Key to Freshwater Algae


I wrote this key to replace the the traditional book-style keys which I have always found cumbersome and slow. This version is a prototype, because I intend for this project to be updated and refined as new information becomes available to make it as useful to researchers and students as possible. I hope to soon make this application available for download on the iPhone/iPad, but for now, it is accessible on these and other mobile devices at this website. See the bottom of this text for more information about contributing to this project.

A bit of a disclaimer: I am not a professional. I am a Botany undergraduate student. My knowledge of algae is not comprehensive, and I have done extensive research on what I have written here, but I claim it to be neither complete nor entirely accurate. All of the drawings done on this site were also done by me. I am not an artist either, so the pictures are intended to draw your attention to the important aspects of the species being described. They are not supposed to look exactly like what you will see under a microscope. Once you have used this guide to identify your algae, I would recommend looking elsewhere for more realistic pictures and information. Several of the sources in the Credits link at the bottom of the page can start you off.

How to use this key

I tried to make the operation of this key as intuitive as possible. If you have a specimen under a standard compound microscope, you should be able to find everything I point out. Simply select the picture with the description that most closely matches what you're looking at. The number of questions you will go through before arriving at a solution varies, just as in the book keys. If you end up in a place that doesn't make any sense, your choices are tracked in the sidebar; simply click the place you suspect you made a mistake, and you will return to that question.

I tried to simplify the language normally found in keys of this kind so that it would be more accessible, but some terminology still had to be used. A link to the glossary is included at the top of the page, but glossary words are written in green throughout the site, and clicking on them will bring up their definitions in a popup window.

Once you reach a solution, you may want to check the links listed under "Compare to" to look at similar genera and make sure you have the right answer. A complete list of all algae in this database can be found at the Genus List link at the top.

Special Notes:

I am going to assume you know how to prepare specimens and use a microscope. If you do not, a quick internet search should give you all the information you need. There are, however, a few tips and tricks for identifying certain kinds of algae, which follow here.

Regarding diatoms:

My descriptions and drawings of diatoms are based only on the appearance of the frustules. If the shape is rather unique, then using living cells shouldn't be a problem, but with many of the more common shapes, you may need to look for details which may be hard to make out with living cells full of chloroplasts. In these cases, it is recommended that you either observe dead specimens or have your sample cleaned. Many protocols for doing this exist; a quick internet search on my part produced this method.

Testing for starch:

Some algae keep energy stores in the form of starch, others use oil. Sometimes very similar looking organisms can be easily distinguished by testing for the presence of starch, as indicated in the key. The use of an iodine solution, such as I2KI will turn starch granules dark purplish black, and has no effect on oil reserves.

Mucilaginous sheaths:

Many questions ask about the appearance of a mucilaginous sheath, either around a colony or an individual cell. Depending on the lighting and specimen, these may be difficult to see. Staining with India ink can help make them more visible, as the sheaths will not stain, and the ink will flow around them.

Immobilizing troublesome organisms:

If your sample keeps swimming out of view before you can identify it, try adding an iodine-potassium iodide solution to immobilize the cells. Do this only after you have the opportunity to observe their motion habits, because some questions refer to them.

Contributing to this key

As I said before, I'm not by any means an expert in this subject. If you find an error in any of my descriptions, have a better picture you think is more representative of the genus, or have any feedback about the website itself, please feel free to email me at I would be happy to consider your contribution or comment.