You should choose a method based on your question, the precision required and your budget. If your purpose was to collect a sample to determine what species were present in an algal bloom, and not for any comparative purposes, it is possible to collect three samples, mix them together in a bucket and then take a sub-sample for counting. This sub-sample will provide an indication of the average counts, but will give no indication of the variation between the samples.
Visual assessment is the least expensive way to monitor phytoplankton -by estimating phytoplankton abundance based on water colour, Secchi depth, area of bloom or from a satellite image. You can also make net collections (20 ^m mesh; see Figure 4.4, Section 4.6), to concentrate rare species. Net collections of phytoplankton are suitable for larger cells, such as some diatoms, but the bulk of phytoplankton in the sea and in rivers is in the less than 20 ^m fraction and even in the less than 2 ^m fraction. Consequently, a plankton tow is regarded as a qualitative measure due to avoidance and particularly extrusion of particles through the mesh.
Quantitative samplers include surface water samples, which are collected by dipping a well-rinsed bucket over the side of the boat. A sample may be collected from the shore or bank with an empty sample jar on a pole. Integrated samples are usually taken from the surface to 3 m depth or more. These samplers can be made from a 2-5 cm diameter PVC or hosepipe (Figure 4.3a). In rivers with extensive rushes and mud banks, a Taylor sphere sampler (TASS) is a simple and ingenious device (Figure 4.3b, Hotzel and Croome 1999). Both samplers are operated in different ways but work on the principle that an integrated sample is taken through the photic zone of the water column. The entire sample is then released into a clean bucket - repeated up to three times - and a 100 mL sub-sample is then removed from the bucket and preserved for phytoplank-ton identification.
Water samples from specific depths can be collected using diaphragm pumps or water bottles, such as 1.7 L Niskin bottles. Water bottle casts ('hydro-cast') can be conducted using a rope over the side of a boat, and a heavy metal 'messenger' then slides down the rope to close the bottle.
At least two replicate water samples should be collected at each station or depth, and their unique numbers recorded on the field data sheet. An extra water sample from each hydro-cast should be retained in case of laboratory mistakes. Label each bottle with a unique identifying number (inside and outside) for the laboratory. A pad of self adhesive labels is useful, such that the same number can be used on the various samples for nutrients, chlorophyll, phytoplankton and zooplankton and the data sheet.
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