Seasonal changes in the activity of phytoplankton and benthic algae in relation to diurnal oxygen pulses were investigated in a 120 cm deep, brackish hypertrophic ditch. A vertical chloride gradient was built up by saline seepage and drain-water effluent. The stable chloride gradient could lead to oxygen stratification near the sediment, and to oxygen gradients towards the water surface. The oxygen gradients were rather unstable, depending on the chloride gradient and the wind velocity.Light was limiting photosynthesis both in summer and in winter. Surface oxygen maxima increased with solar radiation during summer.In summer the diatom Cyclotella caused surface oxygen maxima at light saturation in the late afternoon. Simultaneously, the dominant flagellates Peridinium and Chlamydomonas produced oxygen in dim light, probably choosing their favourite light energy level by vertical migration. Oxygen fluctuations ranged from 0 to 34 mg O2.l-1 in a 100 cm vertical profile above a 20 cm anoxic layer. The amplitude of the diurnal oxygen maxima varied from 10 to 34 mg O2.l-1.In winter the water became very clear. The oxygen gradient was inverted during the day showing a characteristic oxygen maximum above the bottom, produced by benthicAchnanthes colonies.
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