If you’re heading into the field for your first plankton study, the two questions everyone starts with are: how do I actually collect this stuff, and how do I stop it from rotting before I get it under a microscope? Both phytoplankton (tiny floating plants and algae) and zooplankton (tiny floating animals that feed on them) are fragile, and once you scoop them out of the water, they start changing fast. This guide walks through the basics in plain language.
Why Bother With Plankton at All?
Plankton might be microscopic, but it tells you a lot about a water body’s health. Phytoplankton sit at the bottom of the aquatic food chain and respond quickly to nutrients and pollution, so their numbers and species shift with water quality. Zooplankton feed on phytoplankton and are, in turn, food for fish, so they’re a direct indicator of how productive a pond or lake is for fish culture. That’s why almost every water quality or fisheries thesis includes a plankton chapter.
Collecting Phytoplankton
Because phytoplankton are so small and spread out, you can’t just scoop up a jar of water and expect to see much. Instead, you pass a large volume of water, commonly around 100 litres, through a fine plankton net (a mesh size of roughly 25 microns is standard) so that the plankton get concentrated into a small sample at the bottom of the net. That concentrated sample is what you’ll actually preserve and examine.
Collecting Zooplankton
Zooplankton sampling follows a similar idea: water is passed through a net of a similar mesh size, and the plankton collected is retained for preservation. Since zooplankton don’t distribute evenly through a pond, it helps to take samples from a few different depths and combine them, so your final sample reflects the water body as a whole rather than just the surface.
Why You Have to Preserve Samples Immediately
Living plankton cells are delicate. Left untreated even for a few hours, they can shrink, burst, or decompose, and you’ll lose the ability to identify or count them accurately later. Preservation is essentially about “freezing” the sample’s biological state, right there in the field, so what you examine in the lab weeks later still reflects what was actually in the water.
Two Preservatives You’ll Use, Explained Simply
Lugol’s iodine solution is the standard choice for phytoplankton. It’s an iodine-based solution that fixes cell structure and, as a bonus, stains cells a light brown, which makes them easier to spot under a microscope. A simple working example: a small stock solution is made by dissolving iodine and potassium iodide in water, and this stock is then diluted with your sample so the final concentration is roughly 1–2%. Too little and preservation is incomplete; too much and it can distort delicate cells, so getting the dilution right matters.
Formalin is the standard choice for zooplankton (and is also used for phytoplankton in some protocols). It works differently, chemically “fixing” tissue by slowing down the enzymes that cause cells to break down after death. A simple example: mixing 10 mL of concentrated (40%) formaldehyde with 90 mL of water gives a 4% working solution, a typical strength for field preservation. The general rule with formalin is: more dilute for delicate organisms, slightly stronger for tougher ones.
A quick comparison, since students often mix these up:
| Lugol’s Iodine | Formalin | |
|---|---|---|
| Best for | Phytoplankton | Zooplankton |
| Typical working strength | 1–2% | 2–5% |
| What it does | Fixes cells, stains them brown for visibility | Fixes tissue, slows decay |
| Storage | Opaque bottle, away from light | Cool, below 20°C |
A Safety Note Before You Start
Formalin is a hazardous chemical. It’s a strong irritant and a suspected carcinogen with prolonged exposure, so always prepare and handle it with gloves, in a ventilated space, and never pipette it by mouth. Dispose of used formalin through your institution’s chemical waste process, not down the sink. This isn’t usually spelled out in thesis methodology chapters, but it’s worth knowing before your first day in the lab.
Getting Started
Once your samples are collected and preserved, the real work, identifying and counting what you’ve caught, begins under the microscope. That’s a topic for another post. For now, the takeaway is simple: concentrate your sample with a fine net, preserve it immediately with the right fixative at the right strength, and handle formalin with care.
For more on planning your fieldwork and writing it up, see our Thesis Writing Guide and Data & Statistics guide.
References
- Throndsen, J. (1978). Preservation and Storage. In A. Sournia (Ed.), Phytoplankton Manual, UNESCO, Paris.
- Bancroft, J.D. & Gamble, M. (2008). Theory and Practice of Histological Techniques (6th ed.). Churchill Livingstone/Elsevier.
- Suvarna, S.K., Layton, C. & Bancroft, J.D. (2013). Bancroft’s Theory and Practice of Histological Techniques (7th ed.). Churchill Livingstone/Elsevier.