I.) True/False
Essential nutrients are always important limiting factors to production T, F.
Lakes with high N:P loading ratios are more likely to have their primary production N
limited T, F
Decomposition rate of detritus generally decreases as its C:N ratio increases. T.F.
Photosynthetic efficiency of primary producers (amount of biomass produced / amount of
light energy trapped) is low when nutrients are severely limiting because N, P and other
micronutrients are necessary to convert glucose into proteins, nucleic acids and other
important cellular constituents. T or F.
Trophic classifications of lakes usually recognize three categories, oligotrophic,
mesotrophic and eutrophic because the effects of nutrient loading lead to discrete
threshold productivity responses. T or F.
N and P loading to water usually enhances productivity of aquatic macrophytes because
the nutrients eventually accumulate in the sediment, which is the source from which they
are best able to obtain it. T or F.
Plants that do not harbour symbiotic N-fixing bacteria have no access to the largest pool
of N and are forced to live on recycled N. T or F
Estimating the productivity of a rotifer population amounts to estimating the birth rate of
individuals because individuals are born at full size and all of their growth goes into
forming a new egg. T or F.
The presence of Nitrite (NO2-) or N2O in water is a sign that dentrification is likely
occurring T or F
II.) Multiple Choice
1.) Phosphorus is not an important constituent in which of the following?
a) DNA
b) membranes
c) proteins
d) ATP
e) RNA
3) Which of the following an essential nutrient only for diatoms
a) K
b) N
c) P
d) Fe
e) Si
6.) Macrophytes and benthic algae generally outcompete phytoplankton in littoral regions
of oligo- and mesotrophic lakes and rivers because
a) they can outcompete phytoplankton for light
b) they produce substances that are toxic to phytoplankton
c) they are generally more limited by carbon than by N and P
d) they can access N and P from sediments, which phytoplankton cannot
e) they have very low nutrient requirements
Correlational studies can sometimes provide a more accurate picture of the resource that
is limiting to primary producers than experimental manipulations because
a) experiments are often limited in duration by practical constraints and sometimes
only tell us which nutrient is limiting on the short-term
b) experiments often involve simplified treatments that isolate the primary producers
from consumers which is a situation that doesn’t often occur in nature.
c) While experiments can be conducted on a variety of spatial and temporal scales,
they are usually conducted on smaller spatial scales than real systems operate,
making the results an abstraction
d) Many systems, eg large rivers are too spatially complex for clear-cut experimental
treatments to be carried out.
e) None of the above are true
f) All of the above are true
Schindler’s studies on whole lake systems in the Experimental Lakes area of western
Ontario, were unique in that
a) they were the first attempt to manipulate P, N and C simultaneously
b) they completely controlled for all physical factors that could affect the primary
productivity of the ecosystem
c) they were thoroughly replicated and therefore very statistically conclusive
d) they were the first attempt to manipulate the nutrient loading regime of a lake
e) they were the first attempt to study whole lakes in a controlled experimental
framework
The Lake Washington “experiment” influenced our ideas about the functioning of lake
ecosystems because by diverting sewage from the lake and monitoring the responses
a) it showed that Phosphorus was a key limiting nutrient
b) it showed that sewage input to a lake could grossly change the nature of the
ecosystem
c) it showed that environmental science could have a great impact on social
decisions
d) it showed that nutrient diversion always leads to significant improvements in
water quality of a lake
e) both b and c are true
f) both a and b are true
Which of the following is not a reason why nutrient diversions like the Lake Washington
sewage diversion experiment don’t always lead to significant improvements in water
quality.
a) the lake in question may not be limited by any of the nutrients being diverted
b) the internal nutrient load may be so great that the external load diversion may be a
small fraction of the total load
c) the water may be so turbid from clay or other sediment loading that the light
regime is actually limiting to primary producers
d) the water renewal rate may be so slow (water retention time so long) that it takes
many years for a change in the nutrient loading regime to have a significant effect
on the lakes nutrient concentration
e) the non-point source loadings may be so large that the diversion may constitute a
tiny fraction of the total nutrient load to the lake
f) all of these reasons are reasonable possibilities.
Trophic classifications are usually based on either the total Phosphorus content of the
water or the concentrations of Chlorophyll a because
a) P is the only important limiting nutrient that can determine the productivity of the
primary producers
b) This is a convient approach to classification that uses easily measured parameters.
c) P and chlorophyll a are so well correlated that they always lead to the same
trophic status determination.
d) Lakes that are limited by other nutrients like, say N or Si are rare, are too rare or
unimportant to worry about
e) The effects of other limiting nutrients was not known at the time that the P-chl
approach became popular.
Phosphorus loading models have become very popular in lake management because
a) everybody can afford a computer and its cheaper and safer to model lakes from
your office than to go out and make measurements
b) These models force one to think systematically about how the system works and
tend to focus measurements down productive channels
c) Measurements are more likely to be wrong than model predictions
d) Loading models can predict the responses to planned manipulations of the
landscape or changes in effluent treatments, and predict the likelihood that
planned interventions will lead to water quality improvements
e) b and d are both true.
Assuming that a lake has a P budget that is in equilibrium and follows the equation
P*  I  R  I  R . The total P concentration is 60 mg/m3, the lake has an area
V h  ksed  Qo  Vksed
of 10 km2 and a mean depth of 10 m. The annual P load from external sources is 6000 kg
, the ksed is 0.5/yr, and the water discharge from the lake is 5 x 107 m3/yr. The lake
managers predict that a reduction of the annual external load to 3000 kg will bring about
a decline in total P concentration to 30 mg/m3. There prediction will only be correct if
a)
b)
c)
d)
e)
the lake remains unstratified
the water residence time is 2 years
there is no internal P loading from the sediment
there is no dam or wier on the outflow
the answer could never be correct
Aquatic macrophyte communties more often exhibit a negative rather than a positive
growth response to N and P loading because
a) nutrient loading does not usually add significant quantities of nutrient to the mud,
which is the source that macrophytes utilize
b) N and P loading usually enhances phytoplankton growth which reduces the light
penetration to the sediment
c) Macrophytes are more often limited by dissolved inorganic carbon in water than
by N or P.
d) All of these are true
e) None of these are true
Two species of aquatic macrophytes are growing in water containing dissolved CO2 and
HCO3-. Species 1 grows outgrows Species 2 when the pH is low (<7) whereas when the
pH is high (>8) Species 2 outgrows 1. This most likely indicates that
a) Species 1 utilizes dissolved CO2 more effectively than Species 2
b) Species 2 utilizes bicarbonate more effectively than Species 1
c) Species 2 likley employes carbonic anhydrase in accessing its carbon source
d) Both a and b are true
e) All of the above are likely true
Aquatic ecosystems with hard waters, such as those found in Alberta generally have more
aquatic macrophyte biomass and a greater diversity of species because
a) hard waters will contain a larger quantity of dissolved inorganic carbon which is
the main resource that usually limits their productivity
b) aquatic macrophytes require a great deal of calcium
c) hard waters usually are very clear and have good light penetration to the substrate
d) hard waters generally have more dissolved N and P than soft waters
e) all of these answers are valid.
Methane concentrations are usually very low in surface waters of lakes because
a) methanogens are inhibited by light
b) methane is a gas and tends to bubble out of the lake
c) methanogens require H2 which is only found under reducing conditions
d) methanogens are chemolithotrophs and require an inorganic energy source
e) all of the above are true.
