lec 5

Question 1

what is the attenuation coefficient (kd)? intuitively, how can we interpret this?

Answer 1

relationship bw depth and incident light.
equation is
kd = [ln(l0) - ln(lz)] / z where:
- l0 is radiation at depth 0
- lz is radiation at depth z
- z is depth that we want to measure

attenuation coeff indicates ease of light penetrating water –> we don’t have to do all the math to understand that deeper lakes have lower attenuation coeff, since light doesn’t penetrate all the way down.

Question 2

what is a secchi disc? what are its strengths and limitations? what is an alternative we can use?

Answer 2

secchi discs are tools used to measure penetration of visible light. drop it into the water until u can’t see it, bring it back up til u can, rinse and repeat til u got that good mid point.

kd can be estimated as 1.7/z(sd)

+ good for quick estimation
- equation is general, but conversion factor u need fluctuates depending on various conditions such as turbidity and colour

alternative measurement tool is spectroradiometer, which measures PAR.

Question 3

what is the diff bw photic and euphotic zone? what is the compensation depth? what is the aphotic zone?

Answer 3

nothing really, they are just diff names for an area that receives enough light for photosynth > respiration (net positive photosynth = net O2 prod).

compensation depth is where photosynth = respiration.

aphotic zone is where there is more respiration than photosynth, dominated by heterotrophs.

Question 4

how can we tell the diff bw the littoral and pelagic zones?

Answer 4

littoral zone has plants growing in the shore and lakebed, whereas pelagic zone has no growth in the lakebed. to determine, it is best to think in terms of rooted plant growth.

Question 5

what do seasons look like in temperate regions? why does summer and winter differ? what does dimictic lake mean?

Answer 5

OK! so temperate regions have cold winters and hot summers. these oscillations have big implications for what the body looks like.

• winter has pretty uniform cold water w a layer of ice.
• spring/fall [????]
• summer has a steep thermocline, w warm water near surface and cold water towards bottom

lakes stratify (layer) when kinetic energy from wind is not strong enough to mix the entire lake. in winter, water is denser since it is colder, mixing naturally since cold water sinks and warmer water near the surface cools down,

however in summer, water becomes less dense as it heats up . however, it takes more energy for water to heat up as u move further away from peak density (4 degrees), so natural mixing will not occur. kinetic energy of wind only mixes so deep
–> near the surface there will be lots of mixing, but the cold water won’t really get touched.

btw this is called a dimictic lake :)

Question 6

name the three parts of the summer temperature graph. (ok technically 4)

Answer 6

epilimnion: the part that mixes and is warm near the surface
metalimnion: the part that transitions bw warm and cold
hypolimnion: the part that doesn’t really get mixed

THERMOCLINE: the steepest transition bw temperatures per depth increment

Question 7

which is faster: horizontal or vertical mixing?

Answer 7

horizontal! ez :)
[why? idk..]

Question 8

are stratification graphs realistic or is it yet another standard for women to strive for

Answer 8

epi- is typically well defined but meta- and hypo- may not be, and thats okay! u can typically still see most stratification

Question 9

assuming all else equal, of clear and colourful lakes, which will have a deeper epi-? meta-? hypo? why?

Answer 9

clear lakes will typically have deeper epi-, meta-, and presumably -hypo. this is bc light penetrates deeper in clear lakes, whereas in colourful lakes light is reflected and absorbed by colour (DOM/DOC, organisms, inorganic matter, etc).

[I THINK THE IMPLICATION IS THAT LIGHT = HEAT]