Robert G. Wetzel
Table of contents
1.1 Introduction 5
1.2 Size matters - radiation attenuation in relation to loadings of organic matter 6
1.3 Precipitation matters - importance of frequency and intensity of influents 7
1.4 Direct effects of UVR 9
1.5 Allochthonous vs. autochthonous organic matter - key UV-VIS mediated processes regulate heterotrophic utilization 9
1.5.1 Alterations of enzymatic accessibility by the macromolecules 10
1.5.2 Photolysis of humic macromolecules 10
1.5.3 Photolysis of dissolved organic nitrogen and phosphorus compounds 11
1.5.4 Complete photolysis of humic substances to CO and CO2 11
1.5.5 Less direct but important biogeochemical interactions of UVR ... 12
1.6 Recalcitrant organic matter, metabolic stability, and photolysis 13
Solar radiation is the fundamental ecosystem modulator. Nearly all generation of organic matter is photosynthetic and as such the distribution of light in aquatic ecosystems is critical to regulation of major energetic inputs. However, simultaneously specific components of solar radiation, in particular the UV, function as both an accelerator of microbial degradation by enhancing bioavailability of complex organic substrates to microbes and by complete photolysis and oxidation of components of organic macromolecules to CO2 and other inorganic forms of nutrients. Alterations in UV intensities impinging upon and within inland aquatic and coastal marine ecosystems by natural or anthropogenic causes will modify the rates of metabolism and biogeochemical processes associated with these macromolecules. This cascade of effects can greatly modify the functioning of natural ecosystems.
In the subsequent chapters of this volume, detailed evaluations provide a summary of contemporary understanding of the properties of ultraviolet radiation (UVR) in aquatic ecosystems and its effects on aquatic organisms. Here I attempt to provide an overview of the coupling of these properties to emphasize how individual effects of UVR are integrated and, at the ecosystem level, provide a master level of regulation of ecosystem biogeochemical cycling, energy fluxes, productivity, and system evolution.
In regard to these detailed treatments of specific components of solar radiation and their effects, it is useful to emphasize several related universal characteristics of aquatic ecosystems. Namely, ecosystems are biological systems, ecosystems are biogeochemical systems, and the cycling of materials and energy in ecosystems is regulated by a highly variable set of intercoupled physical, chemical, and biological parameters. It is extraordinarily important to evaluate the influences and changes of UVR in the ecological contexts of a highly dynamic, changing environment - dynamic spatial and especially temporal scales. The question then is whether UV effects within the ecosystems are so variable that analyses are chaotic or whether certain stoichiometric analyses allow quantitative predictions of generic system responses to changes in UVR.
The approach taken is to first analyze our present understanding of how UVR influences ecosystem processes and how these processes are intercoupled with other related influences of those processes, such as climatic or atmospheric processes related to UVR. Finally, can one reasonably predict how ecosystems of different characteristics will respond to changes in atmospheric or aquatic conditions that alter UVR.
1.2 Size matters - radiation attenuation in relation to loadings of organic matter
Nearly all UV-C (< 280 nm) is absorbed by the stratospheric gases and by the water of aquatic ecosystems. Although relatively little UV-B (280-320 nm) passes through the stratosphere (Chapter 2), UV-B is highly energetic and an important photactivating agent in waters. UV-A (320-400 nm) is less energetic than UV-B but is absorbed less readily and penetrates more deeply into water. The near UV light in the blue portion of the visible spectrum (400-500 nm) has recently been shown to be functionally similar to the adjacent UV-A radiation in many of the important photochemical reactions influenced by UVR and must be considered in any evaluation of composite effects.
Recent measurements in situ have demonstrated great variability in the penetration of UV-B and UV-A, but penetration has been found to be much greater than was believed previously (Chapters 3, 6, ). When referenced against pure water, the transmission of radiation is reduced drastically with increasing concentration of naturally occurring chromophoric dissolved organic compounds, particularly humic acids. UV-B attenuation depths (za = 1 % of surface irra-diance) range from a few centimeters to > 10 m among a number of waters [2-6], Much (> 90%) of the among-habitat variation in diffuse attenuation coefficients (Kd) could be explained by differences in dissolved organic carbon (DOC) concentrations. Throughout the solar UV-B and UV-A range, Kd was well estimated with a univariate power model based on DOC concentration, particularly in waters of low to moderate phytoplanktonic productivity. The za is strongly dependent on DOC concentrations when below 2 mg C 1_1. In eutrophic lakes, densities of phytoplankton can begin to influence UV attenuation .
Only certain portions of the heterogeneous dissolved organic matter (DOM) absorb solar radiation. In inland waters, phenolic and other aromatic-based humic compounds (fulvic and humic acids), largely of terrestrial and higher aquatic plant origin, form a major component of dissolved organic acids and can constitute some 80% of the total DOM, 30-40% of which is composed of aromatic carbon compounds . Humic substances are the largest component of chromophoric dissolved organic matter (CDOM). Of the soluble part of humic substances, heterogeneous fulvic acids have molecular weights from 500 to 1200 Da and contain many acidic functional groups, primarily carboxylic acids [9-11]. Humic acids are less hydrophilic than fulvic acids and are of greater molecular weight (mean ca. 4000-5000 Da) . Humic substances dominate CDOM and are the most important component in the absorption of solar UV and blue radiation [4,13].
Concentrations of 4-8 mg organic acids liter -1 are common in surface waters and often exceed 50 mg 1 ~1 in organic-rich waters, such as those of wetlands, flood plains of river ecosystems, and interstitial waters of hydrosoils . Concentrations of both CDOM and humic substances commonly decrease along the gradient of fresh-to-coastal-to-oceanic waters.
Because the effects of UVR on aquatic ecosystems are so strongly influenced by concentrations of CDOM, factors that influence the loading rates of CDOM to aquatic ecosystems will influence strongly the selective distribution of UV and its effects on habitats and biota. Two aspects are particularly important in this regard. Firstly, the proportion of the DOM that is derived from higher plant tissues (terrestrial and wetland/littoral sources) that are dominated by chromo-phoric humic compounds vs. that derived form algae, which contain few fulvic and no humic constituents [10,14,15]. The DOM of streams and rivers is almost totally dominated by partial decomposition products of terrestrial and wetland higher plants. Similarly, small lakes receive a high proportion of DOM from terrestrial and wetland sources dominated by higher plant productivity and a high proportion of humic substance residues from partial degradation of structural tissue constituents, particularly lignocelluloses.
Secondly, the morphology of the receiving aquatic ecosystem is imperative because of the direct relationships between lake basin volume to water retention times, dilution of influent DOM, and mixing frequencies into photic zones. Most of the millions of lakes are small (< 10 km2) and relatively shallow, usually <10 m in depth [1,16]. As a result, the frequency of interaction of DOM-entrained water with solar radiation is often high both within stratified lakes and in shallow non-stratified lakes and ponds. Similarly, the frequency with which DOM in water of streams and rivers interacts with solar radiation is also high, particularly among larger stream orders (>3rd order) where the influence of shading from riparian tree canopy is small.
1.3 Precipitation matters - importance of frequency and intensity of influents
Because the penetration of UVR and its effects on ecosystem metabolism and functioning is so strongly influenced by DOM, the rates and timing of loading of DOM to receiving waters is important. Many studies have demonstrated the dominance of allochthonous inputs of terrestrial organic matter, in the form of detrital DOM and particulate organic matter (POM) for material and energy cycling in stream and river ecosystems. Much of that DOM is released from soils into groundwater and from anaerobic processes in adjoining wetlands [e.g., 1,17-19].
The DOM inputs from terrestrial organic matter to streams and lakes results from direct leaching from living vegetation and from soluble compounds carried in runoff from dead plant materials in various stages of decomposition. Very high concentrations of organic matter emanate from wetlands. Inputs of DOM are often directly correlated with precipitation, with high loading rates to receiving waters in the initial flushing stages of precipitation events. DOM loading then declines markedly in the later stages as dilution increases and eventually the discharge volume declines. Similarly, the DOM loading during the initial stages of snowmelt is much higher than subsequently. Although the total loading of DOM is high during these flushing events, dilution is also high. Some of the highest DOM concentrations and resulting UV attenuation occur during periods of low flow in rivers. In stratified lakes, the longer residence time allows for higher rates of photolysis of DOM in the photic zone. As in shallow, nonstrati-fied lakes that mix frequently to the surface layers of high UV insolation, the concentrating effects of water residence time are countered by time available for UV alteration and microbial mineralization (Chapter 4).
The seasonal timing of the DOM loading also affects the effectiveness of UV photolysis and microbial utilization. Obviously, runoff loading events in cold, low light periods of the year will lead to less effective degradation and utilization of the organic compounds by biota of the ecosystem. These altered rates of UV-mediated metabolism will in turn affect rates of nutrient regeneration and subsequent productivity at many biotic levels.
As the DOM is delivered to marine coastal regions by rivers, reduction of transport rates occurs in the estuarine regions with complex hydrodynamic dispersion of water currents. The less dense saline water overlies that of the coastal waters and is exposed to solar photolysis with greater intensity and frequency than the underlying waters. The result is increased rates of partial and complete photolysis, largely by UV radiation, with higher mineralization rates of CDOM to C02 by enhanced microbial metabolism and by direct degradation to CO2. As a result, a significant portion of the residual DOM is non-chromophoric (NCDOM). This relatively recalcitrant NCDOM, constituting perhaps 10-20% of the total DOM, tends to persist in marine environments with appreciable chemical stability and longevity (decades to centuries).
How the loading rates of allochthonous dissolved organic matter to freshwater ecosystems and to continental marine regions are and will be affected by climatic changes is unclear. There are indications among long-term data sets that DOC concentrations are declining gradually in lakes over several decades [e.g., l(p. 779),20,21]. Particularly in oligotrophic lakes where DOC concentrations are often low, UVR penetrates to depths of several meters and can negatively influence organisms by genetic damage, diverting production to increased synthesis of protective pigments, or in high elevations or latitudes where higher plant source materials and DOM loading is low. Organisms in such lakes can be exposed to high intensities of UVR . Even in lakes with higher concentrations of DOM, the long-term trends are often toward slowly decreasing concentrations of DOM .
There is little question that both temperature and carbon dioxide concentrations of the atmosphere are increasing. Rising temperature has also influenced precipitation patterns and has led to large regions in which rainfall and snow accumulations have been reduced . Droughts are a cumulative result of numerous meteorological factors affecting precipitation, évapotranspiration, and other water losses. Droughts usually do not become severe until after long periods of deficient rainfall and unrestrained water use.
DOC in some lakes has declined appreciably over the last quarter century coincident with substantial warming [e.g., l(p. 780),23]. Reduced precipitation and increased évapotranspiration in the drainage basin result in reduced stream flows and lower DOC loading to the streams and lakes. Transparency of lake water to UV photolysis increases under these conditions. Similar reductions in DOC have been observed in streams . The decrease in annual DOC yields of streams occurs in spite of higher concentrations in storm flows following periods of prolonged drought [23,24].
Photosynthesis of algae is clearly inhibited by exposure to natural levels of UV-B and especially UV-A radiation. Physiological and genetic recovery occurs, and as a result a quasi-steady physiological state is found commonly between damage and recover processes [25,26, Chapters 9, 11, and 13]. Many species repair damage to photosystems and DNA during daily periods of darkness. Many species produce UV-absorbing compounds - mycosporine-like amino acids are an important and ubiquitous class of such compounds [27,28, Chapter 10]. Many species have biochemical defenses against toxic end products of UVR, such as radical scavenging by carotenoid pigments and superoxide dismutase (Chapter 15). Some species have limited abilities to avoid intense surface UV by migration to deeper areas.
