Quiz 3 Flashcards by Cassandra Harper

How soluble is oxygen in water? (5)

At maximum saturation, there is only about 14.6 mg/L of oxygen

This is about 14.6 ppm in fresh water

This happens at 0 degrees

Cold water has a higher dissolved oxygen concentration

Boiling water has zero dissolved oxygen

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What are the 3 core environmental factors that control how soluble oxygen is in the water?

Temperature - oxygen solubility declines with increasing temperature

Salinity - oxygen solubility declines with increasing salinity

Atmospheric pressure - oxygen solubility declines with decreasing atmospheric pressure (increasing elevation)

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Why is dissolved oxygen important? (4)

Anything below 5.0ppm becomes stressful for fish

Anything below 3ppm becomes lethal for fish

The presence of dissolved oxygen is critical to sustaining any aerobic life in lakes

Salmonids in particular require high concentrations of dissolved oxygen to survive and reproduce

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Why should we not use % saturation to describe oxygen concentrations in water?

Because is is confusing and misleading - as temperature increases, dissolved oxygen decreases but % doesn’t show all the details

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Orthograde (2)

Oxygen concentration increases with depth (increases at the thermocline then stays steadily high throughout hypolimnion)

Found in cold, deep, oligotrophic lakes

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Why do oxygen concentrations increase with depth in oligotrophic lakes? (4)

Decreasing temperatures

Increased hydrostatic pressure

Therefore % saturation stays the same with depth (related to increase in pressure)

And mg/L increases at the thermocline then steadies out

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Clinograde (3)

Oxygen concentrations decrease with depth due to biological and chemical demands for oxygen

Found in eutrophic lakes

Decreases at thermocline and continues decreasing (unlike orthograde which settles out)

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Dissolved oxygen profiles and seasonal stratification (4)

When combined with seasonal stratification, we get “typical” profiles in oligotrophic and eutrophic lakes

Spring and fall turnover are mixing so oxygen is high in both

Oligotrophic lake - summer stratification looks opposite of temperature/ density stratification (Orthograde)
Winter stratification - steady high oxygen

Eutrophic lake - summer stratification has medium oxygen at surface and decreases very quickly with depth so there is no oxygen in the hypolimnion (Clinograde)
Winter stratification has higher oxygen but decreases gradually with depth throughout hypolimnion (isn’t steady like in oligotrophic lake) = might have to worry

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Metalimnetic oxygen maxima (4)

Variation for dissolved oxygen on typical seasonal profile

Sudden blip of oxygen increase at the thermocline

Also known as a positive heterograde profile

Caused by photosynthetic production from algae which congregate at this depth from density slowing sinking

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Metalimnetic oxygen minima (4)

Variation of dissolved oxygen on typical seasonal profile

Sudden blip of oxygen decrease at the thermocline

Also known as a negative heterograde profile

Caused by decomposition of settling OM and small living organisms as it reaches the denser water of the thermocline where it accumulates and consumes oxygen

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4 types of vertical oxygen profiles found in lakes and reservoirs

Orthograde
Clinograde
Positive heterograde
Negative heterograde

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How is oxygen added to water? (3)

From aeration (splashing, waves)

Diffusion from the atmosphere

From photosynthesizing plants and algae

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Photosynthetic vs decomposition reactions (2)

Photosynthetic reaction of phytoplankton creating oxygen in water

6CO2+12H2O—>C6H12O6+6H2O (+O2 )

Consumption of oxygen by decomposition is the same formula in reverse

C6H12O6+6H2O (+O2) —> 6CO2+12H2O

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Why are eutrophic lakes subject to deoxygenation? (2)

Consumption of oxygen by decomposition (whether natural,sewage, or pulp discharge) is common due to algae blooms and increased decomposers so it can cause large diurnal changes in oxygen concentration

This leads to rises in oxygen throughout the day while algae produce oxygen then reductions throughout the night when algae die and are decomposed

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Where is oxygen depletion most common in eutrophic lakes? (3)

In the hypolimnion because the epilimnion is replaced through the 3 pathways, but the hypolimnion is not

The thermocline acts as a barrier against oxygen diffusion into the hypolimnion from the epilimnion

Internal waves can cause significant mixing of oxygen across the thermocline

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Where does oxygen demand occur? (3)

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Usually occurs in the water column (WOD) and at the sediment water interface (SOD)

Oxygen demand is highest at the sediment water interface

This is because it is where the highest concentration of bacteria and carbonaceous material is location

Sediment layer of oligotrophic lakes (3)

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In oligotrophic lakes there is a thin aerobic sediment layer at the water-sediment interface

As nutrient/organic loading increases, the sediment oxic layer decreases due to biological demands for oxygen

Some chemical reactions use oxygen to create other things like iron, which acts as a “lid” on the top of 10,000 years of sulphides etc. in the sediment

What are all the ways in which oxygen is lost? (4)

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Respiration
Nitrification
Chemical reactions
Degassing and export

Why do you need to be very careful when artificially circulating lakes?

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Because it can lead to fish kills as bacteria respond to increased oxygen availability leading to depletion

Measurement of oxygen demand (4)

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Called BOD5 test - biochemical oxygen demand

It is a lab test for biochemical oxygen demand (BOD)

Measures the oxygen consumed by bacteria as they decompose organic matter

Change in DO concentration is measured over a given period of time in water samples at specific temperature

Standardized at 5 days and 20 degrees in the dark

What are the 2 stages of decomposition in the BOD test?

