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.
(© 2020 The Authors. Global Change Biology published by John Wiley & Sons Ltd.)