LIMNOLOGICAL OBSERVATIONS IN THE UPPER MISSISSIPPI. 
379 
HORIZONTAL DISTRIBUTION IN RIVER. 
As swift streams do not afford favorable conditions for the development of 
plankton organisms we may expect an increase of plankton where water flows slowly 
or is stagnant. The velocity of current is greater in the main channel than near the 
banks and just below the dikes the water is often almost stagnant. Therefore one 
may expect plankton organisms to be more abundant near the banks and between 
the dikes. 
The determinations of the volume of plankton made at different points across 
the river show, however, that such relations do not always exist. The volume of 
suspended material is sometimes greater in the main channel than near the banks. 
Evidently this occurs most often when the material in suspension consists largely 
of detritus and not of plankton. Such a case, for example, was observed at Fairport 
(stations 48, 49) , where it was found that the volume of material suspended in the 
stagnant water below the dike was less than that in the main channel. There were 
very few organisms in the samples that contained detritus almost exclusively. The 
places below the dikes where the water is almost stagnant are evidently unfavorable 
for the development of plankton, as is shown by the fact that samples collected there 
usually contain fewer organisms than can be found in the main channel where the 
current is swift. 
Sometimes, however, the amount of plankton in the shallows close to the banks 
is greater than in the main channel. Thus in the very outlet of Lake Pepin the 
amount of plankton near the left shore was greater than in the other parts of the 
stream (stations 104-106). As shown in Table 14, the amount of plankton near the 
left bank was 33 cm. 3 per cubic meter, whereas in midstream it was 18 and near the 
right bank 21.5 cm. 3 The water near the left bank was stagnant, and the increase 
of plankton was due to the great abundance of copepods forming the greater part of 
the total mass in this sample. 
The amount of plankton in the bays is greater than in the main channel of the 
river, although the total volume of material taken in the plankton net in these bays 
was sometimes less than that taken in the river. Such a case, for example, was 
observed at stations 3 to 5 near New Boston, 111.; the volume of plankton in Stur- 
geon Bay was 5 cm. 3 per cubic meter, and that in the main channel 6.75 cm. 3 , but the 
first consisted exclusively of organisms, and most of the second was formed of 
detritus. 
VERTICAL DISTRIBUTION IN RIVER. 
Plankton organisms are passively carried by running water; therefore their ver- 
tical distribution in the river depends entirely on the current. Results of the deter- 
mination of the volume of plankton taken from different strata show that some- 
times the amount of plankton taken in the deeper strata is greater than that in the 
surface water, while sometimes the vertical distribution of plankton in the river is 
the same as that usually found in the lakes during the warm season; that is, the sur- 
face layers are richer in plankton than the deeper layers. At many stations it was 
found that the vertical distribution of plankton is uniform. This condition occurs 
especially where the current is swift, as, for instance, at the rapids. The vertical 
distribution of plankton in the river is, however, very variable. 
