Filamentous cyanobacteria in the Baltic Sea – spatiotemporal patterns and nitrogen fixation

Summer blooms of filamentous, diazotrophic cyanobacteria are typical of the Baltic Sea Proper, and are dominated by Aphanizomenon sp. and the toxic Nodularia spumigena. Although occurring every summer, the blooms vary greatly in timing and spatial distribution, making monitoring difficult and imprecise. This thesis studies how the spatial variability of Baltic cyanobacterial blooms influences estimates of abundance, vertical and horizontal distribution and N2-fixation. Implications for sampling and monitoring of cyanobacterial blooms are also discussed. An automated image analysis method for identification and measurements of Aphanizomenon filaments was developed. The method gives good comparability with visual microscope counts, and saves 25–75% of the time per sample counted. The monitoring method of taking depth-integrated water samples with a hose was tested, and found to be independent of hose diameter for Aphanizomenon, whereas counts of Nodularia showed large variations between samples, irrespectively of hose diameter. Nitrogen (N) fixation in the Baltic Proper can be estimated from the increase in total N in the upper mixed layer during summer. From 9 stations each side of Gotland every 2–4 weeks, we calculated Baltic N2-fixation in summer 2002 to ∼310 ktons N yr , after correction for atmospheric deposition and sedimentation. This equals ∼one third of the annual, external N load to the Baltic. At the peak bloom, only ∼12% of the fixed N remained bound in the biomass of diazotrophic cyanobacteria, suggesting a large leakage of fixed N to the nondiazotrophic community already during summer. We studied the large-scale distribution of Aphanizomenon and Nodularia at 8 stations each in 2004 and 2005. Variability between stations was large, and could only partly be explained by salinity and winds. Using power analysis, these data sets were used to calculate that ~10–30 replicates are required to detect a 50% difference between years in abundance of Aphanizomenon, while the more spatially variable Nodularia needs ~50 replicates. Aquatic cyanobacteria are known to migrate between surface and deep waters. Experimentally, we tested if such migration would increase growth, by moving Aphanizomenon incubated in dialysis tubes between the surface during day and 12 m or 22 m depth at night. Results suggest that migration is more important for Aphanizomenon towards the end of the summer, when the intracellular P store is low.