Losses and recovery of organic carbon from a seagrass ecosystem following disturbance

Abstract

Seagrasses are among the Earth's most efficient and long-term carbon sinks, but coastal development threatens this capacity. We report new evidence that disturbance to seagrass ecosystems causes release of ancient carbon. In a seagrass ecosystem that had been disturbed 50 years ago, we found that soil carbon stocks declined by 72%, which, according to radiocarbon dating, had taken hundreds to thousands of years to accumulate. Disturbed soils harboured different benthic bacterial communities (according to 16S rRNA sequence analysis), with higher proportions of aerobic heterotrophs compared with undisturbed. Fingerprinting of the carbon (via stable isotopes) suggested that the contribution of autochthonous carbon (carbon produced through plant primary production) to the soil carbon pool was less in disturbed areas compared with seagrass and recovered areas. Seagrass areas that had recovered from disturbance had slightly lower (35%) carbon levels than undisturbed, but more than twice as much as the disturbed areas, which is encouraging for restoration efforts. Slow rates of seagrass recovery imply the need to transplant seagrass, rather than waiting for recovery via natural processes. This study empirically demonstrates that disturbance to seagrass ecosystems can cause release of ancient carbon, with potentially major global warming consequences. 1. Background Seagrass ecosystems are among the most effective carbon sinks on the Earth; they bury organic carbon (often referred to as 'blue carbon') into the seabed at a rate 35 times faster than tropical rainforests [1], and where rainforests bury carbon for decades, seagrasses are capable of storing carbon for millennia [2-4]. However, there is concern that if seagrass ecosystems are disturbed they could leak vast amounts of stored carbon back into the atmosphere, thereby shifting them from carbon sinks into carbon sources [5,6], as has been shown for high-profile terrestrial carbon sinks such as forests, peatlands and perma-frost [7-9]. Importantly, the latter studies show that the rate of carbon loss is much greater than the rate of accumulation. While loss of stored carbon has been demonstrated for other coastal vegetated habitats, such as saltmarshes and mangroves [10-12], there is still major uncertainty regarding whether seagrass ecosystems release soil carbon following disturbance [1]. Fourqurean et al. [13], who provided the first comprehensive survey of seagrass carbon stocks, estimated that present rates of decline in the aerial extent of seagrass ecosystems could result in the release of up to 299Tg carbon per year, assuming all of the organic carbon in seagrass biomass and the top metre of seagrass soils is remineralized (broken down and released

Identifying knowledge gaps in seagrass research and management: An Australian perspective

“…How do changing environmental conditions, particularly anthropogenic pressures, influence microbial communities and how they function in seagrass meadows? Microbes are an important component of nutrient cycling and carbon sequestration in seagrass meadows (Hansen et al, 2000;Macreadie et al, 2015). Research on the effects of added nutrients, increased temperature, organic matter recalcitrance (resistance to decomposition), plant-derived root/ rhizome exudates etc., on microbial metabolism is a priority.…”

Rocking the Boat: Damage to Eelgrass by Swinging Boat Moorings

“…No change in density or canopy height after 10 m. Zostera marina Wales (United Kingdom)A comparison of the impact of “seagrass-friendly” boat mooring systems on Posidonia australis Demers et al, 2013Peer reviewedSwing moorings led to a 9 m radius bar patch in seagrass, screw moorings had little impact. Scar size was 254 m 2 . Posidonia australis Jervis Bay in New South WalesEstimating losses of Posidonia australis due to boat moorings in Lake Macquarie, Port Stephens and Wallis LakeGlasby and West, 2015GrayAmount of damage increases with depth and is typically greater in beds of P. australis than Z. capricorni Posidonia australis and Zostera capricorni Lake Macquarie, Port Stephens and Wallis LakeLosses and recovery of organic carbon from a seagrass ecosystem following disturbanceMacreadie et al, 2015Peer reviewedFifty years after disturbance from moorings limited recovery present Posidonia australis New South WalesImpact of mooring activities on carbon stocks in seagrass meadowsSerrano et al, 2016Peer reviewed1.3 and 0.9 ha of seagrass lost at two different sites due to moorings. Organic carbon stores have been compromised due to these moorings.…”

Reviews and syntheses: <sup>210</sup>Pb-derived sediment and carbon accumulation rates in vegetated coastal ecosystems: setting the record straight

“…Part of this is likely related to the fact that the concentration of C org is not changed, which in reality may actually change since fine sediments, where C org is more efficiently adsorbed, are more easily eroded and OM is remineralized when exposed to oxic conditions during 20 resuspension (Burdige, 2007;Lovelock et al, 2017a;Serrano et al, 2016a) (see simulations 3.2.4 and 3.2.5). Consequently, losses could be significantly larger, as shown in some papers (Macreadie et al, 2013(Macreadie et al, , 2015Marbà et al, 2015;Serrano et al, 2016a).…”