Blue Carbon Storage Capacity of Temperate Eelgrass (Zostera marina) Meadows

Abstract

Despite the importance of coastal ecosystems for the global carbon budgets, knowledge of their carbon storage capacity and the factors driving variability in storage capacity is still limited. Here we provide an estimate on the magnitude and variability of carbon stocks within a widely distributed marine foundation species throughout its distribution area in temperate Northern Hemisphere. We sampled 54 eelgrass (Zostera marina) meadows, spread across eight ocean margins and 36° of latitude, to determine abiotic and biotic factors influencing organic carbon (Corg) stocks in Zostera marina sediments. The Corg stocks (integrated over 25‐cm depth) showed a large variability and ranged from 318 to 26,523 g C/m2 with an average of 2,721 g C/m2. The projected Corg stocks obtained by extrapolating over the top 1 m of sediment ranged between 23.1 and 351.7 Mg C/ha, which is in line with estimates for other seagrasses and other blue carbon ecosystems. Most of the variation in Corg stocks was explained by five environmental variables (sediment mud content, dry density and degree of sorting, and salinity and water depth), while plant attributes such as biomass and shoot density were less important to Corg stocks. Carbon isotopic signatures indicated that at most sites <50% of the sediment carbon is derived from seagrass, which is lower than reported previously for seagrass meadows. The high spatial carbon storage variability urges caution in extrapolating carbon storage capacity between geographical areas as well as within and between seagrass species.

A Synthesis of Blue Carbon Stocks, Sources, and Accumulation Rates in Eelgrass (Zostera marina) Meadows in the Northeast Pacific

“…The highest sediment OC content was found in Southeast Alaska and South Slough, Oregon, while the lowest values were found in the Southern Salish Sea (Figure ). As such, we did not identify a latitudinal trend in the data, consistent with Röhr et al (), who found that latitude explained relatively little variation in OC stocks in Z. marina meadows across the Northern Hemisphere. For example, although the highest %OC values were measured in Alaska (2.32%), some sites in Alaska also had very low values (0.09%).…”

“…The sediment carbon content values reported here fall within the range of those reported from other temperate Z. marina meadows globally. The mean OC content for the entire Pacific Northwest region (0.7%) is within the range of values reported from eelgrass meadows in the Black Sea (0.2%), Portugal (0.6%), and the Baltic Coast of Sweden (0.2%; Dahl et al, ), albeit slightly lower than the average value of 1.4% reported from Z. marina across the globe (Röhr et al, ) and the Gullmarn Fjord in Sweden (2.8%; Dahl et al, ). Our PNW values also align with those reported from meadows on the Atlantic coast of North America of 0.4% (Greiner et al, ) as well as from Denmark (1.7%) and Finland (0.2%; Röhr et al, ).…”

“…Error bars represent standard error. Sources for %OC values are as follows: 1 = Fourqurean et al (), 2 = Green et al (2018), 3 = Röhr et al (), 4 = Röhr et al (), 5 = Dahl et al (), 6 = Greiner et al (), 7 = Postlethwaite et al ().…”

“…Error bars denote standard errors. Sources for OC stock values are as follows: 1 = Lavery et al (), 2 = Röhr et al (), 3 = Röhr et al (), 4 = Postlethwaite et al (). Note that these were the only studies found that calculated OC stocks to 25 cm.…”

“…Organic carbon stocks were calculated by summing the carbon mass from each subsection up to 25‐cm depth, as described by Howard et al (). A depth of 25 cm was selected for carbon stock comparisons to allow inclusion of the greatest number of our cores, and to allow comparisons to carbon stocks from previous studies (e.g., Dahl et al, ; Lavery et al, ; Röhr et al, ). It should be noted that OC values were relatively constant below 10 cm to depths up to 100 cm, which supported both retaining the Sediment Mixed Layer and extrapolating stocks to 1‐m depth for comparison across studies.…”