Bioavailability and compositional changes of dissolved organic matter in urban headwaters

In urban stream networks, the headwaters are comprised of engineered headwaters where particulate organic matter collects during and between storms. During storms, dissolved organic matter leached from these pools is transported to the stream as stormflow connects these ephemeral channels to the network. Throughout the urban network, microbial processing consumes, produces, and transforms DOM, changing its chemical composition and concentration. In this study, we characterized how microbial processing changes the composition and inferred lability of DOM from stormflow samples and leachates of potential DOM sources by pairing optical measurements of DOM composition with measurements of DOC concentration during incubation with a common bacterial community. We found that over 6 days (the approximate residence time of water stored in urban headwater infrastructure) microbial processing significantly altered DOM composition, increasing the chemodiversity of DOM in leachates but not stormflow samples. Particularly in leachates, this initial change in composition was accompanied by little change in DOC concentration. After 60 days of microbial processing, both samples of stormflow and leachates lost more than half their initial DOC concentration on average, and became more similar in composition with indices indicating humic, aromatic DOM generally considered to be recalcitrant. This work provides new evidence that leached organic matter undergoes transient increases in chemodiversity through bacterial action on the DOM pool before further processing leaves behind more homogenous and recalcitrant DOM.