Interactions between dissolved organic matter sources and effects in stream ecosystems

Dissolved organic matter (DOM) is a ubiquitous component of aquatic ecosystems, linking climate and land use to ecosystem structure and function in streams. The operational definition of DOM is any organic compound passing through a particular size filter, and therefore includes a wide dissolved variety of compounds, both simple and complex. DOM acts both directly and indirectly to influence biota. Directly, DOM is a source of energy and nutrients. Indirectly, DOM reduces light transmission through the water column and alters the bioavailability of nutrients and pollutants. DOM effects are due to the aggregate physiochemical properties of the constituent molecules, which vary with DOM source. Ultimately, all DOM derives from photosynthetic activity, which can occur within either the aquatic or terrestrial environment. Because most DOM in streams is terrestrial in origin, DOM properties are often related to terrestrial sources and processing of DOM at the watershed scale. However, DOM processing within aquatic ecosystems may be important as well. My objectives were to determine (1) if watershed characteristics relate to DOM processing and production in aquatic ecosystems, (2) if DOM properties related to watershed characteristics influence DOM processing and interactions with stream biota, and (3) whether DOM alters the toxicity of anthropogenic pollutants. In a survey of 32 streams, I found that the presence of upstream lakes was negatively related to the concentration and light absorbing ability of DOM. While upstream lakes clearly influence DOM properties, my subsequent experiments showed that the photodegradability of DOM and its direct effects on periphyton growth were unaffected by upstream lakes. In-situ experiments showed that photodegradation was less important than bacterial uptake in most streams. I conclude that variation in DOM composition related to watershed characteristics are unimportant to stream communities or the aquatic processing of DOM. I also investigated the role of DOM in altering the toxicity of a class of anthropogenic pollutants (ionic liquids). I studied representatives from two particular classes of DOM molecules, humic matter and low-molecular-weight acid, in addition to natural DOM. I found that the toxicity for one ionic liquid was ameliorated in the presence of natural DOM, but not by either of our isolated components.