Decomposers, Bacteria, and Microbenthos 373 



on each sampling date from the central part of Pond B (no plants). On any 

 date, all the samples were taken within an area of 0.5 m^ In the 

 laboratory, the cores were sectioned into 2-cm layers. Large organisms 

 were microscopically enumerated and identified by placing each section 

 into a petri dish, adding filtered pond water, and then removing the 

 animals one by one. Small zooflagellates, microalgae, and bacteria were 

 counted on the surface of sediment particles with a fluorescent microscope 

 (Fenchel 1970). Most ciliates were identified to species; some small 

 ciliates, zoofiagellates, rotifers, and gastrotrichs were identified only to 

 genus; no attempts were made to identify nematodes, ostracods, or 

 harpacticoids. Counts were converted to biomass using volume estimates 

 of Fenchel (1967) or from linear dimensions and reasonable geometric 

 shapes. 



In order to estimate feeding rates of protozoans in nature, the 

 egestion rates of two species of bacterivorous and two species of algivorous 

 protozoans were determined in laboratory cultures at different 

 temperatures (Fenchel 1975). By assuming that the feeding rate in the field 

 is a continuous process, the actual feeding rate can then be estimated by 

 counting the food organisms in the feeding vacuoles in protozoa 

 immediately after collection from the sediments; the feeding rate is the 

 egestion rate times the actual counts. 



Vertical Distribution and Composition of the Microfauna 



All of the groups were most abundant in the surface sediments and 

 their numbers decreased with depth. One reason for this decrease is the 

 anaerobic conditions below about 2 cm (Chapter 4, Figure 4-11). Only 

 nematodes, which are among the few metazoans that live in anaerobic 

 sediments (Fenchel 1969), were regularly found below this depth. 

 However, some ciliates were found in the 2- to 4-cm sections; these 

 {Caenomorpha medusula, Saprodimum sp. and Metopuses) are known as 

 anaerobic forms from other studies (Bick 1958, Fenchel 1969). 



In these ponds, however, the mechanical properties of the sediment 

 are probably the most important factor which limits the ciliates to the top 

 3 or 4 cm. The upper layers of the sediment consist of large particles (a 

 mean particle diameter of 500 txxn (Figure 4-1)) that are loosely packed 

 and contain large interstitial spaces. In contrast, the deeper sediments are 

 made up of finer particles that clog these spaces and prevent penetration 

 by the ciliates. 



Organisms may also be moved within the sediment by the actual 

 mixing of the sediment itself. This occurs by the movements of chironomid 

 larvae and oligochaetes, by wind action, and by deposition of suspended 

 material (see also discussion in Chapter 5). 



