Re: How do you test for amount of algae

Hi Grace,
Thanks for the question. To answer it properly, I'd need to know some more precise details. For instance, are the algae growing outdoors in a lake, or are you growing them in tubes? And if the second one, are you growing purely algae or would it be a mixture of other organisms too?

Well, here are some ideas to help you with both of those possibilities. The second one first, because it's easier! When microbiologists grow pure samples of cells like algae, they normally measure the growth using what's called the optical density. In other words, you place the sample in a clear tube, put it in a machine called a spectrophotometer and shine a beam of light through it. The cells in the tube scatter the light and the reduction in light passing through the tube gives you a measure of how many cells there are in the sample.

However, I suspect that you want to look at algae in a lake or a pool, outdoors. This is quite difficult to measure accurately, because the water is probably mixed by the wind, so the concentration of algae in one particular place might change from day to day, even if they are increasing over time. If the lake is quite still and the algae are well mixed throughout the water, you might measure the change by seeing how cloudy the water is from day to day. To do this, you can make a disc with black and white segments on it, lower it into the water on a piece of string and measure how deep it goes before you can't see the black and white pattern anymore. Of course, this depends how deep the lake is, and the cloudiness of the water might be affected by other things, like mud or other growing organisms.

Probably the best way to measure algae is by using a microscope. You would have to take a lot of samples from different parts of the lake, then look at them down the microscope. It might be that you see different species of algae, so you might have to choose one of them. It might also be that there are too many to count, so you would have to dilute the sample a few times with water. Then if you knew the volume of water on your microscope slide and the area that you were looking at, you could make a guess of how many cells per milliliter of water there were. You might even be able to use a slide that was divided up into a grid and count the number of cells per square.

So you can see that this is quite a technically difficult thing to do! But I hope this answer gives you a few ideas.

Neil Saunders