Nitrogen fixation and abundance of the diazotrophic cyanobacterium Aphanizomenon sp. in the Baltic Proper

Nitrogen fixation in the Baltic Proper was estimated from the increase in total nitrogen concentration in the upper mixed layer (UML) during summer stratification, using high spatial replication to address the well-known heterogeneity of nitrogen-fixing cyanobacterial blooms. Total nitrogen increase in the UML was found to be 3.2 mol l -1 in both the Western and Eastern Gotland basins. When corrected for atmospheric deposition and sedimentation, basin-wide Baltic nitrogen fixation (excluding the Gulf of Bothnia) was estimated to be ~310 ktons nitrogen in 2002. Of total nitrogen increase, only 6% was estimated to remain in the peak biomass of the dominant cyanobacterium Aphanizomenon sp., indicating large leakage of bioavailable, fixed nitrogen. Peak abundance of Aphanizomenon sp. was estimated to be 16 and 14 m l -1 of filaments in the Western and Eastern Gotland basins, respectively. Results show that the western and eastern basins were similar but comprised considerable spatial heterogeneity at smaller scales. Power analysis was used to estimate the number of replicates required to detect given changes in total nitrogen concentration increase and abundance of Aphanizomenon sp. between 2 yr. This analysis revealed that 10 to 20 replicates are required on a basin-wide scale to detect a 50% change between years by ANOVA.