Light in the Oceans

Question 1

Properties of light

Answer 1

Short wave radiation from sun to surface of ocean in spectrum 300-4000nm, UV to IR
Typical photosynthesis can only make use of wavelengths 400-700nm (PAR)
Irradiance = amount of radiant energy incident per unit or time per unit of area
Rate at which photons arrive at surface is 4000 µmol / m2/ s.
PAR is 40-45% of this

Question 2

Variations in sea surface irradiance

Answer 2

Influenced by:
Time of day
Season
latitude
Clouds
highest daily integrated irradiance is at high lats in summer
Highest mean annual integrated daily irradiances in the sub tropics.

Question 3

Factors influencing sub-surface irradiance

Answer 3

Reflection depends heavily on the angle of the sun:
- if the angle of incidence is between 45 and 90 then <2% is reflected
- if angle is <20 loss by reflection 13-100%
Absorbed or scattered, by water itself or by dissolved substances.
- Particles scatter light and pigments, water and dissolved substances absorb light.
- The net effect is a diffuse light field, which decreases with depth.
- Phytoplankton will absorb light from all directions
Light at each wavelength decreases exponentially with depth.
Rate of decrease with depth (attenuation) is different at different wavelengths.

Question 4

Attenuation of light in the ocean

Answer 4

If we measure downards irradiance E as the flux density of light in the upper surface of a plane parallel to the sea surface, we can describe the reduction or attenuation of light with depth as

Ez =Eo x e^-kz
k= attenuation coefficient (m-1)
z= depth (m)
Eo = irradiance just below surface
Ez = irradiance at depth z

Take natural logs of both sides:
ln(Ez) = ln(Eo) - kz
straight line graph axis = ln % of irradiance by depth
Plot of ln(Ez) against z should be a straight line with intercept of natural log of (Eo) and a gradient corresponding to -k

ln(Ez)= -kz + Eo

Question 5

Absorption by water

Answer 5

Water absorbs strongly in the red and very strongly in the IR
Gives the blue colour

Question 6

Absorption by colour dissolved organic matter

Answer 6

Seawater contains a range of DOM some of which is coloured
DOM is produced and used by microbes and is an important constituent of carbon cycle
CDOM absorbs in the blue
CDOM absorbs strongly in the UV

Question 7

Absorption by particles

Answer 7

Phytoplankton absorb light using a range of pigments
This makes them a key determinant of the colour of seawater

Question 8

Measuring light and attenuation

Answer 8

Secchi disk
Instumentally:
- light measured as Photon Flux Density or Irradiance
- Solar irradiance (PAR) in full sunlight is ~2000µmol/m2/s.
Instruments:
- A flat collector or sensor
- Domed collector (2pi)
- Spherical collector (4pi)

Question 9

Measuring spectral irradiance

Answer 9

Underwater light spectrum can also be measured using multi-wavelength irradiance sensors.

Question 10

Light attenuation

Answer 10

IR and UV are both lost quickly
Red is attenuated fastest followed by green or blue depending on the number of phytoplankton and CDOM in water.

Question 11

Light attenuation in pure water

Answer 11

% water depths:
1% = 300, 0.1% 460
k (m-1) = 0.015
max wavelength (nm) = 450

Question 12

light attenuation in Blue Ocean

Answer 12

% water depths:
1% = 160m, 0.1% 265m
k (m-1) 0.026
max wave (nm) = 500

Question 13

Light attenuation in coastal

Answer 13

% light depths:
1% = 13-27m, 0.1% = 20-40m
k(m-1) = 0.17-0.25
max wave = 550nm