Climate change and the presence of invasive species will threaten the persistence of the Mediterranean seagrass community

The Mediterranean Sea has been experiencing rapid increases in temperature and salinity triggering its tropicalization. Additionally, its connection with the Red Sea has been favouring the establishment of non-native species. In this study, we investigated the effects of predicted climate change and the introduction of invasive seagrass species (Halophila stipulacea) on the native Mediterranean seagrass community (Posidonia oceanica and Cymodocea nodosa) by applying a novel ecological and spatial model with different configurations and parameter settings based on a Cellular Automata (CA). The proposed models use a discrete (stepwise) representation of space and time by executing deterministic and probabilistic rules that develop complex dynamic processes. Model applications were run under two climate scenarios (RCP 2.6 and RCP 8.5) projected from 2020 to 2100 in four different regions within the Mediterranean. Results indicate that the slow-growing P. oceanica will be highly vulnerable to climate change, suffering vast declines in its abundance. However, the results also show that western and colder areas of the Mediterranean Sea might represent refuge areas for this species. Cymodocea nodosa has been reported to exhibit resilience to predicted climate scenarios; however, it has shown habitat regression in the warmest predicted regions in the easternmost part of the basin. Our models indicate that H. stipulacea will thrive under projected climate scenarios, facilitating its spread across the basin. Also, H. stipulacea grew at the expense of C. nodosa, limiting the distribution of the latter, and eventually displacing this native species. Additionally, simulations demonstrated that areas from which P. oceanica meadows disappear would be partially covered by C. nodosa and H. stipulacea. These outcomes project that the Mediterranean seagrass community will experience a transition from long-lived, large and slow-growing species to small and fast-growing species as climate change progresses.

Ecotype-Specific and Correlated Seasonal Responses of Biomass Production, Non-Structural Carbohydrates, and Fatty Acids in Zostera marina

Seagrasses, which are marine flowering plants, provide numerous ecological services and goods. Zostera marina is the most widely distributed seagrass in temperate regions of the northern hemisphere, tolerant of a wide range of environmental conditions. This study aimed to (i) examine seasonal trends and correlations between key seagrass traits such as biomass production and biochemical composition, and (ii) compare seasonal adaptation of two ecotypes of Z. marina exposed to similar environmental conditions on the west coast of Ireland. During summer, plants accumulated higher levels of energetic compounds and levels of unsaturated fatty acids (FAs) decreased. Conversely, the opposite trend was observed during colder months. These findings indicate a positive seasonal correlation between the production of non-structural carbohydrates and saturated fatty acids (SFAs), suggesting that seagrasses accumulate and utilize both energetic compounds simultaneously during favorable and unfavorable environmental conditions. The two ecotypes displayed differential seasonal responses by adjusting plant morphology and production, the utilization of energetic reserves, and modulating unsaturation levels of fatty acids in seagrass leaves. These results underscore the correlated seasonal responses of key compounds, capturing ecotype-specific environmental adaptations and ecological strategies, emphasizing the robust utility of these traits as a valuable eco-physiological tool.

Sources of variability in seagrass fatty acid profiles and the need of identifying reliable warming descriptors

Global warming is expected to have inexorable and profound effects on marine ecosystems, particularly in foundation species such as seagrasses. Identifying responses to warming and comparing populations across natural temperature gradients can inform how future warming will impact the structure and function of ecosystems. Here, we investigated how thermal environment, intra-shoot and spatial variability modulate biochemical responses of the Mediterranean seagrass Posidonia oceanica. Through a space-for-time substitution study, Fatty acid (FA) profiles on the second and fifth leaf of the shoots were quantified at eight sites in Sardinia along a natural sea surface temperature (SST) summer gradient (about 4 °C). Higher mean SST were related to a decrease in the leaf total fatty acid content (LTFA), a reduction in polyunsaturated fatty acids (PUFA), omega-3/omega-6 PUFA and PUFA/saturated fatty acids (SFA) ratios and an increase in SFA, monounsaturated fatty acids and carbon elongation index (CEI, C18:2 n-6/C16:2 n-6) ratio. Results also revealed that FA profiles were strongly influenced by leaf age, independently of SST and spatial variability within sites. Overall, this study evidenced that the sensitive response of P. oceanica FA profiles to intra-shoot and spatial variability must not be overlooked when considering their response to temperature changes.

