title: A Search for Blue Carbon in  Central Salish Sea Eelgrass Meadows
Creators: name: Lutz, Mira D.
Publisher: name: Western Washington University
Year: 2018

description: <p>This study quantified eelgrass (Zostera marina) sediment organic carbon and carbon sequestration rates in Skagit County,Wa, in sites likely to support organic content and sediment accretion, the key components for carbon sequestration. These data may inform eelgrass-specific projects aimed at mitigating climate change through the protection and restoration of “blue carbon” ecosystems. Blue carbon is the carbon stored or emitted by coastal wetlands, which have the capacity to sequester more organic carbon (OC) than equivalent areas of mature forest. This study follows recent research from a single site within Padilla Bay, Wa. (Padilla transect), reporting OC stock approximately three times lower and sequestration rates nearly five times lower than reported global seagrass (all species) averages. The Padilla transect study sampled in the intertidal zone of one established transect in northern Padilla Bay National Estuarine Research Reserve. This site has minimal terrestrial sediment input and low or negative sediment accretion rates. These limitations inspired a search for higher OC storage and sequestration values by expanding the study area to include two bays with active river distributaries and selecting sites likely to support organic content and sediment accretion a priori. Site criteria included location within eelgrass meadows having a combination of similar canopy heights and stem densities but lower elevations, and closer proximities to distributary mouths of active river systems than in the Padilla transect. Study objectives were to: a) quantify OC stocks and sequestration rates in the expanded study area in sites expected to support C sequestration b) compare these values to those from the Padilla transect and to those from widespread Zostera marina studies and global seagrass meadow averages, and c) identify site characteristics associated with higher OC where found. We measured stem density, canopy height, depth below mean lower low water (MLLW), and sediment compaction in the field, and analyzed sediment samples for bulk density, carbon concentration by direct C-analysis and loss on ignition (LOI), sediment accretion rate from gamma ray spectroscopy of 210Pb activity levels, and sediment grain size in the lab. Considering site selection in areas expected to support C sequestration, results may be considered within the upper bounds of the ranges found within our study area. The overall mean % OC± SE over 3 bays was 0.43 ± 0.01 %, with a range of 0.17% to 3.66% (n=20). Overall mean OC density ± SE was 0.0058 ± 0.0001 g cm-3 and ranged from 0.0018 to 0.0479 g cm-3 (n=20). Organic C stock to 50cm ± SE was 27.10 ± 1.40 Mg ha-1 , with a range of 15.23-49.20 Mg ha-1 and SE=1.96 (n=20). The mean carbon sequestration rate ± SE was 43.88 ± 9.19 g C m-2 yr-1 , ranging from 13.90 to 93.04, SE=17.04, (n=7), which should be considered the upper bounds of possible rates in this infauna-rich region. We found higher OC sequestration rates in our study than in the Padilla transect study (F = 8.41, p = 0.01 on 1 and 10 df) however, there were no differences in % OC (F = 0.26, p = 0.62 on 1 and 13 df) nor OC stock (F = 1.86, p = 0.20 on 1 and 13 df) between studies. Like the Padilla transect study, organic C values in our study were also 3-5 times lower than estimated global averages for % OC, OC stock, and OC sequestration rates, but within the range reported by five studies conducted in Zostera marina meadows worldwide. Percent sediment pore space explained most of the variation in OC stock. We conclude that within-meadow eelgrass OC increases with environmental factors contributing to the accretion of fine sediments which increase porosity, that Z. marina meadows in the Pacific Northwest and elsewhere exhibit far lower OC values than global averages for all seagrass species, which may be due to key factors preventing it from thriving in areas conducive to high C sequestration. We recommend that region- and site-specific C values be considered when valuing restoration projects to avoid underestimation of eelgrass area required to offset emissions “purchased” through the voluntary or compliance (cap and trade) C markets.</p>