Project: Collaborative Research: How does salt affect dissolved organic matter (DOM) photodegradation? A view through the lens of carboxyl groups.

NSF Award Abstract
Dissolved organic matter (DOM) includes material from plants and soils in the watershed and from algae, bacteria and animals living in the water. This complex mixture plays key roles within aquatic ecosystems. DOM is a source of nutrients and food for microbes. It acts as a sunscreen in surface waters by reducing ultraviolet radiation. It can also bind trace metals, changing their bioavailability and toxicity. The chemical structure of DOM determines both its ecological roles and its environmental fate. Microbial uptake and respiration and degradation by sunlight are key processes that transform DOM, leading to its eventual oxidation to carbon dioxide. Another key factor in DOM fate is the presence of salt. Salt ions can react with sunlight; the resulting products can alter DOM structures. This project will investigate the role of salt in photochemical reactions. Experiments will be done where salt and sunlight will be added to freshwater samples with high concentrations of brown, land-derived DOM and freshwater samples with relatively clear lake-derived DOM. The study will measure DOM loss and changes in DOM chemical structure. These results will be compared with studies of the original freshwater and samples collected along a river to marine transect. Many inland waters are becoming saltier due to land use changes, road salt use, and climate change. Many coastal waters are undergoing high variability in salt levels due to major storm events like hurricanes. Results from this study will help predict DOM effects on algal growth, trace metal binding, energy for food webs, and interactions with man-made chemicals. This project will provide training opportunities for students; results will be shared with both the scientific community and the public.

DOM in aquatic systems plays key environmental roles. These roles are largely a function of DOM chemical structure, with carboxyl groups in DOM being important in energy transfer in biota, trace metal chelation, and interactions with organic pollutants. The effects of salinity on photochemical changes to DOM structure, including carboxylation and decarboxylation, are, however, surprisingly unconstrained. These salinity effects are important to address now, as climate change is altering the salinity of inland and coastal marine systems. We will investigate DOM photochemistry using a freshwater transect (St. Louis River to Lake Superior) that includes sites varying in DOM source (land plants vs phytoplankton) and structure (more aromatic vs more aliphatic). These samples can be amended with salts of various concentrations and major ion compositions. Thus we can tease apart the roles of ionic strength (how much salt) from the specific effects of halide radical formation (what kinds of salts are present). We will compare these experimental results with photochemical studies on a natural freshwater to saltwater transect (Penobscot River to Gulf of Maine) at a similar latitude and with similar upstream land use patterns. Understanding salinity’s effects on DOM photochemical transformations will be a key step in predicting the effects of climate change on DOM’s environmental roles in aquatic systems. This project includes training opportunities for two graduate students and four undergraduate students. Project results will be shared through presentations and peer-reviewed publications and via Science on Deck research vessel tours and the Coastal Bend Bays & Estuaries Program.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.