Those of you who follow my posts will know that plankton comprise many organisms of different sizes, from viruses about 0.00001 mm to large jellyfish up to a meter in length. It is logical, then, to think that there should be different systems for sampling plankton according to the size of the group of interest. Broadly speaking, however, we can cluster sampling or capture methods into two groups: those focused on microscopic unicellular organisms and those used by larger organisms, such as multicellular zooplankton.
Small unicellular plankton sampling tools
This type of plankton, which includes viruses, bacteria, algae, and protozoa, is usually caught in conjunction with the surrounding water. For this purpose, we have more or less sophisticated devices, from a simple bucket to hydrographic bottles that allow us to take plankton of discrete depths. A hydrographic bottle is nothing more than a cylinder, usually made of PVC or methacrylate, with two airtight lids that can be closed when the bottle is at the desired depth. The most commonly used hydrographic bottle design is called Niskin, by Shale Niskin, who invented it in 1966, but there are others like the Nansen, Van Dorn, and so on. These bottles are usually mounted on a structure called rosette, which surrounds devices that allow us to observe, among other things, in real-time the depth of the bottles, the temperature and salinity of the water, and the fluorescence emitted by the algae that live there. This set of devices is called CTD (conductivity, temperature, and depth) and is essential in any oceanographic cruise. We can also collect seawater with the organisms accompanying it with suction pumps that go down to the depth that interests us. All of these systems are only for sampling microscopic plankton. Copepods, fish larvae, or large crustaceans would be misrepresented if collected by a bottle because their abundance is low, and also because they tend to escape when they detect the bottle, pump, or bucket.
Zooplankton fishing nets
In the title of this section, you will see that I have used the word fishing. The fact is that the mechanisms for catching large zooplankton do not differ much from those used for fish. We use plankton nets, which are usually conical. Depending on our target group, the nets are wider or narrower, longer or shorter, and have a larger or smaller diameter mesh pore. To give you an idea, nets with a pore mesh size of 20 µm (0.02 mm) allow us to capture large phytoplankton and microzooplankton, those of 200 µm (0.2 mm) would be indicated by adult copepods, and those of 0.5 to a few millimeters of pore-size would be used for krill, fish larvae, etc. Depending on the interest and net type, towing can be done vertically, horizontally, or obliquely.
The simple structure I have described to you is the most common and corresponds to nets such as the Juday Bogorov, the WP2, the Bongo, etc., but there are more complex and mechanized nets (for example, the Bioness, or the LHPR, abbreviation of Longhurst Hardy Plankton Recorder) that allow us to fish with different mesh sizes at the same time, sample different depths, and at the same time having a record of the depth and physicochemical parameters of the water. Some devices incorporate video cameras that give images or recordings of everything that enters their field of vision, but of course, they do not capture organisms.
Because of their fragility, certain plankton organisms are complicated to catch intact with a plankton net. This would be the case with gelatinous plankton. If we want to obtain living and perfect specimens of certain species of gelatinous plankton, we have no choice but to get wet and take them out of their environment one by one and very carefully.
Preservation of samples
We already have our plankton samples, so what do we do now? As for the capturing devices, the processing of the samples will also depend on the group to study. The samples can generally be viewed life or preserved for later study. Preservation can be with chemical reagents, such as formaldehyde or Lugol’s solution (basically a modified iodine tincture), by freezing at -20 or -80ºC (depending on the analysis we want to do), or by filtration and drying, or filtration and extraction of pigments in acetone, ethanol, etc. Also, sometimes we need combinations of several of the mentioned techniques, such as fixing, sample filtering, and subsequent freezing. This last technique is used when we want to observe unicellular organisms in an epifluorescence microscope.
If we want to see and classify plankton, the most practical is to use stereoscopic magnifiers or microscopes. However, special machines allow us to process samples faster but lose fine taxonomic resolution compared to a good human specialist. For example, for bacteria we can use the flow cytometer; for algae and microzooplankton, the FlowCam; and for larger zooplankton, the Zooscan. All these devices share the same principle. The sample passes through a tube or is placed on a plate, is stimulated by a light source (laser or normal light), and the emission of photons or images is captured by specialized video or photo cameras. These images or light emissions are often processed by complex computer programs to estimate abundance and primary classification of what is in the water.However, new techniques for biochemical and molecular analysis of samples are gaining popularity recently. These techniques are up-and-coming and can give us an idea of the diversity and some physiological processes in the water. Yet, we are still far from being able to provi