Transcript 2 s -1 PAR - The University of Maine In

Based, in part, on lectures by M. Lewis, MJ Perry , and C. Roesler.
Guest lecture by Emmanuel Boss, Biological Oceanography, 2006
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).
An available form of energy (sometimes damaging).
Used as source of energy by:
•Prokaryotes (with at least 3 different photosynthetic pathways with
different electron donors, Karl et al., Nature, 2002).
•Eukaryotes.
•Multicellular plants (macro Algae).
•Symbiotic algae (e.g. Zooxantella in corals).
•Some Sea Slugs.
http://www.reefkeeping.com/issues/2002-06/bcap/feature/index.php
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’).
Relevant physical characteristics of light:
•Quantized energy (photon) of a given frequency: E=hn
Where n is frequency [s-1] and h=6.6310-34 plank’s constant.
•Distributed over a continuum of frequencies (wavelengths): l=c/n
Where c is the speed of light [m s-1] and l the wavelength [m nm A].
•Polarized (has directionality)  affects vision and camouflage.
•Propagates in vacuum (unlike sound). Slows down in water (changes
wavelength).
n1c1=n2c2, where n is the (real part of the) index of refraction.
•Refract, reflects and diffracts when encountering inhomogeneities:
scatters off organisms and the environment (see later).
Irradiance (W m
-2
-1
nm )
Light distribution, top of the atmosphere:
2.5
Fraunhofer lines
2
(absorption in sun’s atmosphere)
1.5
1
0.5
0
200
400
UV
600
Visible
800
1000
Wavelength (nm)
1200
1400
Infrared
http://rredc.nrel.gov/solar/standards/am0/E490_00a_AM0.xls
•
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.
Light distribution at sea level:
Atmosphere
Kirk 1994, Fig. 2.1, p. 27
Sun light intensity as function of latitude changes
with time of the year.
The cosine effect: E(q)=Ecos(q)*
q
q
instruments.com/cosine.gif
http://www.uwsp.edu/geo/faculty/ritter/
geog101/uwsp_lectures/lecture_earth_sun_relations.html
*Note, from here on E denotes irradiance [W m2], not energy
Solar Radiation Incident on the Ocean
•
Transmission through the atmosphere depends on:
• Solar zenith angle (latitude, season, time of day)
• Cloud cover
• Atmospheric pressure (air mass)
• Water vapor
• Atmospheric turbidity
• Column ozone (important for UV-B)
• Albedo – scattering of light back to the atmosphere from below
l
 photons 
PAR   E0 l  dl 
2

hc
m
s

350
700
•
•
Midsummer Solar Irradiance at 45°N (midday)
• about 400 W m-2 (PAR, energy units)
• 1900 µmol m-2 s-1 (PAR, quanta)
Midwinter Solar Irradiance at 45°N
• about 130 W m-2; 600 µmol m-2 s-1
Visible and UV Irradiance
Typical Spectrum for summer in Maine
Irradiance (W m
-2
-1
nm )
1.5
1
0.5
0
UVB
300
Visible or PAR
UVA
350
400
450
500
550
600
650
700
Wavelength (nm)
•
•
-Visible: 400 to 700 nm
– Also called Photosynthetically Available Radiation (PAR)
ABOUT 45% OF INCIDENT SOLAR RADIATION IS PAR
-Ultraviolet
– UVA 315 (or 320) to 400 nm, UVB 280 to 320 nm, UVC 200 to 280 nm
Examples: Vernal Equinox
Equator — March 21 – noon
60°N — March 21 — noon
2184 µmol m-2 s-1 PAR
901 µmol m-2 s-1 PAR
Sun angle accounts for a 50% reduction. Atmospheric pathlength is also longer.
Diffuse irradiance is enriched in the shorter, scattered wavelengths
Why is the color of the sky and the ‘blue’ oceans blue?
Radiation within the water:
Changes in spectral light
penetration with depth for
different water bodies.
What causes the difference?
‘blue ocean’
L
‘Coastal’
‘Pond’
Radiation within the water:
Attenuation of light with depth
Light attenuates approximately exponentially
z
E0 l , z   E0 l , z e
 k l , z dz

