Note: Read questions from top to bottom – if a question is repeated a second time (or more) I will not have
checked over the answer as thoroughly as earlier versions of the question! Corrections or additions are in red
type. Another disclaimer: although I have looked over these questions and corrected any glaring errors, the
answers contained are not necessarily the very best answers possible. If you come up with other answers, they
could be right too! ~ Karen
Possible Aquatic Biology Final Exam Questions
1. Define a wetland and describe two of the major types of wetland.
According to the US Fish and Wildlife Service, wetlands are transitional zones between terrestrial and aquatic
systems where the water table is at or near the surface or the land is covered by shallow water. Wetland must
support predominately hydophytes, contain substrate predominated by hydric soil, and/or contain substrate that
is nonsoil and saturated or covered with shallow water for part of the growing season each year. Two of the
major types of wetland are bogs and swamps. Bogs display no significant outflow or inflow, accumulate
partially decomposed plant material known as peat, contain acidic water, and are dominated by Sphagnum
Moss. Swamps are wetlands dominated by water tolerant trees or shrubs such as cypress.
2. Discuss the distribution of nitrogen species in eutrophic and oligotrophic lakes and how it relates to the
nitrogen cycle.
In eutrophic lakes the concentration of NO3- is high in the epilimnion due to high rates of external loading
and high rates of nitrification (NH4 + 02  NO3- ). The concentration of NH4 is low in eutrophic epilimnions
due to high rates of nitrification and high biological uptake (assimulation). In the hypolimnion of eutrophic
lakes the concentration of NO3- is low due to high rates of denitrification which occurs under anoxic conditions
(NO3-  N2 (g) ). Nitrification does not occur due to absence of oxygen. In addition, biological demand is low.
Therefore, concentration of NH4 is high. High rates of ammonification, in which organic matter is converted to
NH4, also contributes to high NH4 concentrations.
In the epilimnion of oligotrophic lakes the concentration of both NO3- and NH4 are low. External loading of
nitrogen is relatively low and rates of nitrification are high. Thus, organisms quickly deplete the small NH4
supply through assimulation. NO3- is then exploited as a nitrogen source through assimulative nitrate
reductions. This reduces the epilimneic NO3- concentration. The hypolimnion of oligotrophic lakes display
high concentrations of NO3- . Due to the presence of oxygen in the hypolimnion nitrification occurs. Low
biological demand prevents depletion of NO3- through assimulative nitrate reductions. In addition,
denitrification does not occur since conditions are oxic. NH4 concentrations remain low in oligotrophic
hypolimnions since rates of ammonification are not great and nitrification can still occur.
3. What determines the sensitivity of a lake to acidification? Explain.
Many factors affect the vulnerability of a lake to acidification. The ability of the soils and bedrock in a lake’s
watershed to neutralize incoming acid determines how much acid reaches lake waters. Carbonate bedrock has
greater acid neutralizing capacity than igneous rock. Thus, in watersheds dominated by carbonate rock much of
the acid in runoff may be neutralized before it reaches lakes. The type of bedrock in a lake’s watershed also
determines the acid neutralizing capacity of lake waters. Lakes in watersheds where carbonate rock is common
contain high concentrations of carbonate, and thus have high acid neutralizing capacity. The acid neutralizing
capacity of lakes in regions where igneous rock predominates have much lower buffering capacity, and are thus
more vulnerable to acidification. Landscape position also affects the acid neutralizing capacity of lakes. In
general, lakes lower in drainage systems accumulate more ions, and thus have higher buffering capacity. Thus,
lakes at high elevations are particularly sensitive to acidification. Input of organic acids into lakes can also
effect lake vulnerability to acidification. If a lake naturally receives high acid input from adjacent bogs, extra
inputs of acid could completely deplete its buffering capacity. Finally, neutralizing processes within lakes
affect the response of lake pH to acid input. Oxidations and reductions mediated by bacteria can neutralize
acids.
4. What is the dead zone? What process produces it?
The dead zone occurs on the bottom of lakes around the depth of the thermocline. No benthic life persists in the
dead zone. The dead zone exists due to thermal seiches. During a thermocline seiche the thermocline tilts from
side to side of a lake. On the side of the lake that the thermocline tilts up, hypolimnetic water is abnormally
high. On the other side of the lake epilimnetic water is abnormally low. As the thermocline tilts back and forth
organisms on the adjacent lake bottom would experience changes in temperature as cold hypolimnetic water
and warm epilimnetic water alternately passed over them. Fluctuations in oxygen concentration might also be
encountered, since the hypolimnion of eutrophic lakes is generally anoxic. Benthic organisms are adapted to
specific oxygen and temperature conditions. Thus, benthic life cannot survive in the dead zone where
thermocline seiches cause frequent changes in temperature and dissolved oxygen concentration.
1. Name some adaptations that macrophytes have made to living in aquatic environments
Macrophytes have buoyant stems and leaves to take advantage of the water as a structural support. They also
have the ability to change height and form depending on the depth of the water. For instance, they can be short
and bushy in shallow waters and long and sparse in deep waters. Macrophytes’ submersed leaves have no
cuticle, maximizing CO2 uptake across the entire leaf surface. Finally heterophylly, the differing of leaf shape
depending on whether the leaf is underwater or above the surface, facilitates gas uptake underwater where most
leaves are long and thin, big and broad, or highly dissected.
2. Describe three predator avoidance tactics employed by zooplankton.
Zooplankton exhibit cyclomorphosis, or changes in shape in response to seasonal cues or predators. Examples
are spines or other projections and color changes. They also display diel migrations, moving horizontally and
vertically to minimize predation. Finally, many zooplankton have resting stages in response to heavy stresses or
predation.
3. Describe three classification systems used to organize fish.
Fish are classified by temperature into warm water (lethal upper temperature greater than 26°C) and cold water
(rarely above 24°C). They are also classified by food web role into piscivores, planktivores, and omnivores.
Finally, they can be classified by mouth size into large (piscivorous), small (planktivorous-sight feeder),
upturned (surface forager), or down-turned (bottom feeder). Also acceptable: body shape, taxonomy.
4. Explain the telescoping ecosystem model.
Nutrients loads in streams cannot be explained on the basis of the water alone. Spiral lengths change with
disturbances due to interaction between the stream, hyporheic zone, parafluvial zone, and riparian zone.
1.
Describe 2 paths the nitrogen molecule of ammonia could take to become organic in the epilimnion of
an eutrophic lake.
a. The ammonia could go through nitrification in the oxic epilimnium to become nitrate (NO3),
diffuse into the anoxic hypolimnium, undergo denitrification and bubble to the surface as N2.
There it might be sequestered by the heterocyst of an Alder tree and get fixed into organic
nitrogen.
b. The ammonia might also diffuse immediately into the anoxic hypolimnion, and be assimilated
directly into organic nitrogen by plants (or be assimilated by plants in the epilimnion).
