Long-term carbon storage and its recent loss in an estuarine Posidonia australis meadow (Albany, Western Australia)

• Carbon has accumulated in Posidonia australis sediments over the past 3000 years.
• Seagrasses contributed at least 57% of the total sedimentary organic matter.
• Historical P. australis cover can provide upper-end estimates of carbon stores.
• Meadow disturbance may result in an absence of carbon sequestration.

Oyster Harbour, on the south coast of Western Australia, supports 3.6–3.9 km2 of seagrass meadows, following the loss of approximately 2.8–3.1 km2 in the 1980s. This small area of prevailing meadows hold significant carbon stores accumulated over the past 3000 years. In this study, we sampled three sediment cores from a Posidonia australis meadow and analysed organic matter (OM), organic carbon (OC) and inorganic carbon (IC) contents in bulk sediments, and δ13C signatures of OM. The OM and OC contents (mean ± SE) in the cores were 9.07 ± 0.36% and 2.24 ± 0.05%, respectively. The mean IC content was 3.16 ± 0.17%. δ13C signatures of the sedimentary OM ranged from −10.01‰ to −13.28‰. Using a Bayesian isotopic mixing model, it is estimated that 57–67% of the OM in the seagrass sediments was derived from P. australis detrital matter. The total carbon (TC) stores in 150 cm-thick seagrass sediments averaged 27.92 kg TC m−2 (10.79 kg OC m−2 and 17.13 kg IC m−2). Based on radiocarbon dating, the mean sediment accumulation rate was 0.0494 cm yr−1, which led to a long-term TC accumulation rate of 8.92 g TC m−2 yr−1 (3.45 g OC m−2 yr−1 and 5.47 g IC m−2 yr−1). Based on historical seagrass cover (3.6–6.7 km2 during the 1960s to 1980s), the estimated TC stores in 150 cm-thick seagrass sediments at Oyster Harbour would have been 101–187 Gg TC. The eutrophication-driven loss of seagrasses during the 1980s resulted in the absence of OC accumulation capacity amounting to 280–310 Mg OC (over 29 years). The loss of seagrass area could also have resulted in the release of 37–41 Gg CO2, assuming that all of the OC in shallow sediment is remineralised after meadow disturbance. These results exemplify the importance of seagrasses meadows as important carbon sinks and the potential for losses of carbon stores due to ecosystem degradation.

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