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Isotopic discrimination against 13C during photosynthesis is determined by a combination of environmental conditions and physiological mechanisms that control delivery of CO₂ to RUBISCO. This study investigated the effects of light, flow, dissolved inorganic carbon (DIC) concentration, and its speciation, on photosynthetic carbon assimilation of Zostera marina L. (eelgrass) using a combination of laboratory experiments and theoretical calculations leading to a mechanistic understanding of environmental conditions that influence leaf carbon uptake and determine leaf stable carbon isotope signatures (δ 13C). Photosynthesis was saturated with respect to flow at low velocity (~ 3 cm s−1), but was strongly influenced by [DIC], and particularly aqueous CO₂ (CO2(aq)) under all flow conditions. The non-linear responses of light- and flow-saturated photosynthesis to [DIC] were used to quantify the maximum physiological capacity for photosynthesis, and to determine the degree of photosynthetic carbon limitation for light-saturated photosynthesis, which provided a mechanistic pathway for modeling regulation of carbon uptake and 13C discrimination. Model predictions of δ 13C spanned the typical range of values reported for a variety of seagrass taxa, and were most sensitive to [DIC] (predominantly [CO2(aq)]) and flow, but less sensitive to DIC source [CO2(aq) vs. HCO−1₃]. These results provide a predictive understanding of the role of key environmental parameters (light, flow, and DIC availability) can have in driving δ 13C of seagrasses, which will become increasingly important for predicting the response of these ecosystem engineers to local processes that affect light availability and flow, as well as global impacts of climate warming and ocean acidification in the Anthropocene.

Bergner, J.; Wallin, D.; Yang, S., and Rybczyk, J., 2025. Using drone-captured imagery and a digital elevation model to differentiate eelgrass species: Padilla Bay, Washington. Journal of Coastal Research, 41(1), 105–121. Charlotte (North Carolina), ISSN 0749-0208. There are two primary species of eelgrass at the Padilla Bay National Estuarine Research Reserve, Zostera marina, a native eelgrass, and Zostera japonica, a nonnative. Recently, unoccupied aerial systems (UAS) have been used for eelgrass monitoring and mapping since imagery can be collected frequently and during different seasons. This project, conducted from April to September 2022, utilized UAS imagery, elevation data, and eelgrass vegetation surveys in the intertidal zone to identify regions with Z. japonica–dominant, mixed, and Z. marina–dominant cover. Multispectral imagery, using random forest (2000 trees) classification and eelgrass vegetation survey data, was used to predict eelgrass cover categories. Z. japonica–dominant, mixed, and Z. marina–dominant cover differed spectrally due to speciation and canopy characteristics, but low Z. japonica–dominant cover and exposed mud significantly decreased the accuracy in predicting that cover class in April and May. The overall accuracy predicting Z. japonica–dominant, mixed, and Z. marina–dominant cover was 75% using multispectral data alone. When multispectral imagery was combined with a 1-m-resolution digital elevation model (DEM) with a vertical error of 4.3 cm, the overall accuracy rose to 89%. Accuracy for each cover category rose as well. Most notably, Z. japonica–dominant cover rose from a user's accuracy of 71% to 92%. Z. japonica–dominant cover increased by 0.3 km2 from April to September. Mixed cover slightly increased from April to May, and Z. marina–dominant cover remained relatively consistent through the months. This is the first study to yield highly accurate classification between Z. japonica– and Z. marina–dominant cover, and results can be further improved through additional management of spectral variation.

Seagrasses are marine angiosperms that form highly productive and diverse ecosystems. These ecosystems, however, are declining worldwide. Plant-associated microbes affect critical functions like nutrient uptake and pathogen resistance, which has led to an interest in the seagrass microbiome. However, despite their significant role in plant ecology, viruses have only recently garnered attention in seagrass species. In this study, we produced original data and mined publicly available transcriptomes to advance our understanding of RNA viral diversity in Zostera marina , Zostera muelleri , Zostera japonica , and Cymodocea nodosa . In Z . marina , we present evidence for additional Zostera marina amalgavirus 1 and 2 genotypes, and a complete genome for an alphaendornavirus previously evidenced by an RNA-dependent RNA polymerase gene fragment. In Z . muelleri , we present evidence for a second complete alphaendornavirus and near complete furovirus. Both are novel, and, to the best of our knowledge, this marks the first report of a furovirus infection naturally occurring outside of cereal grasses. In Z . japonica , we discovered genome fragments that belong to a novel strain of cucumber mosaic virus, a prolific pathogen that depends largely on aphid vectoring for host-to-host transmission. Lastly, in C . nodosa , we discovered two contigs that belong to a novel virus in the family Betaflexiviridae . These findings expand our knowledge of viral diversity in seagrasses and provide insight into seagrass viral ecology.