2. Give examples of adaptations that help phytoplankton maintain their critical vertical position.
a. Vacules- bouyant gas structures that help algae float –blue green algae (cyanobacteria)
b. Shape and density – Diatoms
c. Flagella – Dinoflagelate
d. (Clinging – periphyton – periphyton are technically not phytoplankton, although you are right,
being attached to something can keep you in the light)
3. Why might the river continuum concept describe a river running through a deforested watershed poorly?
How might macroinvertibrates (shredders, grazers, collectors, predators) change with the length of this
strange river?
a. The rcc assumes that tree cover and leaf fall will mediate light availability and nutrient input in
low-order streams. Without tree cover there would be full light availability along the entire
length of the river allowing high primary production throughout. Upon the periphyton grazers
would thrive and predators might begin to dominate, downstream. Shredders and collecters
would sadly have little to shred or collect.
4. How do reservoirs differ from lakes?
a. High sedimentation rates (this is true in lakes too, but rivers entering reservoirs usually carry
more sediments than sources entering natural lakes. This statement is good relative to rivers…)
b. Water level change
c. Currents/density flows
d. Low water retention rates
Question 1 (1+2+1+1 points). Would you expect there to be a high or low O2 level in the hypolimnion of an
oligotrophic dimictic lake during summer stratification? Describe two factors that may have influenced this O2
level. Do you expect there to be a high or low concentration of Phosphorus in the hypolimnion water column?
Describe one specific mechanism that can cause this kind of P concentration in the water column.
Answer 1. I expect there to be a high level of oxygen in the hypolimnion because the low decomposition in this
oligotrophic lake has allowed oxygen to remain available in the water. Also, more oxygen can dissolve in the
cold water of the hypolimnion than in warm water. I expect a low concentration of P in the water column of the
hypolimnion. P can be accumulating in the sediments as insoluble FePO4 precipitates.
Question 2 (2+1+2+1 points). What geologic and hydrologic factors affect the amount of suspended solids
(wash load) in a river or stream? How can wetlands influence these factors? What biological factors can affect
the amount of course particulate organic matter (CPOM) and fine particulate organic matter (FPOM) in a river
or stream? What happens to the CPOM and FPOM at dams?
Answer 2. The form/size of substrates (can be large rocks, sand, etc.) and the velocity of the water can affect
the wash load in a stream/river. Small substrates and high velocity advocate a high wash load. Wetlands can
slow the stream velocity by providing a large area for water flow and by providing aquatic plants that create a
resistance to water flow. The amount of CPOM in a stream/river depends largely on the amount of leaf litter
that falls into the water from trees above. The amount of FPOM in the water depends on how actively
“shredder” invertebrates are eating CPOM upstream (this “shredding” yields FPOM). At dams, almost all
FPOM is deposited and most allocthonous CPOM settles out or is processed.
Question 3 (3+2+1+2+1 points). Describe three aspects of a natural watershed that can influence the amount
or quality of runoff entering a water body. What are two ways that anthropogenic development in a watershed
affects the surface runoff? How does it influence sub-surface runoff? Besides runoff, what are two possible
external drivers that determine the dynamics of a lake? If two adjacent lakes are highly coherent, do internal or
external drivers likely determine most of the lakes’ dynamics?
Answer 3. Vegetation can intercept precipitation, decreasing runoff. The type of soil can influence infiltration,
the soaking of water into the soil, which decreases runoff into a water body. The nutrient levels in soil
influences the amount of nutrients that runoff brings into a water body. Also, the slope of the watershed
influences the velocity of runoff. Anthropogenic development in a watershed usually increases the flow of
runoff and the amount of pollution entering a water body. This development also impedes much sub-surface
runoff and deep ground infiltration because the area of impervious surfaces is high.
Two other external drivers are the invasion of some biological species and the activity of humans in the water
(recreation). The dynamics of the two coherent lakes are likely determined by external drivers because they
would have similar external conditions.
Question 4 (8+2+1 points). Discuss the patterns of stratification and mixing in a dimictic lake in the winter,
spring, summer, and fall. Be sure to mention the role of the thermocline and wind. What are surface and
thermocline seiches and when do they occur? How do thermocline seiches affect benthic biota living around
the thermocline?
Answer 4. In the winter, a weak thermocline develops between warmer denser water (4˚C) at the bottom and
cooler lighter water (<4˚C) at the surface. The thermocline is not broken by currents from wind action because
the water is covered by ice. In the spring, ice melts because of increased solar energy. With the exposure of
liquid to wind, the lake begins to mix. Sometime between spring and summer, the solar energy penetrating the
lake increases the lake’s surface temperature enough to create a stratification of water by temperature that
resists mixing by the wind. This stratification, with a strong difference in epilimnion and hypolimnion
temperatures and densities, remains throughout the summer. In the fall, less solar input and continued cooling
via evaporation lessens the difference in temperature between the stratification layers. Along with the lessened
temperature difference comes a lessened density difference. Ultimately, the wind force is strong enough to
break the thermocline and cause the lake to mix.
A surface seiche is the piling of epilimnetic water on one end of a lake, and a thermocline seiche is the tilting of
the thermocline due to a surface seiche. These seiches occur when strong winds are sustained over a lake.
Thermocline seiches can expose benthic organisms that need oxygen to oxygen-depleted waters (causing them
to die).
1. How could the addition of a piscivorous fish to a turbid lake whose (simple, top-down) food web had
previously been dominated by zooplanktivorous fish affect macrophytes in the lake?
Answer: The addition of a level to the food web would reduce zooplanktivorous fish populations, which would
increase zooplankton populations, and decrease phytoplankton biomass. This reduction would decrease
turbidity in the lake. If this reduction were substantial enough, a phytoplankton-dominated lake could switch to
a macrophyte-dominated lake, as positive-feedback loops kick in to maintain low turbidity. This occurs only if
the turbidity decreases enough to allow macrophyte growth.
2. Describe how wetland flooding can remove nitrate from a river system.
Answer: Flooded wetlands give an opportunity for denitrification processes to take place. In this process,
nitrate is converted into nitrogen gas, which is released into the atmosphere. Denitrification requires anoxic
conditions. Flooded wetlands associated with rivers have high water residence times and are frequently anoxic
(at least in the sediments), so denitrification rates are high. (I would add that the alternate flooding and drying
is also important)
3. Dams often have the effect of “resetting” the rivers they are on, that is, making it appear like a lower order
stream than it would otherwise. Explain why this is the case, in the context of the river continuum concept.
Answer: The river following a dam, particularly if the dam releases water from its hypolimnion, often contains
oxygen-poor water. This condition can persist for some time before the river becomes normal again. Also,
increased rates of sedimentation by slowing of water in the reservoir results in removal of much of the FPOM,
and digestion of much of the CPOM. (and settling of inorganic particles as well) Thus, water leaving a dam
tends to be much clearer than one would expect of another river of similar size. The RCC predicts that such
rivers would be low order, so the river past a dam is essentially reset.
4. In what ways does the instillation of a large dam on a river affect the terrestrial environment upstream from
the river?
Answer: One surprisingly large impact is on salmon spawning. A dam makes it difficult, or in some cases
impossible, for salmon to spawn upstream from it. The loss of a salmon die-off that would occur upstream
represents a significant food input to the terrestrial environment that would be missing. Obviously, much of the
land upstream is lost underwater under the resulting reservoir. Less obviously, because water levels in a
reservoir tend to change more than in a natural lake, there is a dead zone along the shore where neither lake nor
terrestrial organisms can survive, because it is neither habitat for any length of time. This results in a loss of
habitat type in the land directly up from the dam. There will be no more riparian area near the water as there
probably was before the dam.