Fats liberate more energy per g, then carbohydrates do because
a) they have less oxygen atoms in their molecules than C atoms
b) the oxidation state of their C atoms is lower than that of sugars
c) each atom of C in a lipid liberates 6 electrons, whereas each C atom in a
carbohydrate liberates only 4
d) a and b are true
e) all of the above are true
Rank the following carbon compounds from most reduced to most oxidized (lowest to
highest oxidation states of C): methane (CH4), Acetic acid (CH3COOH), bicarbonate
(HCO3- ), and ethanol (C2H5OH).
a) methane, bicarbonate, acetic acid and ethanol
b) methane, ethanol, acetic acid and bicarbonate
c) bicarbonate, ethanol, acetic acid and methane
d) methane, acetic acid, ethanol and bicarbonate
e) methane, acetic acid, bicarbonate and ethanol
Thiobacillus, bacteria that obtains energy by oxidizing iron pyrites and other reduced
sulfur compounds to liberate sulphite, sulphate, dissolved iron and other metals and free
protons is considered
a) chemoheterotrophs
b) chemoheterotrophs
c) photoautotrophs
d) chemoautotroph
e) none of these
Halophiles, archaeans that live in highly saline lakes and utilize rhodopsin pigments to
generate ATP from light allowing them to build their biomass from simple organic
compounds are considered.
a) chemoheterotrophs
b) chemoheterotrophs
c) photoautotrophs
d) chemoautotroph
e) a none of these
Pseudomonas, bacteria that obtain their energy by oxidizing organic compounds using
Nitrate as an electron acceptor are considered
a) chemoheterotrophs
b) chemoheterotrophs
c) photoautotrophs
d) chemoautotroph
e) a none of these
The leaves of the aquatic fern Azolla, have sinuses inside which they harbour the
cyanobacteria Anabaena azollae. This relationship is advantageous to the fern because
a) the Anabaena liberates P which the fern requires
b) the Anabaena are photoautotrophs that have heterocyst cells that fix N2 into
ammonia, which leaks into the surrounding water
c) the Anabaena are parasitic and don’t benefit the fern
d) the Anabaena fix N and produce toxic N compounds which inhibit other plants
e) none of these are true
The form of N that can be used by plants with the minimum cost is
a) NH3 or NH4+
b) NO3c) N2
d) free amino acids in the sediment
e) NO2-
Sewage treatment methods aimed at releasing low N sewage back into receiving waters
should try to modify the treatment process to
a) stimulate the growth of Nitrosomonas bacteria
b) try to keep the oxygen level within the range most conducive to Nitrate reducing
bacteria
c) stimulate the growth of N-fixing cyanobacteria
d) stimulate the growth of ammonifying bacteria in the treatment
e) none of these
Lakes with high N:P ratios in their nutrient loads are
a) likely to have their algal communities become P-limited rather than N-limited
b) unlikely to have their algal communities dominated by Aphanizomenon or
Anabaena
c) likely to have their algal communities dominated by toxic cyanobacteria
d) likely to have high dentrification rates
e) all of these are true
Lakes with high human populations in their watersheds are likely to
a) be very eutrophic
b) have high dentrification rates
c) have high ammonia levels in their deeper waters
d) high nitrate levels
e) all of these are true
Mine tailings exposed to air are likely to make the surface waters very
a) anoxic
b) metal rich
c) sulfate rich
d) acidic
e) a and b are true
f) b, c and d are all true
The density of a rotifer population is 600/L and two-thirds are carrying eggs. On average
it takes 4 days for an egg to hatch. The birth rate of the population is
a) 50 per day per L
b) 100 per day per L
c) 1/6 per individual per day
d) 500 per day per L
e) Both b and c are true
If the above Rotifer population is growing at the rate of 1/10 per day per individual what
is the per captita death rate
a) 0.067 /d
b) 0.67/d
c) 0.1/d
d) 0.01/d
e) None of the above
In a pike population the weight at the end of year 2 is 200 g, and the weight at the end of
year 1 is 50 g. The SGR for 1 year old is
a) 1.386 g/g/yr
b) 0.138 g/g/yr
c) 4 g/g/yr
d) Ln 4 g/g/yr
e) Both a and b are true
In a pike population the weight at the end of year 2 is 200 g, and the weight at the end of
year 1 is 50 g. If the biomass of pike in the one-year old age group is 10 kg/ha, the
productivity of this age class is
f) 13.86 kg/yr
g) 1.386 kg/yr
h) 40 kg /year
i) (Ln 4 * 10) kg /yr
j) Both a and b are true

If the productivity of a phytoplankton population is 6000 k J (kilo Joules) /yr per m2, If
sedimentation rate of dead cells to the substrate constitutes 2000 kJ/m2yr, and the
phytoplankton population is dB/dt=0. If the rain of zooplankton fecal pellets to the
bottom is 2400 kJ/m2/yr. What is the assimilation efficiency of the zooplankton trophic
level (assume that they are all feeding on phytoplankton).
