Description: The influence of periodic salinity changes was investigated for 42 days under semicontinuous culture conditions with phosphorus limitation using phytoplankton assemblages from Lake Waihola, a tidally influenced shallow lake. To simulate tidal effects on the phytoplankton community, salinity in the cultures was increased in pulses at different intervals (3.5, 7, and 14 days), and these cultures were compared with those that experienced constant freshwater conditions. Salinity pulses significantly affected competition and succession with a major loss in diversity during the first days of the experiment due to the initial pulse that caused a transition from freshwater to brackish conditions in the cultures. After this initial phase, diversity index (H') and species number (Scorr) decreased less rapidly. The loss in H' and Scorr over time was highest under constant freshwater conditions and lowest in the treatment with an interval of 3.5 days between salinity pulses. At the end of the experiment, the combination of initial loss in H' and Scorr and the time course of H' and Scorr resulted in a U‐shaped relation between the interval length of salinity pulses and both H' and Scorrtemp1.txttemp1.txt. Our results indicate that salinity pulses at intervals of a few days tend to promote phytoplankton diversity. If saline intrusions in coastal freshwater systems occur only at spring tides, this will lead to decreases in diversity and species richness.

Description: The influence of fluctuating light intensities on phytoplankton composition and diversity was investigated for 49 days under semi-continuous culture conditions with sufficient nutrient supply, using phytoplankton assemblages from Lake Biwa, Japan. Light conditions were either periodically changed from high intensity (100 µmol photons m–2 s–1) to low intensity (20 µmol photons m–2 s–1) at intervals of 1, 3, 6 and 12 days, or fixed to constant intensities (permanent high and low light levels). All treatments additionally experienced a day:night cycle of 16:8 h. Phytoplankton abundance increased and reached a saturation level on day 19 of the treatment with permanent high light, but increased continuously until the end of the experiment (day 49) in the treatment with permanent low light intensity. In treatments with periodically changing light intensities, the phytoplankton abundance reached saturation levels between these dates. Under phytoplankton abundance saturation, chlorophytes predominated in the treatment with permanent high light, while either cyanophytes or diatoms were abundant under permanent low light intensity. Treatments with changing light supply had chlorophyte- and cyanobacteria-dominated replicates as well as replicates with balanced proportions of both. Furthermore, species diversity, measured by the Shannon index, was low in cultures under permanent light intensity, while slow fluctuating light at the scale of 3 –12 days resulted in an increased diversity index. These results indicate that species composition and diversity of the phytoplankton were affected by the periodically changing light regime in the order of days, and suggest that temporal changes in weather conditions are a major impediment to competitive exclusion of phytoplankton species in nature.