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Seagrass meadows provide valuable ecosystem services of coastal protection and chemical habitat formation that could help mitigate the impact of sea level rise and ocean acidification. However, the intensification of hydrodynamic forces caused by sea level rise, in addition to habitat degradation threaten the provision of these ecosystem services. With quantitative field measurements of the coastal protection and chemical habitat formation services of seagrass meadows, we statistically model the relationships between hydrodynamic forces, vegetation density and the provision of these ecosystem services. Utilising a high-resolution hydrodynamic model that simulates end of the century hydrodynamic conditions and three scenarios of coral reef degradation (i.e., keep up, remain or loss) we quantify how the environmental conditions within a tropical bay will change given changes to the provision of ecosystem services. Our study shows that increasing hydrodynamic forces lead to a seafloor made up of a larger grain size that is increasingly unstable and more vulnerable to erosion. The loss of a fringing reef leads to larger hydrodynamic forces entering the bay, however, the 0.87 m increase in depth due to sea-level rise reduces the bed shear stress in shallower areas, which limits the change in the ecosystem services provided by the current benthic seagrass meadow. Loss of seagrass constitutes the greatest change in a bay ecosystem, resulting in the sediment surface where seagrass existed becoming unstable and the median sediment grain size increasing by 5-7 %. The loss of seagrass also leads to the disappearance of the unique fluctuating chemical habitat, which leaves the surrounding community vulnerable to ocean acidification. A reduction or complete loss of these ecosystem services would impact the entire community assemblage while also leaving the surrounding coastline vulnerable to erosion, thus exacerbating negative effects brought about by climate change.

Vegetated coastal ecosystems such as saltmarshes, mangroves and seagrass beds are increasingly promoted as sustainable storm and flood defence solutions by reducing wave energy. Yet, there is still intense debate on the ability of root mats to mitigate erosion, with some studies arguing that the direct contribution of roots in preventing sediment erosion is minor, while others consider them of major importance. Here, we hypothesized that the contrasting findings on the role of seagrass root mats in preventing erosion may stem from differences in sediment type. To test this idea, we investigated how root mats of seagrass that thrives in both sandy and muddy sediments mitigate wave-induced cliff erosion using Zostera marina in manipulative flume experiments. Results demonstrate that roots are very effective in reducing cliff erosion rates in sandy sediments. Cliff erosion rates were reduced up to 70% in sandy sediment with high seagrass root biomass. In contrast, cliff erosion rates in cohesive muddy sediments were low and unaffected by seagrass roots. This highlights the important role of seagrass roots in erosion mitigation, which has been overlooked compared to the role of canopies, which has received more attention. We suggest that management strategies should be developed to enhance the stabilization of sandy sediment, such as (1) using species with high belowground biomass, (2) using fast-growing pioneer species and (3) applying temporary stabilising measures.

Seagrass meadows are vital ecosystems in coastal zones worldwide, but are also under global threat. One of themajor hurdles restricting the success of seagrass conservation and restoration is our limited understanding of ecologicalfeedback mechanisms. In these ecosystems, multiple, self-reinforcing feedbacks can undermine conservation efforts bymasking environmental impacts until the decline is precipitous, or alternatively they can inhibit seagrass recovery inspite of restoration efforts. However, no clear framework yet exists for identifying or dealing with feedbacks to improvethe management of seagrass ecosystems. Here we review the causes and consequences of multiple feedbacks betweenseagrass and biotic and/or abiotic processes. We demonstrate how feedbacks have the potential to impose or reinforceregimes of either seagrass dominance or unvegetated substrate, and how the strength and importance of these feedbacksvary across environmental gradients. Although a myriad of feedbacks have now been identified, the co-occurrence andlikely interaction among feedbacks has largely been overlooked to date due to difficulties in analysis and detection.Here we take a fundamental step forward by modelling the interactions among two distinct above- and belowgroundfeedbacks to demonstrate that interacting feedbacks are likely to be important for ecosystem resilience. On this basis, wepropose a five-step adaptive management plan to address feedback dynamics for effective conservation and restorationstrategies. The management plan provides guidance to aid in the identification and prioritisation of likely feedbacks indifferent seagrass ecosystems