1. Explain the Dead Zone. What it is, how it is formed, and how does it relate to biology:
The dead zone is the area of the benthos that is alternately exposed to oxygenated epilimnetic waters and
anoxic (in a eutrophic lake) hypolimnetic waters when the thermocline moves up and down during a
thermocline seiche. It can lead to large differences in temperature and Oxygen saturation at a single point in the
lake. It is very difficult for organisms to tolerate these changes in conditions so there is generally no life in the
dead zone. These fluctuations in the thermocline also expose benthic invertebrates to oxygenated waters by
bringing epilimnetic water lower in the lake.
2. How do dams affect the biology of streams?
Benthic algae levels increase because of the presence of more stable substrates like large boulders and the
increase in light from clearer water due to less suspended solids. Riparian vegetation like cottonwoods decline
because the floodplains disappear. Benthic invertebrates decline if water levels fluctuate over the course of the
day like in some hydroelectric dams. It is very hard for organisms to stay in place in the river with the
abnormal flows (or sudden dewatering strands them on dry river beds). Migratory fish like salmon are affected
because the river is now barricaded by dams.
3. How does human development impact the hydrologic cycle?
Humans take water out of the groundwater and don’t replace it, thereby causing the water table to drop. Urban
environments characterized by water-impermeable surfaces have very high water runoff rates after storms that
drop off quickly. The resulting large discharge means sediment erosion, loss of habitat, and general difficulty
for biotic organisms. There are also no place for water to stop like wetlands or ponds that a natural setting
would have, thereby increasing the influx of water into rivers and streams.
4. Describe the importance of timing in fish spawning.
Fish start out eating plankton, then after having grown (not too much though), they switch to eating smaller fish.
Thus, if fish spawn too late, they will be eaten by fish that have already switched to eating smaller fish. Also,
if fish spawn at the same time as other fish, there will be competition for food sources. If fish spawn too early
in the spring, the cold water will kill their larvae or eggs. Thus, fish need to find the balance between spawning
after temperatures have become warm enough but before their competitors (and/or predators) have spawned.
1.
Explain how floodplains affect the nitrogen cycle within a river. Explain how the nitrogen cycle will
change if floodplains are removed or reduced. What effects will this change have on the nitrogen levels in the
body of water the river flows into?
Answer: Nitrogen in various forms is deposited on a floodplain during a flood. As the water recedes
much of the silt and nitrogen forms that were brought on to the floodplain become anoxic and provide a large
habitat for denitrifying bacteria which are able to convert NO3- to N2. Thus floodplains act as significant sinks
of nitrate in a river system. When floodplains are removed or reduced, as in the Mississippi river, sediments and
nitrates are not trapped and are instead dumped into the receiving body of water such as an ocean, larger river,
or lake. The water in such a body will have higher nitrate levels than before floodplain removal.
2.
What are the two most important characteristics of lakes that are phosphate sources and those that are
phosphate sinks? Why do these characteristics affect the phosphate levels?
Answer: Lakes that are P sinks usually have a hypolimnion that remains oxic and the characteristics that
go along with this: depth and colder. Increasing water residence time also increases P uptake. Lakes that are
sources usually have a hypolimnion that goes anoxic and a short WRT. Whether a lake goes anoxic or not is
vital to the source/sink status because a lake that remains oxic will collect phosphate in its sediments and a lake
that goes anoxic will experience internal loading of P from the sediments. This is because phosphate
precipitation in iron complexes occurs under oxic conditions and bacteria and chemical processes that mediate
phosphate release need an anoxic environment. WRT is important because it allows phosphate more time to be
processed within the lake. If the lake flushes water through quickly then it won’t have time to capture any
phosphate.
3.
Use the River Continuum Concept and the Serial Discontinuity Hypothesis to describe the effects a large
dam might have on downstream sections of a large river.
Answer: Dams generally affect large rivers by reducing the stream order downstream. They generally
do this by:
1) Decreasing turbulence and nutrients to levels more likely to be found in a lower order medium sized
stream. This physical change will greatly affect the biotic dimension of the river downstream. For instance, the
rockier stream bed and clearer water will act together to provide better habitat for periphytic algae. The algae in
turn will provide forage for a more diverse array of grazers and a more limited array of collectors. Macrophytes
will also find the river bed more habitable as well.
2) Decreasing in stream temperature to that more likely to be found in an upstream creek. This will
provide habitat for cold water fauna.
4.
Explain why scientifically sampling the benthos is so difficult.
Answer: Because benthic fauna are extremely habitat specific and the sediments are very different
everywhere you go. This is a problem because the sampling techniques used to sample one particular sediment
type may not be applicable to another, e.g. sand, rock, and mud all require different sampling techniques. If you
have different methods you can’t accurately compare your results. Thus, it’s very hard to conclude for instance
if caddisfly larvea are more abundant on a rocky benthos or a pebbly benthos because one requires you lift each
rock and scrape about, the other that you use an aquatic vacuum to suck up the unsuspecting critters.
1. What are the factors that can increase turbidity in a lake (refer to geography, organisms and lake status)
 High velocity and large amounts of runoff due to on urban setting or surrounding impervious surfaces
 Lack of rooted plants
 Increased nutrients-lots of phytoplankton, this would imply a hyper or eutrophic lake
 High winds=lots of mixing (long fetch, shallow water)
2. Describe the major things that affect nutrient cycling in a stream versus a lake
Stream
 Speed of current
 Depth of river
 Substrate (is it rocks, or clay or sand)
Lake
 Decomposition rate
 Amount of rooted plants(they bring things from benthos back into water column)
 Mixing or no mixing
 Oxygenated hypo or not(will P come back up or not)
3. How would an increase in Phosphorous due to waste disposal rerouting affect the trophic food web of a lake
 Cause an algal bloom
 Cause inc in biomass and possible shading
 That could either cause decrease in phyto or possible inc due to added nutrient
 If assume shading does not affect biomass the inc in turbidity and loss of water clarity=makes is hard for
visual hunting predatory piscivorous fish to catch food, so they would dec
 Big zooplankton would inc because a lot of phyto available and so small would dec
 Planktivoruous fish would inc because not preyed upon and have a lot of zooplankton to eat
4. Name the characteristics of the organisms that can live in each layer of a stratified lake in summer
Epilimnion
 Need O2 (get from surface or have gills)
 Can use light
 Probably not decomposers
 Can have mixing
 Can be rooted or floating
 Warmer temperatures
Thermocline
 Not much can live because varying conditions=dead zone (only if confined to substrate)
 Has to handle varying temp (only if confined to substrate)
 Has to be able to deal with both oxygen and not oxygen (only if confined to substrate)
Hypolimnion
 Can have no light
 Little to no oxygen
 Likes cold temp
 No mixing
 Decomposes possibly (bacteria?)
 Find prey without sight
 Can handle water pressure
1. Explain the hydrology of a forest in contrast to a developed piece of
land and one that has been clearcut.
