The fate of organic matter in natural waters influences microbial activity, food webs, water clarity, and aquatic photosynthesis, among other ecosystem impacts. Organic matter quantity and composition change in response to ultraviolet photodegradation, photosynthetically active radiation (PAR), and microbial metabolism individually. Yet, very few studies have examined how water samples respond to all of these processes individually.
To address this knowledge gap, we obtained water samples from 19 NEON lake, wetland, stream, and river aquatic sites spanning the continental United States. We subjected each sample to the following treatments: 1) incubated in the dark, filtered to 0.2 µm, 2) incubated in the dark, unfiltered, 3) incubated in a solar simulator, filtered to 0.2 µm, 4) incubated in a solar simulator, unfiltered, 5) incubated under PAR without ultraviolet light. This allowed us to estimate the impact of respiration, photodegradation, and photosynthesis + respiration individually for each sample processed. We used the change in total dissolved organic carbon (DOC) to measure net change in carbon production. We used ultraviolet-visible (UV-Vis) absorbance and fluorescence to characterize dissolved organic matter (DOM) composition changes.
We found that the amount of DOC degraded due to respiration increased with the amount of protein-like, low aromatic DOM present within a sample. Photodegradation was conversely positively correlated with the aromatic DOM content in a sample as measured by metrics including specific ultraviolet absorbance at 254 nm (SUVA254). Climatological factors were also important predictors of DOC transformation. Increased solar insolation was related to decreased photodegradation and biodegradation of DOC. Higher rates of photosynthesis were correlated with sites with high forest and grassland watershed land cover. Overall, our results show that DOM composition is more responsive to photosynthesis, respiration, and photodegradation than is DOC quantity.