Effects of Current and Future Summer Marine Heat Waves on Posidonia oceanica: Plant Origin Matters?

Abstract

Marine heat waves (MHWs), prolonged discrete anomalously warm water events, have been increasing significantly in duration, intensity and frequency all over the world, and have been associated with a variety of impacts including alteration of ecosystem structure and function. This study assessed the effects of current and future MHWs on the Mediterranean seagrass Posidonia oceanica performance, also testing the importance of the thermal environment where the plant lives. The effects of current MHWs were studied through a mensurative experiment in a cold and in a warm site (West and North-West Sardinia, Italy, respectively). Future MHWs effects were tested through a manipulative experiment using P. oceanica shoots collected from the cold and warm sites and transplanted in a common garden in front of a power plant (North-West Sardinia): here plants were exposed to heat longer in duration and stronger in intensity than the natural MHWs of the last 20 years, resembling the future scenario. Morphological (total # of leaves, maximum leaf length, and percentage of total necrotic leaf length per shoot) and biochemical variables (leaf proteins, carbohydrates, and lipids) were considered. Plants had similar sublethal responses in both the experiments for most of the variables, revealing that current and future MHWs had similar effect types, but different in magnitude depending on the intensity of the waves: in general, the number of leaves, the maximum leaf length and lipid content decreased, while the leaf necrosis and carbohydrates increased. However, also the origin of the plants affected the results, corroborating the hypothesis that the thermal context the plants live affects their tolerance to the heat. Overall, this study provided evidence about the importance of biochemical variations, such as carbohydrate and lipid levels, as potentially good indicators of seagrass heat stress.

Effects of Field Simulated Marine Heatwaves on Sedimentary Organic Matter Quantity, Biochemical Composition, and Degradation Rates

“…3 m of depth) were similar at the three study sites, with a clear increase in T 1 (after 3 weeks from the initial plume injection) and a partial recovery in T 2 (after 11 weeks; Figure S1A). The simulated MHWs resembled either current or future scenarios according to Hobday et al [3]: the magnitude of the two T anomalies corresponded to conditions observed in the study area during the 2000-2009 decade (+1.4-1.8 • C) and those expected in the future under the worst prediction scenarios (+4.5-5.1 • C) [29] (Figure S1B).…”

“…This creates a marked seawater temperature gradient in the vicinity of the discharge point which is conceivably moderated by season and weather conditions. The historical occurrence, duration, and intensity of MHWs occurring in the last 20 years in the study area have been characterized previously [29]. To run our experiment, we profited from ca.…”

“…Most of the knowledge accumulated regarding the effects of heatwaves on marine ecosystems derived from studies that assessed mass mortality, abundance reduction and changes in community structures of macro-and mega-benthos populations in situ [13,[18][19][20][21][22][23][24]. Other studies carried out manipulative experiments on different marine communities and organisms to examine their response to warming [25][26][27][28][29]. Nevertheless, little research effort has been made to understand how temperature rise associated with MHWs could affect the nutrient and C cycling in coastal sediments [30,31], and to date, little, if any, attention has been given to the effects of MHWs on organic matter gross biochemical composition and degradation rates in coastal sediments.…”

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

“…This indication converges with evidence gained from other seagrass species (Pagès et al, 2010;Llagostera et al, 2016), whereby leaf necrosis is considered as an indicator of irreversible heat-induced damage of the leaf tissue (Beca-Carretero et al, 2020). However, further research is necessary to question to what extent the necrosis should be seen as a sign of leaf senescence or a manifestation of the activation of a process that could improve the survival of the plant, as a higher percentage of necrosis was found after the MHWs in P. oceanica shoots from warmer sites compared to those from colder sites (Stipcich et al, 2022). Overall, shoot density of P. oceanica meadows at the study sites was found to influence morphological traits.…”

“…An important shoot mortality of P. oceanica was estimated in meadows during [2002][2003][2004][2005][2006] where two intense MHWs occurred in Spain (Marbà and Duarte, 2010). Depending on the temperature intensity and heat duration, MHWs can also have sublethal morphological effects on both P. oceanica cuttings (Stipcich et al, 2022) and seedlings (Guerrero-Meseguer et al, 2017), by inducing leaf loss, reducing leaf growth, and triggering leaf necrosis. Seagrasses may be highly susceptible to sudden warming events (Chefaoui et al, 2018) and a functional extinction of P. oceanica by the middle of this century has been hypothesized (Jordà et al, 2012).…”