Forest- precipitation is intercepted by grass and trees. Depending on the substrate there is different amounts of infiltration
and surface runoff (infiltration is greater). As water infiltrates the ground the groundwater is recharged. The groundwater and
the surface water flows into lakes streams or wetlands were it evaporates and restarts the cycle. Evapotranspiration from
trees also accounts for the regeneration of precipitation.
Developed- In contrast to the forest there is no infiltration and surface runoff brings with it pollution from the city to the lakes
and streams. Groundwater regeneration is minimal.
Clear cut- There is a dramatic decrease in the evapotranspiration. there is increase in erosion and nutrient loading in the
streams and lakes. The stream velocity is increased. Nutrient uptake is diminished allowing the nutrients to flow into the
streams and rivers (impact of P).
2. Draw a bottom dwelling fish and a top feeder. What are the major
differences?
Major differences; the bottom dwellers coloration is generally located on it's dorsal side to keep it concealed from predators
from above. They are characterized by barbs that are used to search the ground for anything edible. The mouth is found on
the bottom of the fish. The top feeders coloration is also on it's top to keep it from being seen from above. Below it is a
lighter color to blend into the sky from below. The mouth of a top feeder is pointed up so that it can eat from the surface.
(picture in slide show)
1. Discuss seasonal changes in the temperature and oxygen profiles in eutrophic, dimictic lakes.
At spring turnover the water temperature and dissolved oxygen concentration are relatively constant from
the bottom to top of the lake. Mixing of the entire water column equalizes temperature and dissolved oxygen
levels across the depth profile. As temperatures warm, heating of the top of the lake leads to lake stratification.
An upper, warm epilimnion and lower cold hypolimnion become established. Mixing between the layers does
not occur. (Why not?) The epilimnion displays relatively constant temperature throughout because it is mixed
continually by the wind. The water temperature is also relatively homogenous in the hypolimnion, though
temperatures drop slightly with depth. The hypolimnion and epilimnion are separated by a narrow zone of
maximum temperature change (definition?) called the metalimnion or thermocline. At fall turnover the lake is
again isothermic and displays constant oxygen concentration. In the winter, weak reverse stratification occurs,
with water colder than 4 degrees celcius at the lake surface and water around 4 degrees below.
During summer lake stratification oxygen levels are high in the epilimnion due to high levels of
photosynthesis and atmospheric exposure, and low in the hypolimnion due to isolation from the atmosphere and
high rates of decomposition. Beneath the thermocline oxygen is eventually completely consumed and is not
replenished until fall turnover. Under the ice oxygen remains low near the lake bottom due to decomposition,
and higher at the lake surface (ice surface). Colder water temperature directly beneath the ice allows more
oxygen to dissolve. In extreme situations, lakes may go completely anoxic during the winter as decomposition
consumes oxygen. Lack of photosynthesis and atmospheric isolation prevent oxygen replenishment.
2. Compare the distributions of alga biomass in an eutrophic lake with a surface bloom and in an oligotrophic
lake.
In an eutrophic lake with a surface bloom alga biomass is greatest just beneath the lake surface (often because
these are cyanobacteria that can maintain their depth with air vacuoles; other groups cannot do this and so do
not usually form surface blooms) and decreases with depth. Algae at the surface shades out algae deeper in the
water column. Algae that sink to the thermocline cannot survive because they are beneath the compensation
point. In oligotrophic lakes the majority of the alga biomass is concentrated at the theromocline. The algae
sink and accumulate at the thermocline since they cannot penetrate this density barrier (actually they can, but it
takes awhile, resulting in a backup or accumulation of cells at the thermocline). Due to nutrient limitation alga
does not accumulate rapidly at the lake surface and thus does not shade out alga at the thermocline.
3. How can water discharge from reservoirs affect river biota downstream? How does whether water is
released from the hypolimnion or epilimnion impact changes in river conditions?
Water discharge from reservoirs causes rapid increases in river discharge during seasons when high discharge
would normally not occur. Lotic biota have life cycles adapted to the natural hydrological cycles of rivers.
Thus, seasonal changes in flow patterns can prove detrimental to organisms. In addition, due to water release
from reservoirs stream discharge can fluctuate greatly diurnally. Organisms may not be able to adjust to these
frequent fluctuations. Releasing water from a reservoir can also change the normal thermal fluctuations of a
river. A natural river warms in the summer. However, releasing water from the reservoir’s hypolimnion keeps
the river at a constant cold temperature throughout the year. In coldwater rivers, water released from the
epilimnion may be warmer than usual. Changing the water temperature of a river effects biota since most
species are adapted to specific water temperatures. For example, trout can only survive in cold water with high
dissolved oxygen content. Hypolimneic water released from reservoirs may also have low oxygen content and
high nutrient levels. Low water oxygen content can lead to mortality of species that cannot tolerate low oxygen
conditions. Nutrient pulses may disrupt the natural competitive relationships between species and lead to alga
blooms. Epilimneic water released from a reservoir may be clear or turbid depending on the level of production
in the reservoir and may be low in nutrients due to high epilimneic biological demand. Releasing nutrient
depleted water into a river could negatively affect organisms by increasing nutrient limitation.
4. Discuss the phosphorous cycle in lakes with oxic and anoxic hypolimnions. In which lake is the
concentration of phosphorous in the water higher? How might this affect phytoplankton productivity?
The primary form of dissolved phosphorous in lakes is orthophosphate or PO42-. In the presence of oxygen
orthophosphate reacts with Fe2+ and Fe(OOH) and precipitates out of the water column. However, under anoxic
conditions the iron-phosphorous complexes are soluble and phosphorus is released. Release of phosphorous
from iron-phosphate complexes in bottom sediments is known as internal loading. Anaerobic bacteria may
mediate the release of phosphorous from iron complexes under anoxic conditions. If the hypolimnion of a lake
is oxic internal loading does not occur, and phosphorous in iron complexes is perminently lost to the sediments.
One would expect the concentration of phosphorous in the water to be greater in lakes with anoxic hypolimnion
because internal loading prevents loss of phosphorous to the sediments. In addition, anoxic hypolimnetic
conditions suggest higher lake prodictivity, indicating that phosphorous inputs may be high. Phytoplankton
productivity should increase with phosphorous concentration, and thus also be high in lakes with anoxic
hypolimnion.
1. Explain different mechanisms in a lake that cause nutrients in the benthos to be resuspended in the water
column.
-Nutrients can be resuspended into the water column from the benthos first by turnover, most over during the
fall or spring when the entire lake water mixes (but this isn’t necessarily a physical process in that there aren’t
huge currents circulating around and around the lake during mixing…). Bioturbation is another method;
organisms such as carp may stir around the sediments and thus cause the nutrients to rise above the benthos.
Thirdly, rooted plants often serve as a connection between the benthos and the rest of the lake by taking
nutrients up from the benthos and then releasing them into the water column in some form or another. Lastly, a
simple trophic transfer between organisms such as benthic macrophytes to fish who swim all around the lake
will bring nutrients from the bottom of the lake into the water where they are more available for other
organisms to take advantage of.
