Dr Kerrylee Rogers on her Eureka-prize winning research on coastal wetlands & blue carbon
We hear from Dr Kerrylee Rogers, Associate Professor in the School of Earth and Environmental Sciences at the University of Wollongong in NSW, Australia. Find out more about Kerrylee here and follow her on Twitter @KerryleeRogers_
Why do I study coastal wetlands?
Coastal wetlands (mangrove, saltmarsh, seagrasses and tidal forests) have been recognised as the most efficient natural systems in the sequestration of atmospheric carbon dioxide (CO2). Termed “blue carbon” ecosystems due to their high carbon content and connection to the sea, they are highly productive and the saline, oxygen-depleted soils in which they grow are ideal for the burial and long-term storage of organic carbon. However, these carbon power houses are also extraordinary because of the suite of ecosystem services that they provide: coastal protection; erosion control; flood mitigation; water purification; raw material; maintenance of fisheries;, wildlife habitat; and tourism, recreation and education services.
I have been studying the relationship between coastal wetlands and sea level, and the impacts of sea-level rise and anthropogenic activities on ecosystem service delivery for 20 years. It has become a passion that has taken me to study sites around the world; and I am truly grateful for this opportunity. My travels have allowed me to realise that no study sites are the same, each is different in terms of coastal processes, sediment character, climate and ecology. Most importantly, I have learnt that when I read scientific literature, I am often reading it and through the lens of my own experiences studying coastal wetlands in Australia. That is, I am interpreting research in the context of my own study sites; and I have realised that this can severely narrow your field of view.
Why are Australian coastal wetlands different?
Since undertaking my PhD in 2002-2005 I have not been able to reconcile some of the research in the scientific literature about the substrate evolution and sediment character of coastal wetlands in North America with my own experience of coastal wetlands in Australia. In particular, many coastal wetlands along the Atlantic coastline of North America were adjusting to sea level rise through mineral sediment addition and significant amounts of organic matter addition to substrates. Rates of vertical elevation gain were also substantially higher than what I was measuring in Australia. By contrast, our wetland substrates were significantly more mineral sediment dominated, and rates of vertical elevation gain was lower. In combination, vertical elevation gain largely in the absence of significant organic matter addition and low rates of vertical elevation gain seemed concerning for the long-term resilience of coastal wetlands. But we were not seeing a decline in wetland resilience; rather, vegetated shorelines have remained relatively stable over the aerial photograph record, and where there have been changes, these could be explained by localised factors that altered hydrodynamic energy. These differences remained a curiosity (actually it bugged me that I couldn’t explain them).
Blue Carbon and how coastal wetlands draw down CO2
In 2013, the Blue Carbon Horizons team that I led commenced research on a truly unique field site on the shores of Lake Macquarie, NSW, Australia where underlying subsidence of a mine led to submergence of a coastal wetland at Chain Valley Bay. The ensuing relative sea-level rise of approximately 1 m over a few months, was roughly equivalent to the degree of sea-level rise projected by the IPCC to occur under the highest sea-level rise scenarios by the end of this century. Here we had the opportunity to use this site in an experimental capacity as the subsidence event was analogous to a high sea-level rise manipulation. On the surface, the impact of the rapid relative sea-level rise was dramatic: all of the coastal wetland vegetation zones moved landward. The mangroves occupied a zone previously colonised by saltmarsh with scattered Casuarina trees, and the saltmarsh colonised land that had previously been Casuarina forest. This transition demonstrates that wetland communities are capable of transitioning landward if given the chance.
However, it was the changes beneath the surface that excited us the most. The wetland underwent a rapid trajectory of self-recovery, rebuilding its substrate elevation and sequestering carbon from the atmosphere at the same time. That is, the rate of surface accretion in the impacted wetland doubled following the subsidence, and the proportion of organic carbon in this material was four times that occurring prior to relative sea-level rise. This work from the Blue Carbon Horizons team of researchers demonstrated two potentially significant negative feedbacks: (1) as the rate of relative sea-level rise increases, the rate of marsh elevation gain also increases; and (2) coastal wetlands will act as a negative feedback on atmospheric CO2 concentrations as they draw down more CO2 under future sea-level rise.
We now also had a new hypothesis to consider when trying to reconcile global changes in coastal wetlands: Could the vertical space available for mineral and organic matter accumulation, and carbon storage, be controlled by sea-level rise? We analysed carbon stored in 345 saltmarsh settings across six continents, and found that coastlines subject to gradual sea-level rise over the past few thousand years (North America, Europe) had, on average, 2-4 times more carbon in the surface 20 cm of sediment, and 5-9 times more carbon in the lower 50-100 cm of sediment, compared to saltmarshes on coastlines with a long period of stable sea-level (Australia, South Africa, South America and China). On coastlines where sea level has been rising most rapidly, organic carbon is more efficiently buried as the wetland accretes and is stored safely below the surface. On coastlines where sea level has been stable or falling, less organic carbon is preserved. However, with higher rates of sea-level rise forecast, these latter coastlines may be the sleeping giants of global carbon sequestration, with sea-level rise triggering an increase in carbon accumulation.
Publishing in Nature and winning a prestigious Eureka Prize
We knew we were on to something when the results came in from our analyses at Chain Valley Bay. Having connected to researchers around the world, we were able to leverage our knowledge and data against theirs, and together our Eureka moment came. Our analysis of global carbon storage in wetlands, and potential carbon storage (based on analyses at Chain Valley Bay) was published in Nature, and showed for the first time that sea-level history played a major role in determining the concentration of organic carbon in coastal wetlands. Importantly, we confirmed that this opportunity is only available if we manage coastal wetlands appropriately by allowing them to continue to accumulate sediment and expand laterally as the sea rises. The Blue Carbon Horizons team were nominated and won the Australian Museum Eureka Prize for Environmental Research for this work showing that the capacity of coastal wetlands to store carbon will substantially increase with sea level rise, providing a counter to global warming. Working alongside government, the team’s research is being used to protect and restore coastal ecosystems.
Collaboration, life balance and carbon emissions
Our Eureka moment has made me realise the benefits of connecting with researchers, travelling to other peoples study sites, and collaboration. For me, this has meant that my network of collaborators around the world has increased, and I have the capacity to explore global scale processes with more confidence. However, everything has its advantages and disadvantages. In an age where women are having to juggle teaching, research and governance activities, with families and work-life-balance; where we are often expected to be all things and do it at once; how do we also fit in travel and face-to-face collaboration? At a time when reducing carbon emissions and contributing to carbon drawdown is crucial, can we still justify international travel? These are ongoing struggles for me that remain to be resolved. In the mean-time, I offset my carbon emissions when I travel, engage with other researchers on social media (twitter is my favourite), and do as many conference fieldtrips as possible; and if you don’t mind, I will try to tag along on your field trips too.Posted on: 09/09/2019, by : Shari Gallop