Freshwater Algae Key Information

Instructions for using the key

This key is not dichotomous (dichotomous keys are based on paired choices). Instead, you are offered several choices among matching numbers or letters. Read all the key choices in each section/subsection and look at the annotated examples.

To make a selection, click on the text next to the best description for your specimen. You can scroll through several examples at each level.

If you hover the cursor over the text, additional information will appear (if available).

You can scroll through small images to see annotated examples for each key choice.

When you reach the genus level, the images will expand to include annotations, scientific names, and collection information.

Most of the key choices will take you to genus; species names are only included for easily identified, unambiguous taxa.

One major change in Version 2.0 is that we have omitted scale bars on all the images. Scale bars shown for individual cells can be misleading, especially for beginners. Cells from the same species may differ by more than 10x within a single sample. In addition, specimen processing often causes cell shrinkage. This is particularly problematic with SEM images because the cells may shrink by 40 percent or more during processing. If you need cell measurements for identification, we recommend using a more comprehensive taxonomic guide than this online key.

Examine several healthy specimens before making your selection. Decomposing cells and damaged chloroplasts make identification much harder.

Try to work with live specimens, if possible. Preserved samples are fine if you are familiar with algae, but they are more difficult to work with if you are a beginner.

This key was developed using algae collected in northwest Washington (USA). While there will be considerable overlap with other regions, you will almost certainly encounter specimens that do not match the images in this key.

This key was developed by Drs. Robin A. Matthews, Geoffrey B. Matthews, and Elizabeth A. Matthews. The authors of this key assume no responsibility or liability for any errors or omissions in the content of this site. The information contined in this site is provided on an "as is" basis with no guarantees of completeness, accuracy, usefulness, or timeliness.

All identifications represent our best effort to provide accurate classifications using the available taxonomic literature. The nomenclature in this key follows Algaebase (Guiry and Guiry, 2026), which provides a summary of taxonomic synonyms and citations to source literature containing the diagnostic features for each taxon..

Unless otherwise noted, all digital images in this key were photographed by Dr. R. Matthews. The images and key may be used for noncommercial purposed, with appropriate copyright acknowledgements, under the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.

Questions and comments should be directed to R. Matthews at desmid-dot-lady-at-gmail-dot-com. You are welcome to send suggestions or corrections, but please note that we cannot respond to all emails.

We will attempt to keep this file up to date as changes are made to the key, but it may not include the latest changes.