Sampling the Populations of Phytoplankton and Periphyton | Essay

Sampling technique for phytoplankton:

Phytoplank­tons of a water body can be sampled by following two methods:

(i) In one method, phytoplanktons are obtained by drawing water samples from several depths. Cell counts of algae present in each sample, either normal or concentrated, are made with a Sedgewieh Rafter counting chamber and a Whipple ocular. According to the need, sometime the samples may be concentrated by centrifugation in forest plankton centrifuge. The centrifuged samples are then diluted to a suitable volume (100 to 200 ml) in a volumetric flask.

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During the counting of algal cells, a separate tally should be kept for each species. This will permit an analysis of community structure at each station. For single-celled forms, the number of cells is recorded; while for colonial forms, the number of colonies is recorded.

(ii) In another method, the phytoplanktons of the sample are first fixed by aqueous solution of formaldehyde, then filtered and ultimately counted by quadrate method.

Sampling technique for periphyton:

Epiphyton, the periphyton growing on living plants and animals, can be observed in place on the organism if the substrate is thin or transparent enough to allow the transmission of light. The epiphyton growth on one side of the leaf only is scraped away for study.

Small leaves are examined over the whole area. Large leaves are sampled in strips marked by grids on a slide or by an ocular micrometer. With large aquatic plants a square is cut from the leaf or stem. If the leaf is too thick to handle under the microscope, the periphyton is scraped off quantitatively and is mounted in a counting cell for examination. The results are then related back to the total surface area.

Another method is used for algae growing on such aquatic animals as turtles, molluscs, etc., and on stones. One easy method employs a simple hollow square instrument with a sharpened edge, which is pressed closely or driven into the substrate.

This separates out a small area of given size around which the periphyton is washed away. The instrument is then raised and the periphyton remaining in the sample square is scraped into a collecting bottle.

The periphyton can be counted in a Sedgewich-Rafter cell recording a predetermined number, usually 100 to 1,000, as they appear in the field of view.

Sampling stream-bottom organisms:

Samples of stream- bottom organisms are collected by a modified Surber bottom-fauna sampler (Fig. 4-3). This sampler consists of a brass frame with Stainless-steel side pieces and a current baffler. To this is attached on a removable brass frame a fine net of 74 meshes/linear inch and a coarse net with 19 meshes/linear inch. The latter is fitted in the sampler in front of the line net to produce a cone or cone effect. Flanges on the insert prevent its being forced into the fine net in the rear.

Fig. 4 3. A modified Surber bottom-fauna sampler (after Smith, 1974).

This sampler picks up many small organisms along with fine detritus. At a time, it collects two subsamples with respect to size.

This sampler encloses a specified area of stream (500 cm2), which is the sample unit. Organisms, detritus and trash are scrub­bed free from the substrate, and the current washes it into net. The contents are transferred to a container and are taken back to the laboratory for examination and sorting.

Dendrochronology:

Dendrochronology is the science that cal­culates the accurate dating of past events through the aging of trees. It is based on the variation of growth rings. Growth rings, despite popular belief, are not regular, nor are they all necessarily lay down annually.

Because of the failure of cambium to form a sheath of xylem the entire length of the bole, rings may be omitted, especially near the base. This may be caused by the lack of food manufacture in the crown, by drought, fire, extreme cold, insect out­breaks, and so on. At the other extreme are multiple rings produced by multiple waves of cambial activity during the growing season.

These are caused by temporary interruptions in the normal growth, such as late spring frost, taking place after normal seasonal growth has ended. Thus, the growth rings reflect the interaction of woody plants and their environment.

The fundamental principle of Dendrochronology is cross dating, the correlation of distinctive patterns of growth between trees for a given sequence of years. Because no two plants have exactly the same growing environmental conditions and life history, the simi­larities are relative rather than quantitative.

The relative widths of corresponding rings are the same in relation to adjoining rings. By lining up these similarities, the investigator can establish the relative identity of any ring in sequence and aberrant rings in the individual specimen.

Dendrochronology is used in number of studies—to age trees for management information, to establish dates of past forest fires, insect outbreaks, glaze, periods of suppression and release in the life history of forest trees. It has been involved in hydrological and archaeological studies and even in legal cases involving boundary disputes in which specimens are taken from fence posts and witness trees.

Palynology:

Palynology is the study of plant communities by the analysis of pollen profiles. It involves the collection of samples of peat in bog by peat borer from different depths and microscopic analysis in laboratory of the sample. Carbon-14 dating method can be used for correct assessment of the age of pollen grains in the sample.