Ecology of UV

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Photoinhibition

Reciprocity

Weighting functions

Effects on Phytoplankton

Effects on Protozoa

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The Ecology of UV - An Introduction
prepared by Craig Williamson

UV radiation and ozone

The potential impact of UV radiation on natural ecosystems received little attention by ecologists until significant depletion of stratospheric ozone was observed in Antarctica (the "ozone hole") in the early 1980's (Farman, Gardiner et al. 1985). More recently less severe ozone depletion has been observed in the Arctic and at both south and north temperate latitudes (Herman 1983; Kerr and McElroy 1993; Rex, Harris et al. 1997). These global decreases in ozone have led to concerns about increased exposure of humans and natural ecosystems to damaging UV radiation because UV is strongly absorbed by ozone. UV radiation is the shortest wavelength, and thus highest energy (per photon) solar radiation reaching the Earth's surface. It includes near UV (UV-A 320-400 nm), mid UV (UV-B 280-320 nm), and far UV (UV-C 200-280 nm). The high energy of UV is what makes it potentially very damaging to living organisms.

In spite of the high photon-specific energy of UV, it makes up only about 6% of the solar energy reaching the Earth's surface. About 90% of this UV reaching the Earth's surface is longer wavelength UV-A. The rest is UV-B. No UV-C makes it through the atmosphere. The intensity of the different wavelengths (the spectral composition) of UV reaching the Earth depends on atmospheric conditions, sun angle, latitude, and elevation. Many atmospheric gases and aerosols selectively absorb the shorter wavelength UV-B. Ozone is particularly effective at absorbing short wavelength UV radiation, and as a consequence almost no UV less than 295 nm reaches the Earth's surface. This selective absorption of UV by ozone also leads to changes in the spectral composition of UV with increased elevation due to a decrease in the thickness of the atmosphere through which sunlight must pass. For example, for every 1000 m increase in elevation, the incident 300 nm UV-B increases by about 24% while 370 nm UV-A increases by only about 9% (Blumthaler, Ambach et al. 1997).

On a global scale, the Earth's atmosphere has several layers; the layer closest to the Earth's surface is the troposphere (0~15 km) and the subsequent layer is the stratosphere (~15-50 km). The thicknesses of the troposphere and the stratosphere vary with latitude. The troposphere is about twice as thick at the equator as at the poles, while the stratosphere is much thicker at the poles than at the equator. Most of the ozone in the atmospheric "column" is in the stratosphere. Ozone is also present in the troposphere, and is produced here by the photochemical reactions between UV radiation and smog. This may lead to a "pollution shield" in urban areas where UV may be somewhat lower than in more pristine habitats. Tropospheric ozone is, however, a serious irritant to human respiratory systems and damages other animals as well as plants. For more information an excellent and very readable electronic textbook on stratospheric ozone is available online at: http://see.gsfc.nasa.gov/education/SEES/strat/class/S_class.htm
This Stratospheric Ozone Electronic Textbook was funded, developed, written, and edited by members of NASA's Goddard Space Flight Center Atmospheric Chemistry and Dynamics Branch (Code 916).

Estimating UV Exposure and Impacts in
the Lab and Field

The primary target of UV damage in living organisms is generally considered to be DNA, an important information macromolecule in all living organisms. Most organisms have elaborate defense mechanisms to avoid, reduce, or repair UV damage, but these responses often come at a price. For example, avoiding exposure to UV may reduce access to optimal habitats, and the production of UV photoprotective compounds or repair of DNA damage are likely to have associated ecological and metabolic costs. Some of the goals of the UV-Lakes Project are to find out how sensitive organisms at different trophic levels are to UV damage, how they respond to UV stress, and how UV may thus alter an organism's interactions with other environmental stressors. A second major focus of the project is how climate change may alter both temperature regimes of UV exposure as well as concentrations and optical properties of UV-absorbing dissolved organic matter (DOM) in lakes.

Ecological studies of UV radiation require careful quantification of UV exposure whether working in the laboratory or in the field. In making these measurements it is important to realize that one must consider not only the quantity of UV (the energy content of the UV, usually measured in J m-2) but also the quality of the UV (the spectral composition which measures how much energy there is at each wavelength). There are two reasons for this. First, the potential damage caused by UV is heavily wavelength dependent. For example, for survival in the cladoceran Daphnia, UV at 320 nm is about 100 times more damaging per unit energy than is UV at 370 nm (Williamson, Neale et al. 2001). These wavelength-dependent effects of UV are the domain of spectral weighting functions. Second, there are tremendous variations in the spectral composition of UV in nature as a function of time of day, season, atmospheric conditions, latitude, and elevation to name just a few. Some of these relationships have been discussed in the literature (Caldwell, Robberecht et al. 1980; Williamson, Neale et al. 2001). In aquatic ecosystems the dissolved and particulate substances in the water show strong variations in space and time that may further modify the spectral composition of UV reaching aquatic organisms. Dissolved organic matter (DOM) is a particularly strong absorber of underwater UV in inland waters. A major focus of the UV-Lakes Project is how the relationships between UV, temperature and other environmental variables will be modified by climate change {Williamson and Zagarese 2003).

