Microbial uptake of dissolved organic and inorganic nitrogen in Randers Fjord

Uptake of 15N labeled NH4 C, NO3 , urea and dissolved free amino acids (DFAA) was measured in April and August 2001 at 6 stations along the salinity gradient of Randers Fjord (Denmark) in order to clarify the relative importance of dissolved organic and inorganic nitrogen (DON and DIN) as N sources for the estuarine planktonic microbial community. Although microbial N uptake was generally dominated by DIN (especially NH4 C), DON (both urea and DFAA) also served as an important N source and occasionally dominated uptake. Uptake of 15N labeled algal derived DON was measured in an attempt to mimic uptake of the complex ambient DON pool. Uptake rates for the algal derived DON in April were similar to those for NH4 C and even higher than the summed uptake of NH4 C, NO3 , urea and DFAA in August, suggesting that DON other than urea and DFAA, likely dissolved combined amino acids (DCAA), also served as an important N source. In addition, the relative contributions of heterotrophic bacteria and phytoplankton to total microbial N uptake was estimated from bacterial productivity and measured by inhibiting bacterial N uptake with antibiotics.