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Carbonaceous stage

Nitrogenous stage

What is the carbonaceous stage of the BOD test?

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First stage of decomposition that represents the portion of oxygen demand involved in the conversion of organic carbon to carbon dioxide

What is the nitrogenous stage of the BOD test? (2)

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It is the second stage, representing a combined carbonaceous plus nitrogenous demand, when organic nitrogen, ammonia, and nitrite are oxidized to nitrate

Generally begins after about 6 days as there is a lag effect due to microbial adaptation

Equation of the BOD5 value

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BOD=(IDO-FDO)/(VS/VB)

Where:

IDO = initial DO Oof diluted sample (mg/L)
FDO = final DO of diluted sample (mg/L)
VS = volume of sample (ml)
VB = volume of bottle (ml)
BOD = biochemical oxygen demand (mg/L)

Can also be shown by BOD5=(D1-D2)/P

Where:

D1 = initial DO
D2 = final DO of the sample after 5 days 
P = decimal volumetric fraction of the sample used (sample volume/bottle volume) takes into account dilution - equal to 1 if none

Equation for BOD5 at any time

y=L(1-10^-kt) ``` y = BOD5 at any time L = ultimate BOD K = rate factor (determined by experiment) t = time in days ``` Note: BOD is usually 60-65% of the ultimate carbonaceous BOD

Why wouldn’t we use the BOD5 test for limnology? (2)

BOD5 test is too insensitive to detect oxygen demand in the field, except in highly eutrophic lakes Do not waste your money sending water samples to a lab to find out the oxygen demand, as usually < 5mg/L

What is Areal Hypolimnetic Oxygen Demand (AHOD)? (4)

This is how limnologists calculate oxygen deficits in lakes The concept is that organic material produced in the trophogenic zone sinks into the lower strata, decaying and consuming oxygen as it descends to the lower layers The amount of decay, and thus, oxygen demand, mirrors epilimnion productivity to a point Oxygen deficits can then be determined and a consumption rate can be established

What are the uses of AHOD? (3)

Can be used to examine the effects of discharges into the hypolimnion Can be used for comparisons And for designing restoration activities

What is the actual oxygen deficit?

The difference between the observed oxygen content and the saturation value of a similar quantity of water at its observed temperature, salinity, and atmospheric pressure Eg. Lake at 500m and 7C has 4.7mg/L of oxygen Saturation at 7C is 12.1mg/L, but is reduced to 11.4/L with depth of 500m (ie. x 0.94 due to elevation/pressure correction) Therefore the actual deficit is: 11.4-4.7=6.7mg/L

What is the relative oxygen deficit?

The difference between the oxygen content of the hypolimnion during the summer stratification and that measured at the end of spring circulation Easiest Forget about elevation/pressure correction Eg. Lake at 500m, temp of 7C, contains 4.7mg/L oxygen At the end of spring overturn, oxygen was 11.0mg/L So the relative deficit is: 11.0-4.7=6.3mg/L

What is the absolute oxygen deficit?

Difference between the observed oxygen concentration and the saturation value at 4 degrees, at the pressure of the lakes surface Eg. Lake at 500m, hypolimnetic temp of 7C and contains 4.7mg/L oxygen Saturation is 13.1 mg/L at 4C, reduced to 12.3mg/L at 500m (x 0.94 for elevation/pressure correction) So absolute deficit: 12.3-4.7=7.6mg/L

Areal rate (4)

Mg/m2/d Proposed by Storm and Hutchinson so as to remove morphometric influence from the AHOD based on 4 key assumptions The input of external OM addition is significantly higher than internal A constant fraction of the internal OM sinks into the hypolimnion The sum of the OM decomposed in water column and sediments is proportional to the input of OM onto the hypolimnion Respiration is the only process influencing the rate of oxygen depletion in the hypolimnion

Argument against lake morphometry independence (4)

Proposed by Cornet and Rigler Argued that areal rates of oxygen depletion are not independent of lake morphometry They testing Hutchinson’s 4 assumptions, and found that other factors may influence AHODs Found that residual variation can be removed by including mean depth as an independent variable, therefore deep lakes have higher AHODs than shallow lakes Conclusion: AHOD is strongly correlated with mean thickness of the hypolimnion due to the OM having more time for decomposition as it falls

Why were both views of areal hypolimnetic oxygen deficit true? (2)

Lakes with thicker hypolimnion have: Lower volumetric rates of oxygen demand/depletion per cubic meter (lower VOD) due to dilution of OM Higher rates of oxygen demand/depletion per square meter (higher AHOD) due to depth, as deeper lakes having OM that falls longer and so can continue to decompose

What should be done about the discrepancies in the AHOD method?

Minimize their use and just focus on what fish experience (eg. Oxygen concentration in the water such as mg/L)

What are the possible explanations for Hutchinson’s data being flawed? (3)

Oxygen can be introduced into the hypolimnion by internal waves etc. - lowering AHODs in lakes with shallow hypolimnion If respiration in the water column is more efficient than at the sediment surface, deeper lakes could have higher AHODs than shallow lakes Didn’t have enough data

What are the 3 oxygen deficit sampling designs?

Actual oxygen deficit = saturated - observed Relative oxygen deficit = observed in spring - observed in summer Absolute oxygen deficit =saturates at 4 degrees - observed

Quiz 3 Flashcards

(37 cards)