Seasonal Hydrologic Connectivity and Site-Level Differences Influence Wetland DOM Composition and Photoreactivity

Wetlands are recognized as important sources of downstream dissolved organic matter (DOM) with consequences for biogeochemical cycling and food webs. Wetland DOM is often considered terrestrial in origin, making wetlands important links between terrestrial and aquatic ecosystems. However, the degree of transformation terrestrially-sourced organic matter undergoes within wetland settings has not been extensively studied. Wetlands may simply shunt DOM into receiving waters relatively unchanged or they may export only a fraction of what they receive after altering its chemical composition. The latter is consistent with the widely held assumption that wetlands are biogeochemical hotspots but this is largely based on studies of nutrient transformations. Work is needed to explore the role of wetlands in altering the fluxes and composition of DOM in order to improve understanding of terrestrial to aquatic carbon fluxes and ecosystems-scale processes within watersheds.

While drivers of wetland DOM composition (e.g. land use, climate) have been identified at large spatial scales, site-level variability and seasonal differences are understudied. We characterized DOM chemical composition in 20 small, freshwater, seasonally inundated depressional wetlands within a small area of the Delmarva Peninsula (Maryland, USA) with similar land use and climate. We characterized seasonal differences across wetlands that varied in inundated area, canopy cover, and vegetation. We also assessed DOM photodegradation experimentally to generate a marker of compositional differences and to explore an ecologically relevant process. We found that during times when wetlands were hydrologically isolated, DOM composition and photoreactivity varied among wetlands but seasonal hydrologic connectivity among wetlands and between wetlands and streams homogenized composition and photoreactivity. We discuss the potential consequences for ecosystem processes within the wetlands and downstream. This work demonstrates that site-level differences affect DOM within wetlands, which may inform conservation or restoration project planning, and that hydrologic connectivity in a wetland-dominated landscape may have consequences for aquatic ecosystem processes.