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 à 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- à 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.