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The aim of the present study was to assess the frequency of viral infections in colony-embedded cells of the cyanobacteria Aphanizomenon flos-aquae and Microcystis aeruginosa collected from the brackish Curonian Lagoon. Natural and mitomycin C-treated A. flos-aquae and M. aeruginosa samples were examined for the presence of viruses and lysis by a combination of light-, epifluorescence and transmission electron microscopy techniques. Here we report a lack of evidence for virus infection, progeny formation and cell lysis in colony-embedded cells of A. flos-aquae and M. aeruginosa. These results indicated that viruses contribute little to the mortality of these cyanobacteria when the latter occur in colonies. Consequently, the results supported the hypothesis that colony formation can, at least temporarily, provide an efficient strategy for protection against virus-induced mortality. Finally, assuming that grazing has a negligible effect on colony-embedded cells in the Curonian Lagoon, we propose that most of the cyanobacterial biomass produced is lost from the pelagic food web by sedimentation.

The study was based on CTD-casts performed on 5 consecutive daysat 7 positions along a shallow- to deep-water transect duringthe spring bloom in the southwest Kattegat. The development,dynamics and fate of the sub-surface chlorophyll maximum (SCM)are described. The SCM developed at a constant rate of1.4 mg chl a m-3 d-1,which is equal to a carbon fixation rate of49.0 mg C m-3 d-1. The SCM developed at 6 m depth at low irradiancelevels (10-43.0 µE m-2 s-1). Chl a concentrations reachedup to 23 mg chl a m-3 in the centre of the SCM. Growth ratesin the SCM were estimated at 0.82 d-1. The water column wasonly weakly stratified, with nutrients in excess in the wholeof the water column. The SCM was relocated vertically to depthsbelow the photic zone by wind-induced advection between the bayand the outer Kattegat. There were indications of a horizontalrelocation of the SCM by an internal standing wave. Applicationof high spatial and temporal resolution made it possible to estimatekey SCM parameters, such as growth rates [mg chl a m-3 d-1]and carbon fixation rate [mg C m-3 d-1],on the basis of chl a variations in time and space.