Photodegradation of Natural Organic Matter in Plume Versus Non-plume Waters in Lake Superior

Abstract

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 bioavailable nitrogen and phosphorus species . 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 extent and types photochemistry happening in surface waters of these large lakes. The differences between the photodegradation of natural organic matter from plume-impacted water versus open lake water in Lake Superior were studied by performing irradiations under natural sunlight at 47°N latitude in August and September 2020. Terrestrially impacted samples (both before and after a storm), as well as open water samples were exposed to three days of natural sunlight. Autoclaved 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 water samples from the terrestrially impacted and storm-impacted sites 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. This differed from the whole water samples, where the storm-impacted site experienced the smallest percent change in UV-Vis measurements, most likely because the particulates in this sample limited its light exposure. Except for this site, filtered water irradiations generally experienced lower percent changes and overall changes in UV-Vis measurements compared to whole water samples. An increase in DOC concentration was found in the dark sample for the whole water irradiation of the terrestrially-impacted site taken before a storm occurred, indicating potential desorption occurring when POM is included. Finally, there was also an increase in ammonium concentration in the same aforementioned whole water sample upon light exposure. The photodegradation of organic matter in Lake Superior was mainly affected by site location and whole vs. filtered treatment, and resulted in some ammonium release in whole, terrestrially-impacted samples.