SMS 204: Physics for marine sciences�Today’s topic: light and vision
Instructor: E. Boss
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What is light?
Light: electromagnetic radiation (energy) extending from ~300nm (UV) to ~800nm (IR). Visible light, 400-700nm.
Why should organisms care about (be affected by) light?
An available form of energy (sometimes damaging).
Enables sensing (phototaxis, vision).
Affects physical stratification (warms water).
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Available form of energy near the ocean surface.
Used as source of energy by:
http://www.reefkeeping.com/issues/2002-06/bcap/feature/index.php
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Some ecological ‘behaviors’ associated with light:
Phototropism is plant growth towards a light source.
Photomorphogenesis is the light-induced control of plant growth and differentiation. Certain wave lengths function as a signal causing the generation of an information within the cell that is used for the selective activation of certain genes.
Photoperiodism is the ability of plants to measure the length of periods of light. Certain species (short-day plants) stop flowering as soon as the day length has passed a critical value, while long-day plants begin to flower only after such a value has been passed.
Circadian rhythm is the fact that many function of organism are regulated by the diel cycle. Artificial change of light periodicity often leads to change in the circadian rhythm (e.g. the division cycles of cyanobacteria and diatoms).
Phototaxis is the induction of movement of organisms to or from light. Diel migrations are observed in many marine organisms (think dinoflagellates, zooplankton, visual predators etc’).
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Relevant physical characteristics of light:
Where ν is frequency [s-1] and h=6.63 ⋅10-34 plank’s constant.
Where c is the speed of light [m s-1] and l the wavelength [m nm A].
n1c1=n2c2, where n is the (real part of the) index of refraction.
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The Solar Constant is 1366.1 W m-2. It is defined as the amount of solar radiation on a surface perpendicular to the solar beam, at the outer limit of earth’s atmosphere, at the mean sun-earth distance.
https://www.sciencedirect.com/topics/physics-and-astronomy/solar-spectra
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http://www.uwsp.edu/geo/faculty/ritter/
geog101/uwsp_lectures/lecture_earth_sun_relations.html
Sun light intensity as function of latitude changes with time of the year.
The cosine effect: E(θ)=E⊥cos(θ)*
*Note, from here on E denotes irradiance [W m2], not energy
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Solar Radiation Incident on the Ocean
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UVA
UVB
Visible and UV Irradiance�Typical Spectrum for summer in Maine
ABOUT 45% OF INCIDENT SOLAR RADIATION IS PAR
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Examples: Vernal Equinox
Sun angle accounts for a 50% reduction. Atmospheric pathlength is also longer.
Diffuse irradiance is enriched in the shorter, scattered wavelengths
60°N — March 21 — noon
901 µmol m-2 s-1 PAR
Equator — March 21 – noon
2184 µmol m-2 s-1 PAR
Why is the color of the sky and the ‘blue’ oceans blue?
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Radiation within the water:
Changes in spectral light penetration with depth for different water bodies.
What causes the difference?
‘blue ocean’
‘Coastal’
‘Pond’
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Radiation within the water:
Attenuation of light with depth
Light attenuates approximately exponentially
Note: in an ocean with constant biogeochemistry and optical properties the diffuse attenuation coefficient, k, can still change with:
1. Sun angle (angle of light rays).
2. Depth (competition between absorption and scattering).
WRT PAR, kPAR is certain to change with depth (Morel, 1988, JGR) as different parts of the spectrum attenuate at different rates (e.g. after a few meters very little NIR is left due to water absorption) to contribute to PAR.
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Loss due to absorption and scattering (attenuation)
Φa Absorbed Radiant Flux
Φb Scattered Radiant Flux
Φt
Φo
Incident
Radiant
Flux
Transmitted
Radiant
Flux
Absorption: disappearance of photons along the beam path.
Scattering: redirection of photons away from the beam path.
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The ocean is a dilute medium containing a complex mixture of particulate and dissolved materials
What interacts with light?
water, algae, dissolved organics, non-algal particles (organic and inorganic).
+ Fluorescence.
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Spectral characteristics of absorbing agents in the oceans:
These absorbing agents affect phytoplankton by ‘competing’ on photons (as well as removing potentially harmful ones in the UV).
These absorbing agents affect visual organism by changing the spectrum of available light.
Beer Lambert’s law:
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Phytoplankton chromatic adaptation:
Changing number of pigment complexes, amount of pigments and types of pigments in response to changing light.
Different species adapt to the low light levels by (O(day)):
Different species adapt to high light levels by (O(day)):
Short term adaptations (O(sec-min)):
NB: Macro- and Micro-nutrient availability affects the ability of cells to cope with changes in light.
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Scattering:
Affecting light propagation, refraction, reflection and diffraction
Increases with ‘index of refraction’, a measure of how different the light speed is within the particle.
Increases with size. Mass-normalized scattering has a peak at micron-sized particles.
Angular scattering changes with size. Symmetric when D<<λ and forward peaked with D>λ.
Spectral dependency ~ λ0🡪-4
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Pigment biomass is often not phytoplankton (volume) biomass
Fennel and Boss, 2003. Data from 1989-2000 (C. D. McIntyre)
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Chlorophyll fluorescence is NOT chlorophyll:
Falkowski and Raven, 1997
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Some concepts associated with vision and imaging:
Contrast.
Scattering effects?
Absorption effects?
High contrast
Low
contrast
Pin-hole optics - light travels in straight lines
Eyes: in many organisms, including jellyfish…
SMS 204: Integrative Marine Sciences II
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The human eye perceives photopic parameters, that is, it observes light spectra convolved with the spectral sensitivity of the human eye.
UV NIR and polarized…
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Polarized vision and ecological functions
Secret communication (cuttlefish)
Navigation (Bee’s)
Detection of nearby water surface
Target recognition
Breaking camouflage
Increase detection range (enhance contrast)
This ctenophore plankton can be squid prey. Almost transparent to normal vision (left), it acquires good contrast between crossed polarizer (center), and even better with combined processing (right). �From: http://polarization.com/octopus/octopus.html
Common to crustaceans, cephalopods and some fishes
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Marine birds could use polarization to see through the surface:
Some shrimp send sexual messages through polarized signals
http://oceanexplorer.noaa.gov/explorations/04deepscope/background/polarization/polarization.html
Bikini bottom is not the same without my glasses
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Summary: