Lake Balaton is a large (600 km~2), shallow (z_(mean)=3.2 m) lake in Hungary. Its phytoplankton experience rapidly changing light and temperature. The water usually warms up by 2-5℃ between sunrise and sunset and cools during the night. The mean coefficient of light attenuation is 4 m~(-1) due to permanent resuspension. Planktonic algae circulate in the water column and intercept from very dim to near surface irradiance in a couple of minutes. Our aim was to describe dynamic photosynthesis in this variable environment based on online delayed fluorescence measurements. Delayed fluorescence (DF) is a photon emission caused by backflow of electrons through the photo-system II (PSII) when algae are moved from light to darkness. The intensity of photon emission decays with time, typically in 10-100 seconds. DF intensity is a function of both biomass and the status of the photosynthetic apparatus. (Istvanovics et al. un-publ.) Two measurement methods of DF are common (Gerhardt Bodemer 2000): (ⅰ) registering the kinetics of DF decay after excitation by monochromatic light; (ⅱ) registering initial DF intensity after applying an excitation spectrum between 400-730 nm. DF spectra can be used to distinguish various algal groups since their different pigmentation results in different DF spectra.
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