The Sluice-dock of Ostend, a shallow (1.5 m) marine biotope of 86 ha stays closed during some 6-7 months of the year and is not influenced by the tides. During 24 h cycles the nycthemeral migrations of the zooplankton at 5 depths in a watercolumn of 1 meter were studied in relation to several environmental factors: water temperature, dissolved oxygen, pH and chlorophyll content. The photosynthesis shows a minimum during the night and shows a maximum during the late afternoon, at which time the water-temperature is also maximal. Holo- and meroplanktonic organisms migrate towards the surface around 3-5 o'clock a.m., at which time the chlorophyll content is minimal, hence our hypothesis of nocturnal grazing, this phenomenon perhaps combined with a nocturnal division of phytoplankton cells. Each observed species has its own characteristic behaviour in relation to its negative diurnal phototropism and its own diurnal bottomward migration speed.
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