Chemical speciation and transformation of dissolved nitrogen in Lake Michigan

The Great Lakes have experienced significant ecological and environmental changes due to increasing anthropogenic influences and the introduction of invasive species. However, changes in nutrient cycling pathways in Lake Michigan remain elusive. Water samples were collected between December 2012 and July 2013 along a transect from the Milwaukee River to open Lake Michigan for the measurements of dissolved inorganic nitrogen (DIN, including NO3, NO2, and NH4), dissolved organic nitrogen (DON), and colloidal organic nitrogen (CON). Concentrations of DIN in river waters decreased from winter to spring, while in Lake Michigan, DIN increased from spring/summer to winter, showing a general decrease from river to lake waters, but homogeneous or slightly increase from surface to deep water in Lake Michigan. Within the DIN pool, NO3 is the predominant species comprising >84%. Concentrations of DON also decreased from river to open lake waters, but less variable or slightly decreased from surface to deep waters in Lake Michigan. These variation trends highlighted the importance of terrestrial contribution of DIN and DON to the lake and possible production of DIN in bottom waters. While DIN predominated the total dissolved nitrogen (TDN) pool in both river and lake waters during winter, DON became dominant throughout the entire water column during spring/summer. The imbalance between DON production and DIN consumption during summer suggested that DON could also be derived from particulate nitrogen pool in the water column and other sources. Colloidal organic nitrogen contributed up to 22-56% of the DON pool or 12-32% of the TDN pool in river/coastal waters. Similar to DIN and DON, the abundance of CON also decreased from the Milwaukee River to Lake Michigan, indicating short turnover times of the colloidal N pool and increase the proportion of low-molecular-weight DON in lake waters.