Despite being highly valuable ecosystems, seagrass meadows are threatened worldwide, mostly by human activities. In order to preserve seagrass meadows from collapse, we need to better understand their resilience in a changing environment. By means of various manipulative field experiment in temperate systems, the present thesis addressed seagrass resilience by looking at (i) indicators of resilience as needed for monitoring; (ii) strategies and mechanisms of resilience allowing seagrasses to resist stresses and to recover from disturbances; and (iii) the influence of climatic –latitudinal –gradients as drivers of seagrass traits, resilience and indicators.Indicators of resilienceIndicators are needed in order to monitor seagrass ecosystems and to predict nearness to collapse. In this thesis, we raised the question on the proper interpretation of indicators to estimate seagrass health and resilience. We compared the response of two indicators: i) vegetation cover, which is a traditionally used indicator to asses the seagrass health status; and ii)critical slowing downwhich is a theoretically suggested indicator for seagrass health status in terms of resilience to disturbances, expressed asrelative recovery from disturbance. The two indicators showed an opposite response to disturbance: the higher the cover, the lower the relative recovery from disturbance (Chapter 2). This was however only observed during the seagrass growing phase (Chapter 3). Indeed, we noticed that the timing of a disturbance relative to the seagrass seasonal growth period, were crucial for seagrass resilience, with highest recovery at the start of the growing season, and a decreasing recovery with higher cover (Chapter 3). This conflicting response between indicators was observed independently from wave exposure or nutrient status (i.e. same response even under eutrophication stress). These results do emphasis the need to carefully consider timing of monitoring of the indicators, as it forms a fundamental factor to evaluate indicators in terms of resilience. These findings are likely to extent beyond seagrass, also to other temperate seasonal ecosystems.CHAPTER 9176Strategies/mechanisms for resilienceSeagrasses have the capacity to adapt to their environment by changing their morphological, physiological and mechanical traits. They moreover present different strategies, related to their growth rate (Grime, 1977), resource allocation or ecosystem engineering that make them more or less resilient to stresses and disturbances and preserve them from collapse.In this thesis, we observed that the plant’s responses to stress and disturbances deviated from the response predicted by Grime’s growth strategies, under the influence of ecosystem engineering (Chapter 4). That is, the fast-growing seagrass species was not only a better recoloniser after disturbance as compared to a slower-growing species, but also more resistant to sulphide invasion. This higher resistance was explained by its strong capacity to reduce the stress through ecosystem engineering (i.e. release of oxygen through their root system). Ecosystem engineering hence forms an essential strategy to cope with stressful and disturbed environment by making some seagrass species more resilient. We speculate that this effect depends however on the nature of the ecosystem engineering (i.e. based on physiological or structural traits) as well as the nature of the environment (i.e. stimulating growth or stimulating the formationof physically strong tissues).Another strategy for resilience evidenced in this thesis was related to their resource allocation and more specifically the use of their carbon reserves for resilience. Being photosynthetic organisms, seagrasses can store carbon reserves in their rhizomes during their seasonal growth. These reserves are stored in the form of non-structural carbohydrates and mostly used for growth or to survive the winter period. We showed that, when experiencing short-term stress events, seagrasses use their carbohydrate reserves to recover and regrow, particularly the fast-growing species (Chapter 4 and 5). Overall, we saw that the use of carbohydrate reserves as a resilience mechanism increased depending on the species (i.e. fast growth strategy), the environmental conditions (i.e. exposure to hydrodynamics, nutrient status), the climatic settings (cold winter temperatures and low daily light doses) and the occurrence of short-term stress events during their seasonal growth. It can be speculated thaton the short-term, the use of carbon reserves for regrowth is beneficial for the plants. But on the long-term, the over-consumption of their reserves to recover from disturbance SUMMARY177might be enough to tip the system and lead to collapse, particularly when done at the end of the season (Chapter 5).Being sessile organisms, seagrasses have the capacity to modify their structure or traits to resist the adverse effect of biotic or abiotic stressors such as waves or currents. It is known that the mechanical resistance of seagrass leaves to physical stress depends on their species-specific traits (i.e. growth strategies). In this thesis we also showed that seagrasses could adapt their mechanical traits depending on their distribution along a seasonal or latitudinal gradient but also depending on their environment such as their nutrient status (Chapter 6). In eutrophic conditions, seagrasses presented more brittle leaves, easier to break but also more extensible than in more oligotrophic conditions. This plasticity and adaptation to their local environmental conditions forms hence another important strategy for resilience.The influence of climatic –latitudinal-gradientsSeagrasses can be found in temperate systems along a large gradient of environmental and climatic conditions, controlling the length of their seasonal growth and their population dynamics. With this thesis we demonstrated that these gradients could play an important driving role on their traits such as their reproductive success, carbon reserves, physiological and mechanical traits (Chapters 5 to 7). We also observed that the seasonal dimension of seagrass growth in temperate systems plays a major driving role for seagrass resilience, particularly the winter period (Chapter 5) and the peak of growth (Chapter 3). In fact, depending on their local conditions and distribution along a climatic –latitudinal –gradient, seagrasses present different traits, shaping their resilience to external stresses or disturbances. The northern-European populations may be considered to be in a perpetually colonizing phase with yearly recurrent population initiation by sexual propagules (seeds) (Chapter 7), making them genetically more diverse; and by asexual –clonal –extension of the dormant rhizomes relying on their carbon reserves (Chapter 5). In contrast, the southern, evergreen populations depend much more on clonal propagation, hence being productive all year due to suitable light and temperature conditions for photosynthesis; and relying less on their carbon reserves (Chapter 5) or seed production (Chapter 7), with mechanically stronger leaves (Chapter 6). CHAPTER 9178Conclusions and implicationsOverall, all the mechanisms involved in seagrass resilience as well as the influence of global trends have considerable consequences on the seagrass resilience and the stability of coastal ecosystems in the long-tem. Seagrasses can adapt to their changing environment, as observed along a climatic –latitudinal –gradient. But additional effects of short-term stresses such as e.g. nutrient enrichment inducing eutrophication, might push the system towards its bifurcation point. In that situation, even a small disturbance may induce a meadow to collapse, due to a reduced resistance to stress and recovery potential. The consequences of such collapse are large for the overall health of the coastal ecosystems. In general, a lowered resilience of seagrass meadows can affect their efficiency as ecosystem engineers and thus the positive feedbacks they induce. The maintenance of positive feedbacks for seagrass meadows is essential, as this affects their strategies for resilience (for instance, capacity to reduce sulphide stress through their root system) and their capacity to adapt to climatic gradients.Our findings bear implications for the management of seagrass meadows. In a context of climate change and increasing occurrence of stochastic events (i.e. storms, extreme weather) and human-induced stresses, managers, scientists and stakeholders should consider the importance of: when (seasonal growth), where (latitudinal –climatic gradient) and how (nature of the threat, resilience strategies) seagrass resilience might be affected. Considering these parameters is essential to better preserve temperate seagrass meadows from collapse and to maintainthe stability of their ecosystems |