2. Describe different feeding methods of macroinvertebrates and some possible sources of the food they eat.
-In a basic sense, macroinvertebrates in an aquatic setting may collect their food either by filtering the water or
collecting, graze for their food and either scrape surfaces or chew large particles, or eat prey by piercing them or
completely consuming them (depending on the size of prey). The detritivores that chew large particles will get
their source of food probably from an allochthonous source such as leaf litter; the collectors will probably get
their food from decomposing particles of that matter or from decomposing matter from within the lake; lastly
the carnivores will obtain their food from organisms in the lake most likely.
3. Explain how bacteria in aquatic systems are able to obtain carbon from particles much larger than
themselves. Are fish able to do the same and what allows them to or limits them from doing so?
- Bacteria are obviously very small and have no way to engulf their food. Therefore, bacteria use extracellular
enzymes (enzymes that they release into the environment) or enzymes on their external surface to break
particulate organic matter down into smaller particles of low molecular weight that can then be brought across
their cellular membranes. They can then use those particles for energy or whatever other purpose they need to.
Fish have a very different mechanism of obtaining energy and are limited by the size of their mouth as to what
they can eat. If the particle (i.e. little fish) is larger than a fishes’ mouth, the fish is not going to be able to
consume it. If it fits in the fish’s mouth, the fish will be able to eat it.
4. Explain how a sudden and very large rain storm may affect a lake or river system in an urbanized part of the
landscape.
-When a lot of water is suddenly precipitated on the land surrounding an aquatic system, the first immediate
change will be that there will be increased runoff relative to how much there normally is. If a part of the land is
very erodable, such as a building site, an enormous amount of erosion may occur and lots and lots of sediments
will be brought into the lake or stream. These sediments may “smoother” benthic organisms and increase the
turbidity of the system. If less light can reach into the water body as a consequences, photosynthesizing
organisms may suffer. Most likely, the runoff will also carry pollutants such as oils, salts and fertilizers
especially if the water runs over large expanses of concrete. Also, more organic materials such as leaves will be
brought in which will increase DOC levels and perhaps increase decomposition rates.
Question #1
A forest fire clears the watershed of a lake of its trees, brush and grass cover. Ash covers the landscape. Right
after the fire burns out, a huge storm passes through, dumping a great deal of water on the region in a very short
period of time. What are some of the potential effects on the lake (hydrological and biological)?
-- The loss of all the plants would drastically increase the runoff problem because of a loss of evapotranspiration
and interception. Groundwater recharge rate would be temporarily depressed and runoff would come to the
lake faster and in greater quantities. This runoff would also be carrying ash and eroded soil which could pose a
number of problems. First the particulate matter would shade the lake out for a while possibly causing death of
non-emergent plants and algae and thus further disturbances on up the food chain. Also, this could be a serious
influx of excess nutrients into the lake causing an algal bloom after the sediments subside. (name a few of
these)
Question #2
What would be the likelihood that a piscivore population could survive if transplanted into a small, shallow,
high lake near the headwater?
-- Low in nutrients and high above the other lakes, the headwater lakes tend to support a few macroinvertabrates
and algae. If the lake somehow got stocked with a piscivore, it would be unlikely that there would be a
sufficient planktivore population to support it (energy transfer making smaller prey unlikely to support a large
fish). (However, under these circumstances, piscivore often do resort to alternative prey types like
invertebrates, and then grow very very slowly) There would be a small breeding population and a dismal
outlook and these fish would likely just travel back downstream to more suitable climates.
Question #3
Two stable eutrophic lakes experience a one-time massive infusion of nutrients. One lake is dominated by a
piscivore and the other by a planktivore and both lakes have a stable trophic cascade. What does each lake look
like right after the nutrient inflow?
-- The lake with the piscivore is most likely to be clear and have a rise in zooplankton population and piscivore
population. The lake with the planktivore is more likely to be turbid and have higher algae and planktivore
populations. This is assuming that the nutrient influx would lead to an algal bloom.
Question #4
What kinds of organisms could be living in the hypolimnion of a hypereutrohic lake after a long productive
summer?
-- Any organism that could withstand anoxic conditions or could move from the hypolimnion to the epilimnion
on its own power.
1. Explain the changes in seasonal distribution of different types of phytoplankton, and how it relates to the seasonal
distribution of different types of zooplankton in a dimictic lake.
In Dec. and Jan. numbers of zooplankton and phytoplankton are both very low due to cold temp. and light for
photosynthesis being blocked by snow and ice cover which limit growth. In the early spring when mixing occurs diatoms
predominate because Si becomes present throughout the water column and they can use it in the photic zone (where there
is now more light) to grow. At this point zooplankton populations rise because there is more phytoplankton to feed on. As
the lake becomes stratified at the beginning of the summer and the diatoms sink below the photic zone (because their
frustrules are heavy and they need mixing to stay in the photic zone). By early summer the zooplankton communities have
become more substantial, so there is a short clearwater stage (because the zooplankton are now depleting the
phytoplankton). After the clearwater period nutrients are low, and cyanobacteria are the dominant phytoplankton for a short
while because they have the advantage of being able to fix N. When fall mixing occurs again, diatoms are again able to
utilize Si in the photic zone and become dominant before winter sets in again and all plankton decline.
2. Explain the theory of trophic cascades, and briefly explain how this principle is the basis for the biomanipulation that was
conducted with piscivorous fish in Lake Mendota.
The theory of trophic cascades is that nutrient supply limits algae biomass and production, affecting the entire food chain,
but other variability in biomass throughout the food chain can be explained by the effects of predation. This works because
predators with high enough consumption can control the biomass of their prey, with low enough production of the prey. The
purpose of the manipulation in Lake Mendota (adding piscivorous fish) was to increase water clarity. According to the
trophic cascade hypothesis, since piscivorous fish consume planktivorous fish, they would be able to deplete the population
of planktivorous fish. The result would be that since planktivorous fish consume zooplankton, the zooplankton community
would then flourish. Finally, since zooplankton eat phytoplankton, the phytoplankton community would diminish, thus
decreasing algal blooms and increasing water clarity.
3. Explain why human impact on rivers has caused many lakes to become hypereutrophic.
Rivers around the world have to a large extent been dammed, which causes floodplains to rapidly disappear. Floodplains
have also been removed for agricultural and aesthetic purposes. Floodplains are very important in maintaining the balance
of nutrients in water bodies, because N-fixation and denitrification can only occur in anoxic conditions. When water with
high levels of nitrogen (which is increasingly common now due the presence of N in fertilizers, and it running off of
agricultural fields into surrounding rivers and lakes) moves into a floodplain area, the conditions can become anoxic, and
excess N can be released into the atmosphere. If this doesn?t occur, then excess N will remain in water, and because it
can be a limiting nutrient for algae, can cause big algal blooms and eutrophication.
4. Explain some of the effects that landscape position and the terrestrial environment can have on hydrology and water
quality in lakes.
