A plankton net is a specialized piece of equipment used by scientists to collect microscopic organisms from aquatic environments. Plankton are tiny, drifting life forms, including plant-like phytoplankton and animal-like zooplankton, which form the foundational layer of most aquatic food webs and contribute significantly to global oxygen production. By studying these collected organisms, scientists can gain insights into the health of an ecosystem, monitor water quality, and track the impacts of environmental changes like pollution and climate shifts.
Anatomy and Function of Key Components
The standard plankton net design is characterized by three main components: a rigid mouth, a conical filtration net, and a collection cup known as the cod end. The mouth is typically a metal ring or hoop that holds the net open, ensuring a consistent area for water to enter during deployment. This opening is attached to a towing line, often with a three-point bridle, which allows the net to be pulled through the water column while maintaining stability.
The filtration net itself is a long, funnel-shaped cone constructed from a fine mesh material, most often durable nylon or polyester fibers. The size of the mesh is a precise measure, usually expressed in micrometers (µm), and this size determines the organisms that are captured. For example, a net with a small mesh size, such as 20 to 50 µm, is necessary to collect the smaller phytoplankton, while a coarser mesh, perhaps 64 to 200 µm, is generally used for larger zooplankton.
At the tapered end of the net cone is the cod end, a detachable cylinder or bottle that serves as the final receptacle for the filtered organisms. As the water passes through the fine mesh, the plankton are retained and funneled into this cup, which effectively concentrates the sample. The cod end is securely attached to the net, often with a twisting clip mechanism, and may include a drain valve to allow excess water to escape before the concentrated sample is retrieved.
Deployment and Sample Collection
The methodology for using a plankton net involves carefully deploying the device into the water to collect a representative sample of the organisms present. Researchers must first decide on the towing method, which is typically either a vertical tow, a horizontal tow, or an oblique tow, depending on the scientific objective. A vertical tow involves lowering the net straight down to a specified depth and then pulling it back up through the water column while the vessel remains stationary. This method provides a profile of the plankton distribution across different depths in that specific location.
For a horizontal tow, the net is deployed at a consistent depth and then towed behind a moving vessel, often at a slow speed of around 1 to 3 knots, to sample a wide area. Oblique tows are a combination of these methods, where the net is lowered to depth and then retrieved while the vessel is moving, creating a diagonal path through the water. The controlled speed is important to prevent the plankton from sensing and avoiding the oncoming net.
Once the tow is complete, the net is carefully brought back to the surface, and the sides of the cone are often gently washed down with filtered seawater to ensure all organisms are rinsed into the cod end. The concentrated sample is then removed from the cod end and transferred to a collection bottle. If the sample cannot be immediately analyzed, it is preserved using a chemical solution, such as formalin or Lugol’s solution, for later examination in the laboratory.
Specialized Net Designs for Research
The basic conical plankton net has been adapted into several specialized designs to meet the diverse needs of aquatic research. The Neuston net is specifically designed to collect organisms that live right at the water’s surface. This net design often features built-in wings or a rectangular frame that skims the upper few centimeters of the water column.
Another common specialized design is the Bongo net, which consists of two nets mounted side-by-side on a single circular frame. This system allows researchers to simultaneously sample the same water volume using two different mesh sizes or to collect duplicate samples for comparison.
More complex systems, such as the Isaac-Kidd Midwater Trawl (IKMT) or multinet systems, are used for sampling larger, faster-moving organisms or for collecting depth-stratified samples by opening and closing the net at programmed intervals.