One of the major goals of studying the ecology of UV is to predict how organisms respond to environmental variations in UV exposure related to ozone depletion or other environmental factors. However, one of the greatest challenges in these studies is our limited ability to accurately reproduce both the intensity and spectral composition of sunlight in the laboratory so that we can do well-controlled mechanistic studies. Both the spectral composition and the intensity of most UV lamps are very different from natural sunlight. One exception is the xenon arc lamp which can mimic the spectral composition and intensity of solar radiation quite well. Limitations of xenon arc lamps include their expense, uneven illumination fields, intense heat output, and potential danger of explosion. Diel (daily) variations in the intensity and spectral composition of natural sunlight due to variations in sun angle, cloud cover, or other environmental conditions are also hard to mimic even with xenon arc lamps. In spite of these limitations they are very useful and are part of the spectral weighting function component of the UV-Lakes Project.

In addition to the issue of properly quantifying wavelength and intensity of UV exposures, it is also important to establish whether the effect of a given UV exposure or dose is independent of the dose rate, commonly known as the reciprocity principle. When reciprocity occurs, a low dose rate of UV for a long time causes the same amount of damage as a high dose rate of UV for a short time. Without reciprocity, further studies and special modeling approaches are needed. Establishing effective modeling and predictive capabilities for UV impacts in the absence of reciprocity is another goal of the UV-Lakes Project. A UV exposure facility is being built at Lehigh University to enable us to do more controlled mechanistic studies of acclimation and the temperature dependent effects of UV.


Units of UV Measurement

Understanding the measurement of UV radiation is not as simple as it first may appear. In fact it can be a jungle due to the use of different units and symbols by investigators in different fields. The section below is an attempt to sort out and simplify some of the useful basic terminology and units of measurement for those studying the ecology of UV.

Radiant energy can be measured either as quanta (number of photons), or as energy (Joules), and can be expressed per unit area, per unit time, or both. The basic units, symbols and terms used by the Système International d'Unités (SI) to measure radiant energy are presented in the Table of Radiant Energy Units.

In order to study the impact of UV on living organisms one needs to quantify UV exposure. While studies in photosynthesis usually use quanta (number of photons) to measure photosynthetically active (or "available") radiation (PAR), units of energy (Joules) are more useful in studying the potential damaging effects of UV radiation. The total energy absorbed per unit area during a given exposure time is usually referred to as the dose (energy per unit area). Thus dose is simply the dose rate (energy per unit time and area) multiplied by the exposure time. As a crude analogy, think about rain falling into an empty swimming pool. The harder it is raining, the higher the dose rate. As long as the pool is level, the final depth (= volume per unit area) of water in the pool when it stops raining represents the total dose. This dose may accumulate over the period of an hour, a day, or several days depending on the dose rate (how hard it is raining). The photometrically equivalent terms for dose rate are irradiance if a flat (vectorial) sensor is used or fluence rate if a spherical (scalar) sensor is used. Irradiance is thus the radiant flux (energy per unit time) received per unit area on a surface, and is not to be confused with the term radiance, which is the radiant flux emitted per unit solid angle (steradian) from a point source. The term intensity is often not carefully defined but can be equated to radiant flux (Kirk 1994, page 49). These relationships can be better understood by studying a Table of Radiant Energy Units.

The Biospherical Instruments PUV and BIC radiometers used in the UV-Lakes Project measure irradiance in units of µW cm-2 nm-1. These units can be converted to SI units (W m-2 nm-1) by multiplying the PUV or BIC values by 0.01.


Water Transparency and UV Attenuation

The transparency of natural waters to sunlight is determined by the concentrations of optically active dissolved and particulate substances in the water as well as the optical properties of water itself. The transparency of inland waters is heavily influenced by land use patterns and geomorphology of the surrounding watershed as well as by hydrologic changes related to climate change.