Lakes in surrounding areas with good amounts of vegetation generally will have fewer nutrients than lakes in areas with little
vegetation, because plants use up nutrients in runoff and groundwater that would otherwise reach the lakes. The presence
of impervious surfaces allows pollutants or excess nutrients to runoff directly into lakes, whereas areas with only natural soil
(pervious to water) allow infiltration of water, and rather than contaminating lakes, much runoff can become groundwater
and be filtered. In general lakes with higher landscape position will have lower conductivity than lakes with lower landscape
position, because as water runs down the landscape it collects more and more ions. Lakes with higher landscape position
also usually experience lower ANC (acid neutralizing capacity) and therefore lower pH, because their main water source is
rain, which is often acidifying, and there are fewer buffers present. As landscape position decreases lakes get more water
input from other lakes and rivers, ANC increases, and pH increases, because more buffers are present.
1. Question: Explain the differences you would expect to see in the biota of the littoral zone of a lake – with the
two different scenarios:
a) sunlight penetrates to bottom sediments
b) sunlight does not make it to the bottom of the lake.
Also, what would cause these varying conditions?
Answer: a) cause: clear water; growth of macrophytes which maintains invertebrate population, as it can feed.
There would be lots of oxygen, food sources and biotic interaction in the benthos. b) cause: turbidity; not much
life supported in benthos due to little oxygen or food.
2. Question: Contrast monomictic lakes with polymictic lakes. Name a factor that may disrupt the stratificationmixing pattern of a lake. What factors would change the oxygen concentration?
Answer: Monomictic mixes once a year. (part of answer removed) Dimictic lakes mix twice a year, in the fall
and spring. Disruptions: large wind and currents can cause mixing to occur. Oxygen change: depends on oxygen
present before stratification occurs, length of stratification, volume of hypolimnion and amount of
decomposition and respiration.
3. Question: As a lake manager, what would you worry about causing acidification? What could you do to
increase the ANC (acid neutralizing capacity)?
Answer: Inputs: natural: input of organic acids from catchment (which might be?). Anthropogenic inputs would
be acid rain (due to increased use of coal and oil). To increase ANC one could add carbonate or bicarbonate
mechanically, or do so by re-routing the input stream to run over bedrock that releases these molecules. Possible
make more inputs coming into it, so it is not just affected by acid rain.
4. Question: What would happen to a food web if the presence of zooplankton were severely reduced (how
would it affect the trophic cascade)?
Answer: Because the alternate trophic levels are controlled by predation, a problem would come when
zooplankton were supposed to be in lager concentration. If zooplankton are reduced the secondary consumer
would also be reduced, although it would normally be abundant. Phytoplankton biomass (but not rooted plants)
would increase in abundance, increasing turbidity.
Secondary consumer → primary consumer→ [zooplankton] → phytoplankton
1. Describe the biological differences you might see between a high seepage lake versus a low drainage lake in
this area? Chemical differences? Why?
Low drainage lake: you would generally see more diversity in the lake (more fish species, more
macrophyte species, etc). Chemically, the lake will have a higher conductivity, calcium concentrations,
ANC, and pH.
High seepage lake: biologically, you would generally see less diversity. Chemically, the lake would
have a lower conductivity, calcium concentrations, ANC, and pH.
The reasons for these differences have to do with the water connections in and out of the lakes. High seepage
lakes only get their water from the rain, so they do not get much input of calcium or ions. In addition, because
the lake is isolated, it is hard for plant and animal species to reach the lake, so there will be fewer species
(although in some cases these lakes might serve as refugia for rare species because its less likely that invasive
species will reach them). On the other hand, the low drainage lake has rivers coming in and out as well as
groundwater from lakes higher in the land. This will increase the amount of ions as well as calcium. The rivers
will provide a conduit for plants and animals to expand their territory into the lake.
2. A fair sized wetland was situated to a yet undeveloped lake in Northern Minnesota. Unfortunately, a group
of investors bought up the land and decided to develop the shoreline. To increase the amount of available
shoreline, and their profit, they decided to drain the wetland. What effects will the loss of the wetland have
on the water quality and biota of the lake (this is ignoring the effects of construction and shoreline
development)?
Wetlands act as storm-flow modification and decrease erosion. Consequently, the lake will now have an
easier time of eroding its shoreline as well as experience more flooding. Along with the storm-flow
modification, the wetland also was a sink for heavy metals and sulfur, which will now find their way
into the lake as well as the animals. Less nitrogen will find its way into the lake (?) (ignoring
anthropogenic sources) although more phosphorous and ammonia will find its way. This will increase
phosphorous levels and decrease nitrogen levels, making nitrogen the limiting chemical, hurting many
animals that depend on N for respiration. The wetland was also a source for DOM and POM, making
life harder for invertebrates and bacteria that depend on these levels. Lastly, the wetland removed many
suspended solids; now those solids are going to be in the water column of the lake, decreasing light
penetration into the water column, causing problems for plants depending on that light for
photosynthesis. (Alternatively, the lack of DOC from the wetland may make the water less stained…)
3. We discussed in class and on the take home test how the RCC (River Continuum Concept) explains the
placement of invertebrates along a river. However, when the river is dammed, then the flow of nutrients and
organic matter is stalled at the dam. What effects does the dam have on the RCC?
Because of the dam, much of the FPOM and CPOM are dropped out of the water column, so there is
significantly less organic matter getting below the dam. It could be that the phytoplankton and
zooplankton may replace some of the FPOM and CPOM, but the water has to be released from the right
area in the reservoir for this to help (i.e. somewhere in the thermocline where there is hopefully still
oxygen, but there are dead plankton and zooplankton  they don’t have to be dead to be CPOM or
FPOM!). Consequently, if there is little FPOM and CPOM, the RCC is essentially going to be restarted
at the head of the dam, as the organic matter has to be brought back into the water column by scrappers,
and then consumed by grazers and collectors. However, this will also depend on the rate that water is
released from the dam, which can have its own daily effect on the invertebrates.
4. We know that clear-cutting a forest should have lots of effects on a stream at the bottom of the watershed.
Explain some of these effects and why they are important.
- increase in potassium, phosphorous, nitrogen…. lots of nutrients. This will affect the growth rate of
different organisms and cause blooms in the water column.
- there will be an increase in sedimentation of the river. More sandbars will be formed in slow moving
water, changing the channel of the lake and possibly smothering invertebrates. Also, the
sedimentation will also decrease light availability, making life harder for photosynthetic organisms.
Sedimentation can also smother gravel beds critical for fish spawning.
-
during storms, there will be little interception and water will simply come rushing through the
watershed and cause more flooding than was observed under normal conditions. This could have a
large impact on the biota as they are simply swept downstream when they normally would not have
been.
1.) If the secchi depth of a lake is fairly small, what does that tell us about its trophic state? What can be say about the
amount of oxygen dissolved in the hypolimnion?
We do not have enough information to answer this question, because small secchi depths could be caused by staining (high
DOC), lots of suspended inorganic solids, or lots of suspended organic matter like phytoplankton. If turbidity is caused by
high concentrations of phytoplankton, then we can infer that the lake is eutrophic, and the hypolimnion likely in anoxic for at
least some of the summer. High DOC concentrations or lots of suspended inorganic solids do not necessarily correspond to
a particular trophic state.
2.) A larval fish is transparent when lives in the epilimnion, but moves to a well-lit hypolimnion as it matures. Would it be
more adventageous for this fish to develop into a blue or red fish as it becomes older? Why?
It would be more adventageous for the adult fish to be of the color red, since the hypolimnion is well-lit and red light does
not penetrate as far a blue light does.