UV radiation can impact zooplankton and fish directly in shallow water habitats by damage to DNA and generation of harmful photochemicals (free radicals, reactive oxygen species) [29,30, Chapter 8]. Although many animals can avoid UV-intense habitats, as well as develop photoprotective pigments (carotenoids, cuticular melanin), both of these strategies can alter their susceptibility to predation by other organisms, particularly fish.
1.5 Allochthonous vs. autochthonous organic matter - key UV-VIS mediated processes regulate heterotrophic utilization
Some 90 per cent or more of the total metabolism in aquatic ecosystems is microbial, accomplished by heterotrophic metabolism of bacteria, fungi, and many protists, all of a size less than 100 ¡¿m [1,31]. Therefore, the material and energy fluxes of aquatic ecosystems is totally dominated by metabolism of particulate detritus (non-living) and especially DOM from autochthonous and allochthonous sources. The pelagic open water is but a portion of the whole lake or river ecosystem. In relation to loading and fluxes of DOM, allochthonous and littoral sources are critical because of their chemical differences from that produced by algal photosynthesis.
The modes of senescence, death, and degradation rates of biota are also of considerable importance to rates and pathways of degradation and energetic utilization. For example, the continual slow senescence and release of DOM from a higher aquatic plant is very different from the relatively instantaneous biochemical death and release of DOM from a bacterium or alga. Non-predatory death and metabolism of non-living detrital POM and DOM by prokaryotic and protistan heterotrophs dominate in all aquatic ecosystems.
In providing a synthesis of the ramifications of UV on aquatic ecosystems, a key component is the simultaneous importance of DOM in regulating the distribution and attenuation of UYR as well as the effects that UV has both directly and indirectly on the metabolism, growth, reproductive, and production efficacy of biota. Because these effects of UV are so interactive and coupled, it is difficult to separate them without some redundancy. Several points can be characterized, however, in summary of some of the more detailed discussions in subsequent chapters.
Physical processes, such as partial or complete photochemical modification of organic macromolecules, can result in major alterations in biological availability of portions of complex, heterogeneous dissolved organic compounds. These photochemical processes can result in:
1.5.1 Alterations of enzymatic accessibility by the macromolecules
Polyphenolic organic acids, which occur in great abundance (commonly 4-8 mg 1_1) in many fresh waters, can complex with or induce precipitation of proteins by binding to one or more sites on the protein surface to yield a monolayer that is less hydrophilic than the protein itself [32,33]. This complexation, as well as cross-linking of polypeptide chains with polyphenolic humic substances can lead to enzymatic inhibition or reduction of activity [e.g., 33-35]. More aromatic and condensed humic acid molecules are more rigid and can distort bound enzymes to a greater extent than is the case with simpler compounds, such as fulvic acids [e.g., 36]. The inhibition of enzymes occurs in a classical noncompetitive manner, in which the inhibitor, polyphenol, and substrate bind simultaneously to the enzyme. Furthermore, dissolved humic substances can complex by peptidization and alter biological susceptibility to enzymatic hydrolysis. For example, membrane properties, such as lipid hydrophobicity, can be altered by humic substances and in turn affect enzyme hydrolysis rates and nutrient transport mechanisms [e.g., 37,38], An important ecosystem aspect is that these protein or enzyme complexes can be stored in an inactivated state for long periods, transported within the ecosystems, and later reactivated by partial photolytic cleavage by UVR [1,34,35].
