Based, in part, on lectures by M. Lewis, MJ Perry, and C. Roesler. Guest lecture by Emmanuel Boss, Biological Oceanography, 2006

Transcription

1 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, nm. 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).

2 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.

3 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. i i 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 ).

4 Relevant physical characteristics of light: Quantized energy (photon) of a given frequency: E=hν Where ν is frequency [s -1 ] and h= plank s constant. Distributed over a continuum of frequencies (wavelengths): λ=c/ν 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). n 1 c 1 =n 2 c 2, where n is the (real part of the) index of refraction. Refract reflects and diffracts when encountering inhomogeneities: Refract, reflects and diffracts when encountering inhomogeneities: scatters off organisms and the environment (see later).

5 Light distribution, top of the atmosphere: Irradia ance (W m -2 nm -1 ) Fraunhofer lines (absorption in sun s atmosphere) Wavelength (nm) UV Visible Infrared The Solar Constant is 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.

6 Light distribution at sea level: Atmosphere Kirk 1994, Fig. 2.1, p. 27

7 Sun light intensity as function of latitude changes with time of the year. The cosine effect: E(θ)=E cos(θ)* θ θ instruments.com/cosine.gif geog101/uwsp_lectures/lecture_earth_sun_relations.html *Note, from here on E denotes irradiance [W m 2 ], not energy

8 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 PAR = E 0 ( λ) photons dλ 2 λ m s c h 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

9 Visible and UV Irradiance Typical Spectrum for summer in Maine 1.5 Irr radiance (W m -2 nm -1 ) UVB UVA Visible or PAR 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

10 Examples: Vernal Equinox Diffuse Direct Total Wavelength (nm) Diffuse Direct Total Wavelength (nm) Equator March 21 noon 2184 µmol m -2 s -1 PAR 60 N March 21 noon 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?

11 Radiation within the water: Changes in spectral light penetration with depth for different water bodies. What causes the difference? L blue ocean Coastal Pond

12 Radiation within the water: Attenuation of light with depth Light attenuates approximately exponentially E 0 k ( λ, z ) dz 0, 0 e ~ E0 ( z) = E ( λ, z) λ, z < k ( λ ) > z ( λ z) e L Note: in an ocean with constant biogeochemistry and inherent optical properties the diffuse attenuation coefficient, i k, can still change with 1. Sun angle (angle of light rays). 2. Depth (competition between absorption and scattering). WRT PAR, k PAR 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.

13 Loss due to absorption and scattering (attenuation) Φ b Scattered Radiant Flux Φ a Absorbed Radiant Flux Φ o Incident Radiant Flux Φ t Transmitted Radiant Flux Absorption: disappearance of photons along the beam path. Scattering: redirection of photons away from the beam path.

14 The ocean is a dilute medium containing a complex mixture of particulate and dissolved materials a water b water c water a phytop b phytop c phytop a org part b org part c org part a CDOM c CDOM a inorg part b inorg part c inorg part a b c

15 Spectral characteristics of absorbing agents in the oceans: Beer Lambert s law: a ( λ) ( λ) = total a i 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.

16 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. Dff Different species adapt to high h light levels l by (O(day)): (d 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 h quenching -reduction of fluorescence in cells that t 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?

17 UV exposure is damaging for all organisms due to direct damage to DNA which absorbs around nm. Enhances egg mortality. Can also induce cancer in marine organisms (e.g. fish). Mammals evolved protective strategies such as increased pigmentation. phytoplankton p 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. Other absorbing substances in the water (CDOM, tripton) absorb UV.

18 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 cell chloroplast chlorophyll

19 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 ~ λ 0-4

20 Warning l E The next few slides discuss some VERY COMMON misconception among oceanographers.

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

22 Pigment biomass is often not phytoplankton (volume) biomass 0 All data August 50 all data Aug chl per volume chl [mg m 3 ] biovolume [10 6 mm 3 m 3 ] Fennel and Boss, Data from (C. D. McIntyre)

23 Chlorophyll fluorescence is NOT chlorophyll Falkowski and Raven, 1997

24 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:

25 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

26 Some concepts associated with vision and imaging: Contrast. Scattering effects? Absorption effects? High contrast Low contrast

27 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 pt rs com/files/photopiceffic htm Changes among humans and as function of light history. Some organisms (shrimp) have up to five different spectral receivers.

28 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:

29 Marine birds could use polarization to see through the surface: ActionOptics.htm Bikini bottom is not the same without my glasses Some shrimp send sexual messages through polarized signals

30 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 gprey/food, locating mate, and evading predators. Bulk/individual optical properties and imaging are Bulk/individual optical properties and imaging are common strategies to study biological oceanography.

31 Useful references: Falkowski, P. G., and J. A. Raven Aquatic photosynthesis. Blackwell Science, Oxford, UK. Cambridge University Press. Kirk, J. T. O., Light and Photosynthesis in Aquatic Ecosystems. Mobley, C. D Light in Water, Academic Press. Shifrin, K. S., Physical Optics of Ocean Water. Spinrad R. W., Carder K. L. and M. J. Perry., Ocean Optics. Oxford Univeristy i Press. Wolken, J. J Light Detectors, Photoreceptors, and Imaging Systems in Nature. Oxford University Press.