Various methods exist for sampling the zooplankton of a lake or pond. The most common method is to pull a fine mesh net through the water, either vertically or horizontally, and then collect the animals that have been retained by the net. Another common method is to use a box-shaped device called a Schindler-Patalas trap that can be lowered to a particular depth, triggered to close, and then collect the zooplankton that is captured inside. If the aim is to capture only small zooplankton, such as rotifers, then a whole water sample can be collected, either with a Van Dorn bottle at discrete depths, or with a long tube made of PVC or some other material, which can sample the entire water column. The animals then are settled from the water or sieved onto a very fine mesh.
Photos of a plankton net, Schindler-Patalas trap and integrated sampler.
Each of these sampling methods has pros and cons and the choice of a particular method or set of methods depends on characteristics of the lake (depth, density of algae, etc.) and the zooplankton itself. An additional important consideration is the number of samples required to characterize the zooplankton in a manner that accounts for its variation in vertical space, horizontal space, and time.
How many samples?
The zooplankton can display a high degree of spatial variation, even when considering only the open water (limnetic) region of a lake. This patchiness results from a variety of factors -- including water currents and rapid population growth in locations where phytoplankton biomass is high.
There are statistical methods to identify the number of samples required to accurately characterize the zooplankton at a particular time in a lake. This requires preliminary sampling at many locations and some simple statistics. One problem is that if spatial distribution varies markedly over time, the value of this approach may be limited. However, an intensive preliminary study still can provide a context, so that the investigator may provide appropriate caveats when interpreting results of sampling at a lower intensity.
In reality, main factors that determine the number of samples are: time, money, objectives of the study, and type of lakes to be sampled. For example, if a study aims to generally characterize the zooplankton of a particular geographic region, and requires sampling of many lakes, it only may be feasible to sample one location in each lake or major basin of larger more complex lakes. In elongated reservoirs, samples typically are collected at several sites along the longitudinal axis, because those lakes have zones with distinct water chemistry, phytoplankton, and zooplankton as one moves from the often turbid upstream end to the algal dominated downstream end.
The bottom line is that there is no magic number for how many samples to collect to characterize a lake's zooplankton., but the sampling regime should be grounded in a solid understanding of lake conditions and the questions being asked in the research project.
How frequently to sample?
Zooplankton biomass may vary considerably from month-to-month and between successive years. Rapid increases in biomass (sometimes two orders of magnitude) may occur in just one week in response to a bloom of edible algae and bacteria and/or a rise in spring water temperature. Sudden crashes in biomass may happen just as quickly in response to intense fish predation, collapse of algal blooms, and other factors. Zooplankton taxonomic structure can display the same high degree of temporal variation. As with 'how many samples?' there is no right or wrong answer to the question about sampling frequency. It largely depends on the questions being asked in the research program. If the objective is to characterize seasonal dynamics of zooplankton and explain how they relate to seasonal dynamics of such things as nutrient inputs, phytoplankton biomass, and fish predation, it may be necessary to take frequent samples (monthly or more often). If the objective is to examine long-term (decade or longer) trends, then sampling may occur less often (perhaps quarterly). In any case, the investigator should conduct some preliminary sampling at a relatively high frequency, in order to understand the extent to which infrequent sampling is likely to miss major peaks in zooplankton biomass or important changes in taxonomic structure.
Sampling methods
Three common methods for sampling zooplankton (shown in the photographs above) are net, trap, and tube. Nets are used most often, yet they have serious limitations in regard to obtaining good quantitative data, especially in nutrient and algae-rich waters. Nets are conical devices made of fine nylon mesh that are pulled through the water either vertically or horizontally for a known distance. Animals are captured in a vial or mesh-walled bucket at the bottom of the net and then can be rinsed into a storage bottle for counting. The amount of water from which zooplankton are removed is estimated as length of tow times mouth diameter of the net. However, nets may not actually filter this volume of water. The main advantage to using a net is that samples of large volumes of lake water can be collected quickly. Nets can be obtained with various mesh sizes, depending on whether one wants to collect only the largest zooplankton or the entire size range that occurs in the water.
The most common trap sampler is the Schindler-Patalas trap, obviously named after the two scientists who invented the device. This is a clear plastic box that is lowered to a desired depth in the water column and then quickly closed (upper and lower doors) by pulling upward on the line by which the device is lowered and raised in the water. This traps zooplankton inside the box. When lifted into the boat, the water is allowed to exit a small mesh net that is attached to the lower wall of the box, and zooplankton is collected inside a sampling bucket at the end of that net. This device provides a high degree of certainty regarding the actual volume of water sampled, but if the water column is deep, it may take many samples to collect animals from all depths from surface to bottom.
The third method is a tube, made of common PVC or Tygon. A tube is lowered into the water column, and when the bottom reaches the desired depth (near the sediments), a line is pulled to close the bottom with a rubber stopper or other device. The tube is raised into the boat and the collected water poured through a net to collect the zooplankton. This device also provides a high degree of certainty about volume of water sampled, but it may not be an effective way to sample large animals that occur at a low density, or animals that can detect and escape from a narrow sampling device.
Nets, traps, and tubes will be used to collect representative during the Zooplankton Ecology course, and students will participate in a critical analysis of these three common sampling techniques.
Sample preservation
Preserve freshly collected zooplankton with 40% formalin-sucrose solution that is chilled to around 5 degrees C before use. Add about 2 grams of sucrose (table sugar) to lab grade formalin. Add a sufficient quantity of preservative to the sample to achieve a final concentration of approximately 40% formalin. Formalin preserves the animals by preventing bacterial decomposition. The combined effect of sucrose and chilling the preservative prevents distortion of cladoceran bodies that occurs with non-amended formalin. Keep the preserved samples in near air-tight containers (Whirl-Pak bags or small plastic Nalgene bottles) under refrigerated conditions for best results.
Counting and biomass estimation
Simple counts of zooplankton can be done with a light microscope. For large zooplankton such as Daphnia, which occur at relatively low densities (1 to 100 per liter), the entire sample may be scanned at a low magnification, counting all observed individuals. For small zooplankton, such as rotifers and copepod nauplii, which occur at high densities (>1000 per liter), it is standard practice to count a known percentage of the sample volume at high magnification, and then multiply by total volume / counted volume to obtain the total number of animals in the sample. Once you know the number of animals of each species in a sample, density in the lake is estimated as counts divided by volume of water filtered with the net or collected by the trap or tube.
Biomass estimation requires advanced methods that are covered in the Zooplankton Ecology course.
Grazing rates and diet analysis
This aspect of zooplankton sampling requires advanced methods that are covered in the Zooplankton Ecology course.
Zooplankton production
This aspect of zooplankton ecology requires advanced methods that are covered in the Zooplankton Ecology course.