Annual and diel oxygen regime in two polder ditches

The oxygen regime of two polder ditches and two enclosures within these ditches was studied. Continous oxygen temperature and light measurements were performed for 24-hour periods each month during two and a half year in the ditches and one year in the enclosures. Oxygen concentrations between 0 and 23 ppm were found, with diurnal ranges as large as 18 ppm. Steep gradients between bottom and surface could develop, but mostly disappeared during nightly turnover. The 10-percentile of the surface water measured between 9 and 17 hours was above 3 ppm, fullfilling the Dutch standards for this type of ecosystems. The oxygen concentrations near the bottom, however, could drop to zero and during the night surface concentrations below 1 ppm were measured. Based on average oxygen saturation values it is concluded that in the open water of the ditches oxygen consumption prevailed while in the enclosures oxygen production was most important. Based on the mass balance equation gross primary production and respiration were calculated. Annual average respiration varied between 2.5 and 6.6 g O₂.m^–2.d^–1 and average gross primary production between 3.2 and 4.8 g O₂.m^–2.d^–1. Maximum daily production and respiration were 15.9 and 22.3 g O₂.m^–2.d^–1. These figures classify the polder ditches as highly productive aquatic ecosystems.