PRIMARY PRODUCTION ESTIMATION FROM CONTINUOUS OXYGEN MEASUREMENTS IN RELATION TO EXTERNAL NUTRIENT INPUT

In experimental ditches gross and net production were estimated from c ontinuous dissolved oxygen measurements for individual days over a two year period. There are two series of three ditches: the first is domi nated by benthic algae, and the second by submerged macrophytes. Each series receives three different levels of external nutrient (N,P) load ing, referred to as reference, medium-loaded and highest-loaded level. Generally, there were no vertical oxygen gradients, and a one-box mod el satisfied. For periods with O-2-stratification a two-box model has been developed. This model assumes that primary production only occurs in a well mixed top layer. In both series, the cumulative gross prima ry production over two years was positively related to the level of ex ternal nutrient input. In the ditches dominated by benthic algae, a co ntinuous increase in cumulative net systems production, that was posit ively related to the level of external nutrient input, was observed. I n the macrophyte ditches, the cumulative net systems production was po sitively related to the level of nutrient input during the first half year, but after that showed an irregular pattern. The maximum, areal d aily gross productions during the summer, corrected for temperature, i ncreased from 1990 to 1991 in all ditches dominated by benthic algae, but decreased in all macrophyte ditches. The accumulation of biomass i n the ditches with benthic algae was calculated from the cumulative ne t production, and showed a good agreement with measured values in the reference ditch, but yielded higher values than measured in the medium -loaded and highest-loaded ditches. Reaeration was calculated using a system specific, empirical wind-reaeration relationship. The wind spee d was on average higher during the afternoon than during the rest of t he day. High wind speeds thus coincide with high oxygen saturations, a nd the use of a variable reaeration coefficient during a day results i n higher gross and net production estimates than the use of a constant value throughout a day.