The negative effects of short-term extreme thermal events on the seagrass Posidonia oceanica are exacerbated by ammonium additions

Marine heatwaves recurrence aggravates thermal stress in the surfgrass Phyllospadix scouleri

The surfgrass Phyllospadix scouleri grows in highly productive meadows along the Pacific coast of North America. This region has experienced increasingly severe marine heatwaves (MHWs) in recent years. Our study evaluated the impact of consecutive MHWs, simulated in mesocosms, on essential ecophysiological features of P. scouleri. Overall, our findings show that the plants' overall physiological status has been progressively declining. Interestingly, the indicators of physiological stress in photosynthesis only showed up once the initial heat exposure stopped (i.e., during the recovery period). The warming caused increased oxidative damage and a decrease in nitrate uptake rates. However, the levels of non-structural carbohydrates and relative growth rates were not affected. Our findings emphasize the significance of incorporating recovery periods in this type of study as they expose delayed stress responses. Furthermore, experiencing consecutive intense MHWs can harm surfgrasses over time, compromising the health of their meadows and the services they offer to the ecosystem.

Eelgrass (Zostera marina L.) populations are threatened by high sea-surface temperatures and impaired waters on Nantucket Island, USA

Eelgrass (Zostera marina L.) is a key foundation species that provides multiple ecosystem services to shallow coastal and estuarine systems in the Northern Hemisphere. It is estimated that, over the last century, up to 50 % of all Z. marina habitat has been lost along the east coast of the USA due to factors including light reduction, eutrophication, and physical disturbance. Warming sea surface temperatures are also believed to be exacerbating losses and the future of this ecosystem is unclear. Here, we assess Z. marina meadows on Nantucket, an island system located 50 km off-shore of Massachusetts, by using common indicators of seagrass plant health and environmental quality. Our results show that Z. marina meadows on Nantucket Island are thermally stressed and light-limited during parts of their peak growing season. This suggests that sea-surface temperatures are a pivotal factor, along with cultural eutrophication, in observed large-scale losses of Z. marina and that further degradation could be expected in the future as the climate continues to warm. Methods from this study may be used by managers as a guide to assess seagrass ecosystem status in degrading systems.

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

The response of Posidonia oceanica meadows to global warming of the Eastern Mediterranean Sea, where the increase in sea surface temperature (SST) is particularly severe, is poorly investigated. Here, we reconstructed the long-term P. oceanica production in 60 meadows along the Greek Seas over two decades (1997-2018), using lepidochronology. We determined the effect of warming on production by reconstructing the annual and maximum (i.e. August) SST, considering the role of other production drivers related to water quality (i.e. Chla, suspended particulate matter, Secchi depth). Grand mean (±SE) production across all sites and the study period was 48 ± 1.1 mg DW per shoot yr-1 . Production over the last two decades followed a trajectory of decrease, which was related to the concurrent increase in annual SST and SSTaug . Annual SST > 20°C and SSTaug  > 26.5°C was related to production decline (GAMM, P < 0.05), while the rest of the tested factors did not help explain the production pattern. Our results indicate a persistent and increasing threat for Eastern Mediterranean meadows, drawing attention to management authorities, highlighting the necessity of reducing local impacts to enhance the resilience of seagrass meadows to global change threats.

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.

Experimental thermocline deepening highlights the resilience of the seagrass Posidonia oceanica: An opportunity to investigate shoot adaptability

The deepening of the thermocline, correlated to the rising temperature, can contribute affecting seagrass performance in a changing climate scenario. Here, the effect of the thermocline deepening on the seagrass Posidonia oceanica has been investigated in Cyprus through a manipulative experiment that allowed also testing the effects of the irradiance, origin depth and translocation. P. oceanica shoots were collected from 31 m of depth and transplanted at 12 m under a shading net, simulating the 31 m light conditions. Morphology (i.e. leaf area, leaf necrosis, number of leaves) and physiology (i.e. growth rate) were evaluated. Thermocline and origin depth effects were found with an increase of leaf necrosis, while a translocation effect was highlighted by a decrease in leaf area. No differences in shoot growth rate due to treatments were found. This experiment indicated an overall wide morphological and physiological acclimation of P. oceanica cuttings in coping with future thermocline conditions and it indirectly provides information for restoration efforts.

A Critical Gap in Seagrass Protection: Impact of Anthropogenic Off-Shore Nutrient Discharges on Deep Posidonia oceanica Meadows

In the Mediterranean, anthropogenic pressures (specifically those involving nutrient loads) have been progressively moved to deeper off-shore areas to meet current policies dealing with the protection of marine biodiversity (e.g., European Directives). However, conservation efforts devoted to protecting Posidonia oceanica and other vulnerable marine habitats against anthropogenic pressures have dedicated very little attention to the deepest areas of these habitats. We studied the remote influence of off-shore nutrient discharge on the physiology and structure of deep P. oceanica meadows located nearest to an urban sewage outfall (WW; 1 km) and an aquaculture facility (FF; 2.5 km). Light reduction and elevated external nutrient availability (as indicated by high δ¹⁵N, total N and P content and N uptake rates of seagrass tissues) were consistent with physiological responses to light and nutrient stress. This was particularly evident in the sites located up to 2.5 km from the WW source, where carbon budget imbalances and structural alterations were more evident. These results provide evidence that anthropogenic nutrient inputs can surpass critical thresholds for the species, even in off-shore waters at distances within the km scale. Therefore, the critical distances between this priority habitat and nutrient discharge points have been underestimated and should be corrected to achieve a good conservation status.

