By, ROBERT W. PENNAK
BIOLOGY DEPARTAIENT, UNIVERSITY OF COLORADO, BOULDER, COLORADO
ONE of the most recent lines of investigation in limnology is that of the ecology of the microscopic organisms inhabiting the capillary waters of sandy beaches. During 1936 and 1937 studies in this field were carried out at a number of Wisconsin lakes and the majority of our remarks, which are by no means conclusive, are based on the results of this work. These researches have settled few questions, but have demonstrated the existence of many biological complexities in this interesting environment.
In the mind of the biologist the word "sand" is almost invariably associated with "desert." If, however, we examine the uppermost few centimeters of the exposed sand adjacent to the water's edge, a rich and varied microscopic flora and fauna will be discovered. Figs. 1 and 2 show two such typical Wisconsin lake beaches.
The organisms in the sand, or psammolittoral, as it has been called, are primarily the genera and species encountered in true aquatic environments. Numerically, the rotifers, copepods, and Tardigrada, or "water bears," constitute the majority of the Metazoa. Fig. 3 is an optical section of a small portion of a beach showing the size relationships of the sand grains and these organisms.
In contrast to the chemical and physical conditions prevalent in lake waters, conditions in beaches are subject to rapid and drastic fluctuations. Let us consider first some of the physical factors at work in beaches.
The organisms within a beach are subject to two opposing currents of water: a slow upward current caused by the rise of capillary water in the sand and its evaporation at the surface, and an intermittent, comparatively vigorous downward current resulting from waves and rain beating upon the sand. Nevertheless, in many beaches rich populations of organisms are found in situations over which waves wash only rarely, if ever.
The amount of water in the sand, particularly in the surface layers, varies over a wide range. However, mean values for 30 series of determinations of the water content of the top centimeter of sand show approximately 80 per cent saturation at 100 cm from the edge of the water, 40 per cent saturation at 200 cm, and 20 per cent at 300 cm.
As would be expected, the slope of a beach governs the width of the inhabitable zone of the sand. A beach having a slope of 8° from the horizontal may have a populated zone 150 cm in width, while a beach having a slope of 3° may have an abundance of organisms as far back as 300 cm from the edge of the water.
The size of the sand grains constituting a beach determines the size of the spaces in which the organisms live, but no significant differences, qualitative or quantitative, have been found in the psammolittoral populations which can be attributed definitely to differences in sand-grain size. The proportion of pore space is practically uniform (about 40 per cent of the total volume), whether or not the sand in a beach be fine or coarse, heterogeneous or homogeneous in composition.
Temperatures of the sand are governed by a number of factors. The most important of these are lake-water temperature, air temperature, amount of capillary water and its evaporation, wind, and sunshine. What little we know of beach temperatures support the following conclusions: