Method
There are many methods for taking and analysing samples of freshwater and marine phytoplank- ton, some of which involve specialised electronic equipment for in situ estimates. Analysis of samples is by cell counts. I shall describe only the simplest methods which use very little spe- cialised equipment. You should refer to the technical literature and consider other procedures
(HMSO 1990; Porter et al. 1993; Hotzel&Croome1999).Usingthesametechniquesasfor mapping macrophytes, the phytoplankton can be mapped in the water body. The water body can be sampled using a jar or container on a line although a plankton net can be used to cover a larger area and to concentrate the plankton sample. A plankton net (Figure 4.4) consists of a fine-mesh net (e.g. 30µm) ending with a bottle to hold the plankton. The net frame is usually square or circular, meaning that the sample area of the frame is easily calculated. The net should be small enough to be used easily and sufficiently small that the bottom of the water body is not disturbed (which is a problem especially in shallow streams). The net can be fixed either to a pole or to towing lines. For the latter the net can be towed either by you or by a boat. You can either hold the net against the current or sweep it through the water. The volume of water filtered depends on the area of the net frame, the speed of flow of the water, the speed you tow at and the time in the water. You should attempt to standardise this volume across all samples. Because of the fine mesh the net should be moved slowly (>1 knot) through the water. The phytoplankton will be stratified (i.e. each species exhibits a non-random vertical distribution in the water body) so the net should either be held at the same selected depth for all samples or be moved evenly through all depths. Alternatively, to sample a particular depth of water, a piece of flexible
pipe (e.g. hose pipe) is lowered into the water with a line attached to the lower end. The top end is closed and the lower end is raised up until the pipe forms a U-shape. The whole pipe is taken out of the water and decanted. Phytoplankton are not randomly distributed across a water body, due to variations in flow and nutrients. So samples should be taken at various locations; e.g. at various points across a river and near and far from the shore of a lake.
Thealgalcellswilldecayrapidlyandshouldbepreservedimmediatelyaftercollection.Various fixatives are used, but the commonest is Lugol’s iodine solution. A solution of 20g potassium iodide in 200ml distilled water is made and then saturated by adding 20ml iodine. This is then acidified with 20ml glacial acetic acid. The solution should be kept in a dark bottle to preserve the iodine. You should add 1–2 ml Lugol’s iodine to every 100ml of the sample (remember to allow for this extra volume when calculating cell densities). Densities of species or cell types (a less precise classification) can be determined by counting through a microscope. If densities are low you can concentrate the cells by centrifuging or simplyby sedimentation. The latter involves mixing up the sample to get an even distribution of cells and then removing a measured subsample to a measuring cylinder; narrower cylinders are better so that the final sample is more concentrated. At 20 ◦C cells will settle at a rate of two hours per 1cmdepthofwaterinthecylinderandasedimentationtimeof48hisusuallyrecommendedfor a standard 100-ml cylinder. After centrifuging or sedimentation you siphon off the top 90% of water,shaketheremainderandtakeafurthersubsampleforcounting.Cellscanbecounted,but it may be better to count colonies or filaments of some species. The two simple methods are use of the haemacytometer and use of a counting chamber. The former is more usually used to count red blood cells. It holds a precise volume of liquid in which you can count the numbers of each species. Phytoplankton keys are widely available, but identification of algal cells is a skilled job. Several counts should be made for each sample and averaged to get a density. A Lund counting chamber comprises an ordinary microscope slide with two long, thin pieces of glass or another rigid material glued to the longer edges. This chamber is closed with a coverslip and a measured volumeofthesampleisplacedinthechamber.Cellsarethencountedunderamicroscope.Ifyou wantadensityofcellsyoushouldaccountfortheconcentrationtechniquesused.