THE MICROSCOPIC FAUNA OF THE 

 SANDY BEACHES 



By ROBERT W. PENNAK 



BIOLOGY DEPARTMENT, UNIVERSITY OF COLORADO, BOULDER, COLORADO 



Introduction 



One of the most recent lines of investiga- 

 tion 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 demon- 

 strated the existence of manv 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 tw^o 

 such typical Wisconsin lake beaches. 



The organisms in the sand, or psammolit- 

 toral, 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. 



Physical Factors 



In contrast to the chemical and physical 

 conditions prevalent in lake waters, condi- 

 tions 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 capil- 

 lary water in the sand and its evaporation 



at the surface, and an intermittent, com- 

 paratively vigorous downward current re- 

 sulting 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, par- 

 ticularly 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 jjer 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 popu- 

 lated zone 150 cm in width, while a beach 

 having a slope of 3° may have an abun- 

 dance 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 popula- 

 tions which can be attributed definitely to 

 differences in sand-grain size. The pro- 

 portion of pore space is practically uniform 

 (about 40 per cent of the total volume), 

 wiiether or not the sand in a beach be fine 

 or coarse, heterogeneous or homogeneous in 

 composition. 



Temperatures of the sand are governed 

 by a inimber of factors. The most impor- 

 tant 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 : 



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