Superior growth traits of invaded (Caribbean) versus native (Red sea) populations of the seagrass Halophila stipulacea

The seagrass Halophila stipulacea is native to the Red Sea. It invaded the Mediterranean over the past century and most of the Caribbean over the last two decades. Understanding the main drivers behind the successful invasiveness of H. stipulacea has become crucial. We performed a comprehensive study including field measurements, a mesocosm experiment, and a literature review to identify 'superior growth traits' that can potentially explain the success story of H. stipulacea. We assessed meadow characteristics and plant traits of three invasive H. stipulacea populations growing off the Island of Sint Eustatius (eastern Caribbean). We compared similar parameters between native (Eilat, northern Red Sea) and invasive (Caribbean) H. stipulacea plants in a common-garden mesocosm. Lastly, we compared our field measurements with published data. The newly arrived H. stipulacea plants from St. Eustatius were characterized by higher percent cover, higher below- and above-ground biomasses, more apical shoots, and faster leaf turnover rates than those measured in both native and older invaded habitats. These results were further confirmed by the mesocosm experiment where the invasive H. stipulacea plants grew faster and developed more apical shoots than the native plants. Results suggest that increased growth vigour is one of the main invasive traits that characterize successful invasive H. stipulacea populations in the Caribbean and potentially in other invaded areas. We encourage long-term monitoring of H. stipulacea in both native and invaded habitats to better understand the future spread of this species and its impacts on communities and their ecosystem functions and services.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10530-023-03045-z.

Field development of Posidonia oceanica seedlings changes under predicted acidification conditions

Ocean acidification has been consistently evidenced to have profound and lasting impacts on marine species. Observations have shown seagrasses to be highly susceptible to future increased pCO₂ conditions, but the responses of early life stages as seedlings are poorly understood. This study aimed at evaluating how projected Mediterranean Sea acidification affects the survival, morphological and biochemical development of Posidonia oceanica seedlings through a long-term field experiment along a natural low pH gradient. Future ocean conditions seem to constrain the morphological development of seedlings. However, high pCO₂ exposures caused an initial increase in the degree of saturation of fatty acids in leaves and then improved the fatty acid adjustment increasing unsaturation levels in leaves (but not in seeds), suggesting a nutritional compound translocation. Results also suggested a P. oceanica structural components remodelling which may counteract the effects of ocean acidification but would not enhance seagrass seedling productivity.

Effects of high temperature and marine heat waves on seagrasses: Is warming affecting the nutritional value of Posidonia oceanica?

Primary producers nutritional content affects the entire food web. Here, changes in nutritional value associated with temperature rise and the occurrence of marine heat waves (MHWs) were explored in the endemic Mediterranean seagrass Posidonia oceanica. The variability of fatty acids (FAs) composition and carbon (C) and nitrogen (N) content were examined during summer 2021 from five Mediterranean sites located at the same latitude but under different thermal environments. The results highlighted a decrease in unsaturated FAs and C/N ratio and an increase of monounsaturated FA (MUFA) and N content when a MHW occurred. By contrast, the leaf biochemical composition seems to be adapted to local water temperature since only few significant changes in MUFA were found and N and C/N had an opposite pattern compared to when a MHW occurs. The projected increase in temperature and frequency of MHW suggest future changes in the nutritional value and palatability of leaves.

Field thermo acclimation increases the resilience of Posidonia oceanica seedlings to marine heat waves

Acclimation is a response that results from chronic exposure of an individual to a new environment. This study aimed to investigate whether the thermal environment affects the early development of the seagrass Posidonia oceanica, and whether the effects of a field-simulated Marine Heat Wave (MHW) on seedlings change depending on acclimation. The experiment was done in the field using a crossed design of Acclimation (acclimated vs unacclimated) and MHW (present vs absent) factors. Acclimation has initially constrained the development of P. oceanica seedlings, but then it increased their resilience to the MHW, under both a morphological and biochemical (fatty acid saturation) level. This treatment could be considered in P. oceanica restoration projects in a climate change-impaired sea, by purposely inducing an increased resistance to heat before transplants.

Teasing apart the host-related, nutrient-related and temperature-related effects shaping the phenology and microbiome of the tropical seagrass Halophila stipulacea

BACKGROUND

Halophila stipulacea seagrass meadows are an ecologically important and threatened component of the ecosystem in the Gulf of Aqaba. Recent studies have demonstrated correlated geographic patterns for leaf epiphytic community composition and leaf morphology, also coinciding with different levels of water turbidity and nutrient concentrations. Based on these observations, workers have suggested an environmental microbial fingerprint, which may reflect various environmental stress factors seagrasses have experienced, and may add a holobiont level of plasticity to seagrasses, assisting their acclimation to changing environments and through range expansion. However, it is difficult to tease apart environmental effects from host-diversity dependent effects, which have covaried in field studies, although this is required in order to establish that differences in microbial community compositions among sites are driven by environmental conditions rather than by features governed by the host.

RESULTS

In this study we carried out a mesocosm experiment, in which we studied the effects of warming and nutrient stress on the composition of epiphytic bacterial communities and on some phenological traits. We studied H. stipulacea collected from two different meadows in the Gulf of Aqaba, representing differences in the host and the environment alike. We found that the source site from which seagrasses were collected was the major factor governing seagrass phenology, although heat increased shoot mortality and nutrient loading delayed new shoot emergence. Bacterial diversity, however, mostly depended on the environmental conditions. The most prominent pattern was the increase in Rhodobacteraceae under nutrient stress without heat stress, along with an increase in Microtrichaceae. Together, the two taxa have the potential to maintain nitrate reduction followed by an anammox process, which can together buffer the increase in nutrient concentrations across the leaf surface.