Waves and currents influence not only the spatial distribution of seagrass meadows but also the transport, establishment and survivorship of seagrass propagules and hence the success of seagrass recruitment from sexual reproduction. We quantified the dispersal of propagules of 3 seagrass species (Posidonia oceanica, Cymodocea nodosa and Zostera marina) over substrata of different complexities (sand, coarse gravel and P. oceanica mattes of different shoot abundances) under unidirectional flow in a hydraulic flume. Threshold velocities indicate that Z. marina seeds start to move earlier over a flat sandy bottom (14 cm s−1) than seeds of P. oceanica (20 cm s−1) and C. nodosa (21 cm s−1). Propagule trapping increased with bottom complexity, which was related to the flow reduction that each substratum generated and the boundary layer thickness. Trapping rates were higher in coarse gravel and in mattes with higher abundances of dead shoots, where flow was reduced more than 50%. Over sand, flow reduction was minimal and propagules were not trapped. Furthermore, notable differences between P. oceanica early life stages were observed, with seeds trapped first, followed by seedlings of increasing ages. This result may be related to the smaller frontal area (area exposed to water flow) and higher settling velocity of the younger life stages. Together, our results provide important insights into the drivers of seagrass recruitment, which are of interest for restoration purposes and numerical modelling.

Seagrass is a vital structural and functional element of the marine environment worldwide and is highly valued for its ecological benefits. Monitoring the evolution of the seagrass habitat is essential to understand how this coastal ecosystem changes, and to develop good environmental management practices. For the present study, two remote sensing methods were used to map and monitor Zostera noltei Hornemann, 1832 ( Z. noltei ), in the Merja Zerga lagoon from 2010 to 2020. These methods which are the random forest algorithm and the object-oriented classification, were convenient to provide significant results. The first approach employed Sentinel-2 images from 2018 to 2020, which were used to extract information on changes in Z. noltei (commonly called dwarf eelgrass) distribution and aboveground biomass estimation. The second involved three orthophotography (orthophoto) mosaics from the years 2010, 2016, and 2018, which were analyzed to map the distribution of the species. It was revealed that Z. noltei coverage has increased by 212 ha since 2010, with most of the growth occurring in the center and upstream part of the lagoon. The mean aboveground biomass of dwarf eelgrass in the lagoon was 78.5 DW/m² in 2018, 92.6 DW/m² in 2019, and 115.2 g DW/m² in 2020. The approach used in this study has provided important insights into the dynamic and mean biomass of Z. noltei in the Merja Zerga lagoon. It is therefore a valuable, non-destructive method that uses freely-available Sentinel-2 satellite data.

Restoration of key species in dynamic coastal ecosystems benefits from reduction in environmental stress. This can be realized by promoting positive feedback (intrinsic processes) or by reducing extrinsic negative forcing. In a seagrass (Zostera noltii) restoration project in the south‐western Netherlands, we investigated transplantation success in relation to intrinsic processes (i.e. comparing sods vs. single shoots, transplant size, transplant configuration and transplant density) and extrinsic forcing (i.e. bioturbation by Arenicola marina, desiccation and exposure to water dynamics). In total, 2600 m² of seagrass sods were mechanically transplanted to six intertidal flats over the course of 5 years. In total, 43% of sod transplants (2·25 m²) survived at the long term, whereas single shoot transplants failed within the first 3 months. The use of larger, or more compact (sod), transplant configurations had no long‐term effect on survival, and initial densities did not affect transplantation success either. Reducing desiccation stress increased the transplantation success during the first growing season. Shielding transplants from bioturbating lugworms had a positive effect on long‐term survival. Seagrass abundance in summer was related to spring abundance, whereas winter survival was not related to prior seagrass abundance. At four of the six intertidal flats, transplants gradually decreased in size over time. At the other two, extensive colonization occurred around the transplant areas in some years and was still partly present in 2015. A correlation with the studied environmental parameters was not found. Synthesis and applications. Intrinsic processes favour transplantation development during the growing season, allowing positive feedback. Extrinsic processes favour the development at a longer time‐scale (i.e. reduction in bioturbation, thus breaking the positive feedback of the bare state). Most surprisingly, the starting colonization of two out of six tidal flats could not be related to environmental factors (hydrodynamics, light, emergence time, sediment characteristics, macro‐algae and grazing). Environmental managers can improve transplantation success by restoring the positive feedback, reducing stress, but also via risk spreading by performing transplants over wider areas. They thereby accept the complexity of processes and unpredictable temporal and spatial variation in which transplantation sites turn out to be successful.

