Rahm, L. Linköping University, larra@tema.liu.se

Nitrogen as the limiting factor of the primary production in the Baltic proper has been under debate during the last decades. Based on only a limited number of studies, nitrogen fixation has been assumed the only significant internal nitrogen source. It is assumed that about one fifth of the net nitrogen load to the Baltic proper comes from nitrogen fixation by cyanobacteria. Another alternative is the "direct" utilisation of organic nitrogen. In the present study we hypothesise that nitrogen fixation is the only source. This in order to investigate its potential. If inorganic nitrogen runs out after the spring bloom and if inorganic phosphorus is still left in significant concentrations, then an ongoing decrease in phosphorus may be coupled to the net nitrogen fixation of cyanobacteria. The estimated phosphorus consumption is adjusted for external and internal loads. An estimate of the assumed net annual nitrogen fixation based on the proper Redfield ratio in the surface layer down to the seasonal thermocline is obtained for a number of monitoring stations in the Baltic proper. Typical values of nitrogen fixation are in the range 10 - 130 ?mol/m3/day. A simple integration over the Baltic proper gives an internal load in the range 70 000 - 260 000 ton N/year. Another result is an east-west gradient in fixation rate similar to the inverse nitrogen load.
Day: Friday, Feb. 5
Time: 02:45 - 03:00pm
Location: Eldorado Hotel
Code: SS48FR0330E