Dissolved organic carbon dynamics in a salt marsh creek

Salt marshes are blue carbon systems that sequester carbon at higher rates than many terrestrial ecosystems due to a coupled relationship between high primary production and slow decomposition in anaerobic sediments. Annually, this coupled relationship allows for over 10 Tg of organic carbon to be sequestered in global salt marsh sediments alone, or a storage equivalent of over 55,000 Blue Whales per year. In turn, this storage ability enables salt marshes to help mitigate increasing atmospheric CO2. Despite high primary production in salt marshes and their ability to help mitigate increasing atmospheric CO2, a long-standing question remains in coastal carbon cycling and ecology which asks: Is a fraction of salt marsh produced carbon, prior to sequestration or mineralization, exported (i.e., outwelled) as dissolved organic carbon (DOC) to the coastal ocean? Answering this question of salt marsh DOC outwelling is critical for quantifying the significance of salt marsh carbon outwelling in comparison to total salt marsh carbon storage, total salt marsh primary production, and broader coastal carbon cycling. Because the question of DOC outwelling first arose on the Georgia coast and because the Georgia coast houses some of the most productive salt marshes in the world, this dissertation focuses on analyzing DOC outwelling in Groves Creek, a tidally-driven salt marsh creek on the Georgia coast. Groves Creek was additionally chosen as it lacks a freshwater head and has limited freshwater input, making the analysis of marsh-only DOC fluxes through the estuarine water possible without confounding results from terrestrial DOC input. In Groves Creek and other Georgia salt marsh creeks, DOC is a master variable that controls the light field, initiates photochemical reactions, and provides sustenance to microbes. The dynamics of DOC in these systems are complex as multiple DOC sources, sinks, and patterns of mixing occur. The complexity in salt marsh DOC dynamics plus the failure of past studies to capture export trends in marsh-derived DOC at both high-temporal resolution and across seasons may explain why it remains unclear whether salt marshes generally export DOC (i.e., outwell). Thus, at a Groves Creek study station, this dissertation sought to answer the question of salt marsh DOC outwelling over three research captures. At Groves Creek study station, Chapter 1 captured hydrology (water level, velocity, flow) at 10-minute resolution over 16-months using an in situ Acoustic Doppler Profiler (ADP) deployed in the creek bed over 7 deployments. After data collection, the hydrology record indicated that the ADP instrument was not deployed in precisely the same location of the creek bed for all deployments. Thus, to make hydrology comparable over the entire study, hydrology records required alignment using a novel alignment approach in which non-tidal signals from individual ADP deployments were added to an extrapolated tidal signal based upon three already aligned deployments. Chapter 2 went on to assess DOC concentration at Groves Creek study station at the same temporal resolution and study length as Chapter 1. As no in situ instrument exists that could directly measure DOC concentration, DOC was estimated in Chapter 2 through the use of site-specific machine learning and linear algorithms coupled with optical and other low-to-zero cost predictors (e.g., water level, salinity, local rainfall) collected at high-temporal resolution. Models were trained using 306 discrete lab-based DOC measurements collected as water samples from the study station. These discrete samples served as ground truth. Work from Chapter 2 included the first-ever incorporation of non-linear machine learning to estimate DOC concentration. By combining DOC concentration (Chapter 2) with water flux (Chapter 1), plus measured salinity (Chapter 3), Chapter 3 was able to calculate DOC fluxes at Groves Creek and ultimately assess the long-standing and inconclusive topic of DOC outwelling. Chapter 3 provided the first-ever estimation of both high-temporal (10-minute) and cross-seasonally (16-month) resolved DOC fluxes. Results show Groves Creek is hydrologically complex with ebb-dominated tidal asymmetry and often more water flowing into the main channel than out (Chapter 1). Since the marsh is hydrologically balanced overall, net imported water likely drained the marsh via unsampled flow paths (e.g., smaller channels, overmarsh flow at marsh edge). Concerning DOC estimation (Chapter 2), at seasonal timescales, machine learning (mean absolute error (MAE) 3.7%) modestly improved upon the accuracy of linear methods (MAE 6.5%) but offered substantial instrumentation cost reductions (~90%) by requiring only cost-free predictors (online data) or cost-free predictors in combination with low-cost in situ predictors (temperature, salinity, depth). At intratidal timescales, linear methods proved ill-equipped (median Pearson's correlation coefficient (R) 0.55) to predict DOC concentration compared to machine learning (median R 0.87-0.94), and again machine learning offered a substantial instrumentation cost reduction (~90%). Thus, one of the main advances set forth in this dissertation is a novel, improved accuracy, and lower-cost method to estimate DOC concentrations in complex aquatic ecosystems. The results of this portion of the dissertation, as presented in Chapter 2, are under a second round of review at Limnology and Oceanography: Methods. Chapter 3 marks the culmination of my PhD research by combining hydrologic fluxes (Chapter 1) and DOC estimates from the two top-performing machine learning algorithms (Chapter 2) to estimate net DOC fluxes through Groves Creek and test the hypothesis that salt marshes outwell DOC (Chapter 3). DOC flux results show that cumulative net DOC-flow and DOC-salt relationships were largely conservative, indicating DOC outwelling was not supported over most of the study period at the Groves Creek study station. However, during summer 2014, the conserved DOC-flow and DOC-salt relationships were disturbed with a loss of DOC from the marsh relative to salt and water fluxes. This discursion from conservative behavior marked a short-lived period of DOC outwelling from the marsh creek to the estuary in summer 2014 during which an estimated 5.7 to 42.1 tons of DOC were exported. Although this is a modest carbon flux, the outwelled DOC remains a significant net term in the marsh carbon budget (e.g., up to 12% of the annual organic carbon sequestration in Groves Creek salt marsh) and an important process to capture in mechanistic models of long-term carbon production, export, and storage for marshes and other blue carbon ecosystems. Results also indicate DOC outwelling from salt marshes may occur as a pulse during highly productive summer months. Resolving these hot moments of DOC export at high-temporal resolution across larger salt marsh ecosystems is required to assess the true extent and quantitative significance of DOC outwelling to coastal carbon cycles, coastal ecology, and the carbon budgets of salt marshes--Author's abstract