Natural waters have what are often referred to as both inherent and apparent optical properties. Inherent optical properties (IOP) are a function of the water and optically active substances in it and are NOT influenced by the geometric structure of the light fields since they represent coefficients relative to an infinitesmally thin layer of water. IOP are usually determined in the laboratory and include the following (all units are m-1):

c = a + b

Apparent optical properties (AOP) on the other hand are derived from measurements of natural light fields in a water body with instruments such as the Biospherical PUV and BIC profiling radiometers that we use in the UV-Lakes Project. They account for the geometry of the light fields, including absorption and scattering, and are represented as "K" functions. The most common of these is the diffuse attenuation coefficient for downwelling irradiance (Kd). Irradiance at a given depth (EZ) is a function of the irradiance at the surface (E0), the diffuse attenuation coefficient, and the depth interval (Z) according to the following relationship, where e is the base of the natural logarithms:

Thus the diffuse attenuation coefficient can thus be estimated as follows:

The attenuation depth (Z1%lamda ) for a given wavelength (lamda) is defined as the depth to which 1% of surface light penetrates in the water column, and can be calculated from the previous equation. The solution is:

This same approach can be used to estimate a 10%, 50%, or any other attenuation depth. However, using the 1% attenuation depth provides a convenient parallel to the use of the 1% depth of penetration for PAR used by phytoplankton ecologists to represent the euphotic zone and approximate compensation depth. When estimating any attenuation depths care must be taken to use only depth ranges where the Kd is constant in order to avoid errors resulting from extrapolation into strata with different attenuation properties. The Kd can be increased by dissolved or particulate substances in the water such as dissolved organic carbon or phytoplankton.

There is variation in the depth to which different wavelengths of light penetrate water. UV researchers often report the 1% attenuation depth for 320 nm UV radiation (Z1%320) because this wavelength represents the boundary between UV-B and UV-A radiation and is within the region of peak biological effectiveness when both the spectral irradiance of sunlight and the biological weighting function for damage to organisms are considered. The Z1%320 for inland waters can vary from a few cm to tens of meters depending on water transparency and time of year. For our study lakes in the Poconos of PA, the midsummer Z1%320 values vary from 0.5 m in Lake Lacawac to greater than 15 m in Lake Giles. In Lake Giles the Z1%320 may range from about 3 m in the early spring following ice out to a depth of 17 m in mid July. Similar but less intense seasonal variations in UV attenuation have been observed in Lacawac (Williamson, Hargreaves et al. 1999), and are likely due in large part to photobleaching (Morris and Hargreaves 1997).

In the laboratory spectrophotometers can be used to estimate the absorption properties of natural waters. The foundation of absorption measurements in aqueous media is Beer's law which relates absorbance (D, = optical density) and transmittance (T), as a function of absorptivity (epsilon ), pathlength (r), and the concentration of the absorbing substance (C):

Thus note that absorptivity (epsilon ) is a constant property for a given substance, while absorbance is a function of the absorptivity, the pathlength, and the concentration of the absorbing substance. Laboratory spectrophotometers are generally considered to measure absorbance, or optical density. However, this quantity is more properly termed attenuance (= absorption plus scattering). This difference is trivial in the absence of scattering by particulates. The caveat in working with natural waters is that samples must be filtered before taking measurements of dissolved absorbance.

Note that absorbance is measured with log base 10 while absorption coefficients (a), scattering coefficients (b), beam attenuation coefficients (c), and diffuse attenuation coefficients (Kd) are estimated with the natural log. This means that to convert absorbance to an absorption coefficient, one must convert log10 to ln. For example:

(D = 0.4343 ar), (a = 2.303 D/r)

When making conversions, note that pathlength (r) in a laboratory spectrophotometer is often expressed in units of cm-1 while in the field units of m-1 are more common.


Further reading:
(Hargreaves, 2003)
(Kirk, Hargreaves et al. 1994)
(Kirk, 1994)
(Caldwell, Robberecht et al. 1980)
(Williamson, 1994)


UV, DOM, and Climate Change in
Aquatic Ecosystems

Numerous studies have clearly demonstrated that dissolved organic matter (DOM) plays a primary role in regulating the attenuation of UV radiation in natural waters. Major exceptions to this occur in open oceans and high elevation lakes where the inputs of terrestrially-derived organic matter are so low that phytoplankton play a major role in regulating water column optics. Long-term studies in Canadian lakes by David Schindler and others have demonstrated that hydrologic changes associated with climate change may alter DOM inputs into lakes and thus influence the underwater UV environment. A major focus of the UV-Lakes Project is this regulatory role of DOM in UV attenuation as well as food web dynamics. Below is a brief summary of some of the terminology and concepts that provide the foundations for these relationships.

Dissolved organic matter (DOM) comprises a complex array of carbon containing compounds derived from the decomposition of dead organisms. DOM is usually measured in the laboratory by oxidation of the fixed carbon to carbon dioxide and reported as dissolved organic carbon (DOC). For our purposes here DOC and DOM are directly related and essentially equivalent.

