Overgrazing of Seagrass by Sea Urchins Diminishes Blue Carbon Stocks

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Seagrasses are among the Earth’s most efficient ecosystems for sequestering carbon, but are also in global decline, risking carbon they have accumulated over geological timescales. One contributor to this global decline is seagrass overgrazing by sea urchins; however, it is unknown how this may affect stocks of “blue carbon” by damaging the seagrass root systems that stabilise the carbon-rich sediments of seagrass meadows. To fill this knowledge gap, we used aerial and sonar mapping plus soil carbon measures to investigate a seagrass urchin overgrazing event in Southeast Australia and quantified the concomitant impacts on blue carbon stocks. We found that seagrass loss significantly diminished local organic carbon stocks. The decline was also rapid: areas grazed within the preceding 6 months showed a 35% loss of blue carbon, which continued even after urchins had left the area (46% loss after 3 years). High-resolution 3D sonar reconstructions revealed that urchin overgrazing of seagrass caused erosion of the top 30 ± 20 cm of sediment within the 26,892 m2 barren: the equivalent of 8100 ± 5400 m3 of sediment. To calculate the additional CO2 emissions from this erosion, we assumed between 50 and 90% of the seagrass carbon stock (11.7 ± 1.24 t Corg ha−1 in the top 10 cm) would be remineralised, resulting in the release of between 57.8 and 104 tonnes of CO2 equivalents due to sea urchin overgrazing-induced erosion. This study adds to a growing body of evidence that seagrass loss leads to erosion and concomitant loss of blue carbon stocks.