Capturing the Dynamics of Dissolved Organic Carbon (DOC) in Tidal Saltmarsh Estuaries Using Remote-Sensing-Informed Models

The fluxes of dissolved organic carbon (DOC) through tidal marsh-influenced estuaries remain poorly quantified and have been identified as a missing component in carbon-cycle models. The extreme variability inherent to these ecosystems of the land-ocean interface challenge our ability to capture DOC-concentration dynamics and to calculate accurate DOC fluxes. In situ discrete and continuous measurements provide high-quality estimates of DOC concentration, but these strategies are constrained spatially and temporally and can be costly to operate. Here, field measurements and high-spatial-resolution remote sensing were used to train and validate a predictive model of DOC-concentration distributions in the Plum Island Estuary (PIE), a mesotidal saltmarsh-influenced estuary in Massachusetts. A large set of field measurements collected between 2017 and 2023 was used to develop and validate an empirical algorithm to retrieve DOC concentration with a +/- 15% uncertainty from Sentinel-2 imagery. Implementation on 141 useable images produced a 6-year time series (2017-2023) of DOC distributions along the thalweg. Analysis of the time series helped identify river discharge, tidal water level (WL), and a marsh enhanced vegetation index 2 as predictors of DOC distribution in the estuary, and facilitated the training and validation of a simple model estimating the distribution. This simple model was able to predict DOC along the PIE thalweg within +/- 16% of the in situ measurements. Implementation for three years (2020-2022) illustrated how this type of remote-sensing-informed models can be coupled with the outputs hydrodynamic models to calculate DOC fluxes in tidal marsh-influenced estuaries and estimate DOC export to the coastal ocean. The lateral transfer of DOC from tidal saltmarsh estuaries to the coastal ocean is not well represented in carbon-cycle models. These estuaries are influenced by terrestrial processes, riverine and marsh inputs, and tidal fluctuations that combine to create complex dynamics of DOC concentration and complicate the quantification of DOC fluxes used to compute carbon budgets. Here, we used a combination of field measurements and high-spatial-resolution satellite imagery to develop and test a model predicting DOC concentrations along a tidal saltmarsh estuary in Massachusetts from three environmental variables (i.e., tidal WL, river discharge, and a satellite-derived index of saltmarsh-biomass). A test using an independent data set revealed this simple model can estimate DOC concentrations within +/- 16% of the values measured in the laboratory. A simulation conducted for a continuous period of 3 years (15-min time step) showed that the model produced realistic DOC distributions and captured DOC dynamics adequately in the estuary. The model outputs can easily be coupled with hydrodynamic models to quantify the lateral transfer of DOC from tidal saltmarsh estuaries to the coastal ocean and inform carbon-cycle models. In situ data and satellite remote sensing used to develop and validate a predictive model of dissolved organic carbon (DOC) concentration in tidal saltmarsh estuary DOC dynamics in tidal estuary were predicted accurately (within 16% of in situ DOC) from river discharge, water level, and marsh enhanced vegetation index 2 Modeled DOC dynamics can be combined with hydrodynamic models to calculate DOC fluxes in tidal estuaries and inform carbon cycle models