianchi and Morri, 2003; Borghini et al, 2014), and these rates are predicted to increase as climate change progresses during this century (Somot et al, 2006; Rilov and Galil, 2009). Sea surface temperatures (SST) in the Mediterranean Sea have increased at an average rate of +0.04 ± 0.1◦C per decade, while in the eastern Mediterranean basin these rates were specially high with summer increases of +0.12 ± 0.07◦C year−1; surface salinity has increased at an average rate of +0.015 ± 0.002 per decade, and summer increases of +0.008 ± 0.006 year−1 in the eastern Mediterranean basin (i.e., Axaopoulos and Sofianos, 2010; Borghini et al, 2014; Ozer et al, 2017)
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