The road to seagrass restoration at scale using engineering

The paper addresses how engineering and mechanisation can enable seagrass restoration at the spatial scales needed to deliver nature-based solutions such as coastal protection, water quality improvements, climate mitigation, and biodiversity conservation. Despite rising interest due to widespread seagrass loss and recognition of their ecosystem services, restoration remains costly (estimated US$399,532 per hectare) and labour intensive, with high and often underreported failure rates. Failures often stem from knowledge gaps and inadequate consideration of local environmental stressors and ecological feedbacks (e.g., sediment instability, seed predation, macroalgal overgrowth). Evidence suggests that larger-scale planting increases success by helping overcome adverse feedbacks. The central purpose is to review mechanised and automated approaches across the restoration workflow (seed collection, processing/separation, storage, planting, environmental engineering) to identify promising innovations, their performance, and key knowledge gaps needed to scale restoration effectively and responsibly.

The review synthesises several decades of restoration literature, highlighting early US innovations in the 1990s–2000s (e.g., mechanised seed collection and planting sleds, transplanting machines) that enabled larger-scale actions. Empirical studies indicate that larger-scale planting improves outcomes and that site suitability and stressor removal are critical. The literature on seed-based methods has expanded (notably for Zostera spp.), including Buoy Deployed Seeding (BuDS), seed injection methods, and storage protocols for recalcitrant seeds. Evidence for mechanised transplantation exists for Posidonia and Zostera meadows using sod transplanters, mechanised boats, and semi-robotic systems. Environmental engineering (e.g., sand capping to stabilise fine sediments and improve benthic light) has emerged as a complementary strategy. However, published academic evidence on many engineering innovations is sparse; much innovation is reported in grey literature, project reports, and practitioner networks, with limited quantitative comparisons of techniques or robust assessments of long-term success and generality across species and settings.

The authors focused primarily on Zostera species due to their dominance in the literature, while including other genera where possible. Systematic searches were conducted in Web of Science and Google Scholar using combinations of terms: 'seagrass', 'restoration', 'mechanisation', 'mechanical', 'innovation', 'scaled-up', 'large-scale'. The WoS search yielded 16 papers explicitly referencing mechanised engineering solutions; among these, six duplicated innovations in different contexts, three addressed nurseries, and three addressed sand capping to facilitate restoration. Additional sources included grey literature, online information, social media, personal networks, and extensive details from the authors’ own applied restoration activities. Where possible, the authors extracted/estimated effort metrics for different workflow stages (e.g., person-days per 100,000 seeds) and included diagrams/photos of equipment and methods. The review synthesised approaches across stages: seed collection (manual and mechanised), separation/processing, storage, seed planting (injection, containers), transplanting machinery, nursery development, and environmental engineering.

