ORCID ID

https://orcid.org/0000-0001-8356-1548

Date Awarded

2024

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Virginia Institute of Marine Science

Advisor

Elizabeth A Canuel

Committee Member

Amber K Hardison

Committee Member

Mark J Brush

Committee Member

Zhanfei Liu

Abstract

Estuaries and tidal wetlands are among the most ecologically and economically important ecosystems on Earth. They are well-known for their roles in supporting biodiversity, providing habitats for commercially important fisheries, and protecting shorelines from storms and wave action. These ecosystems are also important components of the coastal carbon cycle due to their role in carbon processing and export. However, these functions are threatened by warming temperatures, landscape alteration, eutrophication, sea level rise, and altered weather patterns. Dissolved organic carbon (DOC) is the least characterized carbon pool but plays a critical role in the aquatic food web as the foundation of the microbial loop. However, characterizing the sources, composition, and fate of DOC is challenging due to its heterogeneity and dynamic behavior in the environment. To better understand DOC dynamics in estuaries and tidal wetlands, this dissertation examined the environmental controls on microbial DOC production and decomposition within surface waters and marsh porewaters along the York River Estuary (YRE), a temperate sub-estuary of the Chesapeake Bay in southeast Virginia, USA. Seasonal field campaigns and laboratory manipulation and incubation experiments of estuarine surface waters and tidal marsh porewaters were conducted under dark (and anoxic for porewater incubations) conditions. Environmental and biogeochemical conditions were also assessed for the estuarine (hydrology, DOC source and composition, nutrients) and tidal marsh (DOC source and composition, nutrients, soil characteristics, microbial communities) study systems. Along the YRE, surface water DOC reactivity was higher during low discharge periods when autochthonous, aquatic sources of DOC dominated. In comparison, DOC reactivity was lower when more allochthonous, terrestrial sources of DOC were more abundant during high discharge periods when precipitation in the Chesapeake Bay region was anomalously high. In tidal marshes along the YRE, spatial variation in salinity and porewater characteristics were related to differences in soil microbial communities primarily involved in methane, nitrogen, and sulfur cycling. These metabolic functions differed in magnitude but co-occurred regardless of location along the estuary, indicating that microbial metabolic processes were not limited based strictly on salinity and biogeochemical gradients. Results from marsh porewater incubations showed complex patterns in DOC loss and production. DOC loss coincided with time periods when labile DOC was more prevalent and sulfate reducers and nitrate-reducing taxa comprised a large proportion of the active microbial community. DOC production, potentially through chemolithoautotrophy, coincided with time periods when refractory CDOM was more prevalent and sulfide oxidizers comprised a large proportion of the active microbial community. Overall, this dissertation shows that DOC composition, hydrology, and microbial community dynamics are primary, interacting controls on the biological fate of DOC in estuaries and tidal marsh porewaters. Human activities and climate change will have profound impacts on the coastal carbon cycle by shifting the forms, magnitude, composition, and reactivity of DOC in estuaries and tidal wetlands. By considering the importance of these controls under a variety of environmental conditions, these results help to better characterize the role of DOC in the coastal carbon cycle as the biogeochemical functions of coastal ecosystems respond to future change.

DOI

https://dx.doi.org/10.25773/v5-y2rt-ge16

Rights

© The Author

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