Research

This quote by Charles Darwin, made on his journey with the ship the Beagle, describes exactly what people feel when they look at a zooplankton sample for the first time. Darwin here describes one of the first or perhaps the first use of a net to sample plankton. Plankton nets are still widely used in zooplankton research, and actually the nets have changed very little after almost 200 years. In the meantime, however, there are many more tools available for zooplankton research! How can you study zooplankton?

Zooplankton research started in the 17th century after the invention of the microscope; first the compound microscopes of Jan Swammerdam and Robert Hooke and then the stronger magnifying single lens microscope of Anthoni van Leeuwenhoek. As early as 1674 van Leeuwenhoek discovered protists and other minuscule aquatic organisms in a lake. He described plankton until just before his death, the last being the veliger larvae of oysters, in 1722. Later, zoologist Martinus Slabber described larvae of invertebrates and all kinds of other plankton in “Natuurkundige Verlustingen” (1778). Below a selection of plates from this work. What can you recognize?

Later pioneers of plankton research were Johannes Müller, who described many planktonic larvae, and his student Ernst Haeckel, famous for his voluminous works with beautiful plates of various organisms. With the discoveries by the British expedition with HMS Challenger from 1873 – 1876 came the realisation that plankton was widespread and even existed in the deep sea, until then considered lifeless. The material from the Challenger Expedition was distributed to many experts, including Ernst Haeckel. It was Victor Hensen who introduced the term “plankton”, and Ernst Haeckel who introduced the term “holoplankton” for organisms that live their whole life in the plankton and “meroplankton” for organisms that also live part of their life on the bottom.

On the RRS Discovery expedition of 1925-1927 Sir Alister Hardy introduced an innovative device, the Continuous Plankton Recorder (CPR), which could be towed behind a ship and automatically collected plankton on a roll of plankton gauze. The CPR is still widely used, many commercial ships are equipped with a CPR. The CPR data make an important contribution to the knowledge on the distribution, composition and occurrence of zooplankton all over the world. The data is freely available to everyone.

Many different techniques have been devised for sampling zooplankton. In essence, these often come down to the same thing, filtering a certain amount of water with plankton through a filter with openings small enough to retain the plankton you want, but letting the plankton and other particles you don’t want pass through, after which you collect the remaining concentrated plankton. For this you can use, for example, plankton nets with larger or smaller openings or mesh sizes. Below are a number of photos of plankton gauze of different mesh sizes.

Three different mesh sizes of plankton gauze photographed through the microscope; 100, 25 en 5 micron.

You can see here that the smaller the mesh size, the smaller the openings in the network. However, the amount of filtering surface of the net is also becoming less and less as thread diameter does not decrease at a similar rate. Nets with a smaller mesh size therefore clog a lot faster than nets with a large mesh size because they block more smaller plankton but also because the filtering surface is getting smaller.

Not all plankton is equally common; of the smallest plankton such as protists and rotifers you may find hundreds to thousands per litre, copepods about one to ten per litre and fish larvae and jellyfish one per thousand litres.  To sample the smallest plankton you need to filter much less water and a few litres is enough, while for fish larvae you need to filter an entire swimming pool to get a good idea of how many and which species of fish larvae occur somewhere. Various kinds of nets and other devices have been developed for this purpose. Here are a few examples.

Besides nets, there are many other ways to collect plankton. Large or fragile animals such as jellyfish are best caught manually by scooping them off the surface with a cup, or underwater while diving or with a (remote-controlled) submarine. There is also always the simplest way: just throw a bucket of water through a sieve.

In recent years, new techniques for sampling plankton have also been developed. For example, by grinding a mixed zooplankton sample in its entirety and checking which organisms the DNA found in the sample belongs to. This is called metabarcoding.

Furthermore, there are various techniques that take photos of plankton flowing past a camera in the water at high speed. In this way you can examine plankton at a high spatial or temporal resolution without having to take thousands of samples! This saves researchers a lot of time and money, and no animals need to be killed for it.

Plankton sampling with nets can be done by lowering the net in a straight line to the bottom (vertically) and lifting it up again, or by dragging the net behind you while sailing at a constant speed (horizontally) or while slowly lifting the net up (diagonally or oblique). There are also all kinds of ways to sample only a certain depth, such as self-closing nets, plankton pumps, niskin bottles or Schindler-Patalas zooplankton traps.

How does this work in practice? As an example below a video of a day of zooplankton sampling on the Oosterschelde and Grevelingenmeer, for my PhD research: