Seasonal and Spatial Variability of Dissolved Carbon Concentration and Composition in Lake Michigan Tributaries

Dissolved organic matter (DOM) is a complex mixture of many compounds and its composition dictates numerous reactions in the environment. Large lakes and marine coastlines receive DOM from watersheds that differ widely in their land cover, with potential implications for both the quantity and composition of carbon inputs. Seasonal variation in DOM quantity and composition may also differ among tributaries and be mediated by land cover. Here, we quantify spatially (i.e., among tributary) and seasonal variation in DOM concentration, DOM composition based on ultraviolet‐visible spectroscopy, and alkalinity across 101 tributaries of Lake Michigan, one of the world's largest lakes, using a synoptic sampling approach. Wetland land cover has the largest effect on DOM, producing high concentrations of DOM that are more aromatic and larger in apparent molecular weight. Seasonal variation is also pronounced, with concentrations and aromaticity of DOM peaking in fall across most tributaries. Watershed lithology and land cover both affect alkalinity, with higher values associated with the geography of carbonate bedrock and urbanized watersheds. Watershed land cover has a larger effect than season on all organic carbon parameters. However, seasonal variation is especially important for DOM composition. This disparity suggests that the environmental processing of DOM within river channels mediates its composition more than its concentrations. Considering the wide range of land cover and lithology around Lake Michigan and other large water bodies, accounting for both spatial and seasonal dynamics is essential for understanding controls on DOM delivery. Plain Language Summary Tributaries carry important inputs of water, nutrients, and contaminants to the Great Lakes. We sampled >100 tributaries of Lake Michigan in each season to determine how the type of land surrounding each tributary affects the carbon it brings to Lake Michigan. We considered organic carbon concentration and composition, as well as alkalinity. Wetlands had the largest effect on organic carbon with tributaries from wetland‐dominated areas having a higher concentration of organic carbon and organic carbon composition consistent with that originating from the land. Both land cover and geology impact alkalinity with the amount of carbonate species underlying the watershed increasing the alkalinity in the tributary. The season of sample collection is significant for organic carbon but not for alkalinity. Ad