The availability of oxygen for the interstitial fauna of sandy beaches

Abstract

The oxygen conditions in four Swedish sandy beaches, three in the Baltic and one in the North Sea, have been investigated at various times during the year. Measurements have been made of both oxygen concentration and oxygen diffusion rate (ODR), the latter with a stationary platinum microelectrode technique, at different distances from the water-line and at various depths in the sand. Laterally along the beach, i.e. along a transect parallel to the water-edge, the conditions at corresponding depths are very similar. Horizontally and vertically, however, the distribution of oxygen shows no clear trend except at the most seaward part of the beach which is the best oxygenated. Even in winter the deep unfrozen strata, covered by thick layers of frost, show clear, though low, ODR values. The availability of oxygen is strongly governed by the physical and chemical properties of the sand: porosity, permeability, percentage air volume, and maxima appear at various depths in the sand. The flow of the interstitial water in the beach is of great importance; the role of waves in determining the availability of oxygen is demonstrated by in situ measurements of ODR. Each separate wave greatly increases the availability in adjacent areas. The dynamics of this process has been analyzed in laboratory experiments, which show the effects of separate drops of water and of changing ground-water pressure upon the oxygen availability of the pore-water. Two examples of faunal distribution in the field along a concentration gradient of oxygen are given. The fauna, consisting of turbellarians and nematodes, was in both cases concentrated in the best oxygenated layers.