Getting started

An invasive Mnemiopsis leidyi comb jelly, caught in the canals of Amsterdam!

The diversity of zooplankton is enormous and includes protists and various different strains of the animal kingdom. Search for zooplankton yourself and discover this wonderful world in your own neighbourhood. On this page I provide advice on how to collect and view plankton yourself. Where are you going to search? Plankton is everywhere but in some places you’ll find more than others. It’s often easier to collect plankton in stagnant or slow flowing waters than in fast flowing ones, because the densities are higher and there’s less sand in the water, making observation and filtering more difficult. Look for a place where you can reach the water easily but above all safely, such as a sheltered bank, a bridge or a floating pontoon in a marina. If you are sampling from a more dangerous place such as a steep quay or a boat, preferably do not go alone and take safety measures such as wearing a life jacket.

There are many ways to collect plankton:

• Scooping from the surface: when there is little wind, you can often see jellyfish and comb jellies swimming. The best way to collect them is to scoop them up directly with a jar or bucket, possibly mounted on a stick. Preferably not with a dip net, because then they will damage quickly when removed from water.
• With a bucket: the easiest way, just scoop up a bucket of water. By making a string on the handle of the bucket you can also sample from high quays and railings. Attention: do not do this from a moving ship as there is a good chance that you will lose the bucket, or even worse, go after it yourself! Tip: tie the other end of the rope to something before you throw the bucket into the water.
• With a plankton net: with a plankton net you can take samples from deeper water, and easily filter and concentrate larger quantities of water. Professional plankton nets are expensive, but they can also be self-made. Relatively cheap plankton nets can be found in shops aimed at (amateur) naturalists. If you want to collect larger plankton such as water fleas and copepods, it is best to use a coarse-meshed net with a mesh size of 200 – 300 μm. If you also want to look at phytoplankton and smaller zooplankton such as shellfish larvae, copepods and worms, or crayfish, it is better to use a fine-meshed net with a mesh size of 70 – 150 μm.
• Light fishing: Light fishing is a great activity to do at night. A lot of plankton comes to the surface at night and/or is attracted to light. By shining a torch on the water, or holding a diving lamp under water, zooplankton is attracted and collects near the lamp. With a bucket or jar you can then easily collect it.
• Snorkeling/diving: Larger plankton such as jellyfish and rib jellyfish are most beautiful in their natural environment, where you can dive or snorkel to see them. Even smaller plankton can sometimes be seen with the naked eye, like when you shine your diving torch about 10 cm in front of your face at a right angle to your face. Especially during a night dive! There is even a special branch of diving called “blackwater diving” where you dive at night in the middle of the ocean on the edge of where the deep sea begins. This is where the most bizarre animals are seen, and it is high on my bucket list to do!

I took a water sample, what now?

Samples taken with plankton nets can usually be put directly into a jar, preferably transparent so that you can get a first impression of what is inside. You can also first put your sample in a larger transparent plastic or glass container to look at it. Transparent rectangular jars for storing food products work well for this. By holding the container up to the light or shining a torch in from the side or bottom, you can often see a lot of plankton, such as copepods and larvae.

Make sure that you label your samples so that you can later find what is in which jar.

A plastic storage bin is an excellent place to store and view plankton.

If the concentration of plankton in the water is low, like in samples taken with a bucket, you can concentrate the plankton. This can be done by carefully pouring the water over a fine mesh sieve. Then, with water filtered through the sieve, you rinse the plankton from the sieve into a jar. Another clever way of concentrating plankton is with a lamp: shine a (pocket) lamp into a corner of your container or bucket. You will often see the plankton swimming towards the lamp, after which you can collect it with a pipette or a smaller container. This is also a great way to separate zooplankton from detritus and algae.

Storage

Zooplankton is most beautiful when viewed alive. To keep the plankton alive for as long as possible, it is best to keep the samples in the dark and chilled, in a refrigerator or cool box. Also make sure that the plankton is not too concentrated because then oxygen becomes limiting fast. If there are any gelatinous plankton in your sample (jellyfish, comb jellies), take them out and store them separately or throw them back, otherwise they will eat the other plankton!

If you want to preserve the plankton for later viewing or reference, you will have to fix it and conserve it so that it does not spoil. Also the identification of zooplankton to species level is often not possible on living organisms because you have to look at very small details like hairs on legs. There are many different fixation and conservation methods for plankton. Many of these use toxic, flammable and even carcinogenic substances such as formalin and can therefore only be used safely in laboratories equipped with safety devices. If you want to preserve plankton as a hobbyist I would conserve it in alcohol, for example 70% ethanol disinfectant alcohol as sold at the drugstore. Alcohol is poisonous and highly flammable, so be careful when storing it. Preserved samples often deform and lose their colour, especially in alcohol, so look at the plankton alive if you can.

Some plankton can also be kept alive in a bowl or aquarium at home or even cultivated, especially with freshwater plankton such as water fleas. Fill a container with some collected plankton, water plants and plant debris and put it away in a light place, but not in full sun.

Freshwater jellyfish from a moat in Zutphen, Netherlands, viewed in a whiskey glass.

The largest zooplankton such as jellyfish and comb jellies can be seen with the naked eye. These are best observed in containers with water. Larger species can be seen in an aquarium or in a narrow transparent plastic or glass tank called a cuvet. The smallest jellyfish can be viewed in a small glass jar. It is just what you have at hand, a whiskey glass works fine too, for example!

Jellyfish often are viewed best against a black background with lighting from the side, for example with a torch or desk lamp.

Microscopes

To take a good look at the smaller plankton you need a microscope. There are different types of microscopes for different purposes.

Stereomicroscope

Stereo microscopes can be used with two eyes at the same time. Just like binoculars, a stereo microscope has a separate set of lenses for each eye. This gives you a 3D effect when you look through them and you can see depth. This is very useful if, for example, you want to manipulate the object you are looking at with tweezers. Stereo microscopes usually have a low magnification factor of 5 – 50 times. Simple stereo microscopes have a fixed magnification, more advanced microscopes allow you to zoom in and out. Viewing plankton under a stereo microscope is very easy, just pour your sample into a bowl or a petri dish and place it under the microscope.

At least as important as the microscope itself is the lighting of the sample. Most stereo microscopes have a lamp in the base of the microscope with a semi-transparent plastic or glass plate above it on which you place your sample. However, this doesn’t work very well when viewing zooplankton; because most animals are transparent they disappear against the light background. You can get a better image by placing your sample on a dark background and lighting it from the side. You can do this with a specially made light source, but these are very expensive. A torch or an LED desk lamp also works fine.  The “JANSJÖ” lamp from IKEA is often used by hobbyists for this purpose. There are also stereo microscopes with special dark field lighting, which means that the object you are looking at is not exposed straight from below, but only diagonally from the side.

Under a stereo microscope you can look at living plankton in a thin layer of water and see how the animals move and filter water. If you want to see even more details, for example to determine the species of plankton, you need a composite microscope.

Compound microscope

A compound microscope has two sets of lenses through which you look at your subject. The first set of lenses is inside the objective. Microscopes usually have a swivel head to fit three to five objectives so that magnifications can easily be switched. Most objectives have a magnification factor of 4 to 100 x.

In addition to the objective, a microscope also has an eyepiece, the part in which the final image is formed that you look at with your eye. Eyepieces also magnify, usually 10 x.

To calculate how many times a microscope magnifies the image you have to multiply the magnification factor of the lens and eyepiece, so a 40 x lens and 10 x eyepiece give a magnification factor of 10 x 40 = 400 times.

For viewing zooplankton you often don’t need such a high magnification at all! I myself have a 5 x, 10 x, 20 x, 40 x and 100 x lens on my microscope and for zooplankton I use the 5 x and 10 x most. 40 x I only use them to look at details such as hairs on the legs of copepods.  100 x I actually never use, this magnification is especially interesting when looking at algae.

Here, too, good lighting is very important. Most microscopes have a condenser, a set of lenses that ensures an even, concentrated exposure. The condenser also has an aperture to change the contrast of the image. Finally, the condenser often has a holder or opening in which filters can be placed to adjust the exposure. With zooplankton, dark field exposure often gives a beautiful image, most of the photos and videos on this site are made with dark field exposure. For this you need to adjust your microscope with a filter. You can often do this yourself, I have done that too.

Compound microscopes can have one or two eyepieces. Two eyepieces works better but is not necessarily necessary.

For plankton, a reversed microscope is also often used, whereby the subject is exposed from above and viewed from below. This is mainly used for dead plankton; because it sinks to the bottom, you can see everything in focus at the same time. It can also be useful because you can manipulate the plankton from above with needles. You do need special objectives for this, especially for high magnifications.

Buying a microscope

If you start by looking at zooplankton, I would first look at a stereomicroscope. These are ideal for viewing live, larger zooplankton. If you want to look at the animals in more detail you can buy a composite microscope later on. The stereo microscope is then still very useful for selecting organisms and making slides to look at under the compound microscope.

On this page a lot of useful advice is given about buying a microscope. New professional microscopes are very expensive. Overall there are two cheaper options:

1. Buy a budget microscope from a company that also sells professional microscopes, like Euromex/Novex or Bresser. I have no experience with this but there are several reviews on youtube that show that the quality of cheap microscopes has improved a lot.

2. Buy a second-hand professional microscope, for example on a marketplace. This is especially recommended if you already know what you are looking for and what to look for when buying a microscope. There are often many options on professional microscopes that you don’t need as a hobbyist, such as fluorescence. Always test the microscope before you buy it! Sometimes secondhand microscopes have been in the attic under a layer of dust for years, which is certainly not good for a microscope. All rotating parts should rotate smoothly. A little dust on the lenses is not a problem, more annoying is mould: star-shaped threaded spots on the lenses.

Whatever you buy, never buy a plastic children’s microscope for a couple of tens of euros. Its handling and image quality are often so poor that there is no pleasure to be had using it or viewing samples, which quickly ends an otherwise fascinating hobby.

The foldscope

These microscopes are all nice and cute, but they cost a lot of money! Luckily there is a cheap option that is very nice to try: the foldscope. This is a paper microscope with one lens, just like van Leeuwenhoek’s very first microscopes. The foldscope only costs a few euros! It magnifies 140 times, and you can easily attach it to your phone, so you can also take pictures. Unfortunately I don’t think they are for sale in the Netherlands yet, but they are for sale in Germany.