5. Light, Heat, & Stratification

Question 1

light attenuation is ___ with depth; loss of light can be linearized by ___ transformation

Answer 1

exponential
log

Question 2

Once we know the light attenuation coefficient (n), it can be used to calculate what?

Answer 2

light remaining at any depth

*higher values of n represent greater light attenuation

Question 3

What is the Secchi Disk?

Answer 3

used to measure penetration of visible light (PAR).
Can be used to estimate the light penetration coefficient using the depth to which the disk can still be seen (zsd) and the formula: n=1.7/zsd

Question 4

How can light attenuation coefficients be measured more accurately than the Secchi Disk method?

Answer 4

submersible spectroradiometer or light meters, which measure light just below the surface and at a known depth

Question 5

The attenuation conversion factor for the Secchi Disk ranges between 0.5-4.0. Why?

Answer 5

Depends on the optical quality of the lake

0.5= turbid
4.0= brown lakes

Question 6

T/F

wavelength-specific attenuation coefficients can be calculated

Answer 6

true

Question 7

In clear lakes, n values (attenuation coefficients) are ____(longer/shorter) for longer wavelengths (red) and ____ for shorter wavelengths (blues)

Answer 7

longer for red
shorter for blue

Question 8

Euphotic/photic zone=

Answer 8

the upper layer of water in which PAR (penetration of visible light) is sufficient to support net positive photosynthesis and O2 production

Question 9

The compensation depth=

Answer 9

defines the lower limit of the photic zone where photosynthesis= respiration

1% incident light level

Question 10

Aphotic zone=

Answer 10

relatively anoxic area dominated by respiration and heterotrophs

Question 11

Littoral Zone=

Answer 11

the region where light at the lake bottom is sufficient to allow rooted aquatic plants and benthic algae to grow

Question 12

The littoral zone is typically defined by:

Answer 12

the 1% incident light level (plants won’t be able to grow at the lake bottom without light, so stops at the compensation depth)

Question 13

Pelagic Zone=

Answer 13

the region where rooted plant growth is not possible. Photosynthesis only occurs in the water column where the light reaches (not on the lake bottom)

Question 14

Light penetration in streams and rivers is largely regulated by:

Answer 14

particulate and dissolved matter caused by mixing

Question 15

Solar radiation in water works to do 3 main things:

Answer 15

1. excite pigments (photosynthesis)
2. Break molecular bonds
3. generate heat*

Question 16

In temperate regions during winter, lakes can freeze when the water throughout is less than __ degrees. This allows the surface waters to decline to __ degrees.

Answer 16

4
0

Question 17

When does lake freezing occur?

Answer 17

On a low wind day once the water throughout is less than 4 degrees.