3.) What is the major form of non-living carbon in higher order rivers and streams? Explain its origin in terms of landscape
position and predominant insect types in the lower order streams.
FPOM is the major form of carbon in higher order streams. It comes from lower order rivers upstream, which have
significant amounts of allochthonous leaf litter input. These leaves are converted from CPOM to FPOM by shredders.
4.) Say you were to dump radioactive isotopes carbon 13 (in the form of CO2) and oxygen 18 (in the form of O2) on the
surface of a lake and monitor their dispersal. Which one would be more likely to disperse the fastest? Why?
Carbon 13 would disperse the fastest because the radioactive CO2 would be fixed into photosynthetic organisms, which
would be consumed by other organisms that can move around much faster than oxygen can (which must rely on diffusion,
which is much slower). hmmmmm
1. How does phosphorous exit and enter a water body? Describe both theories.
• In dealing with the classical view, phosphorous accumulates on the sediments under oxic conditions.
PO43- strongly absorbs to iron aggregate oxyhydroxides (e.g., FeOOH); aggregates accumulate on
sediments, in dead algae cells and other organic matter, and insoluble FePO4 precipitates (Fe3+). In
anoxic conditions, FeOOH-PO4 and FePO4 complexes dissolve, releasing PO43- and Fe2+. The
anoxic sediment PO43- concentration is 5-20 times greater than water column. If water column
remains oxic, Fe3+ precipitates and aggregates on the sediment surface prevent released PO43- from
diffusing upwards from anoxic sediments and/or entering the water column. Modern models suggest
that microbes play an active part in P-cycling.
2. Describe an isotherm diagram of a dimictic lake, and tell what is going on in the water during each
season.
- In the spring the lake is turning over because of the equal temperature and densities. Thus the
lake will mix. In the summer the lake will stratify because of the different temperatures and the
different density. The top layer: epilimnion will be warmer and have a lower density, and will be
oxic, while the bottom layer, the hypolimnion will be colder, have a higher density, and have low
oxygen levels. In the spring the lake is turning over because of the equal temperature and
densities. Thus the lake will mix. In the winter, the lake will again weakly stratify. This is due to
the ice cover and the cold temperatures of the water. It will also have an epilimnion and a
hypolimnion, although the density difference is minor and so there is not a strong separation
between the two layers.
3. Why does Winterkill occur and in what types of lakes do they occur in and what happens to the fish
populations in the lake?
- Winterkill occurs in small, shallow lakes with are anoxic during ice-cover. The lakes go anoxic
during ice-cover because of decomposition and because they have a small hypolimnion. Fish
need to deal with the anoxic conditions and some species of fish will leave the lake, move
towards inlet, move towards the ice/water interface, and/or breathe bubbles. Also smaller fish
can be less tolerant of these conditions.
4. Describe what ANC and how you determine it.
- ANC is a measure of the lakes water’s capacity to buffer ph change. ANC is determined be
titrating a water sample with a strong acid and monitoring the change in pH. When H+ is added,
it is first taken up by carbonate, then by bicarbonate. At this stage the pH is approximately 4.5.
When more acid is added, all the carbonate and bicarbonate is converted to carbonic acid and the
buffering capacity is exhausted. Then the pH is directly proportional to the amount of acid
added.
1) How do dam releases affect nutrients in the water downstream? How would this affect the downstream
biota?
The answer to this question depends upon from where in the reservoir the water is released. If water is released
from the hypolimnion, downstream will be inundated with water with low amounts of dissolved oxygen, high
amounts of organic carbon in the form of dead and decomposing organisms, high amounts of iron-phosphate
aggregates out of which phosphate would precipitate once it was released into the river where there are more
oxic conditions, ammonium, which would undergo nitrification once it was released into the oxic river. The
release of such nutrients would spur growth in the river, causing algal blooms. This would allow less light to
come through the water column and negatively affect the growth of rooted plants and benthic invertebrates. If
water were released from the top of the reservoir, it would be mostly nutrient-poor, although high in oxygen,
except in fall turnover, which would result in pulses of water as described above. The mostly nutrient-poor
water the downstream river would be receiving for the majority of the year would eventually cause a shortage of
nutrients to be available to organisms growing downstream, especially as it got to be later in the summer. The
high amounts of oxygen would be beneficial for animal growth, but the low amounts of other nutrients for the
majority of the year would stunt plant growth.
2) How does a fish such as a largemouth bass change its trophic position from hatching to adulthood?
What are the energetic consequences of these trophic shifts?
When fish hatch they have a low trophic level—they mostly eat zooplankton, which puts them about on level 3,
after phytoplankton and zooplankton. As they grow into juveniles, they increase their trophic level and begin
eating insects or other fishes. This puts them another step up on the ladder. Finally, they become the piscivores
they remain throughout their lives, putting them at the very highest trophic level, except for external predators
such as people. As they rise in trophic level, they gain more energy from their food.
3) How could a cement channelized storm sewer running into a small river affect its functional feeding
groups at the inflow point?
The storm sewer would bring with it a lot of course particulate organic matter, like dead leaves and the like—
these leaves represent a food source of the shredder feeding group. The shredder population would theoretically
increase at the inflow point, which would cause the collector feeding group to increase, as a result of the high
amounts of fine particulate organic matter being produced by the shredders. The increased numbers in these
two guilds would cause predators to flock to this area. However, with this great food source comes other
factors that would have negative effects on the biota at this point. For example, these extra dead leaves and
such will not all be able to be eaten—they will decompose, leading to a possible shortage of oxygen if the water
is not turbulent enough to mix up the water at the inflow point. This could stress all of the functional feeding
groups. Another stress on all feeding groups would be pollution brought in to the river. However, also brought
in would be nutrients such as phosphorus and nitrogen, which would have been runoff from people’s lawn
fertilizer. This could cause a bloom in algal populations, which would be a boon to the scraper guild, which
could then be eaten by predators. But the storm sewer would also bring in a lot of sediments, which would
smother most benthic invertebrates, a major component of the shredder, collector, and scraper guilds. Overall,
it seems like the storm sewer would have a total negative effect on the functional feeding groups at the influx
point in the stream.
4) Your local lake is experiencing chronic extreme algal blooms. How do you fix it?
The first thing to do is to look at nutrient levels in the lake. They will probably be high. If they are, look for
point sources of nutrient input to the lake, such as construction sites, a farm field, or a sewage treatment plants.
Once identified, legislation or other measures can be enacted to reduce the pollution from these specific points.
However, there may not be point sources. If phosphorus loading is a component of the problem, check and see
if there’s any internal loading from an anoxic hypolimnion. If so there are several bandaid measures one could
take, like dredging the sediments, adding chemicals to precipitate out the phosphorus, aerating the hypolimnion,
and such things. One could also take a more ecological approach by restoring wetlands around the edge of the
lake to act as a nutrient sink. One could also manipulate the food web by adding a lot of predatory fish—a
trophic cascade would then occur—the big fish would eat the small fish, the absence of the small fish would
allow the large zooplankton to prosper, which would then eat a lot of algae. Another ecological approach one
could take would be to attempt to get rooted plants to grow—these plants would then take up nutrients which
would otherwise be used by algae.
