Don’t forget your name
Aquatic Biology Exam 2 2002 Most answers require brief responses: be concise but coherent. Point values for
each question are in ( ) next to the question number. Note that longer questions are scattered throughout.
circulation
Phytoplankton biomass
6000
SNOW
ICE
stratification
circulation
SNOW
ICE
3
4
4000
Cyanobacteria(blue-green)
7
Chlorophyceae (green)
Bacillariophyceae (diatoms)
Dynoflagellates
6
Chrysophceae & Cryptophyceae
2
2000
5
1
J
8
F
M
A
M
J
J
A
S
O
N
D
Month of the year
1. (10 total) Above is a simplified seasonal pattern of phytoplankton biomass for a eutrophic, dimictic lake. Note that
physical conditions in the lake are indicated across the top of the figure. Numbers above portions of the graph refer to
different periods during the year (1-8).
a) (3) Diatoms comprise a dominant proportion of the phytoplankton biomass during time periods 3 and 7. What
physical condition of the lake corresponds to diatom abundance? Give two reasons why diatoms only are abundant
in the water column during these periods.
diatoms (Bacillariophyta) are abundant during mixing periods. However, they are heavy because they have
heavy frustules and when summer stratification occurs, they quickly sink out of the epilimnion. Silica (Si) (a
potentially limiting nutrient for diatoms) is also present during mixing periods.
b) (3) Time period 5 corresponds to the Clearwater Phase in the lake. Why is there such a low biomass of
phytoplankton during this time?
Green algae is the most edible algae for zooplankton herbivores, so, although production may be high,
biomass is low as zooplankton eat the green algae.
c) (4) During time period 6, Cyanobacteria are the dominant phytoplankton. Name two characteristics of
Cyanobacteria that allow them to dominate during this time of the year. Explain why those characteristics are useful.
gas vacuoles - flotation
N-fixation in heterocysts - out-compete other species when N is limiting
large - less likely to be eaten by filterers
allelopathy - inhibitory or toxic effect on other species
resting stages - allow repopulation when conditions improve
1
Contribution to primary productivity
2. (4) What is the compensation point? Why do phytoplankton maximize time spent above the compensation point?
Name two methods phytoplankton employ to maximize time spent above the compensation point.
compensation point: depth at which photosynthesis = respiration (controlled by light availability)
Carbon is lost during respiration and so growth does not occur. Carbon is gained during photosynthesis.
phytoplankton maximize time above the compensation point by: reducing sinking rates (small size,
projections, density), being mobile (gas vacuoles or flagella)
phytoplankton
algae
on submersed
emergent or
floating
algae
on sediments
algae
on emergents
submersed
Increasing nutrient loading
3. (6 total) Above is a generalization of the contributions of different primary producers to lake productivity across a
gradient of nutrient loading.
a) (2) What nutrient generally limits growth of these primary producers in lakes?
P (phosphorous)
b) (2) What mechanism is responsible for the loss of submersed macrophytes half way along the nutrient axis?
submersed macrophytes are lost in systems with high nutrient loadings because they are
shaded by phytoplankton (light limitation).
c) (2) Why do emergent or floating macrophytes contribute most to primary productivity at high nutrient loadings?
Emergent or floating macrophytes are not impacted by turbid water (shading) because they are
above or on the surface of the water. They can use the additional nutrients to be more productive.
4. (2) What is the condition of heterophylly in macrophytes? Why does it occur?
Heterophylly- condition in which the submersed leaves are one shape and the floating or emergent
leaves are another shape.
Submersed leaves are thin or highly dissected to increase absorption of CO2; emergent or floating
leaves have a waxy cuticle to reduce evaporation (etc) and resemble terrestrial plant leaves.
5. (4) Contrast autotrophic vs. heterotrophic bacteria
autotrophs: obtain cellular carbon by reducing inorganic carbon (CO2, through photosynthesis) (i.e. make
carbon from scratch)
heterotrophs: obtain carbon by reducing organic substances (i.e. bacteria decompose organic matter and in the
process get carbon from low-molecular weight organic compounds).
2
Turbidity
without vegetation
with vegetation
6. (8 total) The figure to the left shows that, for
a given nutrient level, shallow lakes can be in
either a turbid, phytoplankton-dominated
state, or a clear, macrophyte-dominated state.
