Dissolved organic nitrogen utilization by an Aphanizomenon bloom in Lake Kinneret

The concentrations of dissolved organic nitrogen (DON) and dissolved inorganic nitrogen (DIN) in the epilimnetic waters of Lake Kinneret decreased from 371 to 152 mu g N l(-1) and from 65 to 13 mu g N l(-1),respectively, during September-October 1994, concomitantly with the outgrowth of the cyanobacterium Aphanizomenon ovalisporum. A mass balance estimate based on the cyanobacterial standing stock indicates that the maximum, net N requirement of A.ovalisporum was between 106 and 245 mu g N l(-1). These observations suggest that components of the DON pool were a major direct or indirect source of N for the microplankton and that N fixation was not a significant factor in the4 development of A.ovalisporum. Further support for this idea was given by experiments which showed that monocultures of this organism grew well in mineral media supplemented with organic N compounds (urea, hypoxanthine, lysine, guanine, glucosamine). In general, it is proposed that the DON pool, which is often in relatively high concentrations compared to DIN, must be considered an active participant in N recycling and may serve as an important N source for phytoplankton in many aquatic environments.