Partial photolysis of humic macromolecules, particularly with the generation of volatile fatty acids and related simple compounds that serve as excellent substrates for bacterial degradation [e.g., 39-42]. It is important to recognize that of the total photolytic irradiance, about a quarter of the partial photolysis of organic substrates results from UV-B, about half from UV-A, and about a quarter from the lower wavelengths (400-500 nm) of photosynthetically active radiation (PAR, 400-700 nm). Transmittance and photolytic activity from UV-B and UV-A is restricted largely to the surface waters. In contrast, PAR, although much weaker energetically than UY, penetrates into water to much greater depths. Although photolysis of organic compounds is appreciably less than that induced by UV at surface waters, the photolytic generation of simple substrates is appreciable by PAR as well as by UV [1,43,44], Results of such studies are indicating that an appreciable portion of photolytic generation of some simple substrates is generated by PAR.
1.5.3 Photolysis of dissolved organic nitrogen and phosphorus compounds
Photolytic degradation of dissolved organic nitrogen and phosphorus compounds release inorganic nutrient compounds such as nitrite, ammonia, and phosphate, as well as CO and CO2 [e.g., review of 41,45-47]. Stimulatory effects of increase nutrient availability by such processes clearly occurs [e.g., 47].
1.5.4 Complete photolysis of humic substances to CO and CO2
Photochemical oxidation by solar radiation of natural dissolved organic compounds to both CO and dissolved inorganic carbon (C02 and HC03~) has been known for some years [e.g., 48]. Depending on dissociation and saturation conditions, some excess CO2 will evade to the atmosphere. Previous studies on the photolytic degradation of dissolved organic matter suggested that the dominant photolytic components of solar radiation were UV-B and UV-A, and that PAR was of little consequence. Many of these studies, however, were not performed under sterile conditions, and as a result findings were confounded by nearly instantaneous microbial utilization of organic compounds generated with rapid degradation and generation of C02. Moreover, many of the DOM sources of these studies had been exposed to natural radiation for long (e.g., weeks) and non-comparable periods of light. Contemporary research is indicating that although UV-B and UV-A are significant and can contribute to more than half of photochemical mineralization, PAR is also a major photolytic agent [43,49,50]. For example, from nearly 200 separate photolytic experiments on DOM from different waters and plants under different conditions, the UV-B portion of the spectrum was always most effective in complete photodegradation to C02, but UV-A was also highly effective with small differences from the photolytic capacities of UV-B . PAR is also highly effective in photolytic degradation of DOM to C02 and frequently about a quarter to half of the collective photolysis can be attributed to the largely blue portion of the PAR spectrum. Bioavailability of CDOM may increase , remain unaltered, or decrease from photolysis [47,51,52]. Bioavailability is clearly related to the stages of photolysis and alteration of the dominant components of the heterogeneous natural aggregation of natural organic compounds.
Both partial photolysis to the generation of volatile fatty acids, and the complete photolysis with the generation of large quantities of C02 by PAR are important findings because of the much lower extinction rates of PAR in water in comparison to those of UVR. Photolytic processes, so important to nutrient cycling, are therefore not restricted to the uppermost strata of a few centimetres of aquatic ecosystems, but rather affect much of the seasonally-variable volume of the photic zone.
1.5.5 Less direct but important biogeochemical interactions of UVR
Biogeochemical interactions of UVR upon DOM in aquatic systems are also important, but poorly studied at the ecosystem level. Continued intensive study of natural dissolved organic substances in aquatic ecosystems is resulting in improved understanding of the many ways in which these diverse compounds, particularly humic compounds, can interact with other important metabolic components. Any of these processes will be altered by UV partial or complete photolysis of DOM. Examples are manifold:
(a) Interact with inorganic compounds, particularly in complexation reactions such as chelation [reviewed in 53]. Depending on the concentration ratios of the complexing DOM to inorganic elements, the mode of organic complexation, biological availability and, in some cases, elemental toxicity can be increased or decreased. All of these processes will be altered by UV photolysis ofCDOM.
