When a quantitative analysis of the algal community in a sample is required an algal identification and enumeration will be performed. Counts can be performed on only a single group of algae (i.e. cyanobacteria or just potentially toxin-producing cyanobacteria) or on the entire algal population. Results for each species will be recorded in natural units/mL (cells, colonies, filaments) and cells/mL. Numbers for each group of algae (i.e. diatoms, green algae, cyanobacteria) will be tallied as well as the total sample cell density.

 

Samples will be preserved in Lugol’s iodine solution prior to analysis. For each sample to be analyzed a cleaned Utermöhl counting chamber will be constructed. Depending on the phytoplankton density of the sample, a settling tower of 5, 10 or 25 mL will be used. The tower will be secured to base using a thin film of high vacuum grease. The Lugol’s preserved sample will be shaken for a minimum of 60 seconds to evenly distribute phytoplankton cells and the appropriate volume (1 to 25 mL) added to the settling tower. Samples will be diluted or concentrated prior to settling if necessary. A cover glass will be placed on top of tower and the sample will be allowed to settle in the dark in a vibration-free location. Minimum settling times will be 17 hours for 5 mL samples, 34 hrs for 10 mL samples and 74 hours for 25 mL samples. After settling is complete, the tower will be slid from base of the counting chamber to remove overlying water and the chamber will be covered with a second cover glass. The sample is then ready for microscopic analysis.

 

Enumeration will be performed on a Nikon Eclipse TE200 inverted microscope equipped with phase contrast optics. One ocular will be fitted with a whipple disc, used to define the area of fields to be counted. Specimens that go beyond the left and bottom edges of the whipple grid will not be counted. Before beginning enumeration the slide will be scanned at low power to ensure that distribution of cells is relatively uniform. Only intact, viable cells will be counted. Counts will be made as natural units (cells, filaments, colonies). The goal is to count a total of 400-600 natural units per sample. A minimum of 10 and a maximum of 50 fields will be counted at both 400X and 200X. An additional scan of the entire slide will be made at 100X to count large and/or rare taxa. A maximum of 100 fields combined at 400 and 200X plus the 100X scan will be performed on any sample.

 

For colonial and filamentous forms, number of cells per colony or filament will be recorded (20-30 filaments or colonies for each species if possible, more if necessary) and the mean used to calculate cells/natural unit.