a)
b)
c)
d)
e)
0.40 or 40%
0.60 or 60%
0.35 or 35%
0.25 or 25%
None of these
What is the exploitation efficiency EE (or Consumption efficiency CE) of the
zooplankton trophic level
a) 0.42 or 42%
b) 0.67 or 67%
c) 0.35 or 35%
d) 0.25 or 25%
e) None of these
If the net production efficiency of the zooplankton trophic level is 0.40 (40%) what is the
ecological efficiency () of the trophic level
a) 0.15 or 15%
b) 0.05 or 5%
c) 0.11 or 11%
d) 0.25 or 25%
e) None of these
If the zooplanktivorous fish are consuming zooplankton at the rate of 500 kJ/yr, their EE
(CE) is
a)
b)
c)
d)
e)
0.40 or 40%
0.78 or 78%
1.00 or 100%
0.25 or 25%
None of these
If the zooplanktivorous fish have an assimilation efficiency of 0.70 (70%) and Net
production efficiency (NPE) of 0.20 (20%), the productivity at this trophic level is
a) 90 kJ/yr
b)
c)
d)
e)
70 kJ/yr
100 kJ/yr
280 kJ/yr
None of these
Explain
Explain why a population can have 2o productivity >0 even though its population is not increasing.
Explain how secondary productivity is measured in long-live organisms like fish which have age-structured
populations.
Nutrient addition experiments in stream communities often failed to demonstrate nutrient limitation
Explain the differences that one expects to see in the vertical oxygen gradient in a eutrophic and an
oligotrophic lake. Assume that the lake is deep enough to stratify and form a stable hypolimnion during
the summer.
SGR for t year olds is calculated as Wt+1 = Wt * eSGR. Show that SGR =lnWt+1-lnWt .
Essay Questions
1.)What are the nutrients that most commonly limit primary production in aquatic ecosystems? Explain the
biological basis for the requirement for these nutrients, and explain how nutrient limitation in aquatic
ecosystems is demonstrated.
2.)Outline and explain the use of a single compartment mass balance model for phosphorus in managing lake
eutrophication. Show how the mass balance equations can be derived and used to predict the responses of
lakes to reductions in loading.
3.)Explain why aquatic macrophytes generally do not respond to phosphorus or nitrogen enrichment of either
water or sediment, but hard water lakes usually have higher biomass and diversity of plants than soft water
lakes. Which element does this suggest is actually limiting their primary production in these situations?
Explain.
4.)Outline and explain three reasons why reducing the external phosphorus load to lakes does not always
result in a reduction of phytoplankton biomass and improvements in water clarity.
5.) Outline and explain the use of a single compartment mass balance model for phosphorus in managing lake
eutrophication. Show how the mass balance equations can be used to predict the responses of lakes to
reductions in loading.
6.) The correlational approach to studying lake nutrient relationships, is an example of a scientific approach
commonly used in environmental science often referred to as the comparative method. The approach has
many defenders such as J.Kalff who wrote your textbook
Which of the following ideas do you think he would likely agree with, and which would he disagree with.
