Decomposers, Bacteria, and Microbenthos 375 



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Middle of Pond 



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Jun 



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Jul 



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Aug 



FIGURE 8-11. Numbers of ciliates in Pond B sediments, 1971. 



biomass found here of ciliates (10*^ m"\ 20 to 40 mg m"^) and of 

 micrometazoans (10^ m \ 1000 mg m"^) are lypical of detrital and 

 muddy sediments from temperate freshwater and marine environments. In 

 fine sandy marine sediments, Fenchel (1967, 1969) found up to 20x lO*' 

 m ^ ^ ciliates with a weight of 1 000 mg m " ^ 



Protozoans and micrometazoans were more abundant in the littoral 

 areas of the pond among the emergent vegetation than in the central area 

 (Figure 8-11). This is illustrated by data for ciliates but nematodes and 

 rotifers followed the same pattern (2 to 5 times higher). This distribution 

 reflects the higher numbers of bacteria produced by the decomposition of 

 leaves and excretion of organic matter in the littoral zone as well as the 

 greater depth of the aerobic layer of sediments (Chapter 4, Figure 4-1 1). 



Ciliates in the center area of the pond appeared to be most abundant 

 in 1972 and least abundant in 1971 (Figure 8-12). However, slightly 

 different parts of the pond were sampled each year and a different 

 counting technique was used in 1973. Therefore, the differences between 

 years may not be significant. Rotifer counts were also highest in 1972 

 (3.3 X 10' m~') but lowest in 1973 (0.47) and intermediate (0.83) in 1971 

 (as averages for July of each year). Figure 8-12 does show two peaks in 

 ciliate abundance, one around the beginning and one around the end of 

 July. This is likely in response to the bacterial peaks in the beginning and 

 end of the summer (Figure 8-1) and to the continued increase in numbers 

 of epipelic algae (Figure 5-10) over the summer. Actually, most of the 

 ciliates at the beginning and end of the summer were bacteria feeders 



