Seagrasses provide resources and ecosystem services critical to the health of coastal ecosystems and human populations. They increase water clarity, stabilise sediments and reduce coastal erosion, sequester carbon, and provide habitat and food to marine animals, including commercially important fish and invertebrates.
The global annual value of seagrass services is 11 billion a year, but they are declining at a rate of about 7%. In Australia the reported total loss is almost 300,000 hectares, or 5.5% since the 1930s. Contributing factors include sediment and nutrient inputs, boating and climate change. As human population density along coastlines grows, so will the pressure on seagrasses.
There is a clear and urgent need to restore seagrass to enhance the ecosystem functions and services they provide, and the resistance and resilience of coastal ecosystems to further environmental change. Improved restoration methods are needed to enhance the success of seagrass restoration trials. This requires more basic knowledge of the ecological factors that impact seagrass performance, and applied research to improve restoration methods. One ecological factor critical to restoration success is sediment processes. Sediment processes affect nearly all facets of seagrass restoration, yet have been largely overlooked in restoration projects. This Marine and Coastal Hub project explored how to incorporate sediment processes in seagrass restoration frameworks.
Approach and findings
Opportunities to enhance seagrass restoration
The project team hosted a national workshop and engaged with experts in seagrass biology and restoration to review what is known about sediment processes that influence seagrass health. They identified opportunities to enhance seagrass restoration by explicitly considering sediment processes in planning, site selection, restoration methods, monitoring, and assessment of restoration success. The following key, interdependent areas by which sediment processes influence seagrass health and performance were also identified.
- Hydrodynamics influence sediment properties and microbial community development in ways that can benefit or hinder seagrass restoration.
- Sediment microbial communities control nutrient and chemical cycling for seagrass.
- Seagrass response to sediment processes may be influenced by their life-history and genetics of seagrass species targeted for restoration.
- Interactions with other species can promote and inhibit seagrass survivorship and growth.
An assessment of hydrodynamics is important to selecting the restoration site, strategies, and donor material. In general, high-energy environments (such as shallow sediments subject to high bed shear stress due to wind waves and/or tidal currents) may inhibit seagrass recruitment due to physical disturbance associated with sediment resuspension and smothering. These sites are likely to have coarser sediments, with low organic carbon and nutrient contents. Sulphide stress will be low, but nutrients may be limiting, especially before meadow continuity is achieved. In contrast, low energy environments promote the accumulation of fine sediments and organic matter. Potential sulphide stress may be high and limit the survival of seedlings and propagules. As seagrass meadows develop, the canopy tends to attenuate wave and current energy, promoting particulate trapping which can form an important nutrient input to sustain growth. In some situations, however, this can lead to stagnation of the water column, which can hamper seagrass health (such as large daily oxygen changes, and epiphytic and macroalgal growth).
Seagrasses have intimate connections with their microbial communities which control critical processes such as nutrient cycling and buffering against sulphide toxicity. Microbial community diversity and its beneficial impacts on seagrass health increase due to positive feedbacks associated with oxygen loss to the rhizosphere as seagrasses grow and meadows develop. Understanding how and which microbes (taxonomic or functional) influence seagrass health and their relation to sediment properties such as grain size, and nutrient and sediment chemistry will have major implications for site selection and the identification of suitable donor beds and will greatly improve methods for restoration. For example, site selection may be improved by selecting areas that have sediment properties that support growth-promoting microbes.
The initial phase of seagrass establishment is critical in terms of providing seagrasses with an opportunity to overcome poor sediment conditions. In addition, information on seagrass-microbe interactions should be incorporated in monitoring strategies to check that the manipulation of plants, seeds or sediments does not lead to microbial changes that may negatively affect restoration success.
In the absence of appropriate microbes, promising methods include planting shoots or seeds in biodegradable pots containing appropriate microbial communities. Similar techniques involve taking plugs of seagrass from established beds and transplanting them. This technique will be largely restricted to small scale ventures, however, given the impacts that extensive plug removal may have for donor beds. One potential larger-scale method is seeding sediments with sediment containing preconditioned microbes that can support seagrass survivorship and growth. Seeding areas with ‘good’ sediment may also be a strategy for enhancing the resistance and resilience of seagrass beds that are under stress.
