A Quantitative Survey of the Organic Carbon Content of an Irish salt marsh

Introduction

            Salt marshes are one of Ireland's primary forms of a blue carbon sink. Blue carbon is the term given to carbon which is stored long term (sequestered) in vegetated coastal habitats. It can take the form of salt marshes, mangroves, sea grasses, macroalgae and others. Globally, blue carbon habitats sequester a disproportionately large amount of carbon. For example, although they make up only 3% of the area of global forestry they sequester as much carbon per year as all global forestry¹. Effective sequestration is caused by low decompositional rates, high primary productivity and low oxygen levels in sediments¹. In addition to sequestration, salt marshes provide many other important ecosystems services such as protection from storms and soil erosion, habitats and increased biodiversity². However, salt marshes are currently under increased threat from changing land use and rising sea levels. If sea levels rise by 1 metre by 2100, as is currently predicted, salt marshes would not be able to keep pace³. When a salt marsh dies or is drowned, the carbon stored in its sediments is eroded and the habitat becomes a carbon source rather than a carbon sink⁴.

            This project aims to assess the organic carbon content of an Irish salt marsh and to find an average carbon content per  of sediment and per  of surface area. The data created would be relevant to attempts to assess our national carbon balance. There is also much interest among carbon credit schemes to fund projects that sequester carbon through sustainable, natural methods. Restoration of salt marshes is one such method and it is hoped that the data provided by this project would encourage further restoration projects by providing a snapshot of their sequestration potential.

            The project would also aim to analyse the differences in carbon sequestration potential between healthy, damaged and recovering salt marsh areas. The vegetation type and organic soil depth would be used as a proxy for the status of the salt marsh. This type of analysis could aid in attempts to quantify the damage caused by salt marsh disturbance and the potential benefits of salt marsh restoration.

            The scope of the project would also include an analysis of the commonly used "Loss On Ignition" protocol as compared to the more high definition Elemental Analyser. Loss On Ignition is the standard technique for measuring the organic content of salt marsh soils, however it may fail to take into account the loss of mass incurred due to moisture and other forms of mass loss.

Methodology

            The analysis would be carried out on a salt marsh on Fota Island in county Cork. In the event of restrictions which include an inter-county travel ban but with continued access to the labs, the survey could be carried out on a Dublin salt marsh such as Portmarnock or Bull Island. The project would take place in the late summer of 2021.

·      The aim of weeks 1 would be to survey the vegetation types of the salt marsh and to create an interactive GIS map which the sampling points can be input onto.

·      The aim of week 2 would be to sample a random variety of cores from the salt marsh and to survey the surface vegetation species. The location of each core would be recorded using a differential GPS.

·      The aim of weeks 3 would be to analyse the carbon content of each core in the laboratory. Fixed volumetric samples would be taken from each sedimentary layer within the core. The samples would be dried and then weighed. Each sample would be exposed to temperatures in a furnace which would specifically combust the carbon content but leave the rest of the sample unaltered (called staged loss on ignition). The weight after burning would be subtracted from the weight before burning in order to find the carbon content of the sample. In addition, the sediments contained in the core would be described, classified and any plant remains would be identified.

·      The aim of week 4 would be to analyse a number of samples in an elemental analyser so as to allow a comparison with the Loss on Ignition protocol.

·      The aim of week 5 would be to analyse the data and create an estimate of the average carbon content per  of sediment and per  squared of the marsh surface. The average carbon content of the different salt marsh regions (healthy, damaged and recovering) would also be calculated.

·      The aim of week 6 would be to write up the results and conclusions and to summarise these on an A3 poster.

            Materials needed would include a gouge corer, a differential GPS, a furnace, a syringe for collection of fixed volume samples, a fridge where cores can be stored at 4 degrees prior to processing and access to an elemental analyser. Professor Edwards has certified that access to these materials is available.

            Professor Edwards and post-doctoral researcher Zoë Roseby would assist in the retrieval of cores, proper sampling technique and would guide the lab analysis. They would be onsite during the field analysis carrying out research on sea level rise. My role would be to organise and manage the carbon assessment project, assist in the recovery of cores and to run the lab and statistical analyses. The project involves an interdisciplinary collaboration between myself (a biology student) and the geoscience department (including Professor Edwards).  I would learn geoscience techniques such as coring, lab analysis of cores and loss on ignition protocol. I would use my biological background to aid me in the proper identification of plant species and remains. I would also develop transferable skills in project management, team coordination and the presentation of research findings. The sampling and analysis method described has been adapted from the UNESCO "Coastal Blue Carbon Manual"⁵ which describes an internationally recognised methodology for blue carbon surveys.

            In the case of a level 5 lockdown, the project would become fully online. The aims would be expanded to include an estimate of the carbon storage potential of all Irish salt marshes. Geospatial Information Systems (GIS) mapping would be used to detect and calculate the area of all Irish salt marshes. Marsh extent and distribution data would be taken from CORINE landcover maps. The existing field data on average carbon content from similar sites abroad (such as in Scotland) could be combined with the area calculations to create an estimate of the carbon storage potential. GIS specialist Dr. John Connolly (Dept. of Geography, TCD) has agreed to assist with the technicalities of the project.

Context

            There has been much research into the blue carbon stocks of salt marshes internationally, however, to date, there has been no substantial study on the carbon content of an Irish salt marsh. This area is gaining increasing interest, as evidenced by a recent Science Foundation Ireland grant awarded to Dr. Grace Cott of University College Dublin, to fund research investigating Irish salt marsh carbon content and vegetation. I have established contact with Dr. Cott and she has agreed upon collaboration between our projects, including the sharing of her expertise. My project would provide a novel body of work in a field of increasing scientific interest and societal relevance.

References

1.     Duarte, C., Losada, I., Hendriks, I., Mazarrasa, I., Marbà, N. The role of coastal plant communities for climate change mitigation and adaptation. Nature Climate Change 3, 961–968 (2013). URL = https://www.nature.com/articles/nclimate1970?page=1 (Retrieved 15.01.21).

2.     Purcell, A.D., Khanal, P., Straka, T., Willis, D.B. (2020). Valuing Ecosystem Services Of Coastal Marshes and Wetlands. Land-grant Press. URL = https://lgpress.clemson.edu/publication/valuing-ecosystem-services-of-coastal-marshes-and-wetlands/. (Retrieved 29.1.21).

3.     Callaway, J.C., Borgnis, E.L., Turner R.E., Milan, C.S. (2012). Carbon Sequestration and Sediment Accretion in San Francisco Bay Tidal Wetlands. Published in Estuaries and Coasts. URL = https://link.springer.com/article/10.1007/s12237-012-9508-9 (Retrieved 10.01.21)

4.     Kuwae, T., Kanda, J., Kubo, A., Nakajima, F., Ogawa, H., Sohma, A., Suzumura, M. (2016). Blue carbon in human-dominated estuarine and shallow coastal systems. Ambio, 45(3), 290-301. URL = http://www.jstor.org/stable/45134595. (Retrieved 23.1. 21)

5.     Howard, J., Hoyt, S., Isensee, K., Pidgeon, E., Telszewski, M. (eds.) (2014). Coastal Blue Carbon: Methods for assessing carbon stocks and emissions factors in mangroves, tidal salt marshes, and seagrass meadows. Conservation International, Intergovernmental Oceanographic Commission of UNESCO, International Union for Conservation of Nature. Arlington, Virginia, USA. URL = https://www.thebluecarboninitiative.org/manual/. (Retrieved 10.1.21)