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2nd Newsletter
April 2024


Main aims:

- To replant 1.5 hectares of seagrass meadows in Hong Kong
- To restore 4.5 hectares of seagrass meadows in Hong Kong

To achieve these aims and all the deliverables, we have designed a project whereby the progress each year will influence the activities for the subsequent year. Each year the project needs to address specific questions


- A comprehensive, quantitative assessment of Carbon sequestration from the replanting and restoration of 6 hectares of seagrass meadows that will target biological Carbon capture.

- Half yearly progress reports of activities and achievements throughout the 3 year period of this project.

- Community and stakeholders' engagement through outreach activities, workshops, citizen science, social media and press releases.

- Proof of concept allowing HK Seagrass Restoration Association to apply for larger funding to scale up.

- A science lead project designed to increase knowledge in the field of seagrass restoration and Blue Carbon sequestration that can be used throughout southern China in a proactive way to help naturally sequester CO2 from the environment.

Progress each year:

To achieve these aims and all the deliverables, we have designed a project whereby the progress each year will influence the activities for the subsequent year. Each year the project needs to address specific questions:



a.k.a. The baseline

How much carbon is already in the sediments? Is this carbon from the seagrass or does it come from somewhere else? How do seagrasses and environmental conditions (pollution and typhoons) influence this carbon stock?



a.k.a. Re-wilding

How much carbon can be captured by restored/replanted seagrass? How quickly is this new carbon stored?



a.k.a. Moving forward

Is this new carbon stable or lost back into the atmosphere?

Which process is more efficient at carbon sequestration - replanting or restoring seagrasses?

What do we need to do to scale up?


The HK Seagrass Restoration project has been divided into three phases aiming to build up and consolidate data regarding the potential of local seagrasses to act as blue carbon ecosystems. To achieve the main aim of the project (i.e., replant 1.5 ha and restore 4.5 ha) we need to: understand how much carbon is already in the sediments of local seagrass meadows (Phase one); calculate how much carbon can be captured by restored/replanted seagrass (Phase two) and; finally, with this information determine whether this new carbon is stable or is lost over time (Phase three). As there was a lack of scientific data relating to the carbon stock in local seagrasses, we initially developed models to predict these, as presented in the project proposal. Our original models suggested that carbon stock in sediments (for cores of 100 cm depth) would be on average 90 tonnes of carbon per hectare. Our baseline data from year one, suggests that current carbon stock in sediments (based on the first 15 cm core depth) are on average 33 tonnes of carbon per hectare - equivalent to ~120 tonnes of CO2 per hectare. (These estimates are derived only the first 25 cm of the 1m cores. Once we have analysed the remaining 75 cm depth of the cores we will be able to get a total validation of the stocks.)

Interestingly, these values are higher than those reported in other scientific papers for similar seagrass meadows in the Greater Bay Area and Hainan Island.

For year 2, we will have the total estimate of the stocks and more importantly, the rate at which this carbon is sequestered in local seagrass meadows as well as the stock gains through restoration efforts.
In this second report, we describe the activities and the progress achieved during the second part of the
Baseline Phase  (i.e., knowing what is there in order to accurately assess how much are we adding later on).

This phase provides the scientific data needed to understand the value, in terms of Blue Carbon potential, of the activities planned for years 2 and 3, and the posterior discussion of the possible development of carbon verification and credits.


During this second part of phase one, we have focused on field work and data collection. We have maximised our efforts in sediment core sampling in parallel with seagrass ecological monitoring.


Target locations for this phase of the project were decided according to the seagrass distribution map developed over the last 4 years by JDGE’s team at HKU.


Our work in phase 1 has been focused on Western Hong Kong (Sheung Pak Nai, Ha Pak Nai, San Tau, Yam O; Fig 1 - Fig 2A-H ). As part of Greater Bay Area, these locations receive heavy sediment and nutrient loads from both Pearl River and Shenzhen River.


