Hydrological and Biogeochemical Controls of Seasonality in Dissolved Organic Matter Delivery to a Blackwater Estuary

Abstract

Changes in riverine discharge of dissolved organic matter (DOM) serves as an indicator of linkages between terrestrial ecosystem and receiving aquatic environments. In this study, we test the hypothesis that the seasonal variability of DOM in an estuary fed by a blackwater river is primarily controlled by water discharge and also modified by photochemical and biological processes. We collected surface water samples during 4-week-long field campaigns to the lower Pearl River estuary located in southeastern Louisiana, two during high discharge in spring and two during low discharge in winter and summer, respectively. DOM composition was determined using spectrofluorometric indices and a site-specific parallel factor model, and dissolved organic carbon (DOC) concentrations. Spring samples with low salinity showed higher abundance of terrestrial, humic-like DOM and higher DOC concentrations, indicating the export of flood plain-derived DOM during high discharge. In contrast, summer and winter samples with high salinity had greater proportions of labile DOM and higher biological and fluorescence indices, which may reflect enhanced photochemical and biological degradation during summer and better preservation of labile DOM in winter. Spring DOM displayed highly variable source and quality character, relative to winter and summer samples. This observation suggests that river discharge acted as a more rapid and direct control of spatial variation in DOM and photochemical and biological degradation was responsible for removing this flow-related variation between seasons. The incubation experiments showed that natural light can remove terrestrial and microbial humic DOM, while bacterial degradation was responsible for degrading protein-like DOM. Our results provide new evidence that DOM seasonality in blackwater river estuarine environments is collectively regulated by discharge and photochemical and biological degradation.