Seagrass losses since mid‐20th century fuelled CO(2) emissions from soil carbon stocks

Seagrass meadows store globally significant organic carbon (C(org)) stocks which, if disturbed, can lead to CO(2) emissions, contributing to climate change. Eutrophication and thermal stress continue to be a major cause of seagrass decline worldwide, but the associated CO(2) emissions remain poorly understood. This study presents comprehensive estimates of seagrass soil C(org) erosion following eutrophication‐driven seagrass loss in Cockburn Sound (23 km(2) between 1960s and 1990s) and identifies the main drivers. We estimate that shallow seagrass meadows (<5 m depth) had significantly higher C(org) stocks in 50 cm thick soils (4.5 ± 0.7 kg C(org)/m(2)) than previously vegetated counterparts (0.5 ± 0.1 kg C(org)/m(2)). In deeper areas (>5 m), however, soil C(org) stocks in seagrass and bare but previously vegetated areas were not significantly different (2.6 ± 0.3 and 3.0 ± 0.6 kg C(org)/m(2), respectively). The soil C(org) sequestration capacity prevailed in shallow and deep vegetated areas (55 ± 11 and 21 ± 7 g C(org) m(−2) year(−1), respectively), but was lost in bare areas. We identified that seagrass canopy loss alone does not necessarily drive changes in soil C(org) but, when combined with high hydrodynamic energy, significant erosion occurred. Our estimates point at ~0.20 m/s as the critical shear velocity threshold causing soil C(org) erosion. We estimate, from field studies and satellite imagery, that soil C(org) erosion (within the top 50 cm) following seagrass loss likely resulted in cumulative emissions of 0.06–0.14 Tg CO(2‐eq) over the last 40 years in Cockburn Sound. We estimated that indirect impacts (i.e. eutrophication, thermal stress and light stress) causing the loss of ~161,150 ha of seagrasses in Australia, likely resulted in the release of 11–21 Tg CO(2) (‐eq) since the 1950s, increasing cumulative CO(2) emissions from land‐use change in Australia by 1.1%–2.3% per annum. The patterns described serve as a baseline to estimate potential CO(2) emissions following disturbance of seagrass meadows.