0
~ E0 l , z e  k l  z
LNote: in an ocean with constant biogeochemistry and inherent 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.
Loss due to absorption and scattering
(attenuation)
Fb Scattered Radiant Flux
Fa Absorbed Radiant Flux
Fo
Incident
Radiant
Flux
Ft
Transmitted
Radiant
Flux
Absorption: disappearance of photons along the beam path.
Scattering: redirection of photons away from the beam path.
The ocean is a dilute medium containing a
complex mixture of particulate and
dissolved materials
awater
bwater
cwater
aphytop
bphytop
cphytop
aorg part
borg part
corg part
a
b
aCDOM
cCDOM
c
ainorg part
binorg part
cinorg part
Spectral characteristics of absorbing agents in the oceans:
Beer Lambert’s law:
atotal l    ai l 
i
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.
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)):
•Producing more pigments.
•Producing accessory pigments.
Different species adapt to high light levels by (O(day)):
•Reducing pigmentation
•Producing photoprotective pigments
Short term adaptations (O(sec-min)):
•Migration of chloroplasts to the center of the cell (self-shading)
•Dissipation of excess photons to heat
•Nonphotochemical quenching - reduction of fluorescence in cells that have
recently been exposed to high light levels.
NB: Macro- and Micro-nutrient availability affects the ability of cells to cope
with changes in light.
What are the implications to the use of [chl] as a biomass indicator?
UV exposure is damaging for all organisms due to direct damage to DNA
which absorbs around 260-280nm. Enhances egg mortality. Can also induce
cancer in marine organisms (e.g. fish).
Mammals evolved protective strategies such as increased pigmentation.
phytoplankton have evolved protective pigments as well – some of which are
the microsporin-like amino acids (MAA).
Typical UV-absorption spectrum
of MAA sunscreen analogues.
http://www-med-physik.vu-wien.ac.at/uv/actionspectra/uv_actionspecs.htm#maa
Cynobacteria, Phytoplankton, Macroalgae or Seagrass all produce MAA as
strategy of photoprotection.
Other absorbing substances in the water (CDOM, tripton) absorb UV.
Pigment packaging (Duysen, 1957).
The more pigment molecules are stuffed into a cell the less efficient the
pigments are in harvesting light (light harvesting efficiency goes down).
Effect is more dramatic the larger the cell is.
Sosik & Mitchell 1991
chlorophyll
cell
chloroplast
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<<l and forward
peaked with D>l.
Spectral dependency ~ l0-4
Warning
The next few slides discuss some VERY COMMON misconception among
oceanographers.
The Euphotic zone should but be
given in relative light level.
Euphotic zone: the zone that extends
from the surface to the euphotic
depth. The depth at which light is
reduced to 1% of its surface value
(sometimes 0.1% light level is used).
May occur at depths exceeding 100 m
in oligotrophic open-ocean waters or it
may be a few meters in eutrophic or
turbid waters
Almost all of primary production in the
water column occurs in the euphotic
zone
Plants do not care about relative
photon flux but rather absolute
(Letelier et al., 2004, L&O):
Pigment biomass is often not phytoplankton (volume) biomass
Fennel and Boss, 2003. Data from 1989-2000 (C. D. McIntyre)
Chlorophyll fluorescence is NOT chlorophyll
Falkowski and Raven, 1997
Warning: the observed chlorophyll and photosynthesis (P-E curves)
distribution as function of depth should NOT be thought about in terms of
a single species/culture of phytoplankton.
Species and sub-species (ecotypes) stratify according to light and nutrient
characteristics (e.g. Lisa Moore, USM, for prochlorococus).
Lisa Campbell, TAMU:
Light history of individual cells:
Zaneveld et al., 2001
Vertical excursion influenced by:
Mixing in ML
Internal waves at and below the ML base
jerry.ucsd.edu/ LC_and_IW/LC_IW.html
Some concepts associated with vision and imaging:
Contrast.
Scattering effects?
Absorption effects?
High contrast
Low
contrast
The human eye perceives photopic parameters, that is, it
observes light spectra convolved with the spectral
sensitivity of the human eye.
THE PHOTOPIC LUMINOUS EFFICIENCY FUNCTION
Normalized
spectral response
of individual
photoreceptors
http://www.4colorvision.com/files/photopiceffic.htm
Changes among humans and as function of light history.
Some organisms (shrimp) have up to five different spectral receivers.
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)
Common to crustaceans,
cephalopods and some fishes
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
Marine birds could use polarization to see through the surface:
www.kman.com/ ActionOptics.htm
Bikini bottom is not the same
without my glasses
Some shrimp send sexual messages
through polarized signals
http://oceanexplorer.noaa.gov/explorations/04deepscope/background/polarization/polarization.html
Summary:
•Light is one of the primary determinant of habitat in
the oceans.
•Primary energy source of the biogenic food web.
•Light is also used for ecological functions such as
finding prey/food, locating mate, and evading
predators.
•Bulk/individual optical properties and imaging are
common strategies to study biological oceanography.
Useful references:
Falkowski, P. G., and J. A. Raven. 1997 Aquatic photosynthesis. Blackwell
Science, Oxford, UK. Cambridge University Press.
Kirk , J. T. O., 1994. Light and Photosynthesis in Aquatic Ecosystems.
Mobley, C. D. 1994. Light in Water, Academic Press.
Shifrin, K. S., 1988. Physical Optics of Ocean Water.
Spinrad R. W., Carder K. L. and M. J. Perry., 1994. Ocean Optics. Oxford
Univeristy Press.
Wolken, J. J. 1995. Light Detectors, Photoreceptors, and Imaging Systems in
Nature. Oxford University Press.