Nitrate regeneration coupled to degradation of different size fractions ofDON by the picoplankton in the Elbe estuary

Bacterial conversion of high (HMW > 3,000 Da) and low (LMW < 3,000 Da) mole cular weight DON (dissolved organic nitrogen) was studied along the freshwa ter section of the Elbe estuary during the summer of 1997. Indigenous popul ations of picoplankton were incubated in a flow-through chamber that allowe d a constant exchange of sterile, filtered Elbe water as the culture medium for the microorganisms, which remained within the chamber. Nitrogen conver sion was followed by changes in the concentrations of total and low molecul ar weight DON and dissolved inorganic nitrogen compounds, the uptake of O-2 , and bacterial growth. Along the Elbe estuary, total DON concentrations va ried between 0.69 and 1.1 mg N L-1, of which between 64 and 79% was LMW-DON . Ammonium was a minor nutrient present in the Elbe at concentrations below 0.26 mg N L-1. During incubation in the laboratory between 27 and 64% of t he LMW-DON tvas consumed at rates ranging from 24 to 51 <mu>g N L-1 h(-1). HMW-DON was used only when the degradable LMW-DON pool became exhausted and accounted for between 60 and 100% of the HMW-DON. This produced an increas e in the DON consumption rate between 43 and 79 mug N L-1 h(-1). Nitrificat ion rates were independent of the external ammonium concentration until it decreased to below 1 mug N-NH4 L-1. Most of the N in the nitrification proc ess originated as NH4 regenerated from DON. Between 75 and 100% of the LMW- DON and ammonium consumed was rapidly converted to nitrate. This amount dec reased to between 65 and 85% when HMW-DON was consumed in addition to the L MW-DON. DON and ammonium consumption supported nitrification rates up to 71 mug N L-1 h(-1). The amount of DOC (dissolved organic carbon) degraded was not equivalent to the. C:N ratio of the total dissolved organic matter. Ca lculations based on oxygen consumption for respiration and ammonium regener ation revealed that the substrates used during the incubations contained C: N at ratios of about 3:1. These results suggest that the nitrogen-rich comp ounds had been removed from the dissolved organic matter and subsequently c onsumed by bacteria, while the carbon skeleton remained mostly unaffected b y the degradation processes.