The surface water CO2 budget for the Baltic Proper:: a new way to determine nitrogen fixation

Six cruises were performed in the eastern Gotland Sea in about monthly intervals between March and September 2001 in order to establish a surface water carbon/nitrogen budget and to derive N-2 fixation rates. Balancing the concentration changes of total CO2 and dissolved organic carbon, the CO2 air/sea exchange and the effect of mixed layer deepening yielded the net production of particulate organic carbon (POC) for each time interval. A total net production of 3.5 mol m(-2) was obtained, which is consistent with earlier estimates of the POC export into the deep water. Measured C/N ratios in the particulate organic matter (POM) were applied to calculate the net formation of particulate organic nitrogen (PON), which was used to balance the nitrogen pools and fluxes. Taking into account the changes in dissolved organic nitrogen, the atmospheric input of NO3-/NH4+, and mixed layer deepening, the N-2 fixation was calculated for each period. A total N-2 fixation of 318 +/- 53 mmol m(-2) was obtained, which substantially exceeded any previous estimate for the Baltic Proper (14-260 mmol m(-2)) and which corresponded to almost twice the spring bloom nitrogen consumption. The temporal distribution was characterized by an unexpected peak (108 mmol m(-2)) in late spring that was attributed to the activity of Aphanizomenon. A second maximum (106 mmol m(-2)) was found for the June-July period when Nodularia dominated the planktonic community.