1
Name:
Most answers require brief responses: be concise but coherent. Put your name on every page. Point
values for each question are in ( ) next to the question number. Note that longer essay questions are
scattered throughout.
1(2). What is a secchi disc used to measure?
Depth of light penetration in the water column.
2(6). Briefly define the following terms. How are they characterized according to a temperature profile?
Epilimnion
mixed zone; top layer; similar temperature throughout
Metalimnion
transition zone between meta- and hypolimnion; >1 degree change per m in temperature
Hypolimnion
very little mixing; bottom layer of stratified lakes; relatively cold water
3(6). On the first day of class we talked about four different (but complimentary) methods by which to investigate
aquatic systems (and really any ecological system). List at least three of the four methods. Imagine a situation in
which an investigator must decide between two or more methods. What are some limitations or advantages of
each method you have listed?
1. Experiments- don't always reflect interactions on the scale of whole ecosystem; allow for controlled
manipulations
2. Long-term studies- usually can only study one lake at a time; not very useful for someone completing
their degree in three years; allow the study of ecological processes that occur over long time periods
3. Simulation models - they only reflect what you program into them ; they help determine what you
DON'T know when they don't compare well to natural systems
4. Comparative studies - may not be very mechanistic; time for space substitution
4(4). Name at least one mixing process (water movement) that is unlikely to be important to mixing in a lake with a
very small fetch. What might be important?
Unimportant: surface gravity waves, seiches, etc. Important: convection currents, onshore-offshore
currents.
5(6). Why is water such a kick-butt medium? What makes it so useful? (we talked about several unique
properties of water in class) Explain how one of your water characteristics impacts the biota in a lake or stream.
High latent heat content- moderates the aquatic environment
Surface tension- organisms live on or below the surface
Molecular structure: ice is less dense than water slightly warmer so it floats & the whole lake doesn't freeze
Universal solvent- nutrients dissolve in the water so they are available for biological uptake
Aquatic Biology 2002
Exam 1
2
Name:
6(2). You were snorkeling in the littoral zone this past summer, and noticed sunlight "dancing" across the
sediments. What properties of light and/or water might cause this?
Reflection off the surface (waves changing the angle of reflection)
Refraction of the light when it enters the water
7(4). If you were a small fish and wanted to be inconspicuous at 10 m depth to avoid predation, what color would
you want to be? Red or blue (circle one). Why?
Red because red light quickly attenuates and so you'd appear grey and inconspicuous!
8(4). Why is it that edible algae may have very high production rates, but never accumulate high biomass?
Because it is being consumed faster than it con accumulate biomass.
9(4). This year it was a particularly windy spring, causing a particularly late onset of stratification in Lake Mendota.
Would you predict summer hypolimnetic temperatures be higher, lower, or the same as usual? Why?
Higher than usual because during the prolonged mixing period all the water in the lake would have
warmed up from 4 oC.
Incident light (%)
01
50
Temperature oC
10(2). In the figure to the right, observe that light rapidly goes
0
0
extinct in the shallow surface waters. Briefly explain why
warm epilimnetic temperatures extend to a depth of 5 meters.
5
Light
Mixing of the epilimnion
100
10
15
Temperature
Depth (m)
5
10
15
20
Aquatic Biology 2002
Exam 1
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3
Name:
11(8). Refer to the temperature diagram for Lake Mendota, Madison, WI below:
a. Does the thermocline depth tend to get deeper or shallower during summer?
deeper
b. Roughly when does fall turnover occur?
Early to mid Oct (as it says on the figure!)
c. Is the lake isothermal during July?
no
d. Is Lake Mendota dimictic, meromictic, or monomictic?
Dimictic (it mixes twice a year)
12(4). A question of saltiness: a) Is salt water more or less dense than freshwater? b) Briefly discuss how
accumulation of road salt in lakes might affect lake stratification.
a) salt water is more dense than freshwater
b) salt water would accumulate in the depths of the lake. Development of a dense water mass (salty) will
make mixing more difficult, perhaps causing meromixis.
