Comparison of seasonal changes in fluorescent dissolved organic matter among aquatic lake and stream sites in the Green Lakes Valley

Abstract

The spectral characteristics of whole water dissolved organic matter (DOM) and fulvic acid were studied in samples collected from an alpine lake, a subalpine lake, and a subalpine stream during snowmelt and the summer growing season. Excitation‐emission matrices of whole water DOM and fulvic acid were analyzed by parallel factor analysis (PARAFAC). Allochthonous inputs of terrestrially derived fulvic acid DOM were dominant during snowmelt at the alpine lake, and during both snowmelt and summer at the subalpine sites. At the alpine lake, autochthonous inputs of DOM dominated during the summer phytoplankton bloom, and the spectral characteristics of the whole water DOM diverged from those of the fulvic acid. For example, the quinone‐like fluorophores in whole water DOM at the alpine lake were more oxidized and microbially derived than the fulvic acid fraction during the summer. At the subalpine sites, the seasonal changes in the source and redox state of the quinone‐like fluorophores of the whole water DOM tracked those of the fulvic acid pool. However, at both lake sites there was a greater contribution of amino acid‐like fluorophores in the whole water DOM than the fulvic acid fraction. This trend was not observed at the subalpine stream site. Principal components analysis (PCA) of the PARAFAC components suggests that during snowmelt, the chemical quality of the DOM at the alpine lake was similar to that of the subalpine stream; whereas the alpine site was more similar to the subalpine lake during the summer. Spectral characterization and PCA of the PARAFAC components suggest that nonhumic quinone‐like and amino acid‐like fluorophores were produced in the alpine lake during the summer phytoplankton bloom. Our results show that different types of water bodies produce different seasonal patterns in whole water DOM and fulvic acid quantity and quality.