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1. Picophytoplankton (picocyanobacteria and picoeukaryotes) commuruties in Lake Kinneret were studied from 1988 to 1992. No prochlorophytes were observed in the lake. 2. Picocyanobacteria were a prominent and ubiquitous component of the phytoplankton, being present at all depths throughout the year, with concentrations ranging from 2 X 10-8 X 10^ cells n:il"^ Low cell numbers in winter and spring were followed at the end of the annual dinoflagellate bloom by maximal abundances in summer-autumn in the epilimnion. High cell numbers (> l O"* cells ml'' ) were sometimes also found in the anaerobic hypolimnion. Net growth rates for picocyanobacteria ranged from 0.29 to 0.60 divisions day''. 3. Picoeukaryotes were a very minor constituent of the picoplankton, mostly present in winter and spring, and sometimes at the end of autumn, with concentrations ranging from 44 to 5700 cells ml'^. Higher cell numbers tended to occur in the near surface water layers, hi August-September, picoeukaryotes were found only in the hypolimnion. In December, the occurrence of picoeukaryotes in the deep water layers probably resulted from advection with cold water currents from the Jordan river. Net growth rates for picoeukaryotes ranged from 0.26 to 0.43 divisions day'. 4. Overall, the contribution of picophytoplankton to the phytoplankton standing crop in Lake Kinneret was limited; picocyanobacteria and picoeukaryotes accounted for no more than 7.0 and 0.1% of total algal biomass (semiannual average), respectively. 5. Picophytoplankton cell numbers in pelagic waters were usually similar to those in shallower lake stations. 6. Ficocyanobacteria appear to be an autochthonous population, whereas picoeukaryotes are probably brought annually by the Jordan River and do not maintain themselves in the lake.

Summary 1. We propose that the appearance and establishment of Nostocales (cyanobacteria) species of the genera Aphanizomenon and Cylindrospermopsis in the warm subtropical Lake Kinneret (Sea of Galilee, Israel) from 1994 was linked to changes in climate conditions and summer nitrogen (N) availability. 2. From 1994 to 2009, an increase in frequency of events of elevated water temperature (>29 °C) in summer, and to some extent a greater frequency of lower summer wind speed events, affected water turbulence and water column stratification, thus providing better physical conditions for the establishment of these populations. 3. In recent years, N‐depleted conditions in Lake Kinneret in early summer have promoted the development and domination of Nostocales that could gain an ecological advantage owing to their N2‐fixing capability. 4. Nitrogen fixation rates coincided both with heterocyst abundance and with Nostocales biomass. The N supplied to the lake via nitrogen fixation ranged from negligible quantities when Nostocales represented only a minor component of the phytoplankton community to 123 tonnes when Cylindrospermopsis bloomed in 2005. This high N2 fixation rate equals the average summer dissolved inorganic nitrogen load to the lake via the Jordan River.

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Akinetes are differentiated, thick-walled, resting cells produced by many species of the filamentous heterocystous cyanobacteria belonging to the Nostocales and Stigonematales. They are commonly developed after the end of exponential growth and are normally resistant to low temperatures and to desiccation for extended periods. Akinetes maintain a low level of metabolic activities such as photosynthesis, respiration and de novo synthesis of proteins and lipids. In a previous study we reported that mature akinetes of Aphanizomenon ovalisporum maintained only residual photosynthetic activity. However the cellular abundance of PSI and PSII reaction centres increased in akinetes relative to exponentially grown vegetative cells while the ratio of PSI to PSII remained relatively unchanged. In the present study we provide spectral and biochemical evidence that akinetes lose their phycobilisomes during their development. DAPI stained trichomes of exponentially grown cultures emitted fluorescence signals attributed to nucleic acids and polyphosphate granules. Akinetes that developed in akinete-induced cultures were preferentially enriched with nucleic acids and deprived of polyphosphate bodies, whereas adjacent vegetative cells were loaded with polyphosphate bodies. Examination of akinetes from akinete-induced cultures of A. ovalisporum by spectral confocal laser scanning microscopy indicated reduction of the phycobilisome fluorescence signal. Mature isolated akinetes could be divided into two distinct populations: (i) a majority of akinetes that lost their phycobilisome fluorescence signal, but maintain chlorophyll fluorescence attributed to the reaction centres, and (ii) a minor group of mature akinetes that maintained a high phycobilisome fluorescence signal.