Phytoplankton are often the major primary producers in the open waters of lakes. This exercise will familiarize you with local late summer phytoplankton populations, as well as one common method of estimating primary productivity.

The euphotic zone of Lake Winona extends down to 3 m during late summer.

Phytoplankton and primary productivity will be examined in Lake Winona during mid-September. The following describes the general procedure we will follow to examine the hypothesis listed above.


For phytoplankton analyses:

1) Collect water samples containing phytoplankton from Lake Winona with a Kemmerer water bottle (surface, 1 m, 2 m, 3 m, 4 m, 5 m depths).

2) Return the water samples to lab and prepare wet mounts for viewing with a compound microscope.

3) Using the resources available in the lab, identify the types of phytoplankton present. Do this for water samples from each of the depths.

4) Estimate the relative abundance of the total phytoplankton community at each of the depths (use the average number of organisms per microscope slide, or other similar procedure).

For primary productivity:

1) Collect water samples from 1-m depth intervals (surface, 1 m, 2 m, 3 m, 4 m, 5 m) with a Kemmerer water bottle and fill 3 DO bottles (one brown or "dark" and two clear or "light") with water from each depth.

2) Measure and record the dissolved oxygen in each bottle and return the two "light" bottles and the "dark" bottle from each depth to the depths of collection on a suspension line attached to a buoy.

3) Obtain multiple Secchi disk readings and photometer readings over the course of the next 1 hour.

4) After 1 hour, retrieve the "light" and "dark" bottles, and measure and record their dissolved oxygen content.

5) Calculate primary productivity at each depth. Calculations and their rationale will be explained in lab.

1) Plot net primary productivity vs. depth (this means make a nice, neat figure). Determine the compensation depth (depth where NPP declines to zero).

2) Compare Secchi disk readings and photometer readings with the compensation depth.

3) Compare the primary productivity at the various depths with your relative estimates of phytoplankton abundance.

Return to Limnology Lab

Neal D. Mundahl
Winona State University
Winona, MN 55987-5838