At the end of July, Emma Shuttleworth and I set out on a fieldwork visit to colleagues, Prof Derrick Lai and PhD student Susan Li, at the Chinese University of Hong Kong (CUHK) to explore new research opportunities between our two Universities. The trip was supported by a grant from the Hong Kong Foundation to allow researchers from CUHK and University of Manchester (UoM) to visit each other to develop new and innovative research agendas. The focus for our visit was to look at the how some of my work on carbon cycling (i.e. oxidative ratio) could be applied to sub-tropical mangroves in Hong Kong.
Coastal mangrove wetlands have a higher rate of carbon (C) sequestration than tropical rainforests, making them a vital ecosystem in efforts to reduce anthropogenic CO2 emissions to the atmosphere. Together with salt marshes and seagrass beds, these coastal ecosystems are collectively termed ‘blue carbon’ sinks. Despite their global significance, there are still many unknowns in these systems (e.g. rate of C loss, organic matter properties), and in many areas of the world, they are at risk of degradation from land-use change and other disturbances.
In order to better plan the research project, Derrick and Susan took us on a tour of their research sites in the Mai Po marshes. The Mai Po marshes nature reserve, a stopping off point for many migratory birds, is a truly spectacular location that has many different ecosystems present, including: mud flats, mangrove forest, tidal shrimp ponds, wet and dry reed beds. It is also a Ramsar site and protected site of scientific interest.
Images of the Mai Po marshes. Top left: dense mangrove forests; Top right: open water shrimp pond; Bottom left: floating boardwalk; Bottom right: Eddy covariance tower
The plan is to sample the different carbon pools (e.g. leaves, litter, soil) in the different ecosystems present at Mai Po, to analyse them, using the Geography laboratory facilities at UoM, for their elemental concentrations of C, H, N, O (we’ll also get S at the same time). We will then be able to calculate the carbon oxidation state (Cox) of the organic matter pools, and link to estimates of oxidative ratio (OR) of the different ecosystems. OR reflects the ratio of O2 and CO2 from photosynthesis and respiration in an ecosystem, and it is an important parameter in determining the magnitude of terrestrial C sink of the global biosphere. Recent studies have reported considerable variations in OR among ecosystems such as forests, shrublands, grasslands, and croplands, yet no attempt has been made to quantify OR in mangroves.
The proposed research will not only report on OR variation within mangroves, but we hope to link to ongoing research at CUHK in estimating the complete carbon budget for a mangrove ecosystem. As yet, there are few studies that look at the contemporary carbon budget of mangroves, and fewer still that look at both gaseous and fluvial carbon flux pathways. By linking carbon budgets and Cox/OR properties, we hope to be able to better understand organic matter cycling in these fragile ecosystems. To date only one study (Worrall et al., 2017) has been able to combine a well constrained carbon budget with estimates of Cox/OR, and this was in a blanket peatland catchment in northern England. We hope that the work in the mangroves of Hong Kong will be able to demonstrate the usefulness of Cox/OR as a novel way of looking at organic matter.
We also discussed the potential to look at how much pollution is stored in the mangroves and whether this storage is long-term, or transitory. Using sediment-source fingerprinting techniques, we may also be able to trace the sources of the pollutants – are they sourced from Hong Kong rivers, or from the larger Pearl river to the northwest?
The trip wasn’t all about work, and we managed to have a few days exploring the city and all it has to offer, including Victoria Peak, the Big Buddha and Temple Street Night Market. Derrick and Susan did a great job hosting us whilst we were there, taking us to a number of really great restaurants and generally showing us around.
Research Group Meal. Gareth Clay and Emma Shuttleworth (far left), Derrick Lai (centre purple shirt), Susan Li (far right)
- Autumn 2017: Derrick and Susan will collect samples from the mangroves and ship them to the UK for analysis.
- Winter 2017/18: Lab analysis in Geography labs at UoM
- Spring – Summer 2018: Visit to UK by CUHK team, write up of work in peer-reviewed journal and development of future grant applications.