Limited trait responses of a tropical seagrass to the combination of increasing pCO2 and warming

Understanding species-specific trait responses under future global change scenarios is of importance for conservation efforts and to make informed decisions within management projects. The combined and single effects of seawater acidification and warmer average temperature were investigated by means of the trait responses of Cymodocea serrulata, a tropical seagrass, under experimental conditions. After a 35 d exposure period, biochemical, morphological, and photo-physiological trait responses were measured. Overall, biochemical traits mildly responded under the individual exposure to high temperature and increasing pCO2 values. The response of C. serrulata was limited to a decrease in %C and an increase in the sucrose content in the rhizome under the high temperature treatment, 32 °C. This suggests that this temperature was lower than the maximum tolerance limit for this species. Increasing pCO2 levels increased %C in the rhizome, and also showed a significant increase in leaf δ13C values. The effects of all treatments were sublethal; however, small changes in their traits could affect the ecosystem services they provide. In particular, changes in tissue carbon concentrations may affect carbon storage capacity, one key ecosystem service. The simultaneous study of different types of trait responses contributes to establish a holistic framework of seagrass ecosystem health under climate change.

Sensitivity of Photosynthesis to Warming in Two Similar Species of the Aquatic Angiosperm Ruppia from Tropical and Temperate Habitats

Climate change-related events, such as marine heatwaves, are increasing seawater temperatures, thereby putting pressure on marine biota. The cosmopolitan distribution and significant contribution to marine primary production by the genus Ruppia makes them interesting organisms to study thermal tolerance and local adaptation. In this study, we investigated the photosynthetic responses in Ruppia to the predicted future warming in two contrasting bioregions, temperate Sweden and tropical Thailand. Through DNA barcoding, specimens were determined to Ruppia cirrhosa for Sweden and Ruppia maritima for Thailand. Photosynthetic responses were assessed using pulse amplitude-modulated fluorometry, firstly in short time incubations at 18, 23, 28, and 33 °C in the Swedish set-up and 28, 33, 38, and 43 °C in the Thai set-up. Subsequent experiments were conducted to compare the short time effects to longer, five-day incubations in 28 °C for Swedish plants and 40 °C for Thai plants. Swedish R. cirrhosa displayed minor response, while Thai R. maritima was more sensitive to both direct and prolonged temperature stress with a drastic decrease in the photosynthetic parameters leading to mortality. The results indicate that in predicted warming scenarios, Swedish R. cirrhosa may sustain an efficient photosynthesis and potentially outcompete more heat-sensitive species. However, populations of the similar R. maritima in tropical environments may suffer a decline as their productivity will be highly reduced.

Seagrasses in an era of ocean warming: a review

Seagrasses are valuable sources of food and habitat for marine life and are one of Earth's most efficient carbon sinks. However, they are facing a global decline due to ocean warming and eutrophication. In the last decade, with the advent of new technology and molecular advances, there has been a dramatic increase in the number of studies focusing on the effects of ocean warming on seagrasses. Here, we provide a comprehensive review of the future of seagrasses in an era of ocean warming. We have gathered information from published studies to identify potential commonalities in the effects of warming and the responses of seagrasses across four distinct levels: molecular, biochemical/physiological, morphological/population, and ecosystem/planetary. To date, we know that although warming strongly affects seagrasses at all four levels, seagrass responses diverge amongst species, populations, and over depths. Furthermore, warming alters seagrass distribution causing massive die-offs in some seagrass populations, whilst also causing tropicalization and migration of temperate species. In this review, we evaluate the combined effects of ocean warming with other environmental stressors and emphasize the need for multiple-stressor studies to provide a deeper understanding of seagrass resilience. We conclude by discussing the most significant knowledge gaps and future directions for seagrass research.

Warming intensifies the interaction between the temperate seagrass Posidonia oceanica and its dominant fish herbivore Sarpa salpa

Apart from directly influencing individual life histories of species, climate change is altering key biotic interactions as well, causing community processes to unravel. With rising temperatures, disruptions to producer-consumer relationships can have major knock-on effects, particularly when the producer is a habitat-forming species. We studied how sea surface temperature (SST) modifies multiple pathways influencing the interaction between the foundational seagrass species, Posidonia oceanica, and its main consumer, the fish Sarpa salpa in the Mediterranean Sea. We used a combination of a field-based temperature gradient approaches and experimental manipulations to assess the effect of temperature on seagrass performance (growth) and fish early life history (larval development) as well as on the interaction itself (seagrass palatability and fish foraging activity). Within the range of temperatures assessed, S. salpa larvae grew slightly faster at warmer conditions but maintained their settlement size, resulting in a relatively small reduction in pelagic larval duration (PLD) and potentially reducing dispersion. Under warmer conditions (>24 °C), P. oceanica reduced its growth rate considerably and seemed to display fewer deterring mechanisms as indicated by a disproportionate consumption in choice experiments. However, our field-based observations along the temperature gradient showed no change in fish foraging time, or in other aspects of feeding behaviour. As oceans warm, our results indicate that, while S. salpa may show little change in early life history, its preference towards P. oceanica might increase, which, together with reduced seagrass growth, could considerably intensify the strength of herbivory. It is unclear if P. oceanica meadows can sustain such an intensification, but it will clearly add to the raft of pressures this threatened ecosystem already faces from global and local environmental change.

