Microbial and photochemical degradation of allochthonous dissolved organic matter within river systems and watersheds

Dissolved organic matter (DOM) plays an important metabolic role in aquatic ecosystems by serving as an energy and nutrient source to bacteria and algae. A significant amount of DOM enters aquatic systems from the terrestrial environment. Currently, it is not known how the reactivity of DOM from different land use varies or how the reactivity of the riverine DOM will change as watershed land cover is altered. Understanding the effects of watershed land use on riverine DOM lability is important because it is the DOM lability that determines whether it contributes to eutrophication and hypoxia. This dissertation addresses these issues by focusing on the biological and photochemical reactivity of allochthonous DOM in rivers and watersheds at several spatial scales, with the overall goal of understanding the role of these processes in DOM cycling and the connection with watershed activities. The first three sections of this dissertation examine the relative importance of microbial and photochemical processes in degrading DOM and the bioavailability of DOM from different land uses and in rivers with varying land cover distributions. Results indicate that at the molecular scale, microbial processes are more efficient than photochemical ones at degrading DOM inputs from natural and anthropogenic land uses. The bioavailability of the dissolved organic carbon (DOC) and nitrogen (DON) from the natural and anthropogenic land uses examined is not markedly altered following exposure to light, suggesting that microbial processes may be more important in affecting the quantity and quality of DOM exported from rivers. At the land use scale, wetlands can be an important source of bioavailable DOC and DON to rivers. The lability of the wetland DOM changes seasonally and is affected by anthropogenic activities. At the landscape scale, individual watershed land uses influence riverine DOM composition and bioavailability. The fourth study examines how nutrients and trace elements affect the metabolic role of DOM and the overall processing of organic matter in estuaries. At this watershed scale, DOM fuels estuarine metabolism; however, its metabolic role changes with the addition of nutrients and trace elements as the balance between whole system gross primary production and respiration is altered.