457 
That the greater part of the plankton lost, or all of it, was settled 
out or consumed by larger organisms, rather than that it perished be- 
cause of any failure of the food supply (particularly nitrogen), is forci- 
bly suggested by two or three considerations: that the losses took place 
at the greatest rate during the hot season, when the current was least and 
settling easiest, the rate of loss in August 1910 being 98% ; and that in- 
stead of there appearing any evidence that the losses were due to dim- 
inution in food, both the total limnetic nitrogen and the nitrogen in the 
form of nitrates increased down-stream both in the spring and midsum- 
mer—autumn months in 1914, the nearest year for which we have nitro- 
gen figures. I note here that incomplete studies on the succession of 
Algae, Protozoa, Rotifera, and Entomostraca in some of our down- 
stream series of plankton-catches suggest that a good part of the loss in 
plankton below Havana in the spring months during rising pulses of En- 
tomostraca may be due to internal consumption, within the plankton 
population itself. In May, 1899, in fact, these four groups of micro- 
organisms showed a progression in reaching their maximum abun- 
dance, each at a station farther down stream. In the circumstances the 
presumption seems strong that each pound of Cyclopidae or Rotifera 
taken near the mouth of the river represents several pounds of smaller 
plankton species eaten farther up stream. (See tables, pp. 458, 459.) 
5. COINCIDENCE OF RICHER AND POORER PLANKTON AND Bortrtrom- 
Fauna REACHES IN THE RIVER BELOW CHILLICOTHE 
The fact that there are shown, on a basis of the plankton and bot- 
tom-fauna figures (1899—1915), such close coincidences between the 
location and extent of the richer and poorer plankton and bottom-fauna 
reaches between Chillicothe and the mouth of the river is not, I think, to 
be taken too quickly as in itself dependable evidence that the bottom 
fauna is to any certain and large extent a simple function of the vol- 
ume or weight of plankton above it. Not only, however, does it appear 
that both in its bottom fauna and its plankton stocks the sixty odd miles 
of low-sloped river channel between Chillicothe and Havana is far 
richer on the average than the lower river reaches, but the decrease 
down stream, on a broad scale, is in each case found to be progressive, 
and in fact in substantially similar ratios, if the comparison is made with 
the midsummer plankton figures. (Table, page 460.) The finding, on the 
contrary, in August 1913, in a local section of low-sloped channel in the 
lower river, of a rich plankton-consuming population of Sphaeriidae 
that was apparently not far from as rich as the best found in 1915 in 
the middle Illinois Valley district suggests that the very general lack 
of a suitable substratum for small Mollusca in the channel of the TIli- 
nois below Lagrange may have more to do with the decrease of the 
bottom fauna in the lower river than the decrease in the stock of plank- 
ton above it. Other influences that may have some bearing on the aver- 
age very poor showing made by the bottom fauna in the lower river in 
1915 will be taken up farther on. 
