Mapping Subtidal Seagrass Bed (Zostera marina) Habitat Suitability in Scotland

This research article, written by Le Huang is based on her Master’s dissertation project for MSc Marine Systems and Policies (2020/21) at the University of Edinburgh. Specifically, her dissertation project mapped seagrass bed habitat suitability in Scotland. This project was conducted under the supervision of the University of Edinburgh, and NatureScot. This article summarises the importance, methodology and outcomes of her ecological model as well as including some interesting facts about seagrasses.

Photo by Benjamin L. Jones taken from Unsplash 

Seagrass beds are considered a priority for conservation, mainly because they can provide an assortment of ecosystem services. For example, seagrasses are important in preventing coastal erosion, providing habitats for marine organisms,and being a significant source of  blue carbon. Blue carbon refers to carbon stored in ocean and coastal ecosystems, otherwise known as ‘carbon sinks’. The seagrass beds within the coastal habitats can sequester more carbon per unit area than terrestrial forests. Τhis means that the seagrass beds, by capturing atmospheric carbon dioxide, a primary greenhouse gas, play an important role in mitigating climate change. In the meantime, seagrass beds, as critical carbon sinks, remove carbon dioxide in the air, and help approach carbon neutrality, i.e., the state of net-zero carbon dioxide emissions. 

However, European seagrass beds have been suffering from dramatic habitat loss due to the seagrass wasting disease (associated with the pathogen Labyrinthula zosterae), and since the 1930s, and the historical degradation has not recovered yet. In addition, other factors such as coastal development, pollution, and climate change are also likely to threaten the seagrass beds in the future. According to the Marine Life Information Network (MarLIN) portal, seagrass beds are moderately sensitive to global warming, marine heatwaves, and sea-level rise. The ecological importance of seagrass beds as essential parts of marine ecosystems, and the risks facing them have led to increasing interests in their conservation. 

The subtidal seagrass beds in Scotland are located within Scottish inshore waters (within 6 nautical miles from shore) and are dominated by a seagrass species, known as the common  eelgrass (Scientific name: Zostera marina Linnaeus 1753). Given their inshore location, Z. marina meadows can be easily spotted by scuba divers. Therefore, any ocean lover planning to dive particularly in European coastal zones may be fortunate enough to come across the scene of ‘underwater grasslands’ composed of seagrass meadows, featuring numerous dark-green ribbon-like leaves and inhabited by diverse creatures.

The distribution of seagrasses in the ocean floor is not random, but dependent on numerous factors. Firstly, in order to survive, seagrasses require sufficient light for photosynthesis. As a result, they are usually limited within the depth of 10 meters (for Z. marina). Apart from depth and light availability, wave exposure can be another important environmental variable that also drives the distribution of seagrasses. Firstly, high wave exposure may cause physical harm to the seagrass leaves and rhizomes (the growing underground stem with lateral shoots, and adventitious roots). In addition, strong waves can hinder light penetration through raising sediments from bed to water columns, which normally refers to the term of wave-driven sediment resuspension. Furthermore, the coarse sediments (e.g., coarse sand, gravel, pebbles, shingle and cobbles), associated with high wave exposure, have lower nutrient concentrations to support the seagrass growth. Therefore, seagrass beds generally favor moderately sheltered environments. Notably, the small and patchy meadows, and seedlings are especially sensitive to powerful waves, due to a lack of extensive rhizome systems. Since seagrasses need to anchor themselves to the seafloor, bathymetry characteristics such as slope, and sediment roughness, are also associated with habitat suitability. Steep slopes, and hard rocky substrata are not conducive to the anchoring of seagrass roots. Instead, seagrasses usually prefer soft sediments, i.e., muds and sands, which are also abundant in nutrients. In a nutshell, seagrasses show preferences towards shallow, and sheltered waters, as well as gentle slopes, and soft sediments.

With these factors that influence seagrass distribution in mind, one can explore locations of suitable seagrass habitats in Scotland, to then help determine the optimal sites for restoration work. Habitat suitability maps are constructed using predictive species distribution models (SDM). The input files used to build species distribution models included seagrass presence records (e.g., the coordinates of habitat locations), and some environmental datasets (i.e., environmental statistics for each pixel within the study area). For the seagrass distribution models, depth, relative wave exposure, slope, and seabed roughness are potential environmental variables of interest, as they can influence the distributions of seagrass beds. Using presence records, and these environmental datasets to train and test (‘build’) the models, predictive distribution data for seagrass habitats was generated.

A seagrass habitat suitability map was constructed for Scottish inshore waters through filling each pixel of the Scottish inshore map with a predictive distribution value. The higher the value, the greater the likelihoods of presence. So these values represented the habitat suitability index. The pixels were displayed in gradient colours, according to their predictive distribution values, making it possible to visually determine the most likely locations of the suitable seagrass habitats. The predictive distribution values for seagrass beds tended to decrease in deeper waters (distant from coastlines) and/or on steep slopes. In addition, seagrass beds are less likely to appear where the waves are extremely strong. Moderate roughness (i.e., the degree of irregularity of the surface) of the seabed was favourable for seagrass beds. This is likely because the extremely rough or smooth substrata are not conducive to the anchoring of seagrass roots. 

Seagrass Bed Habitat Suitability Map in Scottish Inshore Waters 
from Le Huang’s MSc Dissertation
‘Mean High Water Spring’ refers to the highest level that spring tides reach on average over a period of high water heights occurring at the time of the spring tides

In order to enrich their data selection, conservationists have taken initiative to involve the public through an innovative citizen science platform known as Seagrass Spotter. This platform belongs to Project Seagrass and encourages the public to submit sighting photos of seagrasses and their locations through a mobile application or online websites. 

In a nutshell, seagrass beds are important to provide a variety of ecosystem services, as well as to help achieve carbon neutrality, and mitigate climate change. By running predictive distribution models, habitat suitability maps can be generated to assist restoration work. Lastly, public awareness and involvement has played a significant role in the conservation of seagrasses and its influence should not be overlooked. 

Le Huang is an MSc student from Marine Systems and Policies at the University of Edinburgh. She is particularly interested in mapping habitat suitability of marine ecosystems. The research interests drive her to pursue higher studies in the future. Find her on LinkedIn @Le Huang.

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