Blue carbon, estimation of the carbon stock and restoration

Blue carbon is the atmospheric carbon captured and stored by marine and coastal ecosystems, mainly mangroves, seagrasses, tidal- and saltmarshes   ^1–3  .

Carbon sequestration

Blue carbon ecosystems are capable of higher carbon sequestration per unit area than terrestrial forests, even though their global area cover is one to two order of magnitude smaller   ^4,5  .

Depending on the type and state of the ecosystems, their sequestration rates can be two to forty times higher than tropical forests, with carbon being stored for millennia instead of decades   ^5–8  .

Ecosystem services

Blue carbon ecosystems provide numerous ecosystem services including  ^8–11  :

• coastal protection against floods and storms
• providing habitats and nurseries for diverse species
• supporting healthy fisheries
• improving water quality and
• providing tourism and recreational benefits

Despite all the services and benefits blue carbon ecosystems are some of the most threatened ecosystems on earth   ^8,12  . Globally, habitats are being lost at an estimated rate of 1–7% annually   ¹³  . In Australia, approximately 52–78% of mangroves, 50% of saltmarshes and 20–26% of seagrass meadows have been lost since European settlement   ⁹  .

Due to their proximity, the ecosystems are subjected to anthropogenic influences such as land-use changes, clearing and degradation   ¹⁰  .

Mangroves store carbon both above- and belowground in their biomass and soils in different carbon pools.

On average, mangroves in Australia can sequester 4.8 Mg ha  ^-1   annually   ¹⁴  . The range of estimated carbon stocks lies between 662-2139 Mg ha  ^-1   for mangrove forests in Australia  ²  .

In NSW, the total mangrove carbon stock has been estimated to approximately 5 billion tonnes   ⁹  . This is equivalent to emissions from 250’000 households   ⁹  .

The Brunswick River, located in the Byron Shire in the Northern River Region of New South Wales (NSW) Australia, holds immense cultural and heritage value, whilst providing ecosystem services including habitat to all three blue carbon ecosystems, nursery grounds, recreational and commercial activities, and aquaculture   ¹⁵  .

A non-intrusive carbon stock methodology was developed for the Grey and River mangrove. This method was then applied on a degraded riverbank of the Brunswick River.

Study area

The mangrove carbon stock estimation and vegetation survey was conducted on a 200 m inner bank stretch after Mullumbimby.

Field procedures

A total of 10 quadrant plots of 25 m  ²    each were surveyed over the course of two days in November 2022.

The following main components were recorded:

• Vegetation and vegetation cover
• Dead and downed wood
• Pneumatophores
• soil texture and
• tree measurements

Data analysis

After the field days, the mangrove carbon stock was estimated and the species diversity calculated.

Each of the mangrove ecosystem components were analysed and allometric equations were applied to estimate the above- and belowground biomass of the mangroves. The biomass was then converted to carbon and the total mangrove stock was estimated.

Results

A total mangrove carbon stock of 50.84 ± 2.41 Mg ha  ^-1   was estimated for a degraded riverbank area. The aboveground carbon pool total was estimated to a total of 44.28 ± 2.39 Mg ha  ^-1  , and the total belowground carbon pools to 6.56 ± 0.31 Mg ha  ^-1  . Tree carbon pool contributed the highest fraction towards the total mangrove carbon stock, with an estimated 44.09 ± 2.40 Mg ha  ^-1  .

Approximately 96% of this carbon stock is stored in the Grey Mangroves, with 43.40 ± 2.00 Mg ha  ^-1   stored above ground and 5.68 ± 0.31 Mg ha  ^-1   belowground in the roots.

The carbon dioxide concentration was estimated to 185.59 CO  ₂  e. This is equivalent to 23.4 home’s energy use for one year, or 22’575’659 smartphones charged   ²²  .

A total of 27 different plants were found on the survey site. Of these, 25 species from 19 different families could be fully identified. 15 species were native and 10 naturalised.

On average, the species richness was 7 per plot. The mean Shannon-index was 1.338 and the mean evenness 0.802.

A native fine-leaved Tuckeroo was found on plot eight, which is listed as “vulnerable” on the conservation status NSW. Of the naturalised species, four were identified as invasive. The invasive species found were Camphor laurel, Coastal morning glory, Mickey Mouse Plant and Glossy nightshade.

Restoration projects have been commenced by the Byron Shire Council and other authorities   ²⁰  . However, as large areas are affected by degradation and riverbank instability, local organisations such as Positive Change for Marine Life (PCFML) and have started to restore smaller riverbank areas, which are not within state or council land   ^15,21  .

To make the choosing of potential restoration sites of degraded riverbanks easier, maps were generated from existing data layers. These were gathered and analysed to fulfil the criteria catalogue developed for the search of potential restoration sites on the Brunswick River.

Mapping result

Multiple potential restoration sites which avoid major bureaucracy and are on degraded riverbanks exist on the Brunswick River. The potential restoration sites are located between Mullumbimby and the Pacific Highway Bridge. Most of the potential restoration sites without bureaucracy contain blue carbon potential priority areas as mapped by the NSW DPI Fisheries.

It needs to be highlighted here that this map only includes riverbank and riparian areas where mainly mangroves are located, and thus any blue carbon potential found in the waters of the river, or beyond the riparian area are not included. This includes any potential seagrass and salt- and tidal marshes. Therefore, the potential blue carbon habitats on the Brunswick River are not limited to the extent of the map.

This story map was created as a summary of the Bachelor Thesis "Blue carbon, estimation of the carbon stock and restoration on the Brunswick River New South Wales" for the Bachelor's degree Natural Resource Science of the Zürich University of Applied Sciences 2023.

Alea Roth-Douglas

Acknowledgements

I wish to I acknowledge the Arakwal people, traditional owners of the Bundjalung land and waters where we gathered data for my thesis and pay my respects to Elders past, present, and emerging. I recognise their connection to Country and role in caring for and maintaining Country over thousands of years.

Thank you to Positive Change for Marine Life, especially Lauren Morgan, for providing access to data, information and helping with the organisation and data collection of the field trips. Without the cooperation, this thesis would not have been possible.

Thank you also to Vasukee Kasinathan and Morgan Roskelley for assisting with the vegetational surveys.

Thanks also to Ubuntu Foundation, Northern Rivers Community Foundation, Foundation for Rural & Regional Renewal and Southern Cross Credit Union for supporting PCFML.

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