How would I go about estimating the population of my daphnia culture?

[Sack Boy]

The culture is in a standard 10 gallon tank (20"x10"x12"). I started with about 200 daphnia (<1mm to 3mm). I would like to know where I am at right now.

[jim mcnamara]

Estimate by sampling:

If you can safely assume:
daphnia are dispersed thru the 2400 in^-3 volume more or less randomly - ie. 90% of them are not parked on the walls or the bottom of the tank then

This is the poor man's version:
find a small clear container that has a water tight top. A 10ml test tube is great. Measure the volume of water it holds. Empty the water out.

Cover the container and open it underwater. Count the daphnia. Repeat this several
times - 10 - 15 times. Retreive the samples from different places in the tank. Record the results for each count.

Calculate the mean -
sum all of the numbers then divide that total by the number of samples. Let's pretend you got 8.2 as the value of mean. This is a large number but it does not matter.

Your tank has approximately 39.3 liters of volume. So if your sample container
was 10ml (that is .010L) then your population estimate is:

8.2 * ( 39.3 / .010 )
or
8.2 * 3930 = 32226 daphnia