The term chromophoric refers to molecular structures that absorb light. The chromophoric dissolved organic matter (CDOM) typically found in aquatic ecosystems strongly absorbs in the UV and blue regions of the solar spectrum. It makes natural waters appear yellow to brown to black in color. Interestingly, the absorption of sunlight by CDOM leads to the alteration of the structure of CDOM through photolysis and photobleaching. Photolysis, the process by which CDOM undergoes structural changes through exposure to certain spectra of light. Photolysis frequently results in a decrease in the molecular weight and aromaticity but not necessarily the quantity of DOM. The term photobleaching refers to the process by which the chromophoric structure of dissolved organic matter is degraded by absorbed light energy. By definition, photobleaching results in a loss of absorbance (color fading) of CDOM and is frequently associated with an alteration of optical parameters such as spectral slope (ln absorbance or ln DOC-specific absorbance versus wavelength over a specific wavelength range) and molar absorptivity (more accurately called DOC-specific absorbance since it changes with wavelength) of dissolved organic matter.

Photolability is the potential for CDOM to undergo photolysis or photobleaching. More technically, photolability is the potential for CDOM to undergo structural changes as a result of absorption of high-energy photons (typically in the short wavelength blue and UV regions of the solar spectrum). In contrast, biolability is the relative potential for DOM to be assimilated by heterotrophic bacteria. Biolabile DOM generally has a lower molecular weight and is structurally simpler than more recalcitrant DOM that is by definition resistant to microbial breakdown. Exposure of bulk DOC to UV generally increases its biolability, decreases photolability, and increases water transparency to UV. These changes in turn have important implications for the availability of fixed carbon to microbial food webs as well as for the exposure of aquatic organisms to increased damage from the higher levels of underwater UV radiation.

References

Blumthaler, M., W. Ambach, et al. (1997). "Increase in solar UV radiation with altitude." Journal of Photochemistry and Photobiology B: Biology 39: 130-134.

Caldwell, M. M., R. Robberecht, et al. (1980). "A steep latitudinal gradient of solar ultraviolet-B radiation in the arctic-alpine life zone." Ecology 61: 600-611.

Farman, J. C., B. G. Gardiner, et al. (1985). "Large losses of total ozone in Antarctica reveal seasonal ClOx/NOx interaction." Nature 315: 207-210.

Hader, D. P. and M. Tevini (1987). General Photobiology. New York New York, Pergamon Press.

Hargreaves, B. R. (2003). Water column optics and penetration of UVR. UV effects in aquatic organisms and ecosystems. E. W. Helbling and H. E. Zagarese. Cambridge, UK, Royal Society of Chemistry.

Herman, K. M. (1983). The common aromatic biosynthetic pathway. Amino acids biosynthesis and genetic regulation. K. M. Herman and R. L. Somerville. London, Addison-Wesley: 301-321.

Kerr, J. B. and C. T. McElroy (1993). "Evidence for large upward trends of ultraviolet-B radiation linked to ozone depletion." Science 262: 1032-1034.

Kirk, J. T. O. (1994). Light & photosynthesis in aquatic ecosystems. New York NY, Cambridge Univ Press.

Kirk, J. T. O. (1994). "Optics of UV-B radiation in natural waters." Arch. Hydrobiol. Beih. Ergeb. Limnol. 43: 1-16.

Kirk, J. T. O., B. R. Hargreaves, et al. (1994). "Measurement of UV-B radiation in two freshwater lakes: an instrument intercomparison." Archiv fur Hydrobiologie Beihefte Ergebnisse der Limnologie 43: 71-99.

Morris, D. P. and B. R. Hargreaves (1997). "The role of photochemical degradation of dissolved organic carbon in regulating the UV transparency of three lakes on the Pocono Plateau." Limnology and Oceanography 42: 239-249.

Rex, M., N. R. P. Harris, et al. (1997). "Prolonged stratospheric ozone loss in the 1995-96 Arctic winter." Nature 389: 835-838.

Williamson, C. E., and H. E. Zagarese, editors (1994). "Impact of UV-B radiation on pelagic freshwater ecosystems." Archiv fur Hydrobiologie Ergebnisse der Limnologie 43: 1-226.

Williamson, C. E., B. R. Hargreaves, et al. (1999). "Does UV play a role in changes in predation and zooplankton community structure in acidified lakes?" Limnology and Oceanography 44: 774-783.

Williamson, C. E., P. J. Neale, et al. (2001). "Beneficial and detrimental effects of UV radiation on aquatic organisms: implications of variation in spectral composition." Ecological Applications 11: 1843-1857.

Williamson, C. E. and H. E. Zagarese (2003). UVR effects on aquatic ecosystems: A changing climate perspective. UV effects in aquatic organisms and ecosystems. E. W. Helbling and H. E. Zagarese. Cambridge, UK, Royal Society of Chemistry. 1: 547-567.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

last modified on Feb 12, 2009