Positive effects of high salinity can buffer the negative effects of experimental warming on functional traits of the seagrass Halophila ovalis

Coastal ecosystems, and especially estuaries, are subject to environmental fluctuations that can be amplified by anthropogenic changes. Under a future scenario of global warming, temperature and salinity are likely to be altered and the persistence of macrophyte-dominated ecosystems can be compromised, particularly native or local seagrass communities. This study examined the response of the local seagrass Halophila ovalis to the joint effect of a short-term salinity increase and a transient temperature stress, through two mesocosm experiments. Warming caused a decline in F_v/F_m, TNC content in leaves and plant growth, and increased dark respiration, revealing clear detrimental symptoms of heat stress on plant metabolism and performance. Salinity increase in isolation favoured ramet survival. However, in combination with warming, salinity had a positive effect on Gross P_max. This suggests that increased salinities might dampen the negative effects of high temperatures, buffering, to some extent, the impact of global warming in temperate estuaries.

Too hot to handle: Unprecedented seagrass death driven by marine heatwave in a World Heritage Area

The increased occurrence of extreme climate events, such as marine heatwaves (MHWs), has resulted in substantial ecological impacts worldwide. To date, metrics of thermal stress within marine systems have focussed on coral communities, and less is known about measuring stress relevant to other primary producers, such as seagrasses. An extreme MHW occurred across the Western Australian coastline in the austral summer of 2010-2011, exposing marine communities to summer seawater temperatures 2-5°C warmer than average. Using a combination of satellite imagery and in situ assessments, we provide detailed maps of seagrass coverage across the entire Shark Bay World Heritage Area (ca. 13,000 km² ) before (2002 and 2010) and after the MHW (2014 and 2016). Our temporal analysis of these maps documents the single largest loss in dense seagrass extent globally (1,310 km² ) following an acute disturbance. Total change in seagrass extent was spatially heterogeneous, with the most extensive declines occurring in the Western Gulf, Wooramel Bank and Faure Sill. Spatial variation in seagrass loss was best explained by a model that included an interaction between two heat stress metrics, the most substantial loss occurring when degree heating weeks (DHWm) was ≥10 and the number of days exposed to extreme sea surface temperature during the MHW (DaysOver) was ≥94. Ground truthing at 622 points indicated that change in seagrass cover was predominantly due to loss of Amphibolis antarctica rather than Posidonia australis, the other prominent seagrass at Shark Bay. As seawater temperatures continue to rise and the incidence of MHWs increase globally, this work will provide a basis for identifying areas of meadow degradation, or stability and recovery, and potential areas of resilience.

Species-Specific Trait Responses of Three Tropical Seagrasses to Multiple Stressors: The Case of Increasing Temperature and Nutrient Enrichment

Seagrass meadows are declining globally. The decrease of seagrass area is influenced by the simultaneous occurrence of many factors at the local and global scale, including nutrient enrichment and climate change. This study aims to find out how increasing temperature and nutrient enrichment affect the morphological, biochemical and physiological responses of three coexisting tropical species, Thalassia hemprichii, Cymodocea serrulata and Halophila stipulacea. To achieve these aims, a 1-month experiment under laboratory conditions combining two temperature (maximum ambient temperature and current average temperature) and two nutrient (high and low N and P concentrations) treatments was conducted. The results showed that the seagrasses were differentially affected by all treatments depending on their life-history strategies. Under higher temperature treatments, C. serrulata showed photo-acclimation strategies, while T. hemprichii showed decreased photo-physiological performance. In contrast, T. hemprichii was resistant to nutrient over-enrichment, showing enhanced nutrient content and physiological changes, but C. serrulata suffered BG nutrient loss. The limited response of H. stipulacea to nutrient enrichment or high temperature suggests that this seagrass is a tolerant species that may have a dormancy state with lower photosynthetic performance and smaller-size individuals. Interaction between both factors was limited and generally showed antagonistic effects only on morphological and biochemical traits, but not on physiological traits. These results highlight the different effects and strategies co-inhabiting seagrasses have in response to environmental changes, showing winners and losers of a climate change scenario that may eventually cause biodiversity loss. Trait responses to these stressors could potentially make the seagrasses weaker to cope with following events, due to BG biomass or nutrient loss. This is of importance as biodiversity loss in tropical seagrass ecosystems could change the overall effectiveness of ecosystem functions and services provided by the seagrass meadows.