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Coastal systems often depend on foundation species such as seagrasses that are supported by self-facilitation. Seagrass meadows, however, are threatened worldwide due to climate change and local human impact, disrupting self-facilitation leading to system instability. Florida Bay is a large seagrass dominated coastal ecosystem that suffered from multiple seagrass mortality events over the last half century driven by hypoxia, high water temperatures, hypersalinity, and high biological oxygen demand. These conditions reduce the amount of photosynthetically-derived oxygen in the plant causing sulphide intrusion into meristematic tissues. Using a bay-wide sampling design and long-term monitoring trends of seagrass condition, we investigated the current state of the meadows, sediment characteristics (e.g., organic matter, sulphide, nutrients) and discuss how climate stressors interact with plant and sediment oxygen dynamics. Our survey revealed that at sites where seagrass had been previously denuded by die-off, the dominant seagrass Thalassia testudinum had not recovered, while the pioneering seagrass Halodule wrightii recolonized the impacted areas. Organic matter and sulphide levels were higher at the impacted sites, apparently a persistent characteristic of the formerly dense T. testudinum meadows in central and western Florida Bay. These sediment conditions promote sulphide intrusion of T. testudinum belowground tissue under anoxic conditions. Plant oxidation initially buffers sulphide intrusion, but disruption of this oxidation mechanism due to changing environmental conditions results in widespread mortality and seagrass community collapse. Climate change cannot be fully mitigated by local management, however, attempts can be made to control critical salinity and oxygen levels by increasing freshwater input, reducing hypersalinity and aiming to keep the internal seagrass oxidation feedback intact. Our study shows that the Florida Bay seagrass ecosystem is still recovering four years post die-off and continues to be susceptible to future climate change and system degradation. [ABSTRACT FROM AUTHOR]

Coastal vegetated ecosystems including mangroves, seagrasses, and salt marshes are often shaped by positive plant–environment feedbacks. Plants improve their own living conditions with increasing patch size and density by attenuating hydrodynamics and stabilizing sediments. As these habitat modifications are critical for survival and growth, the positive density‐dependent nature of these feedbacks can lead to establishment thresholds for young plants in absence of mature conspecifics. Although feedback strength is known to depend on hydrodynamic exposure and plant traits (e.g. stiff versus flexible stems), it remains unclear how 1) opposing morphological plant traits affect establishment in contrasting environments, and 2) whether trait plasticity influences establishment success. Here, we investigate this by transplanting two tidal species with opposing growing strategies – Spartina anglica forms tussocks of stiff stems while Zostera noltii forms patches of stress‐avoiding flexible shoots – from two different donor sites in eight experimental locations. Results show that the survival and growth of both species was most successful at field locations with diverging environmental characteristics, while overall survival was highest for Z. noltii. Mainly, S. anglica survival was highest at locations with high organic matter and silt content and higher elevation relative to the tidal amplitude. In contrast, Z. noltii survival was highest at locations with larger grainsize and lower relative elevations. Furthermore, despite initial differences in plant traits between the two donor sites of Z. noltii, we found no effects of donor origin. Contrastingly, we found a significant effect of donor origin on S. anglica growth, even though transplants from the two donor sites showed no initial trait differences. Collectively, these results suggest that the stress‐tolerance strategy of S. anglica hampers establishment in exposed conditions, whereas the stress‐avoiding Z. noltii appears to be more susceptible to stress from desiccation and silty sediments. [ABSTRACT FROM AUTHOR]

Coastal ecosystems globally face pressures, with natural coastal habitats being replaced by engineered structures. While hard structures for navigation‐purposes and coastal defense can negatively impact native communities, they can also be applied in ecological restoration as artificial reefs. This way substrates may facilitate establishment of biogenic (shellfish) reefs and provide habitat heterogeneity in soft‐sediment ecosystems. In a 1.5‐year experiment, we introduced six different types of natural or biodegradable hard substrates in a subtidal soft‐sediment system. We compared the substrates with surrounding soft sediment and evaluated differences among substrate types using biodiversity indices, community composition analyses, and food web indicators. This offers a comprehensive understanding of how the introduced hard substrates affect ecosystem complexity. Overall ecological complexity indicators were higher in the second year of the experiment. We found a significant increase in species richness, community composition, and link density (number of feeding interactions per species) on hard substrates compared to the surrounding sediment. However, Shannon diversity index and food web connectance (measure of food web saturation) did not differ between substrates and the surrounding sediment. Although differences among types of hard substrates were small, certain species were uniquely associated with specific substrates with wood and granite performing best. While there was no establishment of epibenthic shellfish reefs within 1.5 years, we conclude that introducing hard substrates effectively enhances ecological complexity in soft‐bottom coastal ecosystems within that timeframe. We recommend the use of substrates with a historical ecological relevance in the system for scaling up introduction of substrates for ecological restoration. [ABSTRACT FROM AUTHOR]

In soft‐bottom marine ecosystems, bedform variation is induced by wind‐ and tidal‐driven hydrodynamics. The resulting megaripples, sand waves and sandbanks form a spatially and temporally heterogeneous seafloor landscape. The strong physical forces imposed by the migration of these bedforms are important determinants for the occurrence of different macrozoobenthic species. Quantifying the effect of these forces can help in differentiating natural‐ and anthropogenically induced physical stressors. However, large‐scale mapping of seabed morphology at high resolution using multibeam echosounder is challenging, costly and time‐consuming, especially in shallow seas, prohibiting wide swaths. Instead, their bathymetry is typically studied using single‐beam transects that are interpolated to bathymetric grids with a relatively coarse resolution (20 m). However, this leaves out information on smaller scale (<20 m) bedforms that can be ecologically relevant. In the Dutch Wadden sea, a shallow tidal system, we characterized bedform variation at high resolution using single‐beam data for the first time. We calculated a 2‐D Terrain Ruggedness Index (TRI) at sub‐meter resolution along the single‐beam transects and interpolated the results to a full 3‐D grid. We then validated the result by relating TRI to independently modeled hydrodynamic parameters and to the distribution of macrozoobenthic species. We found that TRI successfully integrates the variation of tidal‐driven bed shear stress and wave‐driven orbital velocity. In addition, we found TRI to be a good predictor of the occurrence of macrozoobenthic species. The inferred small‐scale bedforms provide valuable information for separating the relative importance of natural dynamics versus anthropogenic disturbances such as dredging and bottom trawling activities. We discuss that by repurposing already available single‐beam data in this way, bedforms can be characterized at high resolution without the need for additional equipment or mapping campaigns, yielding novel input to decision‐making on marine management and conservation. [ABSTRACT FROM AUTHOR]