A case study on the relative organic carbon content response to intertidal sediment disturbances

While research in climate mitigation solutions have led to great interest in Blue Carbon Ecosystems (BCEs), such as mangroves, seagrass meadows, and tidal marshes, there has been a growing understanding that other marine environments could be critical to our understanding of BCEs. These “emerging” BCEs, include tidal flats and marine sediments. While receiving less attention than “traditional” BCEs, unvegetated sediments store significant amounts of carbon, globally accounting for an estimated 3,117,000 Mt of organic carbon (OC) within just the top 1 meter of sediment. The potential contributions of emerging BCEs to carbon sequestration are undoubtedly important, but physical disturbances, such as bottom trawling, dredging and climate-change-related weather events such as storms, risk transforming these carbon sinks into carbon sources. Understanding how physical disturbances affect sediment carbon storage and release is paramount to holistic, practical, and successful protection and management of coastal carbon resources.  

This study investigated the relative carbon loss from intertidal sediments with simulated physical disturbance to different depths over 9 weeks. Disturbance was applied to 20 intertidal sediment plots in the Tay estuary, Scotland weekly. The depth of disturbances varied across plots, including 2cm (surface), 10cm, 20cm depths, alongside controls (no disturbance) with 4 replicate plots of each. In addition, complete homogenization of sediment to 30cm depth was performed in the lab to simulate a single large mixing event. In-tact cores were extracted from all plots on weeks 2, 5 and 9 to understand cumulative effects. Each core was capped with a custom-built lid attached to an EMG-5 portable gas analyser to measure CO₂ flux from the sediment. Each core was then artificially eroded under flow in the laboratory, with the labile and refractory fractions of the particulate carbon quantified from the eroded material. Cores were sliced to generate sediment profiles of different carbon fractions, and to quantify OC:N ratios. 

The single large mixing event simulated in week 9 (sediment homogenised to 30cm) resulted in a significantly higher loss of both labile and refractory carbon during erosion, while the loss of labile and refractory carbon was reduced in plots disturbed to 20cm depth compared to controls. CO₂ flux data was variable across the weeks and treatments. While less pronounced, the changes in the carbon and nitrogen composition of sediment bed profiles, together with the resuspension of carbon under flow suggest that repeated disturbances may alter the subsequent loss of labile and refractory carbon to the overlying water column after disturbance events. More importantly, the disturbance from a single large mixing event can lead to significant subsequent losses of both labile and refractory carbon from the bed. Further investigations into the flux of dissolved and particulate carbon due to bed disturbance are required to understand the impact of physical disturbance on mudflat carbon dynamics.