LIMNOLOGY PROBLEM SET
This problem set will count for 15% of your final grade. A hard-copy of this is due on Wednesday, November
20 on the Canvas course site.
Provide a written answer with proper grammatical structure for each of the questions below. Also show any
necessary calculations, formulas or graphics that you need to answer each of the questions. Pay special attention
to the units data are presented in! You should be able to find all of the relevant information in the class notes
and in your textbook.
1) (15 pts.) The light intensity versus depth profiles for two lakes are listed in the table below. Calculate the
extinction coefficient (η) for both lakes using all of the data provided (i.e., you need to calculate the slopes
of these curves). Give three explanations for the observed differences in light attenuation in these lakes.
Light Intensity (µEin m-2 s-1)
Black Lake
Tan Lake
Depth (m)
0
1
2
3
4
5
920.0
558.0
338.4
205.3
124.5
75.5
1304.0
966.0
715.7
530.2
392.8
291.0
2) (15 pts.) One method of estimating primary production in lakes is by incubating lake water in situ in both
"light" and "dark" bottles and comparing the dissolved oxygen concentration in those bottles to an initial
concentration. Using the values below, calculate the rates of net primary production, gross primary
production, and respiration for Sixteen Lake and Lake McMurray. Comment on the relative productivities
of these lakes. Is respiration related to the primary production rate? Does this make sense?
Dissolved Oxygen Concentration (mg O2/L)
Sixteen Lake
Lake McMurray
Initial
Light
Dark
9
11.5
7.4
9.5
10.1
7.6
Incubation Time (hr)
4
6
3) (15 pts.) The Washington Department of Fish and Wildlife (WDFW) is considering stocking planktivorous
rainbow trout into two alpine lakes in the Cascades. They want to know if the lakes have enough
zooplankton to sustain fish populations and so they hire you to provide an estimate of the zooplankton
community biomass for each of these lakes. After sampling and counting, you find the following densities
of certain zooplankton species. Using the provided information on individual species mass, calculate the
volumetric (mg m-3) and areal (mg m-2) zooplankton community biomass for each lake. Pay attention to
the units you use for your answer. Furthermore, the WDFW asks you to predict what the zooplankton
community will look like 5 years after stocking planktivorous fish. Specifically, comment on which species
you believe will increase or decrease (and explain why).
Density (number L-1)
Blue Lake
Maiden Lake
Average
individual mass
(µg)
cladocerans
Daphnia pulex
Bosmina longirostris
Holopedium gibberum
0.2
0.5
1.3
7.2
5.8
0.8
6.1
0.5
6.2
copepods
Diaptomus ashlandii
Tropocyclops prasinus
Mesocyclops edax
0.2
0.1
0.3
2.5
0.8
2
3.2
1.3
10.2
120.1
102
7.2
12
10.2
0.2
0.004
0.008
0.3
15
20
rotifers
Kelicotia longispina
Keratella cochlearis
Asplanchna spp.
LAKE MEAN DEPTH (m)
Bonus (5 pts): Explain why estimating biomass of zooplankton in a lake might not be a good indicator of the
ability of the zooplankton community to support fish production.
4) (15 pts.) The city of Pleasantville obtains its water supply from nearby Skunk Lake. The hypolimnion of
Skunk Lake is chronically anoxic and has a relatively high concentration of hydrogen sulfide (H2S) that
makes drinking water smell like rotten eggs. To minimize the odor problem, drinking water is drawn from
an intake that is located at one end of the lake and about 3m above the normal thermocline depth. Residents
notice that their drinking water often smells bad following big storms, and that this odor problem is
especially bad on a periodic basis that peaks a couple of times a day after a strong wind storm.
Explain what physical phenomenon is occurring in the lake to cause this periodic water odor problem.
During the summer, the epilimnion temperature is 25°C and 10 m thick, and the hypolimnion is 10°C and 15 m
thick. The lake is 10 km long. Can you estimate the number of hours between peaks of bad smelling water in
Pleasantville’s water supply following a wind storm? (there are several water density calculators on the web;
assume salinity=0)
(15 pts) The following data were collected by a phycologist studying the phosphorus (P) uptake kinetics of three
species of phytoplankton. From these data, estimate Vmax (the maximum P uptake rate) and K (the halfsaturation constant for uptake) for each of the algal species.
Assume that these three species are the only species of phytoplankton in a lake that has very low grazing rates
by zooplankton. Which species is predicted to outcompete the others if P concentrations in the lake are very low
and homogeneous (i.e., < 1 ug/L)? Why? Which species is predicted to outcompete the others in a highly
heterogeneous lake where P is very patchily distributed? Why?
P concentration
(µg P/L)
Species A
Uptake rate (µg P/h)
Species B
Species C
0.1
0.5
1
3
5
7
9
10
0.1
2.6
3.8
5
5
5
5
5
0.1
1.2
2.5
5.5
6.8
7
7
7
0.1
0.7
1.4
5
7
8
8.5
8.5
5) (15 pts) A hydrologist measures all of the known inflows and outflows of water from Pea Soup Lake. She
also measures the phosphorus (P) concentration in each of these inflows or outflows. The data are given
below.
Based on these data, can you estimate whether the lake level is currently changing? In what direction?
The hydrologist also calculates a P budget for the lake based on the inflow and outflow data and discovers that
there is a major imbalance between the P that comes into the lake, and the P that leaves the lake. Calculate this
imbalance (i.e., how much P is entering and how much P is leaving the lake?). What in-lake processes might
explain why the P budget based on inflows and outflows alone is not balanced?
INFLOWS OR SOURCES
Water flow
P concentration
(m3/h)
(µg P/L)
Rainwater
Stream 1
Stream 2
Stream 3
Groundwater
NOTES:
1 m3 = 1000 L
1 mg = 1000 µg
4.5
1.3
3
12.7
1.5
0.6
5
17
8
19
OUTFLOWS OR LOSSES
Water flow
P
(m3/h)
concentration
(µg P/L)
Evaporation
Evapotranspiration
River
Groundwater
5.1
10.0
3.0
1.3
0
0
4
3.5