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Extreme diurnal oxygen changes - highest peak

Oxygen time series of an IGB-station monitoring a channelized stream outflowing off Lake Maxsee east of Berlin, germany, in June 1994. The blue line shows measured oxygen data (highest oscillation I ever measured: Who found more in open free flowing freshwaters ?). The red line is water temperature, resolution 2 minutes both.
From sunrise until noon oxygen rises fast because of high primary production of macrophytes (mainly Ceratophyllum demersum). In the afternoon, the rise slows down suddenly. Some minutes later, within bright sunlight late afternoon, community respiration (and low output to the atmosphere) exceeds primary production. At midnight already, oxygen has decreased nearly down to zero, where it will remain the rest of the night.
This day gross primary production was 14 gO2/(m²d). Respiration from sunset to midnight was 17 gO2/(m²d), later 10 gO2/(m²d) until sunrise and oxygen concentration near 0 mg/l.

The stream is channelized, flowing through a marsh. The water outflowing the lake is rich in nutrients and suspended phytoplankton. The monitoring station (Hydrolab Datasonde 3) was sent to the stream bottom 1.6 km downstream the lake outlet. Flowing velocity was 5 - 7 cm/s. Ceratophyllum demersum had about 5 kg/m² FW. Seston concentration in the lake was 15 mg/l DW on an average. After flowing 1.6 km, it was only 11% of the lake value. And further 10 km downstream flowing through dense macrophyte stands, snorkeling I measured 6 to 8 m visibility.

The days before ?

 Michael Böhme, Boehme@gmx.de

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