Adequate sampling of the euphotic zone or detrital for phytoplankton is a subject which is outside the scope of the presen method. Once obtained, the final sample (generally 500 ml-5 litera in volume) is filtered through a small piece of clean 0.3-mm mesh nylon netting to remove the larger zooplankton. For open sea samples filtration of small volumes through a 0.15-mm mesh net will still not retain significant amounts of phytoplankton. The required volume of this filtrate abould be measured by a polyethylene measuring cylinder into a polyethylene bottle. Two or three drops (ca.0.1-0.2 ml) of magnesium carbonate suspension (see Sect. E.2) are added. The sample may then then be stored in a cool dark place for a maximum of about 8 hr. It is desirable, however, that samples be filtered through a membrane falter at the time of collection.
Membrane filter can be stored by folding them in half (with the plankton innermost) and storing them in the dark in a desiccator frozen to -20 C but only for a few weeks. This procedure almost always leads to low results and makes the extraction of chlorophyll more difficult; filters should be extracted without delay if at all possible.