STREAM POLLUTION 
369 
and since the studies of these forms constitute part of the routine background for 
the interpretation of the pollution hazards, the field methods employed in connec- 
tion with the entire pollution study operations have been given. 
Water Samples 
At each station water samples were collected by means of a brass sampler of the 
general type described by Kemmerer, Bovard, and Boorman (1923) and transferred 
at once without aeration to 300-cubic-centimeter glass-stoppered, self-sealing 
magnesium-citrate bottles. The first or top water samples were taken at a depth 
of from 1 to 2 feet, the actual surface water (the first few inches below the surface) 
being avoided in the general sampling because the dissolved gases in the water in 
immediate contact with the air do not present a true picture of the dissolved gases 
in the main mass of the water in the stream (this fact is discussed more fully under 
dissolved oxygen). Whenever the depth was greater than 3 feet a second set of 
water samples, bottom samples, were taken about 3 inches above the floor of the 
stream. Intermediate samples were collected between the top and bottom at levels 
from 5 to 10 feet apart wherever the depth of the water justified such sampling. 
Mud Samples 
Bottom mud samples were obtained with a Peterson dredge properly weighted 
to bring up about one-half cubic foot of bottom from an area approximately 1 foot 
square. These mud samples were divided into aliquot parts, some of which were 
dried (see fig. 3) or fixed with various reagents for chemical determinations, and others 
were sieved through a series of Monell screens for qualitative and quantitative 
studies of the bottom fauna. 
Plankton 
Plankton counts were made from the catch obtained by pumping slowly 1 
cubic meter of the water under consideration through a standard silk bolting cloth 
plankton net, supported in the water and provided with a glass trap bottle of 250- 
cubic-centimeters capacity in which the organisms accumulated uninjured. On 
some of the river lakes, as Lake Pepin and Lake Keokuk, a trawl supported by a 
boom and operated over the side of a cruiser was used both for plankton and the 
larger surface animals. 
For purposes of quick field diagnosis of water conditions only net plankton 
were used, although in the detailed studies of plankton both nano- and macro- 
plankton were determined. 
Fresh- Water Mussels 
At the stations where fresh-water mussels were to be taken a heavy mussel 
dredge (see fig. 4) operated from a catamaran was used. This dredge, which was 
found to be very effective to a depth of 8 or 9 inches in the mud of the river floor, 
brought up approximately 1% cubic feet of river bottom with the contained organ- 
isms. Quantitative as well as qualitative studies were possible, therefore, from these 
dredgings. 