Moreover, these locations are also influenced by nutrient loadings from the surrounding urbanization and from wastewater management areas. We are currently assessing if these particular environmental conditions influence the Blue Carbon potential of these local seagrass meadows.

Figure 1.   Distribution of the sampling sites in Western Hong Kong.
Figure 2.   Overview of the sampling sites in New Territories (A-F) and Lantau Island (G-H).

Sediment sampling includes surface (top 5-10 cm) sediment collection and deeper 1m sediment coring (Fig 3 & 4). We have prioritized high spatial and temporal resolution at each site in order to reduce intra-meadow variability and to get a more accurate assessment of the overall carbon stocks.

Figure 3.   Example of the experimental design layout for the sampling of cores in Lantau Island.
Figure 4.   Example of the experimental design layout for the sampling of cores in New Territories.

Sediment cores sampling

The 1 m sediment samples were collect using an auto-sampler (height = 100 cm, Fig 5), with multiple replicate cores taken repeatedly across the various meadows and through the year.


Sampling points were randomly selected by walking towards the middle of the meadow, at least 3 m from the edge, and randomly throwing 0.5 *0.5 cm quadrats in a clockwise the orientation.

Sediment cores were sliced into 10-12 different depth sections (Fig 5-6): 0-2 cm, 2-5 cm, 5-10 cm, 10-15 cm, 15-20 cm, 20-25 cm, 25-30 cm, 30-40 cm, 40-50 cm, 50-60 cm, 60-80 cm, 80-100 cm depending on the compression status.


All slices were individually sealed in zip-lock plastic bags and stored at −80 ◦C after returning to the lab at HKU for processing of stable isotopes and sediment characteristics.


Considering that at each location and for each season we get 14 sediment cores, that means we have ~165 sediment samples in total per site. This is replicated across all sites and seasons in phase 1.


From a scientific point of view, this both makes our project one of the most robust studies in terms of both replicating samples and total number of sample taken.

Figure 5.   Sediment core sub-sampling strategy (refer to Howard et al. 2014)
Figure 6.   1m sediment coring using an auto-sampler (SDI Vibecore Mini, US).
                   The cores were sliced into 10-12 different depth sections depending on the compression as                       described above.
Table 1.   Laboratory analysis for the collected sediment samples.

Laboratory work

The sub-samples of the 1m sediment cores were dried at 50℃ until a constant weight (Fig 7).
The dried samples were ground in an agate mortar and packed for following analysis.
HKSRA Newsletter_2_Sediment drying and grounding for further analysis_2.jpg
HKSRA Newsletter_2_Sediment drying and grounding for further analysis_3.jpg
HKSRA Newsletter_2_Sediment drying and grounding for further analysis_4.jpg
Figure 7.   Sediment drying and grounding for further analysis.

Ongoing work and Preliminary findings from Phase 1:

This data is still under development and analysis. The preliminary results presented here correspond to first 15 cm of the 1m sediment cores. This sub-section gives us a good idea of the carbon stored during the last 2-5 years. We will complete the total assessment of the overall Carbon Stock during the first semester of 2024. The assessment of actual sequestration rates will be developed during Phase 2.

HKSRA Newsletter_2_Table 2.jpg
Table 2   Summary of the mean values of sediment carbon density (SCD, g cm-3), C and N content                      (%), carbon stocks and CO2 equivalent in sediment core fractions (at 5 cm intervals) for two                    species of seagrasses in Hong Kong. Data separated by dry and wet seasons. Statistical                              significance (p<0.05) is derived from two-way ANOVA analysis using season (dry and wet)                      and species (H. ovalis and H. beccarii) as fixed factors (ns = non-significant)

The sediment (in the top 15cm) carbon stocks showed significant variation between seasons, whereas the carbon stocks in the seagrass biomass varied between seasons and species (Fig. 9).


