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California Salt Marsh Accretion, Ecosystem Services, and Disturbance Responses In the Face of Climate Change


Coastal salt marsh ecosystems in California are at risk from projected rates of sea-level rise (SLR) of up to an order of magnitude higher than rates seen over the past 6,000 years of stable sea levels (Griggs, Cayan, Tebaldi, Fricker, & �rvai, 2017). With rates of this magnitude, salt marsh area, already limited by land use changes in the 19th and 20th centuries, could be completely lost by 2100 (Thorne et al., 2018). To better understand how California salt marshes are adapting to modern acceleration of SLR, over 100 sediment cores were collected from 13 salt marsh sites, ranging from Humboldt Bay to Tijuana River Estuary. Sediment accretion rates over the past several hundred years were measured using radiocesium, radiolead, and radiocarbon dating on 32 cores. Valuation of the carbon storage, an ecosystem service known as blue carbon provided by salt marshes, presents an opportunity to help preserve and restore sites threatened by SLR through carbon credits (Bear, 2017; Callaway, Borgnis, Turner, & Milan, 2012; Mcleod et al., 2011), but there are many questions which much be addressed before this can become a reality for the state of California (Macreadie et al., 2019). A standardized protocol for estimation of carbon content from loss-on-ignition (LOI) was developed with an emphasis on quantifying error and uncertainty in carbon measurements for blue carbon purposes. Using a conversion between soil organic matter and soil organic carbon shown to be effective for California salt marshes, carbon content was estimated through LOI analysis of 61 sediment cores. The impact of climate change in these ecosystems was further explored in the first documented record of a fire in a Pacific coast salt marsh at Mugu Lagoon.

California salt marsh sediment accretion averages at 2.93 � 1.9 mm yr-1, which is lower than average rates from regions such as the US Gulf and East coasts. Rates of accretion and relative SLR (RSLR) show a non-linear relationship with highest accretion occurring at rates of RSLR from 2 – 6 mm yr-1. Linear relationships between SLR and accretion are comparatively weak, but are stronger in the low elevations of salt marsh habitat. Salt marshes in the state annually sequester about 0.08% of state-wide annual greenhouse gas emissions and store about 23% of one year’s emissions in their soils (as compared to 2016 emissions). Because of limited area, these habitats will not serve as an effective mitigation strategy at the state level, but loss of this habitat may release up to 27 � 0.3 Tg stored carbon, potentially valued at about $1.4 billion (using an estimate of $15/tonne CO2 �equivalent). Preservation of current habitat through facilitation of sediment accretion will have the largest positive impact on carbon storage and sequestration, as well as protect salt marsh habitat from being lost to SLR. Analysis of the persistent effects of a recent marsh fire at Mugu Lagoon demonstrates that drought-stress may slow California salt marsh response to disturbance by one or more growing seasons and highlights the uncertain impacts of climate change on system function. This dissertation provides important baseline data for salt marsh sediment accretion, salt marsh carbon stocks and sequestrations rates, recommends best practices for use of LOI as a measure of soil organic carbon, and examines ecosystem recovery under multiple stressors. This work can be used in vulnerability assessments, ecosystem models, and valuation of ecosystem services for California salt marshes.

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