Dissolved organic carbon dynamics in lakes and streams in Northern Wisconsin

Abstract

Dissolved organic carbon (DOC) is naturally occurring; however, various aspects of global climate change are increasing anthropogenic DOC in freshwater systems. Here we focus on lakes and streams in the Northern Lakes and Forests region of Wisconsin. This study aimed to 1) determine DOC concentration and composition in these systems, 2) compare DOC dynamics between system types and each month sampled, and 3) determine if relationships exist between DOC and nutrient quantities and microbial community production. This study found that DOC ranged from 2.62 - 61.35 mg/L, with no differences in DOC concentrations between the system types or months sampled. However, DOC composition differed greatly between system type and months, with lakes having more autochthonous carbon and streams having more allochthonous carbon (p < 0.001, ANOVA). There were differences in DOC composition between lake and stream sites at the beginning of summer (May) compared to the end (September) of summer (p = 0.001, ANOVA). While there were no significant differences in chlorophyll production between the system types or months, heterotrophic bacterial secondary production was significantly higher in streams compared to lakes during the months of May (p = 0.023, ANOVA) and July (p = 0.021, ANOVA). The relationship between color:chlorophyll predicting algal DOC contribution corroborates the DOC composition results, with lakes having a higher degree of autochthony than streams, yet most sites were still dominated by allochthonous signatures. Most sites had more CO2 saturated in the water relative to the air, indicating that these sites are dominated by heterotrophy that is supported by excess allochthonous inputs. There were positive linear relationships between DOC and color in lakes (p < 0.001, Regression) and steams (p = 0.048), as expected in the Northern Lakes and Forests region and indicates a presence of DOC molecules associated with browning trends globally.