Molecular tools to investigate microbial communities and functions offer an additional tool to determine the health of seagrass beds and should be part of large-scale monitoring programs (see Hub projects 1.5 and 1.6). If seagrass health/microbe relationships are known, they may be useful for detecting stressed beds even if loss of seagrass is not yet evident. Techniques are being researched that may allow the screening of sediments for beneficial/harmful microbes that could be early indicators of seagrass performance and the ecological status of a site. Standardised, best-practice methodology for sample collection and processing, culturing protocols, primers, and bioinformatics pipelines is also central to developing a framework for incorporating seagrass-sediment interactions in restoration, management and monitoring. Large consortia and associated databases, such as BioPlatforms in Australia and the Mangrove Microbiome Initiative and Earth Microbiome Project worldwide, would facilitate standardisation, sequence deposits and metadata.
Seagrass life histories and genetics
Seagrass life-histories and genetics will have important implications for restoration, including site selection and selection of donor material. For example, genomic analyses can assist in identifying and matching genotypes from donor meadows to environmental conditions at restoration sites. Transcriptomic studies of gene expression also provide opportunities for restoration genetics as they allow for the identification of the genes underlying responses to specific environmental stressors. Ensuring restoration material contains genetic variants that allow for adaptation to future projected environmental conditions will be critical for positive long-term management outcomes. In addition, some fast-growing species such as Halophila may be less reliant of microbial interactions and may be used to prime disturbed sediments with good microbes, or by improving below-ground sediment chemistry to support the restoration of slower growing, longer-lived species such as Posidonia and Amphibolis. Further experimental research is needed to ensure the genotypes or functional groups used in restoration trials match local environments.
Interactions with other organisms
Positive and negative interactions with other organisms will also influence restoration success. For example, bioturbating species may be distributed in areas where they are absent to enhance sediment oxygenation and chemical cycling. Alternatively, restoration may be inhibited by species such as sand dollars that may bury seeds to disturb shoots. A move towards whole ecosystem management (a focus of Project 1.6), rather than managing individual habitats explicitly, acknowledges such factors. One aspect that shows promise is co-restoration of seagrass with oyster reefs which may stabilise sediments and increase organic inputs conducive to seagrass growth.
There is a need for increased community ownership and participation in successful restoration projects. This project reviewed the results of three restoration trials involving collaboration with Indigenous and community groups. These are: use of sediment filled hessian tubes for seed and seedling capture (Malgana Rangers, UWA) and scaling up seed collection for seed-based restoration (Seeds for Snapper, OzFish Unlimted, UWA) in Western Australia and; assessing fragment collection techniques and the effects of sediment quality manipulations on engendered Posidonia australis in Botany Bay (Gamay Rangers, UNSW).
Community engagement with recreational fishing groups (OzFish) and Indigenous peoples (Malgana Land and Sea Rangers; Gamay Rangers) was successful across all three on-ground projects conducted in New South Wales and Western Australia. Common successful elements were community involvement in the collection and distribution of propagules, logistical support and infrastructure deployment. There were also opportunities for community groups to get involved with the science of restoration, including the scientific training of Indigenous students, and the training of divers in data collection. The community activities also highlighted the potential use of seeds for large scale restoration and to maximise diversity.
People and agencies involved in planning, funding, researching, managing and assessing seagrass restoration projects now have a consolidated evidence base indicating that sediment processes are at the heart of many feedback processes influencing seagrass health.
This provides a foundation for building on existing knowledge through developing experimental strategies aimed at understanding the role of sediment processes in seagrass health, and ultimately advancing the field of seagrass restoration.
New South Wales Department of Planning and Environment
University of New South Wales
The University of Western Australia
Malgana Land and Sea Rangers
Technical report – Project 1.8
Inclusion of sediment processes in restoration strategies for Australian seagrass ecosystems
Gribben PE, Jongen R, Ferguson A, Marzinelli EM, Verges A, Kendrick GA (2022) Inclusion of sediment processes in restoration strategies for Australian seagrass ecosystems. Report to the National Environmental Science Program Marine and Coastal Hub. University of New South Wales.