Concentration and turnover of dissolved free polyamines on the South Coast of Lake Erie

Dissolved free polyamines (DFPAs) have been suggested as an important source of reduced nitrogen in marine systems, but their importance in the cycling of dissolved organic nitrogen (DON) in freshwater systems has yet to be assessed. This study measured the concentrations, turnover and uptake rates of DFPAs and compared them with those of dissolved free amino acids (DFAAs) in samples from 21 sites along eight coastal transects on the south shore of Lake Erie (LE), a Laurentian Great Lake. DFPA compounds were consistently detected in all samples and their concentrations (16.9-191.5 nmol L-1), turnover (1.6-3.2 d(-1)), and uptake rates (3.2-192.0 nmol L-1 d(-1)) were all significantly higher than those reported in marine environments. On the other hand, corresponding DFAA measurements (45.1-822.2 nmol L-1, 3.9-11.4 d(-1), and 35-1690 nmol L-1 d(-1), respectively) were within the range of reported values in marine environments. The DFPA to DFAA ratios in LE samples ranged between 0.21 and 0.65, which were much higher than what have been reported in marine environments (0.01-0.07). Within the bacterioplankton community, particle-associated (PAB) and free-living (FLB) proportions contributed equally to the turnover of DFPAs and DFAAs in LE. PAB and FLB also contributed equally when using requested DFPAs to fulfill their carbon (BCD; 3.1-7.9%) and nitrogen (BND; 13.5-18.5%) demands. Overall, our results suggest that DFPAs is a significant component of labile DON pool in freshwater systems, where they serve more as nitrogen than carbon sources to freshwater bacterioplankton.