Optical changes associated with cyanobacterial bloom termination by viral lysis

Optical changes that accompanied a collapse of the populationof filamentous cyanobacteria from a shallow, eutrophic lakewere studied in laboratory-scale enclosures (LSEs). The experimentalconditions are known, from previous work on these systems, tocause a dramatic collapse of the dominant algal or cyanobacterialspecies, which in turn can be associated with viral activity.Within 2 weeks of continuous addition of nutrient-rich growthmedium, near-complete collapse of the dominant population occurredover the span of a few days. The collapse was repeatedly andreproducibly observed and was primarily characterized by a markedincrease in water transparency. Scattering of light decreasedby 80%, absorption decreased by 20–80%. There was highsimilarity in optical changes between several experiments, carriedout in different seasons. An increase of dissolved materialand submicron-sized particles (SMP) that showed chlorophylla (Chl a) absorption was observed during the collapse. The phycocyanin(PC): Chl a ratio and phaeopigment : Chl a ratio proved to begood indicators of the observed collapse. Reflectance spectrathat were modelled using a constant volume-scattering functionindicated that mass mortality of this magnitude can be detectedin natural systems using current remote sensors.