Based, in part, on lectures by M. Lewis, MJ Perry , and C. Roesler.

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).

Guest lecture by Emmanuel Boss, Biological Oceanography, 2006

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=hWhere is frequency [s-1] and h=6.6310-34 plank’s constant.

•Distributed over a continuum of frequencies (wavelengths): =c/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).

Light distribution, top of the atmosphere:

InfraredVisibleUV

0

0.5

1

1.5

2

2.5

200 400 600 800 1000 1200 1400Irra

dia

nce

(W

m-2

nm

-1)

Wavelength (nm)

Fraunhofer lines(absorption in sun’s atmosphere)

• 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.

http://rredc.nrel.gov/solar/standards/am0/E490_00a_AM0.xls

Light distribution at sea level:

Kirk 1994, Fig. 2.1, p. 27

Atmosphere

instruments.com/cosine.gifhttp://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()=Ecos()*

*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

• 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

700

35020 s m

photons d

hcEPAR

UVAUVB

0

0.5

1

1.5

300 350 400 450 500 550 600 650 700

Irra

dia

nce

(W

m-2

nm

-1)

Wavelength (nm)

Visible and UV IrradianceTypical Spectrum for summer in Maine

Visible or PAR

• -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

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

0

20

40

60

80

100

120

140

160

180

200

300 400 500 600 700

Wavelength (nm)

Diffuse Direct Total

0

20

40

60

80

100

120

140

160

180

200

300 400 500 600 700

Wavelength (nm)

Diffuse Direct Total

Equator — March 21 – noon

2184 µmol m-2 s-1 PAR

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’

‘Coastal’

‘Pond’

L

Radiation within the water:

Attenuation of light with depth

zkdzzk

ezEezEzE

z

,~,, 0

,

000

L

Light attenuates approximately exponentially

Note: 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)

a Absorbed Radiant Flux

b Scattered Radiant Flux

to

IncidentRadiant Flux

TransmittedRadiant 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

aCDOM

cCDOM

ainorg part

binorg part

cinorg part

a b c

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:

i

itotal aa

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.

Cynobacteria, Phytoplankton, Macroalgae or Seagrass all produce MAA as strategy of photoprotection.

http://www-med-physik.vu-wien.ac.at/uv/actionspectra/uv_actionspecs.htm#maa

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.

cellchloroplast

chlorophyll

Sosik & Mitchell 1991

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<< and forward peaked with D>.Spectral dependency ~ 0-4

Warning

The next few slides discuss some VERY COMMON misconception among oceanographers.

Plants do not care about relative photon flux but rather absolute (Letelier et al., 2004, L&O):

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

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:

Vertical excursion influenced by:

Mixing in ML

Internal waves at and below the ML base

jerry.ucsd.edu/ LC_and_IW/LC_IW.html

Zaneveld et al., 2001

Some concepts associated with vision and imaging:

Contrast.

Scattering effects?

Absorption effects?

High contrast

Lowcontrast

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

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.

Normalized spectral response of individual photoreceptors

Polarized vision and ecological functions

Secret communication (cuttlefish)Navigation (Bee’s)Detection of nearby water surfaceTarget recognitionBreaking camouflageIncrease 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

www.kman.com/ ActionOptics.htm

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

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.