PART III: RESULTS AND DISCUSSION 

 Aquatic Communities 



Plankton — Mississippi River 



We were not able to find any surveys of the plankton in the study 

 reach of the Mississippi prior to 1930. Consequently, we have no way 

 of comparing plankton populations before and after construction of the 

 navigation dams. We can hypothesize that the navigation dams, by making 

 portions of the Mississippi River more lake-like, may have increased 

 the population of truly planktonic forms which live in the water column, 

 as opposed to forms which live attached to the substrate and appear in 

 the water column as the result of scouring. 



An excellent survey of phytoplankton and zooplankton in the study 

 reach was conducted in 1974 by Colbert et al. (1975) , and they also 

 provide a review of other recent plankton studies. Colbert et al. 

 (1975: Table 2) took plankton samples from both main channel and side 

 channel habitats at 13 locations in the Mississippi between river miles 

 201.3 and 302.2. Samples were taken at the beginning of July, 1974 

 during a period when water levels were declining from flood stage and 

 again in mid-September, after water levels had been stable at approxi- 

 mately the 31-year average level since the end of July (Colbert et al. , 

 1975: Figure 3). 



They found that the density of phytoplankton in the main channel 

 increased more than 100 times between July and September, although 

 nutrients (nitrogen and phosphorus) were higher in July (Colbert et al. , 

 1975: 51, 53). The increase in phytoplankton during the period of stable 

 water levels was attributed to decreases in: (1) turbidity, (2) current 

 velocity, (3) mechanical destruction of algae, and (4) dilution of 

 algal populations by direct rainfall and surface and groundwater runoff 

 (Colbert et al. , 1975: 48, 51). 



An isolated bloom of green algae (primarily Tetraedron minimum ) 

 occurred at river mile 215.5 in September. The density of green algae 