13(2). What is the bacterial-mediated process that removes N from the lake or floodplain? Under what
condition(s) does it occur?
Denitrification occurs under anoxic conditions
14(4). Apple Lake and Orange Lake are similar in surface area, depth and volume of water. However, Apple Lake
is situated in a region where the bedrock is primarily granite while Orange Lake is situated in a region where the
bedrock is primarily carbonate rock (like limestone). Which lake would you expect to have a higher conductivity?
Which lake would have "soft water"?
Orange lake would have higher conductivity
Apple lake would have "soft water"
Aquatic Biology 2002
Exam 1
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Name:
15(12). On one figure, draw and label the depth profiles for concentrations of NO3-, NH4+, dissolved O2 and
temperature in a temperate-zone (e.g. Minnesota) eutrophic lake in August. Also label stratification layers. Don't
Increasing
concentrations
NO3-
Temp (oC)
Epilimnion
O2
Metalimnion/thermocline
NH4+
hypolimnion
Increasing
Depth
worry about axis values, just patterns. Explain the patterns for NO3- and NH4+
(8 pts for the figure in total; 4 for explanation of the patterns)
NO3- is abundant in the epilimnion because nitrifying bacteria transform NH4+ to NO3- through the process of
nitrification (O2 present). Algae in the epilimnion also assimulate NH4+ quickly. In the hypolimnion there is little
demand for NH4+, it is a by product of ammonification (mineralization/decomposition), and it is often absorbed to
sediments or particles. In the anoxic hypolimnion, NO3-disappears is rapidly transformed to N2 through the process of
denitrification.
16(4). a) Contrast conservative and dynamic ions. b) Name a dynamic ion that we talked about in class (using
names or chemical nomenclature) and explain why it is dynamic.
a) conservative ions remain in the lake at about the same concentrations year-round and from year to
year while the concentrations of dynamic ions will vary due to biological demand…
b) we talked about both Ca++(calcium) and Si (silica), both which vary in concentration over the course of
a year due to biological demand. (N, P also ok)
Aquatic Biology 2002
Exam 1
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Name:
17(6). Contrast thermocline seiches and surface gravity waves. What areas of the lake do these water
movements impact? What organisms might they impact (give your rational)?
Thermocline seiches are caused by strong winds piling water up on one side of a lake, which tilts the
thermocline down. When the wind lets up, the epilimnion sloshes back to the other side, and this induces a
rocking motion in the thermocline. Thermocline tilting results in horizontal currents, entrainment of epilimnetic or
hypolimnetic waters and erodes weak thermoclines (transporting nutrients across the thermocline). In contrast,
water in surface gravity waves only move in an elliptical or circular motion as a wave passes. This doesn't do too
much in deep water, but in shallow water, if the circular movement comes in contact with sediments, and if the
orbital velocity is great enough, sediments can be resuspended.
18(2). How do Langmuir spirals influence the spatial distribution of planktonic particles?
Langmuir spirals influence the spatial distribution of planktonic particles (zooplankton, algae) by trapping lighter
particles at the water surface (like the ice cakes we saw in the photo) and suspending heavier particles beneath
the surface, between opposing circulation spirals.
19(2). What is "fetch"? Why is it important?
Fetch is the greatest distance across a water body that wind can blow. It will influence the maximum height of
waves and the amount of mixing that can occur in a lake.
20(4). Why are nitrogen and phosphorus considered "limiting nutrients"?
These are nutrients that are relatively scarce in the environment but are both required for plant growth. Because
they are scarce, the lack of one will prevent plant growth, even if the other is present in great enough quantities.
21(2). Merry Lake and Greyweather Lake are of similar depth, size, and winds regimes. Merry Lake is located in
central Minnesota. Greyweather Lake is located in central Florida. Which of these two lakes is more likely to fully
mix at some point during the year. Briefly explain why. The figure below might be helpful in a general sense.
Merry Lake. The colder temperature results in
a smaller density differential. Warm lakes are more difficult to mix.
Aquatic Biology 2002
Exam 1