On some large farms, technicians make frequent estimates of phytoplankton type and abundance. Such analyses are labor intensive, and it is often difficult or impossible to interpret results. A more simple approach is to observe water color and to estimate turbidity from Secchi disk readings. Phytoplankton abundance can change rapidly, so measurements should be made daily. No one knows for certain the best color for shrimp pond water. Most pond managers prefer a dull green, yellowish green, or brownish green color because these colors are associated with desirable green algae and diatoms. Color may be used to distinguish turbidity caused by suspended soil particles from that of phytoplankton. Color changes also suggest changes in plankton community composition. Secchi disk visibility is a measure of plankton abundance because visibility increases when plankton abundance declines and decreases when plankton abundance rises. Secchi disk visibilities of 25 to 35 cm are considered desirable by most shrimp farmers. Higher visibilities suggest inadequate plankton, scarce natural food organisms, and a threat of macrophyte infestation. Lower visibilities indicate excessive plankton and potential problems with low dissolved oxygen (Boyd & Fast, 1992).

When using water color and Secchi disk visibility for management purposes, the analyst should standardize his observation methods. Measurements should be made at the same time each day by the same person using the same procedure (Boyd & Fast, 1992).

Nutrient measurements are too difficult to make and interpret for routine practical use. It is more feasible to use phytoplankton abundance as an indirect measure of nutrient availability, because phytoplankton responds to nutrient concentrations. Nutrient limitations are suggested when phytoplankton abundance declines (Boyd & Fast, 1992).