Nutrients
6a) (2) What is the critical turbidity level indicated by the dashed horizontal line in the figure?
The turbidity level above which rooted macrophytes will not grow.
6b) (4) What mechanisms maintain these two alternative stable states (name two mechanisms per state)?
Turbid state is maintained because there are no aquatic plants to cause sedimentation of particles (or
prevent resuspension of particles), there are few herbivorous zooplankton present because there are no
aquatic plants to take refuge in (and so there is a lot of phytoplankton), and because the water is turbid,
rooted plants and benthic algae, which normally compete for nutrients with phytoplankton, cannot grow.
Alternatively, in the presence of aquatic plants in the clear state, wave action is controlled, sedimentation of
suspended particles occurs. Rooted macrophytes and bentic algae compete with phytoplankton for
nutrients, and macrophytes serve as refuge for herbivorous zooplankton. (some combination of these
reasons)
6c) (2) What steps might a lake manager take to "flip" a lake from the turbid state to the more desirable clear-water
state?
We discussed draining the lake to lower the water level so that rooted plants would get enough light to
start growing and manipulating the fish communities (getting rid of carp, for instance). Reducing nutrients
works, to a point, but its very difficult to reduce nutrients low enough that a turbid state is no longer an
alternative.
7. (4) Researchers have observed, across many different lakes, a pattern of higher concentrations of bacterioplankton in
lakes with higher concentrations of phytoplankton (measured as chlorophyll-a). What is the basis for this pattern (i.e.,
how do phytoplankton and bacteria interact?)?
Phytoplankton are decomposed by bacteria who incorporate phytoplankton carbon into their cells. In
the process, bacteria release nutrients that are used by the phytoplankton for growth.
3
8. (4) What is DOC? What part of DOC is labile (easily utilized by bacteria)?
DOC is dissolved organic carbon; that is, organic carbon that is small enough to fit through a standard filter.
DOC consists of low molecular weight molecules (sugars, etc) and high molecular weight molecules (ex).
Bacteria are able to easily utilize only the low molecular weight molecules.
9. (6 total) Currently the microbial foodweb and the "macro" foodweb are thought to be functionally separate.
a) (1) What is the source of carbon in the macro foodweb?
Inorganic carbon (CO2) fixed by photosynthesis (plants/algae)
b) (1) what is the source of carbon in the microbial foodweb?
Organic matter/carbon (DOM, POM, DOC, POC)
c) (2) Why is microbial carbon not utilized in the macro foodweb?
This is essentially an issue of size: bacteria are so small that their carbon is almost entirely respired by the
time you are only a few trophic levels away ( for example: bacteria  protozoa  dinoflagellates 
copepods  large predaceous zooplankton  small fish  big fish). Also, Bacteria respire 80-90 % of the
carbon they take up.
d) (2) What is the function of the microbial foodweb for organisms in the macro foodweb?
Right now scientists feel that the microbial foodweb influences the macro foodwebs primarily through the
recycling of nutrients, not fixing of carbon.
10. (4) Define internal loading of phosphorus. Why is internal loading of phosphorus a concern for lake managers?
Internal loading of P is the recycling of P from the sediments into the water column. It happens extensively in
eutrophic lakes with anoxic hypolimnia, which are usually the lakes that managers are trying to remediate. In
this case, even if lake managers are able to reduce or stop the input of P into a eutrophic lake, the internal
cycling of P ensures that plentiful P will be present for many years to come (10’s to 100’s of years). (many
people also talked about short-term algae blooms, etc)
11. (4). Fill in the two inorganic carbon species
are missing from the graph to the left.
1. H2CO3 or CO2(dissolved)
that
2. HCO3-
12. (6) Why is the carbonate buffering system critical for aquatic organisms? How does it work?
The carbonate buffering system helps regulate (buffer large changes in pH) the pH of water by absorbing H+
ions (to prevent acidification) or disassociating (to prevent high pH). As measured in lab by ANC titrations.
4
13. (6) What were three consequences of experimental acidification of Little Rock Lake?
Changes in zooplankton biomass and species composition
Increase in the concentration of toxic substances such as methyl-mercury, Al, etc.