Simultaneous triple stable isotope analysis of carbon, nitrogen and sulphur was employed to study the temporal variation in the food web of a subtidal eelgrass Zostera marina bed in the western Baltic Sea. Samples of 3 potential food sources (eelgrass, epiphytes and seston) and consumer species were collected biweekly from March through September 2011. Temporal variation of stable isotope compositions was observed in primary producers and consumer species. However, variation between replicates, particularly omnivores, often exceeded variation over time. The high degree of omnivory among the generalist feeders in this eelgrass community allows for generalist feeders to flexibly switch food sources, thus enhancing food-web stability. As coastal systems are subject to seasonal changes, as well as alterations related to human disturbance and climate, these food webs may retain a certain resilience due to their plentiful omnivores.

Simultaneous analysis of carbon, nitrogen and sulphur stable isotope ratios was applied in this pilot study to examine the food web of a Zostera marina L. system in the western Baltic Sea. Samples of 3 potential food sources: eelgrass, epiphytic algae and seston, as well as 69 consumer species were collected during the growing season of Z. marina from March to September 2011. The measured delta super(13)C values of epiphytes were close to delta super(13)C values of eelgrass, impeding a clear distinction of those 2 carbon sources, whereas seston delta super(13)C values were clearly different. This frequently encountered problem was solved by the additional use of delta super(34)S, which resulted in easily distinguishable values for sediment and seawater derived sulphur. The combination of delta super(34)C and delta super(13)C values made a separation of carbon sources possible and enabled the allocation of potential food sources to consumers and a description of their trophic relationships. The results of stable isotope ratio analysis of this eelgrass community strongly indicate a food web based on epiphyte and seston production. delta super(15)N values show a food web consisting of large numbers of generalists and a high degree of omnivory amongst the consumer species analysed. This implies an occupation of every trophic position possible, which is supported by an even distribution of delta super(15)N values. Previously described eelgrass food webs may have to be re-evaluated by considering sulphur stable isotope ratios in order to provide a clear picture on primary carbon sources.

Latitudinal gradients in species interactions are widely cited as potential causes or consequences of global patterns of biodiversity. However, mechanistic studies documenting changes in interactions across broad geographic ranges are limited. We surveyed predation intensity on common prey (live amphipods and gastropods) in communities of eelgrass (Zostera marina) at 48 sites across its Northern Hemisphere range, encompassing over 37° of latitude and four continental coastlines. Predation on amphipods declined with latitude on all coasts but declined more strongly along western ocean margins where temperature gradients are steeper. Whereas in situ water temperature at the time of the experiments was uncorrelated with predation, mean annual temperature strongly positively predicted predation, suggesting a more complex mechanism than simply increased metabolic activity at the time of predation. This large-scale biogeographic pattern was modified by local habitat characteristics; predation declined with higher shoot density both among and within sites. Predation rates on gastropods, by contrast, were uniformly low and varied little among sites. The high replication and geographic extent of our study not only provides additional evidence to support biogeographic variation in predation intensity, but also insight into the mechanisms that relate temperature and biogeographic gradients in species interactions.

In a seagrass restoration project, we explored the potential for enhancing the restoration process by excluding antagonistic engineering interactions (i.e., biomechanical warfare) between two ecosystem engineers: the bioturbating lugworm Arenicola marina and the sediment-stabilizing seagrass Zostera noltii Hornem. Applying a shell layer underneath half of our seagrass transplants successfully reduced adult lugworm density by over 80% and reduced lugworm-induced microtopography (a proxy for lugworm disturbance) at the wave-sheltered site. At the wave-exposed site adult lugworm densities and microtopography were already lower than at the sheltered site but were further reduced in the shell-treated units. Excluding lugworms and their bioengineering effects corresponded well with a strongly enhanced seagrass growth at the wave-sheltered site, which was absent at the exposed site. Enhanced seagrass growth in the present study was fully assigned to the removal of lugworms' negative engineering effects and not to any (indirect) evolving effects such as an altered biogeochemistry or sediment-stabilizing effects by the shell layer. The context-dependency implies that seagrass establishment at the exposed site is not constrained by negative ecosystem-engineering interactions only, but also by overriding physical stresses causing poor growth conditions. Present findings underline that, in addition to recent emphasis on considering positive (facilitating) interactions in ecological theory and practice, it is equally important to consider negative engineering interactions between ecosystem-engineering species. Removal of such negative interactions between ecosystem-engineering species can give a head start to the target species at the initial establishment phase, when positive engineering feedbacks by the target species on itself are still lacking. Though our study was carried out in a marine environment with variable levels of wave disturbance, similar principles may be expected to apply to other ecosystems that are inhabited by ecosystem engineers.