ECOLOGICAL SIGNIFICANCE OF DISSOLVED ORGANIC MATTER COMPOSITION AND REACTIVITY IN DEPRESSIONAL FRESHWATER WETLANDS

Abstract

Dissolved organic matter (DOM) plays a central role in the biogeochemistry of aquatic ecosystems and is an important flux of carbon (C) from terrestrial to aquatic systems. Wetlands are rich sources of DOM to downstream waters, but the origins of wetland DOM and its role in biogeochemical processes in wetlands and downstream are not fully understood. To better understand the role of wetlands in mediating the movement and transformation of organic matter between terrestrial and aquatic ecosystems, I characterized the chemical composition and the microbial and photochemical reactivity of wetland DOM in a depressional wetland setting in the interior Delmarva Peninsula. I used laboratory experiments to understand DOM reactivity. I characterized sensitivity to photodegradation, concluding most wetland DOM was somewhat sensitive though site differences affected sensitivity. In another experiment, wetland DOM showed little biodegradability, but C losses to microbes were enhanced after photodegradation. This suggested photochemical and biological degradation may have interacted to influence wetland DOM composition within wetlands and in downstream waters. I also found terrestrial sources of DOM (plant and soil leachates) were more biodegradable than wetland surface water. I concluded wetland DOM was largely comprised of leftover material from previous microbial metabolism in soils or wetland water. To characterize wetland DOM and explore its environmental influences, I undertook a field sampling campaign of 22 wetlands over 18 months. Samples were characterized using a suite of DOM measurements, and variability in these data was modeled using water level, regional air temperature, a proxy for site canopy cover, estimated photosynthetically active radiation, and others. DOM varied considerably seasonally and among sites, and modeling suggested that complex seasonal and site-related interactions influenced DOM, not including water level. This research indicates that depressional freshwater wetlands accumulate and process DOM, some of it likely originating from soils and some within wetlands, but spatial and seasonal variability lead to DOM variability. Wetland DOM exported to downstream waters has intrinsically low biodegradability, though this may be enhanced by photodegradation downstream. This research may be useful for efforts to improve representation of depressional freshwater wetlands in mineral soils in C cycle models and inform policy concerned with wetland biogeochemical functions and connections with downstream waters.