Defend your answer with examples in a 200 word essay.
a)While the approach has drawbacks in that it is often difficult to interpret patterns mechanistically, it can
lead to broader appreciation of patterns in nature.
b) Correlations cannot demonstrate causation so the patterns observed must be interpreted with caution
c) With the application of sophisticated statistical methods, and data on a wide variety of systems,
sophisticated empirically based sciences can be developed, even though mechanistic understanding is
limited.
e) Since the method involves the development of correlations and patterns derived from cross-system
comparisons, that can be used predictively (eg the P-chl relationship) there is no need to have a conceptual
understanding of the processes involved.
Practice Questions MidT II—2nd set
Essay questions
1.) During the 1960’s Lake Michigan underwent a dramatic reduction in transparency following the
collapse of the salmonid populations (mainly lake trout). This lead to a heated debate between those
that favoured a bottom-up explanation and those that favoured a top-down one. Briefly outline both
of these arguments and the evidence that they were based on.
2.) Fish yields from natural aquatic ecosystems tend to be from 5-50 kg/ha, but yields from
aquaculture can be several orders of magnitude higher depending on latitude and the degree of
human intervention. Describe the ecological limitations that keep the yield from natural systems
low, and the types of human interventions that are used to overcome these ecological limitations in
more intensive aquaculture operations.
3.) Explain why the presence of unchecked predators can shift the composition of prey communities.
Outline and give examples of the types of changes in community composition or behaviour that fish
predators can cause.
4.) Brooks and Dodson demonstrated that alewife, which had recently invaded coastal lakes, had
considerable impact on the size-structure of zooplankton communities. Explain how this affect
occurs, and explain the size-efficiency hypothesis. Besides the size structures, outline other effects
on zooplankton communities that can result from zooplanktivorous fish.
5.)The zebra mussel (Dreissena) accidently gained access to the Great Lakes in the 1980’s and has
rapidly transformed the nature of aquatic ecosystems in eastern North America. How did this animal
invade the system? What are the adaptations that allowed it to impact the foodwebs of lakes in
eastern North America? What do we need to do to prevent the invasion and spread by such
organisms?
6.) Outline the changes that take place along a river system from shaded first order
streams, down through the midreaches, and on down to the lower reaches of a river
system, in terms of the physical environment, the primary producer community and the
invertebrate and fish communities, and explain the mechanisms behind the patterns that
you have described.
Interpret the following graph
Explain
Explain why periphyton or aquatic macrophytes tend to be more abundant in the midreaches of a river than in headwater streams or the lower reaches. Assume that the
watershed is a forested one.
Explain why aquatic plant detritus tends to decompose more rapidly that terrestrial plant
detritus.
Explain why detritus tends to break down much more rapidly in litter bags that are open
to animals than in litter bags that are not, even though the animals’ contribution to total
respiration is quite low.
Briefly outline an experiment that can allow you to test whether or not the size of the
animals that have access to a sample of detritus, will affect the rate of decay.
Explain why upstream reaches of a stream tend to have more macrodetritivores where as
lower reaches tend to have more collectors (microdetritivores).
Explain why macrodetritivores tend to predominate in forested headwater stream,
whereas microdetritivores tend to dominate in lower reaches.
Zooplankton species from lakes with zooplanktivorous fishes are usually very small and
rather transparent compared to the larger and more darkly pigmented forms that
predominate in fishless lakes. Explain
Technologically efficient gear has contributed to undermining the effectiveness of
fisheries management
Hybrids collected at higher elevations tend to have fewer rainbow trout alleles than those
collected lower down.
Introduced rainbow trout tend to select lower elevation habitats that are more productive,
and are able to outcompete native cutthroat trout from such reaches, however rainbow
trout alleles are not confined to these lower reaches.
