Quantitative periphyton samples collected by certain sampling methods, such as the foil-replicate or PVC cylinder methods, usually contain a large volume of stream water relative to the biomass of algae in the sample. If CHL and AFDM are not to be determ ined, the sample is preserved immediately and labeled; record the volume of the sample on the field data sheet and on the sample label. If CHL and AFDM determinations are required, shake the original periphyton sample vigorously and pour a measured volum e of algae-water mixture into a separate container for filtration. Record the volume of the remaining periphyton sample (the ID subsample) on the sample label and on the field data sheet; add preservative before transporting the sample to the District laboratory. Record the total volume (the original sample volume) of the periphyton sample on the field data sheet.
When field activities have been completed, excess stream water is decanted from the ID subsample to reduce the size of the container needed for subsequent transport and storage of the sample. Allow periphyton in the ID subsample to settle to the bottom of the container for a sufficient length of time (a minimum of 1 hour for each vertical centimeter of sample in the container). Water may be decanted from the sample by using a filtering flask, to which are attached (1) a vacuum pump and (2) a length of tubing with a pasteur pipette tip attached to the distal end of the tubing. A vacuum is drawn on the filtering flask and the pipette tip is inserted into the sample. Decant the samples until approximately 2 cm of water remains above the periphyton-water interface (proper disposal of preservative solution is essential). Reconstitute the sample and pour it into a (smaller) ID sample container. Place a sample label onto the container, and record the initial and final (decanted) volumes of the ID sample, as well as the name of the person who processed the sample, on the sample label and field data sheet.
Filtration for the optional determination of CHL and AFDM involves a stepwise procedure using equipment listed below.