- Costs and scaling: Restoration averages US$399,532 per hectare. Scaling up requires reducing labour intensity through mechanisation and improving reliability by aligning techniques with local environmental conditions. - Evidence for scale: Larger-scale plantings have higher success, in part by overcoming negative ecological feedbacks (e.g., sediment resuspension, predation, macroalgal overgrowth). - Seed collection: Mechanised approaches (e.g., bow-mounted harvesters, cutting sleds with hydraulic blades and nets) can greatly increase throughput. For species with near-sediment or small spathes (e.g., Nanozostera noltii), new methods (e.g., careful suction dredging) may be needed; by-catch risks require precautionary measures and potential AI-assisted avoidance. - Seed separation/processing: Innovations include low-cost IBC-tank systems with mechanical stirring for small batches (~≤50,000 seeds) and large recirculating aquaculture system (RAS)-based funnel tanks (total ~8,000 L) with cooling, filtration, aeration, and gravity drainage to automate separation over 4–6 weeks. BuDS can combine on-site separation and deployment, though success varies with hydrodynamics (lower in high-tide areas). Quantitative evidence linking separation method to seed viability and germination remains limited. - Seed storage: Recalcitrant Zostera seeds store best in high salinity (often ≥35–50 ppt) at low, non-freezing temperatures (~3 °C typical in practice), with flow, darkness (to reduce premature germination), and low density to mitigate fungal growth. Copper treatments can reduce oomycete infections and may improve germination for some taxa. - Seed planting depth and methods: Optimal Zostera marina seed burial is ~2–5 cm. Methods include manual Dispenser Injection Systems (DIS), mechanised DIS wheel buggies for rapid intertidal injection (~320 seeds m² in ~20 s; ~26× faster than hand DIS), and towed sleds delivering seed–matrix slurries via peristaltic pumps. In some US sites, simple seed broadcasting performed well due to low predation; elsewhere, predation and hydrodynamics necessitate burial or protective matrices. - Seed containers: Hessian/burlap bags, clay seed balls, and pots can protect seeds but require substantial hand preparation. Mechanised deployment innovations include pipe drops from boats, long-line placement systems, and a towed “potato planter” sled that furrows and buries containers, improving emergence. A heavy-engineering “Waffle” device to place 64 bags simultaneously initially failed to ensure burial; vibration plate upgrades are being tested. - Substrates/matrices: Uniform fine matrices facilitating injection flow (Greek yoghurt consistency) are key; artificial mixes (e.g., specific grades of fire clay powder with seawater) avoid clogs and have shown successful germination. Gelatin matrices may attract predators. Kettering loam improved germination and seedling growth in trials; microbiome inoculation benefits remain uncertain. - Transplant machinery: Systems include the ECOSUB1 towed vehicle for Posidonia sods (0.25 m², 0.5 m deep), a mechanised planting boat (for Halodule, tested for Zostera), and emerging robotic seabed walkers (e.g., ReefGen) for standardized shoot planting. - Nurseries: Land-based nurseries for seed and plant production are growing; they can reduce pressure on donor meadows, support selective breeding, and provide climate-resilient stock. - Environmental engineering: Sand capping stabilizes muddy sediments and improves benthic light, facilitating recovery. Hydrodynamic modifications (matting, shields) can ameliorate flow-related feedbacks. Restoring freshwater inflows may be more impactful than planting in some systems. - Efficiency gains: Mechanisation reduces person-days vs. manual approaches by an average of ~6.4× (seed collection 8×, processing 5×, separation 8×, planting 5×, seed bag planting 5×). Table 1 provides comparative person-day estimates for specific methods (e.g., DIS wheel ~0.3 person-days per 100k seeds; towed sled ~0.5; manual hand DIS ~2.2).

Mechanised and automated solutions substantially reduce labour demands and can increase consistency across the restoration pipeline, addressing a central bottleneck to scaling. However, outcomes remain constrained by biological and environmental factors: seed viability, microbiome dynamics during separation and storage, optimal substrates and additives, species-specific planting depths, predation, hydrodynamics, and sediment stability. The review shows that methods must be matched to site conditions; broadcasting works where predation is low, whereas injection or protected containers are needed elsewhere. Environmental engineering (e.g., sand capping, flow modification) can remove feedback barriers to recovery and may obviate or complement planting. The findings emphasize coupling engineering with improved biological understanding (microbiome management, coatings, hormones, nutrients) and flexible regulatory frameworks that allow iterative methodological trials. Subtidal mechanisation without reliance on divers (e.g., autonomous or tethered robots, towed injectors) is a key frontier. Overall, scaling success depends on interdisciplinary integration of engineering, ecology, and site-specific management of stressors.

Mechanisation across seed collection, separation, storage, planting, and transplanting substantially increases throughput (on average ~6× fewer person-days than manual operations), offering a viable path to large-scale seagrass restoration, especially for Zostera meadows. Despite progress, restoration remains characterized by high failure rates due to biological bottlenecks and environmental feedbacks. Priority directions include: developing sustainable methods for near-sediment seed collection; optimizing planting media and seed coatings/additives; creating remote subtidal planting tools independent of SCUBA; elucidating microbiome dynamics during separation and storage; determining optimized separation parameters; enabling more flexible licensing for trials; advancing ecological engineering to overcome feedbacks; and expanding mechanised approaches to additional genera beyond Zostera, Posidonia, and Thalassia. Continued interdisciplinary research and innovation, coupled with robust quantitative evaluation across diverse settings, are essential to achieve reliable, scalable restoration.

- Evidence base skewed toward Zostera spp., with limited data for many other seagrass genera and regions. - Few academic publications detail mechanised innovations; substantial reliance on grey literature, practitioner experience, and authors’ own projects may introduce bias and limit generalizability. - Limited quantitative, comparative studies on key steps (e.g., seed separation methods’ effects on viability/germination; subtidal DIS performance; long-term outcomes of different deployment devices). - Site-specific environmental factors (hydrodynamics, sediment type, predation) strongly influence success, complicating cross-site extrapolation. - Regulatory constraints and variable permitting affect feasibility and scalability of mechanised seed collection and subtidal operations. - Long-term, large-scale performance and ecosystem service recovery for several mechanised approaches remain insufficiently documented.