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1. Populations of marine megaherbivores including green turtle (Chelonia mydas) have declined dramatically at a global scale as a result of overharvesting and habitat loss. This decline can be expected to also affect the tolerance of seagrass systems to coastal eutrophication. Until now, however, simultaneous effects of topdown control by megaherbivore grazing and bottomup control by nutrient input have not been tested experimentally.2. We therefore investigated the interacting effects of nutrient (N and P) addition and mimicked green turtle grazing on seagrass and epiphyte productivity, seagrass biomass and nutrient contents in exclosures at a pristine seagrass site in the Indo-Pacific region (Kalimantan, Indonesia).3. Grazing almost doubled leaf biomass production rates, while nutrient addition (N+P, slow-release granules) did not have an effect on these rates. Rhizome biomass was, however, strongly reduced by nutrient addition. In contrast to phosphorus, tissue nitrogen contents increased after nutrient addition, showing that nitrogen was not limiting primary productivity. Epiphyte growth was, however, strongly correlated with high water column P concentrations, indicating an indirect negative effect of eutrophication when turtle grazing would be absent. We calculated that green turtle leaf grazing leads to substantial exports of N and P, at rates of at least 8% of the standing stock per day equalling the daily seagrass production, up to 13 (N) and 1.4 (P) mg m(-2) day(-1).4. Synthesis. By combining our quantified effects with literature data, we propose a conceptual model of seagrass functioning under megaherbivore leaf grazing and eutrophication. In tropical seagrass systems with high green turtle grazing pressure, grazing alleviates the negative effects of eutrophication by the stimulation of seagrass production and concomitant nutrient uptake, the increased export of nutrients and the indirect prevention of low below-ground biomass. Similar to the role of terrestrial megaherbivores, these strong top-down controls show the pivotal role of green turtles in current coastal systems, which is lacking in systems where their numbers have greatly declined. These marine megaherbivores do not only drive structure and functioning of their foraging grounds but also increase the tolerance of seagrass ecosystems to eutrophication.

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Contains fulltext : 35149.pdf (Publisher’s version ) (Open Access) 148 p.

Contains fulltext : 201202.pdf (Publisher’s version ) (Open Access) Radboud University, 01 maart 2019 Promotores : Herman, P.M.J., Hendriks, A.J., Bouma, T.J Co-promotor : Katwijk, M.M. van 192 p.

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

Contains fulltext : 155841.pdf (Publisher’s version ) (Open Access) Radboud Universiteit Nijmegen, 08 maart 2016 Promotor : Herman, P.M.J. Co-promotor : Bouma, T.J. 142 p.

Contains fulltext : 135165.pdf (Publisher’s version ) (Open Access) Radboud Universiteit Nijmegen, 12 december 2014 Promotor : Herman, P.M.J. Co-promotores : Bouma, T.J., Katwijk, M.M. van 237 p.

Contains fulltext : 131904.pdf (Publisher’s version ) (Open Access) Radboud Universiteit Nijmegen, 04 september 2014 Promotores : Herman, P.M.J., Ziegler, A.D. Co-promotor : Bouma, T.J. 169 p.

Contains fulltext : 101061.pdf (Publisher’s version ) (Open Access) Radboud Universiteit Nijmegen, 22 maart 2013 Promotores : Herman, P.M.J., Roelofs, J.G.M. Co-promotor : Bouma, T.J. 138 p.

Contains fulltext : 113022.pdf (Publisher’s version ) (Open Access) Radboud Universiteit Nijmegen, 18 december 2013 Promotores : Herman, P.M.J., Webb, E.L. Co-promotor : Bouma, T.J. 176 p.