Polynomial Visual Identification Key (PVIK)

is a modern digital tool for phytoplankton species identification.

Identifying plankton can be challenging: many species look almost identical.

PVIK is a multi-character visual identification key that allows several morphological features to be assessed simultaneously. Unlike a traditional dichotomous key, PVIK does not require users to follow a predetermined sequence of choices. Instead, multiple characteristics can be combined to rapidly narrow down the possible taxa.

The idea behind PVIK dates back to 1997, when Satu Zwerver developed Excel-based identification tools that allowed multiple morphological features to be used simultaneously. In 2025, this concept was developed into a dedicated application with the help of a programmer, Daniel Blackburn. Now we have more than 30 years of professional experience in plankton identification brought into an intuitive digital environment.

While preparing a presentation about PVIK, I discovered Jerome H. Friedman’s work on pairwise classification from 1996 and recognised a conceptual similarity: both approaches use multiple pieces of information to distinguish between alternatives. However, PVIK is not based on Friedman’s algorithm; the concept behind PVIK was developed independently for plankton identification.

Although PVIK is primarily designed for professional identifiers, it can also be used by anyone with sufficient taxonomic experience to recognise the groups covered by the key. Species are organised at genus level, keeping the identification process focused and manageable.

New groups are being added continuously, with approximately 15 new groups planned each year from 2026 onwards.

PVIK provides a clear visual overview of morphological size ranges. This is a deliberate advantage: rather than working through a long sequence of text-based choices, you can immediately see how species compare in size and where your specimen fits within the group. Drawings and photos of algae will be added in future versions – don’t worry, they are on the way!

PVIK is kept very simple for the time being. The key itself works fine on its own, but if you want to read more information about the background, the groups or new features you can read it here on the homepage. For this reason it is not possible to have direct links to specific groups in PVIK from this homepage, but one has to got to the app and there choose the desired group from there.

Why use PVIK?

Because it is fast. But why is it faster?

Start with a smaller group

PVIK starts with a much smaller number of species than most traditional identification keys. Species are divided into morphological groups that typically contain only 15–20 species.

That makes a considerable difference. For example, traditional keys for Closterium, such as those by Růžička and Coesel, may require you to work through 70 or more species.

Instead of starting with a large and sometimes intimidating list, PVIK gives you a smaller, more manageable group from the outset.

Use several characters at the same time

Traditional dichotomous keys are essentially linear: at each step, you are required to choose between two alternatives.

PVIK works differently. You can combine several morphological features simultaneously, making the identification process considerably faster and more flexible.

Select the appropriate group, enter the length and width of your specimen, and then refine the result using additional morphological filters. Within seconds, PVIK produces a shortlist of species that match the characteristics you have observed.

The final identification should, of course, still be verified against the relevant taxonomic literature. PVIK is not intended to replace standard identification literature. Its purpose is to save valuable time by reducing the number of plausible candidates to a manageable few – while also helping you learn and compare species more effectively.

How does PVIK work?

1) Start at genus level

PVIK begins at genus level. This means that the user needs some basic taxonomic experience: you should already be able to recognise, for example, a Closterium or a Phacus.

Within each genus, species are divided into smaller morphological groups based on easily recognisable characters. For example:

• Closterium species with parallel sides
• nearly round Phacus species

Each group typically contains 15-20 species – still challenging, but manageable. You see the whole list of species in a specific group on the right side.

2) Enter the size

First, select the appropriate group of algae using the menu in the top right-hand corner.

You can then enter the size of your specimen using the sliders or by clicking directly on the graph. For fine adjustment, you can also use the arrow keys.

In most groups, length is shown on the y-axis and width on the x-axis. There are exceptions where another measurement is more useful for identification. For example, in the cyanobacterial genus Dolichospermum, the y-axis represents akinete length rather than cell length. This is because akinetes are considerably more informative for identification than the vegetative cells. This approach allows PVIK to combine most useful measurement of the vegetative cell – its width – with the highly variable size of the akinetes.

You can also choose not to use size as a filter by deactivating “Filter by measurements”. This allows you to see which morphological features are associated with which species, making the tool particularly useful for learning and training.

This option can also be useful when identifying cyanobacteria without akinetes. If no akinetes are visible in your specimen, you can simply switch off the measurement filter and continue using the other morphological characters.

3) See the size distribution at a glance

The graph shows the size distribution of the entire group, allowing you to see the ranges of all species simultaneously.

At a glance, you can see:

• species with exceptionally large or small dimensions
• clusters of overlapping species
• where your specimen falls within the overall size distribution

This visual overview also helps you develop an intuitive feel for the size differences between species. Over time, that familiarity makes future identifications easier.

4) Narrow down the candidates

As soon as you enter the measurements, PVIK generates a preliminary candidate list by highlighting the species that could have that size.

In a matter of seconds, you might go from 20 potential species to just 4 or 5.

Two shades are used to indicate the reliability of the size match:

• Darker shading indicates that the measurement falls within the normal reported range of the species.
• Lighter shading indicates an outlier – a size reported in the literature outside the normal range, usually given in brackets.

Not all species have an outlier range. Such ranges are common in genera such as Closterium and Dolichospermum, where considerable size variation can occur.

5) Combine multiple features

The next step is where PVIK really comes into its own.

You can apply multiple morphological filters simultaneously to further narrow down the candidate list – again, within seconds.

Instead of repeatedly going back and forth through a dichotomous key, you can simply select the features you observe in your specimen and let PVIK combine them to identify the most likely candidates.

This is the core advantage of the system: several characters can be considered at the same time rather than one after another.

Learn while you identify

The same tools can also be used for training.
Explore the different morphological features and see which species possess them and which do not. Adjust the measurements, activate and deactivate filters, and explore how the candidate list changes. You can also deactivate “Filter by measurements” and see which morphological features are associated with which species, making the tool particularly useful for learning and training. Do use the graph for learning the size differences in species.

In other words: play with the key and learn.

The more you use it, the more familiar you become with the morphological variation within each group – and the easier it becomes to recognise species in your routine work.

Get involved!

The next version of PVIK is now online – please take a look!
Five groups are currently available to try free of charge, and more are being added all the time. You can follow the development on the Groups page.
In autumn 2026, a larger version will become available to subscribers for €36 per year (0% VAT). If you subscribe part-way through the year, you will only be charged for the remaining months.

Is there a group you’d like to see?
Would you like to have a PVIK identification key for a particular phytoplankton group?Or perhaps there is a morphological filter you would like to see added to one of the groups already available?

Get in touch!
I can add your group or filter to the development list, or we can work together to create the next PVIK group.

PVIK – built for identifiers.
Try PVIK and discover a faster, more flexible way to identify phytoplankton.

The app is written by Daniel Blackburn, Startle Company.