Evaluating biomass of Baltic filamentous cyanobacteria by image analysis

The biovolume of heterocystous cyanobacteria in the Central Baltic Sea was determined in natural, mixed-species samples by image analysis. Epifluorescence microscopy combined with computer-mediated image analysis was used to distinguish phycobiliprotein autofluorescent filaments of Anabaena, Aphanizomenon and Nodularia from other phytoplankters collected on membrane filters. A detailed description is given of the method based on the automatic measurement of single filament morphometry. Manual interactions and mathematical corrections were necessary during image processing to overcome filament overlapping and curvature on filters as well as the coiling of Nodularia spp.. The autofluorescence patchiness of Aphanizomenon sp. made it necessary to trace its dimensions manually. The data obtained were used to estimate variations in the vertical distribution of the biomass of the 3 cyanobacterial genera over a diel cycle.