Is there a succession in the autotrophic picoplankton temperate zone lakes

Abstract

The seasonal periodicity in phytoplankton species composition as well as its causes were extensively studied in Lake Constance located on the northern fringe of the European Alps. Algal species and their predators turn up and disappear in specific phases of the season. However, in the microbial loop small coccoid cyanobacteria of the Synechococcus type dominate the autotrophic picoplankton throughout the year. In this size class, a recurrent pattern of seasonal differences in abundance is observed. Synechococcus spp. isolated from this environment were genetically diverse and genotypes were shown to differ in growth rates with variances enhanced by exposure to stress. Furthermore, small predators from the same environment exhibited differential feeding preferences for single Synechococcus strains. These results indicated that genotypes may have adapted to highly specific niches. Phylogenetic analyses of the 16S rDNA and the 16S-23S ribosomal spacer region (ITS-1) showed that this adaptation could have occurred during a recent radiation. Further analysis of the population was impeded by a low plating efficiency, scarcity of morphological characteristics and lack of divergence in 16S rRNA sequences. For discrimination and quantification of genotypes on basis of ITS-1 target sequences the application of fluorescent TaqMan probes in Taq nuclease assays (TNA) was examined. Ecology of photoautotrophs in temperate zone lakes In 1986 the plankton ecology group (PEG) headed by U. Sommer proposed a model that describes the seasonal events which occur in the phytoplankton and zooplankton of temperate zone lakes [12]. Briefly, the most important factor for phytoplankton growth is the seasonal pattern of stratification and overturn. In deep lakes such as Lake Constance (surface area 500 km 2 , maximum depth 254 m, mean depth 100 m) winter overturn resets the community close to zero each year. Algal mass growth starts with the onset of stratification allowing algae to stay permanently in the euphotic zone. Under continuously stratified conditions, small rapidly-growing algae will soon decrease the water transparency, deplete nutrients, and allow the build-up of grazer populations. The spring bloom is terminated when grazing exceeds algal reproduction leading to a rapid decline of algal biomass. After this clear-water phase, different species will rise and constitute the highly diverse summer phytoplankton. Changes in the summer population are controlled by successive depletion of different nutrients and a replacement of more edible algae by poorly edible species.