The annual cycle of growth and reproduction of planktonic blue-green algae in the Salopian meres

Abstract

The physical and, chemical factors which affect sporulation, and the growth and survival of vegetative populations of blue-green algae were examined in two eutrophic kataglacial lakes of the Ellesmere (North Salop) group of meres. The importance of individual factors were examined under controlled conditions using natural populations or uni-algal isolates from the meres. The induction of akinete formation, which, in all the cases examined, proceeds once the alga forms a surface bloom, is thought to be due to the extreme conditions, particularly of solar radiation, which prevail at the surface during the summer. Winter blooms did not sporulate. Nutrient deficiency, particularly of orthophosphate, is not considered to be a critical factor. This is supported by the finding that blooming algal material is not depleted of phosphorus, that alkaline phosphate activity was low, and that nutrient deficiency under controlled conditions did not induce sporulation of the species predominant in the meres. Nitrogenase activity (acetylene reduction) and the photosynthetic rate (oxygen evolution) were observed to fall in the period subsequent to bloom formation. The reduction of measured net photosynthesis may be correlated with the increase of bacterial numbers. The activity of light-damaged material was lower than that of unaffected populations. It appears that there must be a low cellular carbon to nitrogen ratio before akinete differentiation can begin, indicating that a reduction of photosynthetic activity relative to that of nitrogon assimilation is required. Akinetes probably do not form a significant overwintering mechanism in the lakes investigated. Numbers of akinetes found during the winter were low. The bloom populations probably develop from growth of the overwintering planktonic vegetative populations during the spring and early summer. Cornparisons of planktonic, sedimenting and benthic algal material indicates that germination of akinetes shortly after their maturation may give rise to the greater part of the overwintering vegetative populations. Physical rather than chemical facters appear to control the population sizes and life cycles in the environments studied.