PLANKTON NET DIAMETER SELECTION

The diameter of a plankton net is determined primarily by the sampling objective, the volume of water that must be filtered, the type of tow, and the available towing speed. A larger diameter filters more water in less time but also creates greater drag and requires a stronger towing system.

Factors to Consider:

1. Target Organisms

---

2. Required Sample Volume

The amount of water filtered is determined by the mouth area and the tow distance.

The mouth area is:

$$A=\pi {\left(\frac{D}{2}\right)}^2$$

where:

• A = mouth area (m²)
• D = net diameter (m)

Approximate filtering area by net diameter:

Because mouth area increases with the square of the diameter, doubling the diameter increases the filtering area by four times.

---

3. Tow Type

---

4. Water Conditions

Small diameter nets

• Better in shallow streams
• Easier to tow
• Less drag
• Better in dense vegetation

Large diameter nets

• Better offshore
• Filter much larger water volumes
• Preferred for low plankton densities
• Require stronger towing gear

---

5. Towing Speed

Larger nets create substantially more drag.

Typical towing speeds:

---

Rule of Thumb

---

How to Choose

Consider the following questions:

1. What organisms are you targeting? Larger organisms generally require larger mesh sizes, but not necessarily larger diameters.
2. How much water must you sample? If plankton densities are low, a larger mouth diameter helps filter more water in a practical tow length.
3. Where are you sampling? Small nets are ideal for ponds, rivers, and shallow lakes; larger nets are better suited to open-water environments.
4. Can you tow the net effectively? The vessel or sampling platform must be able to handle the increased drag of larger nets.

Recommendations by Application

If you're selecting a net for commercial aquaculture, environmental monitoring, or fisheries research, a 30 cm (12 in) diameter net is often the most versatile choice. It provides a good balance between filtered water volume, ease of handling, and compatibility with common flow meters and towing equipment.