CONCLUSIONS

Our results thus corroborate the existence of environmental microbial fingerprints, which are independent from the host diversity, and support the notion of a holobiont level plasticity, both important to understand and monitor H. stipulacea ecology under the changing climate.

Projected Rapid Habitat Expansion of Tropical Seagrass Species in the Mediterranean Sea as Climate Change Progresses

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.

Seagrass fatty acid profiles as a sensitive indicator of climate settings across seasons and latitudes

Zostera marina is a dominant meadow-forming seagrass in temperate regions in the northern hemisphere. Here, fatty acid content and composition, and pigmentation, in leaves were evaluated across temporal (April, July, November -2015 and January-2016) and latitudinal (Greenland to southern Spain) environmental gradients. Content of total fatty acids (TFA) in samples collected in Ireland during warmer periods (summer) was 2-3 times lower than in winter and exhibited a lower proportion of polyunsaturated fatty acids (PUFAs), which have high high-nutritional value relative to saturated fatty acids (SAFA). The latitudinal comparison (Greenland to southern Spain) revealed a clear reduction in the proportion n-3 PUFAs and an increase in n-6 PUFA and SAFA, which correlated with the rise in temperature towards southern locations, which correlated with the rise in temperature towards south. Results indicate that future warming may negatively affect its lipid nutritional value. These results demonstrate the capacity of seagrasses to adjust their lipid composition to achieve optimal membrane functionality, suggesting the potential use of FA as an eco-physiological indicator of global change conditions. The results also suggest that future warming may negatively affect the lipid nutritional value of seagrasses.

Halophila stipulacea descriptors in the native area (Red Sea): A baseline for future comparisons with native and non-native populations

Halophila stipulacea is a small tropical seagrass species native to the Red Sea. Due to its invasive character, there is growing interest in understanding its ability to thrive in a broad range of ecological niches. We studied temporal (February 2014 and July 2014), depth (5, 9, 18 m) and spatial (NB and SB) related dynamics of H. stipulacea meadows in the northern Gulf of Aqaba. We evaluated changes in density, morphometry, biomass, and biochemical parameters alongside the reproductive effort. In both sites, maximal growth and vegetative performance occurred in the summer with a marked increase of 35% in shoot density and 18% in biomass; PAR reduction with season and depth induced a significant increase of 28% in leaf area. Sexual reproduction efforts were only observed in July, and the density of plants carrying male or female flowers decreased significantly with depth. The favorable growth responses of H. stipulacea plants observed in the N-enriched NB site suggests their capacity to acclimate to human-disturbed nearshore environments.

Effects of an experimental heat wave on fatty acid composition in two Mediterranean seagrass species

Global warming is emerging as one of the most critical threats to terrestrial and marine species worldwide. This study assessed the effects of simulated warming events in culture on two seagrass species, Posidonia oceanica and Cymodocea nodosa, which play a key role in coastal ecosystems of the Mediterranean Sea. Changes in fatty acids as key metabolic indicators were assessed in specimens from two geographical populations of each species adapted to different in situ temperature regimes. Total fatty acid (TFA) content and composition were compared in C. nodosa and P. oceanica from natural populations and following exposure to heat stress in culture. After heat exposure, individuals of C. nodosa and P. oceanica adapted to colder temperatures in situ accumulated significantly more TFA than controls. For both species, the proportion of polyunsaturated fatty acids (PUFA) decreased, and the percentage of saturated fatty acids (SFA) increased significantly after the heat treatment. These results highlight that populations of both species living at warmest temperatures in situ were more thermo-tolerant and exhibited a greater capacity to cope with heat stress by readjusting their lipid composition faster. Finally, exposure of seagrasses to warmer conditions may induce a decrease in PUFA/SFA ratio which could negatively affect their nutritional value and generate important consequences in the healthy state of next trophic levels.

Seagrass ecosystem services - What's next?

Seagrasses, marine flowering plants, provide a wide range of ecosystem services, defined here as natural processes and components that directly or indirectly benefit human needs. Recent research has shown that there are still many gaps in our comprehension of seagrass ecosystem service provision. Furthermore, there seems to be little public knowledge of seagrasses in general and the benefits they provide. This begs the questions: how do we move forward with the information we have? What other information do we need and what actions do we need to take in order to improve the situation and appreciation for seagrass? Based on the outcomes from an international expert knowledge eliciting workshop, three key areas to advance seagrass ecosystem service research were identified: 1) Variability of ecosystem services within seagrass meadows and among different meadows; 2) Seagrass ecosystem services in relation to, and their connection with, other coastal habitats; and 3) Improvement in the communication of seagrass ecosystem services to the public. Here we present ways forward to advance seagrass ecosystem service research in order to raise the profile of seagrass globally, as a means to establish more effective conservation and restoration of these important coastal habitats around the world.