In the wet season, the sediment carbon stocks in H. ovalis (~32. Mg C ha-1) and H. beccarii (~34 Mg C ha-1) meadows were 1-fold and 1.7-fold higher compared to the dry season. Higher sediment carbon stocks in the wet season resulted in higher CO2 equivalent storage of sediment organic Carbon (Corg) in wet season in both species. The CO2 equivalent of sediment Corg in H. ovalis (~116 Mg CO2 ha-1) and H. beccarii (~125 Mg CO2 ha-1) was

1-fold and 1.7-fold higher in wet season than dry season.


In H. ovalis meadows (~6 ha), the contribution of sediment carbon stocks (~191 Mg C) across dry and wet season was 99.9% higher than biomass (~0.54 Mg C), with a CO2 equivalent of ~698 Mg CO2 and ~2 Mg CO2, respectively (1 Mg of C or CO2 is equivalent to 1 Ton). Similarly, in H. beccarii meadows (~11 ha) the contribution of sediment carbon stocks (~286 Mg C) was 99.9% higher than biomass (1.56 Mg C), with a CO2 equivalent of ~10,489 Mg CO2 (for sediment stock) and ~4.62 Mg CO2 (for biomass). These values represent the first 15 cm and we expect higher Carbon concentrations once we complete the 1m core assessment.

Figure 9.   Plots of biomass and sediment carbon stocks and the respective CO2 equivalent of H. ovalis                     and H. beccarii and associated sediment (SD) in dry and wet seasons in Hong Kong.                                   Statistical significance(p<0.05) was derived from two-way ANOVA analysis using season                         (dry & wet), and species (plant tissues & sediment) as fixed factors. (p<0.0001***,                                     p<0.001**,p<0.05*) . Not significant (ns). Above-ground (AG), below ground(BG)

There is good potential for carbon storage in seagrass sediments in Hong Kong. The questions are now about the source of such Carbon (Phase 1) and the rates of accretion (Phase 2) and if these increase due to restoration (Phase 3).


As you will see in Figure 10, there are different signatures of organic carbon accumulated in the sediment of the seagrass meadows. Some of these are from plants such as seagrasses and mangroves, while others are from marine algae.


This is a good signal as it suggests that the Carbon accumulated is from local sources (autochthonous) and thus, the role of our seagrasses for blue carbon (and future carbon credits) is well supported.

Figure 10.
Sources of the organic carbon stored in seagrass sediments across seasons.

Outreach activities and training workshops

Science is fundamental for the successful development of this project. However, the long-term projection and its future translation/application for carbon credits, requires the engagement of different stakeholders, including NGOs and the Government. Therefore, we have been actively involved in the science communication and knowledge exchange of seagrasses as Nature-based solutions to help mitigate climate change and promote biodiversity. Included below are some examples of activities that we developed over the second half of phase 1.

The Nature-Based Solutions for Climate Forum

Nature-Based Solutions (NBS) involve the protection, management, and restoration of natural and semi-natural ecosystems to effectively address climate change-related challenges whilst providing benefits to both people and nature. These solutions have the potential to contribute up to 30% of the cost-effective mitigation required to limit global warming to 1.5°C by 2030.

Additionally, NBS offer multiple other socio-economic and environmental co-benefits. The Nature-Based Solutions for Climate Forum, co-organised by The Nature Conservancy (TNC) and Civic Exchange, was held on 27th October 2023 with resounding success. The event witnessed the attendance of over 90 in-person participants, including representatives from government, related industries, and interested corporates.


This Nature-based Solutions-themed forum provided a platform for stakeholders to delve into the transformative potential of NBS and their role in climate mitigation and adaptation.

Prof. Juan Diego Gaitan-Espitia was one of the keynote speakers. He discussed the potential of seagrass restoration for blue carbon sequestration and highlighted the negative effects of human activities on global biodiversity, including seagrasses. He presented preliminary results from seagrass restoration efforts in Hong Kong, emphasizing the positive trends in biodiversity recovery observed at both macro and micro levels.


The presence of micro-organisms capable of reducing heavy metals in the sediment and the increased soil capacity for carbon storage with seagrass growth were significant findings. Prof. Gaitan-Espitia also shared ongoing initiatives, including pilot studies and the use of indoor and outdoor nurseries to support seagrass survival.

