Seasonal periodicity, heterocyst differentiation and sporulation of planktonic Cyanophyceae in a shallow lake, with special reference to Anabaena solitaria

Abstract

The periodicity of eight species of Cyanophyceae is recorded over two seasons in Sawley Dene, a 9·5 ha lake in North Yorkshire. The succession of blooms of heterocystous forms followed by colonial coccoid species is similar to that found in the epilimnion of many temperate eutrophic lakes that stratify in summer, though the Yorkshire lake is too shallow for thermal or chemical stratification to occur. The periodicity is explained by the long retention time of the lake, variable column stability and summer nitrogen deficiency, conditions that favour the larger self-regulating cyanophytes. Cell degeneration, heterocyst frequencies and akinete differentiation are discussed in relation to nitrate-nitrogen concentrations and population maxima. In the decline phase after a bloom, heterocystous forms show cell degeneration and akinete formation whereas the coccoid forms undergo sinking with little cell degeneration since intact colonies are necessary for overwintering in the sediment. Forty per cent of the akinetes of Anabaena solitaria germinate shortly after maturation and it is suggested that these akinetes serve to remove the alga from the upper and potentially damaging layers of the lake rather than persisting as long-term resting spores. Germinated akinetes may well constitute the major overwintering stage.