Plankton are microscopic organisms, including algae and protozoans, that float or drift in great numbers in fresh or salt water, especially at or near the surface, and serve as food for fish and other larger organisms. Phytoplankton are the microscopic plants and zooplankton are the microscopic animals.
Plankton can be sampled in a number of ways:
Water-sampler: Water samplers are used to collect seawater samples for phytoplankton analysis in situations where direct contact with the water body cannot be made e.g. from the deck of a large vessel. The sampler is mounted to a weighted hydrowire and closed at the desired depth by a dropping weight. The sampler collects a vertical segment of water. Following recovery of the sampler, the water sampler is discharged into a 10mm net and the retained phytoplankton is collected in a container and preserved for later identification.
Sample bottle: Water samples for phytoplankton analysis can be collected directly into a sample bottle where contact with the waterbody can be made e.g. streams, accessible rivers and lakes and from a RIB or inflatable. The sample bottle is simply lowered by hand into the water and left to fill. Where samples from below the surface are required, a sample bottle that can open at the required depth can be used. The water sample is poured into a 10mm net and the retained phytoplankton is collected in a container and preserved for later identification.
Tube sampler: This method is used when a vertical cross section of the upper 4 metres of the water column is required. A PVC tube with a ball valve at the base is quickly inserted into the water to obtain a water-column sample. The contents of the tube are poured through a suitable sized net and the procedure is repeated until a sufficient quantity of water has being filtered. This method can be used for both phytoplankton and zooplankton. The plankton retained on the net is collected in a container and preserved for later identification.
Towing nets: Sampling is performed from a moving vessel with the net towed at the required depth behind the vessel. The towing speed is critical for the collection of the plankton. As the net passes through the water column, plankton is retained on the net. The collected plankton is then stored in a container and preserved for later identification. This method is suitable for both phytoplankton and zooplankton.