Living Shorelines: Coastal Resilience with a Blue Carbon Benefit

<div><p>Living shorelines are a type of estuarine shoreline erosion control that incorporates native vegetation and preserves native habitats. Because they provide the ecosystem services associated with natural coastal wetlands while also increasing shoreline resilience, living shorelines are part of the natural and hybrid infrastructure approach to coastal resiliency. Marshes created as living shorelines are typically narrow (< 30 m) fringing marshes with sandy substrates that are well flushed by tides. These characteristics distinguish living shorelines from the larger meadow marshes in which most of the current knowledge about created marshes was developed. The value of living shorelines for providing both erosion control and habitat for estuarine organisms has been documented but their capacity for carbon sequestration has not. We measured carbon sequestration rates in living shorelines and sandy transplanted <i>Spartina alterniflora</i> marshes in the Newport River Estuary, North Carolina. The marshes sampled here range in age from 12 to 38 years and represent a continuum of soil development. Carbon sequestration rates ranged from 58 to 283 g C m^-2 yr^-1 and decreased with marsh age. The pattern of lower sequestration rates in older marshes is hypothesized to be the result of a relative enrichment of labile organic matter in younger sites and illustrates the importance of choosing mature marshes for determination of long-term carbon sequestration potential. The data presented here are within the range of published carbon sequestration rates for <i>S</i>. <i>alterniflora</i> marshes and suggest that wide-scale use of the living shoreline approach to shoreline management may come with a substantial carbon benefit.</p></div

Total carbon stock by marsh age.

<p>Data represent averages of total organic carbon (0–30 cm), from cores of known age. Error bars show maximum and minimum values from replicate cores. Points without error bars (13, and 38 yrs., AM-N and PM-N) represent single cores.</p

Depth profiles.

<p>Comparisons of: a) soil bulk density, b) soil percent organic matter, and c) soil carbon density in core of known age from each site that is closest to 0 m NAVD88 elevation.</p

Sampling Locations.

<p>Samples were collected from PIE, PIW and PIN in 2012 and 2013 for analysis of belowground biomass/elevation trends. Cores were collected in 2014 from all sites except PIN for analysis of soil carbon.</p

Carbon sequestration rate.

<p>Rates were calculated as: (total carbon stock—background)/marsh age, for cores of known age from each marsh. Error bars show maximum and minimum values from replicate cores from each site. Points without error bars (13, and 38 yrs.) represent single cores.</p

Elevation and carbon characteristics of individual cores collected in 2014.

<p>Bold font represents cores that were collected from regions that were known to be planted at the time of marsh creation (cores of known age). Sites are designated as high (H), mid (M), or low (L) based on their relative elevation within each site. N = natural reference marshes. Values of carbon density are reported for the top 5 cm of each core. Total C stock was calculated as the sum of bulk density times % organic matter for each 5 cm interval. Sequestration rate was calculated as total C stock divided by marsh age.</p><p>* Marsh age not determined.</p><p>** Core only extended to 20 cm depth.</p><p>^† These sites have not been colonized by <i>S</i>. <i>alterniflora</i> for entire duration of marsh age, thus sequestration rates may be underestimated.</p><p>Elevation and carbon characteristics of individual cores collected in 2014.</p

Organic matter, carbon and nitrogen profiles.

<p>Cores were collected at mature (~ 38 yr. old) and young (< 5 yr. old) regions of the same marsh. The presumptive sediment surface at time of planting was calculated by assuming a rate of surface elevation increase equivalent to the locally measured rate of sea level rise (see text for details).</p

Live aboveground biomass by elevation.

<p>Total aboveground biomass measured at the collection site of each soil core.</p

Belowground biomass production by depth.

<p>Total amount of belowground biomass (> 2 mm) by depth interval in ingrowth bags after one year of growth in: a) low, and b) high elevation cores. Cores were 10–30 cm in total depth.</p

Conceptual model of carbon burial and turnover in a newly created marsh.

<p>At each time step a new “cohort” of carbon is added to soil as BGB. Each new cohort is represented by a different color. The decrease in size of a given cohort over time represents remineralization of the labile and semi-labile fractions. This remineralization continues until only the recalcitrant material remains. The result is that over time the bulk reactivity of the soil decreases as does the time-averaged carbon sequestration rate. Note that in this conceptual model, the amount of new carbon being input each year is constant. In a natural marsh, biomass, and therefore new carbon inputs, will fluctuate annually. As a result carbon stock is likely to fluctuate over time but will show a general upward trend over long time scales.</p