Biogeochemical and Physical Controls on the Microbial Degradation of Dissolved Organic Matter Along a Temperate Microtidal Estuary

Dissolved organic matter (DOM) is the foundation of the microbial loop and plays an important role in estuarine water quality and ecosystem metabolism. Because estuaries are influenced by DOM with different sources and composition, changing hydrologic regimes, and diverse microbial community assemblages, the biological fate of DOM (i.e., microbial degradation) differs across spatiotemporal scales and between DOM pools. To better understand controls on DOM degradation, we characterized the biogeochemical and physical conditions of the York River Estuary (YRE), a sub-estuary of the Chesapeake Bay in southeast Virginia (USA), during October 2018 and February, April, and July 2019. We then evaluated how these conditions influenced the degradation of dissolved organic carbon (DOC) and nitrogen (DON) and chromophoric dissolved organic matter (CDOM) by conducting parallel dark incubations of surface water collected along the YRE. Compared to other sampling dates, DOC reactivity (Delta DOC (%)) was over two-fold higher in October when freshwater discharge was lower, temperatures were warmer, and autochthonous, aquatic sources of DOC dominated. Delta DOC (%) was near zero when allochthonous, terrestrial sources of DOC were more abundant and when temperatures were cooler during higher discharge periods in February when precipitation in the Chesapeake Bay region was anomalously high. DON was up to six times less reactive than DOC and was sometimes produced during the incubations whereas Delta CDOM (%) was highly variable between sampling periods. Like Delta DOC (%), spatiotemporal patterns in Delta DON (%) were controlled primarily by hydrology and DOM source and composition. Our results show that higher freshwater discharge associated with prolonged wet periods decreased estuarine flushing time and increased the delivery of allochthonous DOM derived from terrestrial sources into coastal waters, resulting in lower rates of DOM degradation especially under cool conditions. While these findings provide evidence for seasonal variation in DOM degradation, shifting environmental conditions (e.g., increasing temperatures and precipitation) due to climate change may also have interactive effects on the magnitude and composition of DOM exported to estuaries and its subsequent reactivity.