Assessment of Posidonia oceanica traits along a temperature gradient in the Mediterranean Sea shows impacts of marine warming and heat waves

Abstract

Global warming and Marine Heat Waves (MHWs) are having large-scale impacts on the seagrasses and their effects on the Mediterranean endemic Posidonia oceanica need to be properly defined. This research aimed to sharpen the knowledge on P. oceanica response to the warming by correlating the shoot morphology and the productivity of the seagrass to temperature conditions and MHW occurrence. Two correlative studies, along a Mediterranean temperature range (sites at the same latitude), were conducted: one explored the associations of summer 2021 Sea Surface Temperature (SST) and MHWs on P. oceanica morphology and the other used a reconstruction technique (lepidochronological analysis) to relate past temperature conditions and MHW occurrence to proxies of seagrass productivity for the corresponding periods. The results showed that the mean summer temperature affected the P. oceanica leaf necrosis and that MHWs occurrence changed the morphology of the plant by lowering the leaf area and increasing leaf necrosis. Interesting results were also found relating the past thermal environment, since rhizome biomass has been negatively affected by the number of MHWs and rhizome length by the temperature range. This research provides fundamental insights about P. oceanica changes linked to warming and MHWs, identifying the potential plant indicators assisting future experimental and modelling studies. To the best of our knowledge, this is the first time a reconstruction technique is used to evaluate the effects of past MHWs on the morphology and productivity of a plant species.

Decline of seagrass (Posidonia oceanica) production over two decades in the face of warming of the Eastern Mediterranean Sea

“…Nonetheless, our dataset encompassing longer, and larger scale variability showed that populations living close to the warm edge of their distribution are indeed impacted by the increase in annual SST and SST aug . This hypothesis is further supported by a previous study showing a negative relationship between P. oceanica production and the number and duration of MHWs per year, and a decrease in phenological traits with increasing summer temperature across a longitudinal gradient (from Italy to Greece and Cyprus; Stipcich et al ., 2022a), suggesting that eastern populations can be also affected by the fast warming at this part of the basin.…”

“…A deeper understanding of the impact of warming on P. oceanica meadows extending along the Greek Seas is crucial, given that their extent represents 71% of the total extent estimated for the P. oceanica meadows of the Eastern Mediterranean Sea (Pergent‐Martini et al ., 2021). However, the lack of historical baselines of seagrass dynamics in Greece (Gerakaris et al ., 2021) and the scarce and rather localized information on the potential response of these meadows to warming (Bennett et al ., 2022b; Stipcich et al ., 2022a,b) limits our comprehension on the impact of warming in this region and, thus, our capacity to design specialized mitigation measures.…”

“…The iconic endemic seagrass of the Mediterranean Sea, Posidonia oceanica (L.) Delile, is particularly vulnerable to thermal stress, showing extensive damage (Ontoria et al ., 2019a; Pansini et al ., 2021; Stipcich et al ., 2022a) or even massive mortality (Díaz‐Almela et al ., 2009; Marbà & Duarte, 2010). The Mediterranean Sea is warming on average 20% faster than the global ocean (Lionello & Scarascia, 2018), with a cumulative increase in mean SST of 1.3°C over the last 40 yr (Pastor et al ., 2020), and an increase of 0.056°C yr −1 in maximum (i.e.…”

“…In addition, over the past decades, MHWs have become increasingly common, and a fourfold increase in their intensity is projected by the end of this century for the basin (Darmaraki et al ., 2019). Nevertheless, information on the regression of P. oceanica due to warming is strongly biased towards the Western Mediterranean subregion, while very limited data exists for the Eastern Mediterranean (Bennett et al ., 2022a; Stipcich et al ., 2022a), where warming is particularly intense, likely underestimating the magnitude of impact across the Mediterranean Sea. The increase in SST has been sharper in the eastern basin with a rate of 0.048°C yr −1 compared with 0.036°C yr −1 in the western part (Pisano et al ., 2020), which together with the forecasted intensity and frequency of MHWs in the former (Soto‐Navarro et al ., 2020; Garrabou et al ., 2022) poses relatively higher risks to marine habitats in the eastern basin.…”

Signs of local adaptation by genetic selection and isolation promoted by extreme temperature and salinity in the Mediterranean seagrass Posidonia oceanica

“…light (Dattolo et al, 2017), water quality (Maxwell et al, 2014), nutrients (Pazzaglia et al, 2020), salinity (Tomasello et al, 2009), warming (Marín‐Guirao et al, 2018), among others] and over a wide range of spatial scales [e.g. between sites of the same region (Maxwell et al, 2014), between regions (Tuya et al, 2019), along with depth gradients (Dattolo et al, 2017), latitudinal gradients (Jahnke et al, 2019; Ruocco et al, 2022) and between seas (Nguyen et al, 2020; Pansini et al, 2021; Stipcich et al, 2022)]. It is important to note that conclusions on local adaptation on seagrasses have been derived not only from population genetic data, but also from the comparison of phenotypic responses to environmental stressors among populations.…”