Lake Survey
Location: Allen Lake, Irish Hills, Michigan
Time: 10 am – 3 pm
Introduction
Every lake has a distinct set of characteristics determined by its latitude
(temperate zone vs. tropics), geological setting (limestone vs. granite bedrock), shape,
depth, biological community, and especially its watershed. Limnological surveys are
very useful for determining these characteristics and repeated surveys over time can help
determine whether the lake environment is changing due to human impact on the
watershed, regional changes (acid rain), or introduced species.
The physical, chemical, and biological characteristics of a lake vary seasonally in
fairly predictable manner. We will be visiting Allen Lake in its summer stratified state in
which we will find distinct differences between the surface water layer (epilimnion) and
the bottom water layer (hypolimnion). In order to determine these differences, we will
create a vertical profile of the water column by taking measurements at several different
depths near the center of the lake. We will also sample the littoral zone along the edge of
the lake for aquatic invertebrates and fish species.
Bring with you
Lunch, water bottle, hat, sunscreen, bug spray, notepad
Activities
Open Lake Sampling (from pontoon boat)
Students will work in pairs and rotate through using the YSI probe, the Van Dorn
sampler, the plankton net, and Ekman dredge. We may not have time for every student to
perform every measurement, therefore take note and be prepared to share your data.
Physical Characteristics
Light profile:
Use meter to measure light levels. Record your data in a notebook.
Air (in the sun)
0 m (just below surface on sunny side of boat)
0.5 m
1m
1.5 m
2m
3m
4m
5m
This data will be used later to calculate the extinction coefficient and
compensation depth
Secchi Depth: Each student should determine and record Secchi depth.
Temperature and Turbidity:
This data will be collected using the YSI multiprobe. The hand unit stores
the data, which will be provided to you later as an Excel file. Using the YSI, record data
just below the surface, at 1 m and continue at increments of 2 meters all the way to the
bottom or until you run out of cable.
*If the cable does not reach the bottom, how can you obtain data from deeper depths?
Chemical Characteristics
*Dissolved Oxygen (mg/L)
*Conductivity (uS)
*pH
*Total Dissolved Solids
*in situ chlorophyll fluorescence
*These measurements are recorded on the YSI.
Total Phosphorus
Alkalinity
These measurements require water samples. Use the Van Dorn sampler to
collect water samples at 1m depth and at a depth in the hypolimnion about 2 m
above the bottom. Label and fill 1 L sample bottles and put these samples on ice.
Samples will be kept frozen until analysis at a later date.
Biological Characteristics
Zooplankton
If the zooplankton net is equipped with a flow meter, first record the meter
reading. Then lower the net to about 2 meters above the bottom and then pull it to
the surface at a rate of about 1 ft per second. Record depth of tow and flow Rinse
the net with a rinse bottle and concentrate the contents in the mesh cup. Preserve
zooplankton in formalin for later analysis. The zooplankton net collects from a
given depth to the surface. How could you use the net to determine differences in
zooplankton between the hypolimnion and the epilimnion?
Phytoplankton
Use the YSI measurements to determine the depth where in situ
chlorophyll fluorescence is greatest. Use the Van Dorn bottle to collect a water
sample from this depth. Pour sample into a 125 ml dark plastic bottle and add
1.25 ml of Lugol’s Solution as preservative.
Benthos
Collect 4 “grab” samples using the Ekman dredge: 2 from the bottom in a
shallow area of the Lake where dissolved oxygen is high (how would you
determine oxygen?) and 2 samples from near the center of the lake. Set the spring
on the Ekman dredge (USE CAUTION!) and lower it to the bottom. Trigger
dredge with a messenger and pull to surface. Empty dredge into a sieve bucket
and strain sediments for benthic invertebrates. Preserve invertebrates in each grab
in a jar with ethanol (one jar/grab) for later analysis.
Shoreline Sampling
Chemical characteristics: Use hand-held pH, Conductivity, and dissolved oxygen
probes to make measurements among macrophytes and bare shoreline. Record in
notebook.
Biological characteristics: Use dip nets to sample aquatic invertebrates from
various nearshore habitats (macrophytes, sandy bottom, among rocks, etc).
Preserve invertebrates in containers with ethanol for later analysis. Use seines to
collect fish along shoreline. Fish may be kept temporarily in aquaria for ID and
observation and then released. Compile a list of all fish species found.
Lab Report
Prepare a short laboratory report (word-processed; about 2 pages excluding tables
and graphs) using your field notes and the results of your field measurements. The report
should, at a minimum, contain the following information.
(1) Introduction
Briefly introduce the topic and the goals of this lab. Using one or two sentences for each
measurement, state the purpose of each measurement that you made – what can this
measurement tell you about the lake? (This will require some reading ahead in the
textbook!).
(2) Materials and Methods
When appropriate, refer to this handout (no need to repeat this information in report) and
to verbal instructions by the instructors and TA. Be sure to describe all modifications
you applied. Note the depths of water samples, plankton tows, benthic grabs, dimensions
of Ekman dredge, diameter of plankton net, flow meter readings, etc. (You will need this
information later).
(3) Results
YSI data
Transfer the YSI data file to your own computer. Use MS Excel to create and print out
figures that profile temperature, pH, DO, conductivity, turbidity, and fluorescence with
depth. Place depth on the y-axis (with zero at the top) and the parameter of interest on
the x-axis. Make notes on the figures to point out features of interest (thermocline,
epilimnion, hypolimnion, maximum fluorescence, etc.) Make sure that the x and y axis
of each figure are labeled complete with units. Give each figure a number “Figure 1” etc.
and refer to this number when discussing your results.
Light
Use data from the light meter readings to create a figure similar to above illustrating
irradiance (I) vs. depth. Include an arrow to note the secchi depth on this figure. Indicate
the 1% irradiance depth (depth that receives 1% of light that was recorded at depth = 0).
Also plot the natural log of light ln(I) against depth. Use Excel to fit a straight line
through the points.
(4) Discussion
Examine your figures and explain any trends that you notice. How do the physical and
chemical parameters change with depth? Discuss the differences between the epilimnion
and the hypolimnion in terms of temperature, DO, pH etc. How would you explain these
differences? Do you notice any relationship between fluorescence readings and the depth
of the thermocline? If so, what do you think this means?
If we were to visit this lake again in November, how do you think these patterns might
change?
Similarly, discuss any differences between the littoral and pelagic (epilimnion) zones in
terms of measurements from probes. We will examine differences in biotic communities
between locations at a later date.