Multiple Choice
While the dry matter content of detritus tends to decrease over time, the protein content
usually increases to a maximum before declining. This reason for this pattern is that
a) proteins are more difficult for animal detritivores to attack
b) microbes (bacteria and fungi) have a higher protein content than the un
conditioned plant matter and become an increasingly large fraction of the detrital
mass during the decay process, but then tend to be selectively eaten by animal
detritivores.
c) carbohydrate and lipid material are converted into protein during the decay
process, which are then selectively eaten by detritivorous animals.
d) Both a and b are correct
e) Both b and c are correct
Detritivorous animals stimulate the breakdown of detritus even though their contribution
to total respiration is low. The reason for this is
a) breakdown of most detritus is anaerobic and is carried by methanogens
b) animals have much lower metabolic rates than microbes like bacteria and fungi
c) the animals tend to break down intact detritus into small particles which are in
turn more readily decomposed by bacteria and fungi.
d) Animal metabolism raises the temperature which allows the bacteria and fungi to
decay detritus more rapidly
e) both a and b are correct
dB
 rB  (b  m) B , describes the rate of change of B (biomass of a
dt
population) with time, as a function of birth/growth (b) and mortality (m) processes. This
equation describes what type of growth process
a) exponential growth or decay
b) sigmoidal growth process where B approaches a carrying capacity
c) geometric growth
d) where the per capita birth and death rates are independent of the population
biomass
e) both a and d are correct
The equation
dB
 rB  (b  m) B and (2) Bt  B0 t , where   1  r 
dt
(B is biomass and b and m represent per capita birth/growth and death respectively with respect
to time t) differ in that
a) equation (1) describes a population where per capita rates of change are
independent of B whereas in (2) they are dependent on B. sigmoidal growth
process where B approaches a carrying capacity
b) equation (1) involves a carrying capacity whereas (2) does not.
c) Equation (1) describes a population that changes continually in time whereas in
(2) the changes occur in discrete jumps
d) both a and d are correct
e) none of the above are true.
The two equations (1)
In a population where per capita birth rate decreases and per capital death rate increases
with increasing B, there will be a value of B where the two lines intersect. When B
exceeds this value
a) the population has reached the inflection point (i)
b) The population has reached r0
c) the population has reached its maximum sustainable yield (MSY)
d) the population will decline
e) per capita death rate will exceed per capita birth
f) both d and e are true.
dB
 B
 r 0 B 1   , where
dt
 K
r0=the intrinsic rate of increase, and K is its carrying capacity. If such a population is at
its carrying capacity and exploited at the rate C. Its B will
a) remain constant
b) change at the rate r0
r0K
c) approach
4
d) approach K/2
e) decline
f) increase
For a population whose biomass (B) follows the growth curve
Explain why attempting to maintain a constant yield near the MSY is an extremely risky
management approach.
Sunfish have a deep-bodied (“panfish”) form that has been interpreted as an adaptive
response for defense against gape-limited predators. The cost of this adaptation is likely
a) reduced swimming speed is as a result of increased intertial drag
b) the large dorsal and anal spines, and their supporting skeletal elements, are likely
very energetically costly
c) the body is likely not flexible enough to generate much muscular propulsive
force.
d) Both a and b are likely true
e) a, b, and c are all likely true.
Bythotrephes are a large carnivorous zooplankter that have recently invaded North
America. Zooplanktivorous fish at first eat it, but the large tail spine, which can puncture
the stomach, or at least fill it up with non-digestible spines that tend to stay there, create
large problems. Some studies show that fish do “learn” to avoid eating them. This is
actually a very ecologically costly solution because
a) Bythotrephes is a competitor with these fish, and not eating them just strengthens
their competitive position
b) not eating them just allows them to become more abundant
c) fish have to expend time and effort to distinguish Bythotrephes from palatable
zooplankton
d) a, b, and c are all likely to be true
e) not eating them seems a reasonably efficient solution.
While zooplankton evolve vertical migration patterns as a defensive response against
predators, these responses are costly because
a) it requires a lot of effort for a tiny animal to move up and down several 10’s of
meters every day
b) moving down during the day, can put them into colder water with much less
phytoplankton for them to feed on
c) being able to feed in the more productive surface waters only during the night,
will sharply reduce their productivity
d) a, b, and c are all likely to be true
e) none of these are likely true.