Assessing chromophoric dissolved organic matter (CDOM) distribution, stocks, and fluxes in Apalachicola Bay using combined field, VIIRS ocean color, and model observations

Understanding the role of estuarine-carbon fluxes is essential to improve estimates of the global carbon budget. Dissolved organic matter (DOM) plays an important role in aquatic carbon cycling. The chromophoric fraction of DOM (CDOM) can be readily detected via in situ and remotely-sensed optical measurements. DOM properties, including CDOM absorption coefficient at 412nm (ag412) and dissolved organic carbon (DOC) concentrations were examined in Apalachicola Bay, a national estuarine research reserve located in the northeast Gulf of Mexico, using in situ and satellite observations during the spring and fall of 2015. Synoptic and accurate representation of estuarine-scale processes using satellite ocean color imagery necessitates the removal of atmospheric contribution (~90%) to signals received by satellite sensors to successfully link to in situ observations. Three atmospheric correction schemes (e.g., Standard NIR correction, Iterative NIR correction, and SWIR correction) were tested first to find a suitable correction scheme for the VIIRS imagery in low to moderately turbid Apalachicola Bay. The iterative NIR correction performed well, and validation showed high correlation (R2=0.95, N=25) against in situ light measurements. A VIIRS-based CDOM algorithm was developed (R2=0.87, N=9) and validated (R2=0.76, N=20, RMSE=0.29m−1) against in situ observations. Subsequently, ag412 was used as a proxy of DOC in March (DOC=1.08+0.94×ag412, R2=0.88, N=13) and in November (DOC=1.61+1.33×ag412, R2=0.83, N=24) to derive DOC maps that provided synoptic views of DOC distribution, sources, and their transport to the coastal waters during the wet and dry seasons. The estimated DOC stocks were ~3.71×106kg C in March and ~4.07×106kg C in November over an area of ~560km2. Volume flux (out of the bay) almost doubled for March 24 (735m3s−1) relative to November 4 (378m3s−1). However, estimates of DOC fluxes exported out of the bay from model-derived currents and satellite-derived DOC were only marginally greater in March (0.163×106kgCd−1) than in November (0.124×106kgCd−1) and reflected greater DOC stocks in the fall. The combination of satellite-, field-, and model-based observations revealed the strong linkage between the Apalachicola River plume, a major source of DOM, and the overall hydrodynamic forcing that controlled distributions of CDOM abundance, DOC concentration, stocks, and fluxes in the bay. [Display omitted] •A VIIRS-based CDOM empirical algorithm is presente