Multi-decadal trends and influences on dissolved organic carbon distribution in the Barataria Basin, Louisiana from in-situ and Landsat/MODIS observations

摘要

Barataria Basin, a deltaic marsh-estuary system in the northern Gulf of Mexico (nGOM), is an important source of dissolved organic matter (DOM) to the coastal ecosystem. To assess DOM dynamics, monthly measurements of dissolved organic carbon (DOC) concentrations and colored DOM (CDOM; an optical proxy for DOC) absorption were obtained over a three-year period (2008-2011) along a transect encompassing the upper, middle and lower regions of the Barataria Basin estuary. While CDOM absorption coefficient at 355 nm (a(g)355) generally decreased and spectral slope at 275-295 nm (S275-295) increased along the transect from the upper to lower basin, deviations were observed associated with hydrologic and meteorological influences. An application of wavelet analysis to the time-series of a(g)355 and various freshwater sources revealed a spatiotemporal resonance of ag355 with freshwater input from the Mississippi River (MR), the controlled freshwater discharge from the Davis Pond freshwater diversion, and rainfall in the lower, middle and upper basins, respectively. Field measurements and Landsat/MODIS data spanning across different seasons between 2008 and 2011 were used to develop optimized a(g)355 empirical algorithms for the high river flow, cold front, and normal conditions. Power law models relating a band ratio (green/red) of a satellite sensor to a(g)355 showed good performance for the MODIS-Aqua (R-2 similar to 0.81-0.83, N = 581) and Landsat-5 TM (R-2 similar to 0.72-0.79, N = 304) imagery. a(g)355-DOC relationships (R-2 similar to 0.77-0.84, N = 306) applied to historical Landsat-5 TM (1985-2011) and MODIS-Aqua imagery (2002-2012) showed strong environmental influences on multi-decadal CDOM and DOC spatiotemporal trends. Satellite-derived DOC agreed reasonably well with in-situ measured DOC (averaged R-2 = 0.71; RMSE = 2.13). While the time-series DOC showed seasonality and decreasing gradients from the upper to lower basin, an overall increase in DOC was observed