1. Why do managed marshes demonstrate extremely different hydroperiods than their natural counterparts?
Name some of the differences between managed and natural marshes observed throughout the course of the
year, and possible reasons for such patterns.
Water levels for a marsh depend on the time of year, the ratio of precipitation to evaporation, and water
sources and drainages. In terms of differences between natural and managed marshes, it’s been observed
that managed ones generally have a higher baseline water level throughout the year, and while natural
systems tend to peak around April, managed systems have maximum water levels in August. Possible
explanations for this behavior include man-made dams obstructing water flow, different vegetation and
ecosystem life in managed systems selected for certain species, or the manipulation of water levels for
human needs – for example, draining more water during the hot periods of the summer to match demands.
2. How does man-made channelization of braided rivers affect the biotic communities? Specifically address
benthic algae and invertebrates, riparian vegetation, and fish.
Because the channelization of rivers essentially changes the RCC numbers, the various species groups in the
river are affected. Benthic algae usually increase in abundance because there is clear flowing water coming
down the channel, which means more light and potential for photosynthesis. If there is a change in water
flow, some invertebrate species that are dependent on flow at specific times may become less abundant,
while others could change their drifting periods to the wrong time of the year. Riparian plants are likely to
decrease in abundance and diversity because of habitat loss, and similar results may be seen in fish. Due to
changing habitat and temperature, fish may be forced to adapt to new conditions or migrate.
3. How do dams affect oxygen levels downstream? What does that do to the organisms in the river, and what
sort of adaptations may come in handy for such conditions?
If the dam is large enough to build a big reservoir with an anoxic hypolimnion, the water released
downstream could have very low oxygen levels. In terms of the biota, certain fish can live only with a
certain amount of oxygen in the water. Once a level below 2 mg/L is reached, essentially no fish can live.
Therefore fish that are able to migrate or tolerate low oxygen conditions may be the only ones to survive.
Other species like macroinvertibrates would survive if they had other adaptations like siphons or hair to
catch surface air. Large macrophyte beds would not be able to live right downstream of the dam, because
there would be little oxygen to take up during photorespiration.
4. Go through a possible scenario for a molecule of carbon working its way through an aquatic system, starting
in a terrestrial leaf that falls into a headwater stream and ending in a top-predator on the food chain.
Carbon starts out in the leaf and falls to the steam in autumn as part of allochthonous nutrient input.
Macroinvertebrates go to work shredding the leaf into smaller pieces, or fine particulate organic matter.
This FPOM can be digested outside of bacteria cells and broken down into dissolved organic carbon. From
here, the carbon may be utilized by bacteria and then filtered up to the macro-foodweb, passing from
zooplankton to primary predators to secondary predators such as piscivous fishes. Admittedly, the
probability of C going from bacteria to the macroinvertebrates is kind of low (it is thought that cycling
within the microfood web is more likely) and the mechanisms are not fully known, but there is a connection
with outside inputs eventually working up through the foodweb. An alternative route would be for the
FPOM to be consumed by a macroinvertebrate collector, the macroinvertebrate eaten by a fish and so on up
the food chain.
1. Explain the nutrient spiraling method and any problems associated with it.
This method is based on uptake and retentions of nutrients by biota. It is measured by spiral length which
relates to the time that nutrients are retained by organisms in the benthic zone. One problem seen with this
method is it does not incorporate all habitats of the entire stream.
2. How does zooplankton protect themselves from predators?
They will alter their appearance like spines or other projections in order to become inedible. They will also
change colors to become more cryptic. They will move to different areas of the water column depending on
where predators are present.
3. Name some effects dams and reservoirs have on river systems.
The transfer of CPOM and FPOM is interrupted because there is a much higher sedimentation rate in the
reservoir. This makes water clarity downstream much better which increases the amount of benthic
primary producers present. The change in flow over the year effects organisms downstream because the
environment is full of extremes like temperature, O2 concentration and particulate matter. Migrating fish
are unable to move upstream to spawn so they experience a low fitness.
4. Define ANC.
It is the measure of a lakes water capacity to buffer pH change. It tells you the concentration of carbonate
and bicarbonate in the water column since they neutralize acids.
1. A company decided to erect its newest factory on the shores of Lake Wilsonia, a lake of medium depth and
fetch situated in a steeply sloped, forested watershed located in northern Wisconsin. They believed the sight of
the lake and surrounding wilderness would energize their workers into performing at a higher level. Early
March they began construction by clear-cutting the entire forest to make way for the factory, parking lots, a gas
station and a lunch café. Mid-March the company ran into a legal battle and the land was left undeveloped until
early August. Describe the biotic and abiotic effects the newly clear-cut watershed will have on Lake Wilsonia.
March-august is rainy and warm:
Incr in run-off – no uptake by terrestrial plants
Incr in nutrients – no uptake by terrestrial plants
Incr in turbidity post rain – no roots (or interception)  erosion
Loss of terrestrial habitat which might have housed animals important to macrophyte reproduction and many
other species that interact with the aquatic ecosystem
Incr in algae – algae blooms  decr in macrophytes
Incr algae  incr in zoop  initial incr in zooplanktivores and small fishes  incr in piscivores
Then, due to loss of breeding (or refuge) habitat [decr in macrophytes] decr in zooplankivores and small fishes
 decr in piscivores (this process might take many years because larger fishes are long-lived)
2. What will effect finishing the planned developments have on Lake Wilsonia and why?
Pavement (and roof-tops, etc) is an impervious surface and leads to increased storm runoff which will carry
many pollutants from the factory, parking lots, roads and gas station. The concrete will keep surface water from
reaching the groundwater recharge points – leading to a drop in groundwater levels. Essentially, by finishing the
project they will be compounding the problems.
3. Draw a top-of-the-water-column temperature curve, diatom population curve, limiting nutrient level curve
and a cladoceran population curve for a dimictic lake on the axis below.
high
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low winter
spring
summer
fall
- the temperature curve will start low and increase beginning near spring, have a wide peak in the summer and
decrease toward fall
- the diatom population curve will start low and peak in spring, decline during the summer and peak again in the
fall
- the limiting nutrient levels curve will start high in the winter, begin declining in the spring, reach a wide low
level in the summer and begin to increase again near fall
- the cladoceran curve will start low and begin to increase near the end of the diatom peak in spring, increase
through and peak in summer and decline again just as the diatom population levels rise in the fall
4. As a stream ecologist you know that dams disturb the hydrology, river characteristics, biota and water
regimes downstream of the dam. Name three effects of dams on stream ecology.
- Water pulses upset biota, there is a loss of diversity, the velocity, depth and possible temperature variability is
too great
- Water being released may be much colder, lower in O2 and higher in nutrients than the river if it is released
from an anoxic hypolimnion, or much warmer and possibly more turbid (from phytoplankton) with fewer
nutrients than in the river if it is released from the epilimnion.
- Increase in sedimentation before the dam leads to clearer water with a loss of DOC with leads to more benthic
algae downstream of the dam
- Control of the water level leads to a loss of flooding which leads to a loss in the riparian vegetation that
depends on floods
- A large wall in the river impacts fish migration