Plankton consists both of plant and animal organisms, ranging in size from a few micrometers (thousands of a millimetre (mm)) to a few centimetres or larger. Generally speaking, the plant plankton (phytoplankton) are smallest, and zooplankton tends to be bigger, although they still span a wide range of sizes, which may appear small to us as humans, but actually reflect a huge difference in body volume. For example a cylindrically shaped organism that is 3mm long has a body volume 27 times bigger than a similarly shaped organism that is 1mm long. In this post, we discuss the methods used to collect zooplankton, and what you need to consider when you have collected your sample.
The most commonly used method for collecting plankton is using a net. There are other novel techniques, like cameras, holographs etc possible, but most are expensive, and sometimes require large ships to deploy. A net is relatively cheap and easy to put over the side of a boat.
In addition, the net collects plankton from a large volume of water, filtering the organisms together to get enough to count. Although plankton is highly abundant, you have to sample quite a large volume to make sure you catch a representative sample and also collect species that occur infrequently. By attaching a flowmeter to a net, it is possible to estimate the volume of water that has passed through a net, and so by counting the organisms caught in the net, a concentration per volume of seawater can be inferred. For example, if a net is towed for a period where 2 cubic meters of water has passed through the net, and you count 200 organisms in your sample, you can express this as a concentration of 100 organisms per cubic meter. This makes it possible to compare the counts of organisms from samples where the volume was not exactly the same.
A plankton net is built up of a mouth area, a mesh cone and a cod-end. The mouth area is usually made from a metal ring or another rigid material, so the mouth opening of the sample is always the same. The cone is made from a material where the mesh-size is known so you know what size of animals will be retained in the net. As the net is pulled (towed) through the water, the material retained by the mesh will be forced down into the cod-end (like a sock). The cod-end is typically also a mesh material, often formed into a bag that can be detached from the net so the collected sample can be washed out and brought back to the laboratory.
As a consequence of the diversity of sizes within the animal (zoo-) plankton, it is virtually impossible to sample the entire size range from fractions of a millimetre to large animal plankton like larval fish and jellyfish. If you focus on sampling very small organisms, you are limited to sampling with a very fine net. The fine net will catch more particles in a small volume of water, but will also become clogged with particles trapped in the mesh cone. If you tow the net through the water too fast, and the net clogs, you can end up compromising sampling efficiency, splitting the mesh, and losing your sample.
Larger and more coarse-meshed nets can be used to collect larger zooplankton organisms. Generally you need to consider both the size of the mesh and your towing speed to try and match the sampling to the species groups you are targeting with an analysis. Some of the larger zooplankton can detect approaching nets and swim fast enough to avoid some nets, so to catch these, you will have to tow the net faster, and most likely increase the mesh size of the next to avoid clogging and damange during a faster tow.
Nets are commonly deployed in two different ways; vertical and towed. The vertical tow is a straight down and up deployment of a net while the ship is stationary. This method is relatively quick, and collects a modest volume of water. A towed plankton net deployment is more like deploying a fishing net, dragging it while the ship moves forward. Using this method, you can collect a higher volume of water, and the net is moving faster through the water, so reducing sampler avoidance for some zooplankton.
While it may seem at first glance that it is simply a matter of sampling as large a volume of water as possible, this is not very practical. If you have very abundant animals in a sample, there is a huge amount to count under a microscope afterwards. Taxonomic analysis is time consuming and require expert training, so you do not want to waste this limited resource by collecting samples that takes all week to count, if a smaller fraction can tell you the same information.
At the Stonehaven site, both types of deployments are used to collect zooplankton. A vertical bongo net with two cones are lowered from a crane to 5m above the bottom of the sea (at 45m depth) and straight back up again. The two cones have different meshes fitted – one with a 200µm mesh (or 0.2mm between the threads) and one with a 68µm mesh. So in a single deployment, two different size fractions of the zooplankton community are sampled. The same type of net is also deployed on the west coast zooplankton monitoring site in Loch Ewe.
In addition, a larger, and coarser meshed (350µm) net is towed as well. The ship moves at approximately 2.5 knots against the current while towing the net. This net has a depth sensor attached so staff on the ship can tell how deep the net has gone while paying our wire and moving forward. The net is brought down to approximately 5-10 m above the seabed (40-45 m depth), back to the surface and then the dip is repeated as part of the same deployment. This deployment is called a “double-oblique” tow which collects material from the whole water column (in profile it would look like a “W”). The dip is done twice to ensure enough water travels through the net to catch some of the rarer species as well.
Once samples of zooplankton are collected and preserved in the field, they are brought back for a taxonomic analysis (a topic we will cover in a later post), and finally the count of organisms are converted to a concentration per cubic meter or per litre to be able to compare samples (as mentioned above). However, it is not just the concentration of organisms that is important, but also what is known as “metadata” about the sample. What mesh was used, when was the sample collected (so you can find out about tidal state, weather etc.), what direction did the tow go (you note start and end positions, so you can determine this), how long did the tow last (duration), and every sample is numbered uniquely, so it can be found in databases, and even in the physical storage location if anything needs to be double-checked.