Abstract
Optical proxies, especially DOM fluorescence, were used to track terrestrial DOM fluxes through estuaries and coastal waters by comparing models developed for several coastal ecosystems. Key to using optical properties is validating and calibrating them with chemical measurements, such as lignin-derived phenols - a proxy to quantify terrestrial DOM. Utilizing parallel factor analysis (PARAFAC), and comparing models statistically using the OpenFluor database (http://www.openfluor.org) we have found common, ubiquitous fluorescing components which correlate most strongly with lignin phenol concentrations in several estuarine and coastal environments. Optical proxies for lignin were computed for the following regions: Mackenzie River Estuary, Atchafalaya River Estuary, Charleston Harbor, Chesapeake Bay, and Neuse River Estuary. The slope of linear regression models relating CDOM absorption at 350 nm (a350) to DOC and to lignin, varied 5 to 10 fold among systems. Where seasonal observations were available from a region, there were distinct seasonal differences in equation parameters for these optical proxies. Despite variability, overall models using single linear regression were developed that related dissolved organic carbon (DOC) concentration to CDOM (DOC = 40×a350+138; R2 = 0.77; N = 130) and lignin (Σ8) to CDOM (Σ8 = 2.03×a350-0.5; R2 = 0.87; N = 130). This wide variability suggested that local or regional optical models should be developed for predicting terrestrial DOM flux into coastal oceans and taken into account when upscaling to remote sensing observations and calibrations.
Highlights
Terrestrial organic carbon (OC) flux from rivers and estuaries into coastal oceans is on the order of 0.2 Pg C yr−1 and constitutes a major part of the oceanic carbon cycle (Raymond and Spencer, 2014)
We do not extend our analysis of relationships beyond linear regression models as Fichot and Benner (2012) have done for DOC-normalized results; rather, we focus on the basic chromophoric dissolved organic matter (CDOM), DOC, and lignin measurements that can be used in a first order optical algorithm for satellite retrievals of properties related to ocean color (Mannino et al, 2008; Tehrani et al, 2013)
This work has narrowed the knowledge gap between CDOMDOC and CDOM-lignin relationships that have been developed for large rivers and coastal waters by focusing on estuaries, which are notoriously heterogeneous
Summary
Terrestrial organic carbon (OC) flux from rivers and estuaries into coastal oceans is on the order of 0.2 Pg C yr−1 and constitutes a major part of the oceanic carbon cycle (Raymond and Spencer, 2014). In terms of estuarine and coastal ocean biogeochemistry, this work has mainly been restricted to large rivers and to delta front estuaries (Raymond and Spencer, 2014) Both CDOM and lignin have been studied widely in estuaries separately, but rarely together (e.g., Osburn and Stedmon, 2011). This paucity of information complicates an understanding of how estuarine processes modify terrestrial DOC during transport to coastal waters
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