304 



M. Butler etal. 



Pond J 



Pond Si 



FIGURE 7-3. Relative abundances of major macrobenthic 

 invertebrate taxa in the central sediments of Ponds J, G 

 and Q averaged over 1975 and 1976. C — Chironomini, 

 Tt — Tanytarsini, Tp — Tanypodinae, Or — Orthocladiinae, 

 Po — Podonominae, Ol — Oligochaeta. 



wind-generated currents. Peaty areas appear to be former stands of plants 

 with rhizomes still present but no longer growing. In the emergent 

 vegetation habitat, the water is shallower than in the pond centers and 

 circulatfts less. As a result, the water temperature at midday may be 

 several degrees warmer in the plant stands than in the pond centers. 

 Because of almost constant winds, complete exposure of the ponds, and 

 low water temperatures, there is no depletion of oxygen above the 

 sediments in the center although there may be a slight decrease among the 

 plants on especially warm days. Sediments in the center of Pond J have a- 

 median grain size (wet sieved) of 500 ^lm and 95% of the particles were 

 larger than 6.2 ixm. The sediments in Pond J lose 68% of their dry weight 

 on ignition, but other ponds have up to 84% organic matter. Their water 

 content is very high and the interface between sediments and water is often 

 indeterminate. There are slight differences in fauna between the types of 

 sediments. 



