Coastal Carbon Opportunities: vegetation dynamics at Mutton Cove, a site of recent tidal re-connection

Abstract

Coastal vegetated ecosystems accumulate large amounts of organic carbon and can store it in their sediments for thousands of years. These carbon-rich coastal habitats may be extremely valuable for mitigating the effects of increased atmospheric greenhouse gas concentrations, as they are disproportionately significant in their capacity to store carbon. Saltmarshes and mangroves are often found adjacent to one another in the intertidal zones of temperate coastlines. They form a natural ecotone; but this natural boundary is dynamic and influenced heavily by tidal inundation. Small changes in inundation such as restriction through the construction of a seawall, or increased inundation due to sea-level rise, will lead to shifts in the ecotone as different conditions will favour different coastal vegetation communities. The aims of this task of our Coastal Carbon Opportunities project were to: 1) use novel and innovative methods for collection of high spatial resolution remote sensed survey data of coastal wetland vegetation dynamics using drones and other methods; and 2) assess potential impacts of vegetation changes on carbon storage dynamics. We achieved both these aims and provide novel, drone-based mapping data that demonstrate methods for monitoring tidal wetland vegetation cover and health, as well as linking historic changes at the site with carbon gains and losses. Specifically, we investigated the management history and vegetation dynamics at a site of remnant coastal wetland (mangrove and saltmarsh) called Mutton Cove on the Le Fevre Peninsula, north of Adelaide. The site was originally a natural system of tidal creeks fed by the Port River and was dominated by mangroves, like the mangrove and saltmarsh ecosystem still found on nearby Torrens Island. A seawall was built around Mutton Cove in the early 1970s, which led to drying of the site, subsidence, complete loss of mangroves and minimal retention of only stranded, patchy saltmarshes. The site remained in this degraded state for the next few decades, until restoration was undertaken in the early 2000s, which facilitated controlled tidal flows through pipes in the seawall. This led to limited tidal inundation of the site, which favoured saltmarshes over mangroves, which flourished. In 2016, the seawall was breached by an extreme water level caused by a storm surge and coincident high tide, which has not been repaired. In the years since 2016, the saltmarshes have died back, due to their low elevation in the subsided site and the now considerable amount of water coming in through the seawall breach. At the same time mangroves have continued to grow along the edges of the main creeks, as well as gradually starting to colonise areas further from the creeks, particularly the lower lying parts of the site. We have used a suite of data to document and measure the tidal inundation and vegetation changes at the site through time. These include historical aerial images; sediment elevation tables; water level loggers; on-the-ground vegetation and elevation surveys; and remote sensed imagery collected using drones. Using digitised historical imagery to generate maps, we recorded a significant historical shift from mangrove to saltmarsh at the site, along with considerable disturbance and degradation after the seawall was built. The impacts of these changes can be detected in the sediment core samples we collected as part of Task 1 of the Coastal Carbon Opportunities project. The core sample result showed a loss of carbon stocks and a reduction in carbon accumulation over the last 70-years, coinciding with the period after the seawall was built (early 1970s) when the site became dry and tidal vegetation cover reduced. We used the on-the-ground surveys and drone-based imagery to detect clear and broadscale saltmarsh die-back at the site over the last 18-months, associated with the uncontrolled inundation resulting from the seawall breach (also recorded using the water level loggers). We have combined these results with information from the literature to better understand the carbon storage implications of the recent changes, since the seawall breach. We propose that there is likely some short-term loss of carbon at the site linked to saltmarsh die off. This is due both directly to the loss of saltmarsh vegetation (above-ground carbon) as well as mobilisation of sediment associated with vegetation loss leading to some emissions of carbon from the below-ground carbon store. However, if mangroves can colonise (as they are already starting to do), we estimate that the site will ultimately have the capacity to accumulate greater amounts of carbon than before, because of the increased above-ground biomass and carbon storage potential of mangroves compared to saltmarshes.