Abstract
Worldwide, seagrass meadows accumulate significant stocks of organic carbon (C), known as “blue” carbon, which can remain buried for decades to centuries. However, when seagrass meadows are disturbed, these C stocks may be remineralized, leading to significant CO2 emissions. Increasing ocean temperatures, and increasing frequency and severity of heat waves, threaten seagrass meadows and their sediment blue C. To date, no study has directly measured the impact of seagrass declines from high temperatures on sediment C stocks. Here, we use a long-term record of sediment C stocks from a 7-km2, restored eelgrass (Zostera marina) meadow to show that seagrass dieback following a single marine heat wave (MHW) led to significant losses of sediment C. Patterns of sediment C loss and re-accumulation lagged patterns of seagrass recovery. Sediment C losses were concentrated within the central area of the meadow, where sites experienced extreme shoot density declines of 90% during the MHW and net losses of 20% of sediment C over the following 3 years. However, this effect was not uniform; outer meadow sites showed little evidence of shoot declines during the MHW and had net increases of 60% of sediment C over the following 3 years. Overall, sites with higher seagrass recovery maintained 1.7x as much C compared to sites with lower recovery. Our study demonstrates that while seagrass blue C is vulnerable to MHWs, localization of seagrass loss can prevent meadow-wide C losses. Long-term (decadal and beyond) stability of seagrass blue C depends on seagrass resilience to short-term disturbance events.
Highlights
Seagrass meadows, salt marshes, and mangrove forests have been recognized as important sinks for carbon (C) on a global scale (Mcleod et al, 2011)
This is the first study to directly quantify the impact of a marine heat wave on sediment C stocks in a seagrass meadow
Using long-term data from a landscape-scale restoration project that spanned the period before, during, and after the marine heat wave (MHW), we report that inner meadow sites experiencing extreme shoot density declines of 90% during the MHW showed net losses of 20% of C in upper sediments (0–5 cm) over the three following years
Summary
Salt marshes, and mangrove forests have been recognized as important sinks for carbon (C) on a global scale (Mcleod et al, 2011). Numerous processes will affect the vulnerability of sediment C following disturbance and vegetation loss, including sediment deposition and erosion, redox oscillations from physical and biological drivers, and plant-microbe interactions (Spivak et al, 2019) These interacting processes occur over varying spatial and temporal scales and impact the preservation of existing C stocks and the continued accumulation of new organic C inputs (Belshe et al, 2017). No study to date has empirically measured seagrass sediment C stocks before and after a temperature-induced dieback, leaving uncertainty about the spatial and temporal effects of temperature-induced seagrass loss and subsequent recovery on sediment C stocks We address this knowledge gap using long-term data collected at the Virginia Coast Reserve Long-Term Ecological Research site (VCR). As part of a long-term (18 years) study, we continued measurements for 3 years after the initial loss of shoots to understand the patterns in seagrass C loss and re-accumulation over time
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