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Abstract 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 (ΔDOC (%)) was over two-fold higher in October when freshwater discharge was lower, temperatures were warmer, and autochthonous, aquatic sources of DOC dominated. ΔDOC (%) 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 ΔCDOM (%) was highly variable between sampling periods. Like ΔDOC (%), spatiotemporal patterns in ΔDON (%) 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.
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Composition and Bioreactivity of Dissolved Organic Matter Leachates From End Members in a Mountain to Prairie Transitional River Valley
Abstract River organic matter transformations impact the cycling of energy, carbon, and nutrients. The delivery of distinct dissolved organic matter (DOM) sources can alter aquatic DOM cycling and associated biogeochemical processes. Yet DOM source and reactivity are not well‐defined for many river systems, including in western Canada. Here, we explore DOM cycling in the mainstem of the Oldman River (stream order 6–7), a heavily regulated river network in southern Alberta (Canada). We compared seasonal river DOM content, composition, and bioavailability with nine endmember leachates from the river valley using optical properties and incubations to estimate biodegradable dissolved organic carbon (BDOC). River DOM composition was most similar to terrestrial soil leachates, followed by autochthonous DOM leachates. River DOM bioavailability was low (BDOC = 0%–16.6%, mean of 7.1%). Endmember leachate bioavailability increased from soils (BDOC = 23.9%–53.7%), to autochthonous sources (fish excretion, macrophytes, biofilm; BDOC = 49.9%–80.0%), to terrestrial vegetation (leaves, shrubs, grass; BDOC > 80%), scaling positively with protein‐like DOM content and amount of leachable dissolved organic carbon (DOC), and negatively with aromaticity. Seasonally, DOC concentrations changed little despite >15‐fold increases in discharge during spring. River DOM composition shifted modestly toward soil‐like endmembers in spring and more bioavailable autochthonous end members in autumn and winter. Low DOM bioavailability in the river mainstem and low DOC yields shown in previous work point to limited internal processing of DOM and limited bioavailable DOM delivery to downstream habitats, possibly due to upstream flow regulation. Our observations provide important insights into the functioning of western Canadian aquatic networks.
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- Award ID(s):
- 2053048
- PAR ID:
- 10517841
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Biogeosciences
- Volume:
- 129
- Issue:
- 6
- ISSN:
- 2169-8953
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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