Saltmarshes as a source of chromophoric dissolved organic matter (CDOM) to Southern California coastal waters

Abstract

To determine chromophoric dissolved organic matter (CDOM) sources in Southern California coastal waters, optical properties of a river outlet and adjacent tidally flushed salt marshes were monitored (dry season; June‐July 2001). Average absorption coefficients doubled at ebb vs. flood tides (4.8 ± 1.5 vs. 2.1 ± 0.9 m‐1; 300 nm), suggesting significant salt marsh CDOM inputs into coastal waters. Average spectral slopes were not statistically different for any sites or tides (0.010 ± 0.002 nm−1), consistent with salt marsh CDOM dominating coastal waters. Three‐dimensional fluorescence excitation‐emission matrices (EEMs) at ebb tide showed contributions from terrestrial, protein, and marine humic‐like peaks, suggesting production and output of these materials from the marsh. A marine humic signal at the river outlet during an offshore upwelling event indicated an additional sporadic nonmarsh marine humic‐like source. EEMs of six common salt marsh plant leachates showed protein, terrestrial, and marine humic‐like peaks. To estimate CDOM photodegradation in the marsh, fluorescence intensity decays from photobleaching experiments were fit to first‐order kinetics. Most humic peaks degraded with a half‐life of t1/2 = 10‐20 h, which overlaps the estimated residence time of water in the marshes (∼12 h). Most protein peaks were resistant to photodegradation, suggesting that the low levels of protein vs. humic‐like material measured in natural waters was due to rapid bioutilization of proteinaceous material. The rapid photodegradation of plant leachate humic material and the low spectral slopes for the field sites suggest that marsh sediments would be an important source of CDOM.