Abstract
The oxygen regime of three fresh-water sandy beaches in southern and middle Sweden was investigated. Microdistribution of interstitial mesofauna was examined at the same time, and the influence of oxygen availability measured as oxygen diffusion rate (ODR) upon the microdistribution was noted. The ODR is influenced by different environmental conditions. Sudden changes in the ODR are caused by rapidly changing factors, e.g. movement of interstitial water in parts of the beach influenced by wave action. Normal diel temperature variation in the beach implies that surface ODR values, in those parts of the beach where the sand surface is wetted by waves or by capillary action, may show a diel variation. Changes in oxygen concentration of the interstitial water are gradual and therefore sudden changes in ODR caused by this factor are unlikely. Ground-water pressure, porosity of the sand and amount of detritus undergo sudden changes only at the water line. Strata of coarser sand may display higher ODR than strata of fine sand in the vicinity, owing to better drainage. No direct correlation between redox potential and ODR can be found, although strongly reducing Eh values may imply oxygen depletion. In parts of the beach where the superficial layers are moist, a strong correlation can be found between oxygen availability and mesofaunal development. In these parts moisture content, grain size and other environmental factors are often regularly distributed vertically, while a strong gradient in ODR is found, with higher values in the superficial layers. This is reflected in a strong concentration of animals in the uppermost layers. In the middle beach animals may congregate in areas where the ODR values are higher than in the surrounding layers. In this part moisture content limits the distribution of most mesofaunal groups upwards. Values of ODR reported as limiting for harpacticoid copepods are found in deeper layers of the beach without seeming to limit these animals' distribution. Pe3NoMe
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