Impact of the inoculum size on the population of Nostocales cyanobacteria in a temperate lake

Abstract

Akinete formation enables cyanobacteria of the order Nostocales to survive adverse conditions (e.g. winters in temperate zones) and is assumed to be a prerequisite for the invasion of tropical species like Cylindrospermopsis raciborskii into the temperate zones. We chose C. raciborskii for a 4-year study of the seasonal patterns of all life cycle stages in the pelagic and benthic zones of a shallow eutrophic lake in NE Germany. Akinete production started when C. raciborskii populations reached maximum cell abundance and water temperatures started to decrease, resulting in an increase in the number of akinetes at the sediment surface. The number of akinetes on the sediment (inoculum for the next year) was population size dependent. It decreased slightly in winter but dropped sharply from end of April to June/July, reflective of germination. Thus, germination started at temperatures of about 13°C. Typically, first filaments appeared in June and populations peaked in August. However, the size of the population in summer was not determined by the size of the inoculum. Consequently, it is assumed that population development is determined by growth conditions and loss processes after germination.