Dissolved organic matter (DOM) is a major driver of lake biogeochemistry and water quality through its influence on microbial metabolism and light attenuation. Aromatic DOM is delivered primarily via rivers where it reduces light availability and can alter vertical thermal structures through the absorption of solar energy. Further, photodegradation of aromatic DOM converts it into more labile forms which can fuel microbial metabolism. Despite the importance of DOM, plume dynamics and their ecological impacts remain difficult to quantify due to spatiotemporal variation in DOM source inputs and rapid in-situ processing. As a result, accurate descriptions of spatial gradients and the impacts of DOM delivery require high-resolution measurements capable of capturing both plume structure and rapid processing.
To quantify DOM gradients and processing signals across tributary plumes in the Laurentian Great Lakes of North America, we utilized high frequency UV-Vis sensing. Nearshore to offshore transects were sampled in proximity to tributaries in Lake Huron and Lake Michigan using a towed, submersible ultraviolent nitrate (SUNA) sensor to compare plume extent and DOM optical characteristics in both lakes. In a different application, we analyze fixed location SUNA data from buoys in the western basin of Lake Erie during the months preceding harmful algal blooms (HAB) onset between years of different bloom severity. We quantified DOM optical properties using UV absorbance at 254nm (a254) to characterize aromatic DOM pools, and the a254:a350 absorbance ratio to estimate relative DOM lability.
Across transects, tributaries displayed distinct DOM optical signatures and plume structures, with Lake Michigan tributary plumes showing greater average DOM aromaticity than those in Lake Huron. Diel and spatial variability in a254 across lakes display patterns consistent with photochemical processing and mixing of distinct water masses. In Lake Erie, a254:a350 ratios were higher prior to HAB onset in the year with more severe blooms, indicating a greater proportion of more labile DOM and suggesting a possible priming perquisite in which preseason DOM quality can enable greater bloom intensity.