Blue carbon hotspots: Distribution and abundance of blue carbon in Victoria

The recent discovery of the ability of ‘blue carbon habitats’– mangroves, saltmarsh, and seagrass – to bury carbon at rates over 40 times faster than their terrestrial counterparts has shed new light on opportunities... [ view full abstract ]

The recent discovery of the ability of ‘blue carbon habitats’– mangroves, saltmarsh, and seagrass – to bury carbon at rates over 40 times faster than their terrestrial counterparts has shed new light on opportunities to reduce greenhouse gases in our atmosphere. Importantly, it has also revealed another critical ecosystem function of these habitats that may be lost when disturbed. While we are gaining a better understanding of the capacity of blue carbon habitats globally, we still lack an understanding of how carbon stocks vary spatially, particularly in southern Australia. This study aimed to quantify sediment blue carbon stocks across the five Victorian coastal catchment regions to answer the following questions: 1) How much carbon is stored in blue carbon sinks in Victoria? 2) How do carbon stocks vary across habitats and locations? 3) Where are carbon ‘hot spots’ and are they under threat?

Sediment cores were collected at over 100 blue carbon habitat locations along the entire Victorian coast. The sampling distribution covered over 2,000km of coastline, all major estuaries, and many remote locations. Sediments were dried and weighed to calculate dry bulk density, and analyzed for organic carbon. Carbon density measurements, in conjunction with state-wide habitat mapping of saltmarsh and mangroves, and regional seagrass mapping, were used to estimate total blue carbon sediment stocks in Victoria.

Based on an estimated 76,000 hectares of blue carbon habitat in Victoria, we calculated over three million tonnes of organic carbon in the top 30cm of the sediment. Saltmarsh habitat consistently had the highest carbon stocks, followed by mangroves, then seagrass. Carbon stocks varied among locations, and appear to be higher in upstream locations within an estuary than those further downstream. The West Gippsland catchment region had the highest total carbon stocks, followed by the Port Phillip and Westernport catchment region. Carbon hotspots were classified by carbon density and habitat patch size, and identified through geospatial modelling.

We suggest blue carbon stocks be incorporated into evaluations of ecosystem services so the value of these habitats as carbon sinks can be considered when prioritizing areas for conservation.