COMPARISON OF NITROGEN FIXATION RATES ACROSS THE LAURENTIAN GREAT LAKES (LGL)

Nitrogen fixation (N_fix) is an important microbially mediated process that converts dinitrogen (N₂) gas to ammonia (NH₃) using the nitrogenase protein complex (Nif). Nitrogen fixation is a linchpin of the nitrogen cycle that re-mobilizes N lost to denitrification processes and potentially a significant source of N to the LGL. Diazotrophy is pervasive in marine environments but has been overlooked in freshwater systems and may aid cyanobacterial bloom longevity. Thus, we sought to quantify whether diazotrophy 1) occurs in the LGL and 2) the quantify the rates of N_fix. Using the acetylene reduction assay, we detected significant differences in N_fix rates between the LGL. Additionally, rates were depth dependent, variable between stations, and displayed nearshore vs. offshore differences. Similarly, nifH gene diversity suggests many microbes are capable of N_fix. Together this suggest that N_fix may be overlooked but a significant source of N in the LGL.