Increase in the elephant snot algae
Increase in the depth of light penetration (water clarity)
14. (4) Why are the eastern portions of the US and Canada more prone to acidification than the western US and
Canada?
Bedrock type, west to east air flow (bringing with it SO4 and NOx), higher human population overall
15. (8) Explain the processes (mechanisms) by which soluble phosphate moves from the anoxic sediments to the water
column in a eutrophic lake in the summer. Consider both sediments in the littoral zone (overlaid with oxic water) and
sediments in the hypolimnetic areas of the lake. I am looking for 2 processes for each sediment type.
There are at least two answers for the hypolimnion and epilimnion. In the hypolimnion, (1)
inorganic ferrous phosphate is soluble in an anoxic hypolimnion, (2) the organic decay of dead algae
and bacteria releases soluble P.
In the epilimnion, (1) macrophytes transfer nutrients, including P, from the sediments to the
water column when the macrophytes senesce, and (2) mixing (bioturbation) or bubbling of gasses
brings soluble P up from anoxic sediments.
16. (2) Name two factors that affect the solubility of oxygen in water.
Temperature, air pressure, salinity
17. (6). What happens to the concentration of dissolved oxygen in the hypolimnion of an eutrophic lake over the course
of the summer? Explain why.
Over the course of the summer, the amount of dissolved oxygen in the hypolimnion of an
eutrophic lake drops to zero through the biological uptake (e.g. respiration or decomposition or the use
of O2 by organisms). Because the hypolimnion does not mix or contact the atmosphere, very little
dissolved oxygen reaches the hypolimnion to replenish the dissolved oxygen concentrations (i.e.
diffusion of dissolved oxygen across the metalimnion is very slow). Because it is a eutrophic lake, we
assumed that photosynthesis is not occurring in the hypolimnion and so again, oxygen is not
replenished.
18. (4) In order to measure the biological uptake of phosphorus in a section of Boulder Creek, WI, Cailin Orr (our guest
speaker on 9 Oct) said that she was going to add both phosphate and a conservative ion tracer (salt, NaCl) to the water.
She was then going to measure the concentration of phosphate and salt in the water downstream of the addition point.
Why did Cailin proposed adding both phosphate AND salt? What is meant by "conservative ion tracer"? Why did she
need to add the tracer in addition to the phosphate?
The conservative ion tracer is not taken up by organisms, so should arrive at the bottom of the reach in the sme
concentration that Cailin added it in. If it is more dilute, that means there is groundwater input into the stream
reach, and Cailin needs to take the diluting effect of groundwater into account when calculating the amount of
phosphate used by organisms (or otherwise taken up in the stream reach). (if the salts become more
concentrated, that might indicate that there is groundwater recharge occurring in the stream).
5
19. (4 total) Based on the information given in
Cailin Orr's lecture about nutrient cycling in streams
and the figure and equation to the right, answer the
following questions:
a) (2) Does uptake length (Sw) lengthen or shorten
as Spring Creek becomes the Lyman Lakes?
shorten
b) Would you expect the Lyman lakes to retain more or less nutrients than Spring creek? What assumptions (i.e.
regarding the equation for Sw) do you have to make to decide?
Lyman lake should retain MORE nutrients
Uptake rates (U) remain similar (I gave credit for almost any assumption)
20. (8 total) Your assistant has unearthed some old data sheets missing some crucial information, like the lake name,
and the time of year that the samples were taken. Because your research group is running long-term experiments on
several lakes of differing trophic status in the area, this information could be potentially very valuable. The only data on
the sheets are values for SRP (very low concentrations), Total P (very high concentrations), and the depth at which the
samples were taken (2 meters).
a) (2) What is SRP and what does it measure? (we used this method in lab)
Soluble reactive phosphorus: measures phosphate in the water, plus a little organic phosphorus that gets
broken down.
b) (2) What does Total P measure?
The total amount of P in the water: organic P (like what is in zooplankton) and phosphate
c) (2) In what type of lake (i.e., trophic status) do these samples likely come from?
Eutrophic because there is lots of total P
d) (2) What time of year were the samples probably collected? Why?
The summer, because more inorganic phosphorus (Phosphate) is incorporated into organisms (the amount
of free phosphate is very low (SRP)).
6