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Daily variations of the phaeoplast arrangements in cortical cells of Dictyota dichotorna were investigated in field experiments carried out at Helgoland (North Sea). Thallus transmittance changes caused by phaeoplast movements were used as a measure and determined with a microphotometer. The transmittance of samples exposed to daylight in the sea was measured every hour; as reference the transmittance level of the low intensity arrangement was used. In addition photomicrographs of the corresponding phaeoplast arrangement were taken. To investigate the dependence of the phaeoplast arrangement on water depth, several thalli were exposed at different depths and measured at noon. Results of earlier laboratory experiments were comparable to present results obtained in the natural habitat. According to the time of day the phaeoplasts were in the low intensity arrangement at low fluence rates and in the high intensity arrangement at high fluence rates, although the thalli were floating and, hence, continuously changed their orientation to the light direction. Down to a depth of 4 m the phaeoplasts moved to the high intensity arrangement at noon. At greater depths fluence rates were too low to induce t h s movement. Transient changes in the fluence rate of daylight caused by cloud cover did not induce significant changes of the phaeoplast position.

Daily changes of the phaeoplast arrangement in the brown alga Dictyota dichotoma as studied in field experiments