During his talk, Prof. Gaitan-Espitia emphasized the urgent need to address the decline in biodiversity caused by human actions such as exploitation, habitat loss, pollution, invasive species, and climate change. He highlighted seagrasses as essential for carbon sequestration, acting as blue carbon ecosystems that store carbon in sediments for thousands of years.


The global decline of seagrasses, including in Hong Kong, where local extirpation and concerns about recovery have been observed, was a cause for alarm. Prof. Gaitan-Espitia concluded by emphasizing that seagrass restoration not only improves ecosystem services like biodiversity and carbon sequestration but also contributes to climate change adaptation and mitigation, making it a crucial aspect of conservation efforts.

The Nature-Based Solutions for Climate Forum successfully brought together stakeholders from many diverse backgrounds, fostering collaboration and knowledge exchange. The event concluded with a shared hope that the insights and ideas generated during the forum would contribute to meaningful actions in tackling the climate crisis.

Over 90 in-person participants, including representatives from government departments (AFCD, DSD), related industries, and interested corporates.

Talk to School about technologies for seagrass and mangroves restoration and monitoring

During the second semester of 2023, the seagrass team was invited to be involved in a Landscape Architecture knowledge exchange program organized by the Department of Architecture, HKU. In the talk, we emphasized the importance of landscape architecture for the design of sustainable cities and land reclamation, which is the main driver of seagrass declines in Hong Kong and the subsequent loss of carbon stocks. We presented examples of how Hong Kong coastal development is threatening seagrasses and how their protection can be aligned to the Government plans for Zero Carbon Emissions by 2050. Following the talk, the workshop was divided into working groups to provide short introductions on the technologies applied in our work, including (1) Remote sensing (2) Sediments for Stable Isotopes (3) Environmental and physiological data collecting (Chl a, irradiance, temperature/light sensors) (4) DNA. During the session, the students actively engaged and showed interest in the equipment and technologies presented. This interactive activity offers a unique opportunity for our seagrass team to share our knowledge and expertise in seagrass and mangrove research with a broader audience.

Restoration moving on !

The team has consolidated the indoor and outdoor seagrass nurseries for the start of the restoration phase. We have successfully established plants with fruits, seeds and flowers in the lab! These plants have been used for trials of transplantation in different areas of Lantau Island (see Fig 13 and 14).

Figure 11   Facilities at SWIMS; Seagrass nursery system running with healthy Seagrass; Seagrasses in                       the nursery are healthy and showing outstanding growth, even after only a few days.
Figure 12   Photos of different seagrass structures showing seeds, flowers and fruits were taken in                                2023 using 40X magnification. (A) New leaves emerged from the seeds of H. ovalis. (B)                          Seeds remained inside the leaf of Z. japonica. (C) Flower of H. ovalis. (D) Fruit of H. ovalis.
Figure 13   The team practising different techniques of seagrass restoration in Lantau Island.
Figure 14   Transplantation schematic for restoration sites.
                    Left: Three transplantation sites across tidal gradient.
                    Right: Transplantation design across each transplant material with planting patterns shown                                     with transplanted cores: A) line; B) dense; and C) bullseye

We appreciate your support to the project and will keep you updated about the progress in this exciting new Phase Two! Stay tuned!

The HKSRA core team


© 2024 by Hong Kong Seagrass Restoration Association.

References for the sampling protocol applied in this project
Howard, J., S. Hoyt, K. Isensee, M. Telszewski, and E. Pidgeon. 2014. Coastal blue carbon: methods for assessing carbon stocks and emissions factors in mangroves, tidal salt marshes, and seagrasses.
Howard, J. L., A. Perez, C. C. Lopes, and J. W. Fourqurean. 2016. Fertilization changes seagrass community structure but not blue carbon storage: results from a 30-year field experiment. Estuaries and Coasts 39: 1422–1434.

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