Macrobenthos 



333 



m 



a> 



Q. 



a> 



in 

 o 



in 



c 



a> 



E 



400 



300- 



200 



100 







— 1 — 1 — I — I — I — I — 



S • Org. 7 mm TL 

 MO Org. 17mm TL 

 L A Org. 31 mm TL 



"1 — I — r 



T — I — r 



25 



15 

 Temperature, °C 



FIGURE 7-17. Movement of three sizes o/Lepidurus arcti- 

 cus (in order of total length) at different temperatures. All 

 animals were acclimated at 10°C. The units of movement 

 refer to the lines on a I -cm grid and mean and ranges are 

 given for each observation. 



pond due to the considerable variation in prey density (Kangas 1972) and 

 to the various prey's differing abilities to escape this predator. Some 

 detritus is ingested by Lepidurus but assimilation is likely to be low. For 

 example, small Lepidurus kept in the laboratory, with only surface 

 sediments for food, did not grow nearly as fast as animals in the field. 



Activity 



The movement o{ Lepidurus was tested in a 14.5-cm-diameter shallow 

 container with a gridded bottom. An observer counted the number of lines 

 that each animal crossed in 5 minutes. Actual measurements of the path 

 showed that each crossing equals 0.73 cm of path. 



Although the ranges frequently overlapped, the general picture was of 

 increasing activity for larger animals and increasing activity at higher 

 temperatures (Figure 7-17). Over the 5° to 15°C temperature range, the 

 means increased 2.7 times for the 7-mm animals, 5.3 times for the 17-mm 

 animals and 5.8 times for the 30-mm animals. Field observations, using a 

 gridded Plexiglas sheet placed above the sediments, showed a movement of 

 9.6 cm min \ Similar animals in the lab moved 7.3 cm min ' at 5°C and 

 26 cm min ' at 10°C so that the laboratory data appear to be reasonable. 

 Therefore, we can next calculate the amount of sediment surface area that 

 can be disturbed each day by the animals in each square meter (Table 7-7) 

 from the movement per day and the width of the carapace. The disturbed 

 area in each square meter was 40.9 m^ for the 33-day interval or 1.2 m "^ 



