PHOTODEGRADATION OF NATURAL ORGANIC MATTER FROM TERRESTRIALLY-IMPACTED VERSUS OPEN WATERS IN LAKE SUPERIOR

Natural organic matter (NOM) undergoes direct and indirect photodegradation under ultraviolet (UV) light exposure, is an important source of energy for the aquatic food web and affects how much light can penetrate a water column. Photodegradation of NOM can lead to photobleaching of colored dissolved organic matter (CDOM), the release of low-molecular weight (LMW) organic species and the release of inorganic nutrients. Photomineralization of NOM can produce carbon dioxide and carbon monoxide, removing organic carbon from the system. Recent storm events of greater intensities and frequencies have caused increased amounts of runoff, including dissolved and particulate natural organic matter, in the Laurentian Great Lakes region. This increased runoff may change the properties of photochemistry happening in surface waters of these large lakes. The differences between the photodegradation of natural organic matter from terrestrially-impacted water versus open lake water in Lake Superior were studied by performing irradiations under natural sunlight at 47°N latitude in August 2019. Terrestrially-impacted waters were exposed to 3.5*10⁶ J/m² from 250-700 nm wavelengths, and open water samples were exposed to 4.0*10⁶ J/m². Whole-water and filtered-water samples from before, during, and after the irradiations, along with matching dark controls, were analyzed for total and dissolved organic carbon, total and dissolved nitrogen, total and dissolved phosphorus, soluble reactive phosphorus, ammonia, and UV-Visible spectroscopy proxies (spectral slope ratios, CDOM absorbance, and SUVA₂₅₄). Irradiated filtered and whole water samples from the terrestrial-impacted water exhibited larger percent and overall changes in spectral slope ratios and greater losses of colored dissolved organic matter (CDOM) absorbance relative to open water samples. There was an increase in ammonia in the whole water, terrestrial-impacted sample, but otherwise no significant changes relative to dark control aliquots, were found in TOC/DOC concentrations for filtered and whole water samples as a function of irradiation.