(b) Interact with other organic compounds, such as peptidization, and alter biological susceptibility to enzymatic hydrolysis. For example, membrane properties, such as lipid hydrophobicity, can be altered by humic substances and in turn affect enzyme hydrolysis rates and nutrient transport mechanisms [e.g., 37,38]. In a most interesting interaction, humic substances can complex with proteins, particularly enzymes both freely soluble and membrane-bound, with non-competitive inhibition [54,55]. Enzymes can be stored for long periods (days, weeks) in this complexed, inactive state, be redistributed in the ecosystem with water parcel movements, and reactivated by partial photolytic cleavage by UVR [31,34,35,43].
(c) Alter chemical properties such as redox and pH. A predominance of humic acids can result in an organic acidity that can influence, and at times exceed, inorganically derived acidity form natural or anthropogenic sources [reviewed in 1]. Exposure of natural dissolved organic matter to UV can form reduced reactive oxygen species, particularly hydrogen peroxide (H202) [56,57; Chapter 8]. H202 has a half-life of several hours in natural waters and can radically alter redox cycling of metals .
(d) Microbially reduced humic substances can, upon entering less reduced zones of sediments, serve as electron donors for the microbial reduction of several environmentally significant electron donors . Once microbially reduced, humic substances can transfer electrons to various Fe(m) or Mn(iv) oxide forms abiotically and recycle the humic compounds to the oxidized form, which can then accept more electrons from the humic compound-reducing microorganisms. The interactions of UVR on these highly reactive processes in shallow waters, particularly littoral and wetland areas, are unclear, (e) Change physical properties such as selective modifications of light penetration. The well-known selective attenuation of light by CDOM [cf. 1] can further modify biogeochemical cycling in numerous ways. Such modification of the light regimes can alter rates of photosynthesis, hormonal activities, and migratory distribution and reproductive behaviors. Absorption of UYR by humic substances can protect organisms from genetic damage as well as modify macromolecules and enhance bioavailability of organic substrates.
1.6 Recalcitrant organic matter, metabolic stability, and photolysis
The commonly observed incomplete photolysis of DOC is critical to accelerated utilization of these macromolecules, but is clearly not mandatory. Portions of the complex DOM pools, including fractions of humic and fulvic acid compounds, are degraded, but total degradation rates are clearly slow. Chemical organic recalcitrance of DOM is instrumental in providing a thermodynamic stability to metabolism within lake, reservoir, wetland-littoral land-water interface, and river ecosystems [1,31,54,60-62]. The chemical recalcitrance is a "brake" on ecosystem metabolism, and that brake is critical for maintenance of the integrated stability of heterotrophic utilization of synthesized or imported organic matter and energy. U VR can alter the effectiveness of that chemical recalcitrance "brake".
Most of the detrital organic pool, both in particulate and dissolved phases, of inland aquatic ecosystems consists of residual organic compounds of plant structural tissues. The more labile organic constituents of complex dissolved and particulate organic matter are commonly hydrolyzed and metabolized more rapidly than more recalcitrant organic compounds that are less accessible en-zymatically. The result is a general increase in concentration of the more recalcitrant compounds, commonly exceeding 80% of the total, with slower rates of metabolism and turnover. These recalcitrant compounds, however, are metabolized at rates slowed and regulated in large part by their molecular complexity and bonding structure.
In every detailed annual organic carbon budget of lake and river ecosystems, organic matter generated by phytoplankton will not support all of the heterotrophic metabolism of the ecosystem. At least several fold support of the total metabolism is by organic subsidies from the land-water interface communities and allochthonous production. From the standpoint of metabolic stability, it is particularly important that most of the organic carbon is dissolved and relatively recalcitrant, widely distributed within the inland waters. The chemical recalcitrance of this dominating DOM ameliorates the violent metabolic and growth oscillations so characteristic of the pelagic biota components of the ecosystem when resources are available in abundance. In addition, much of the POM formed in the dominating land-water interface regions of lake, river, and estuar-ine ecosystem, is displaced to reducing, anoxic environments of the littoral and profundal sediments. The DOC, largely of higher plant origins, provides the stability and is the currency for the quantitatively more important detrital pathways in aquatic ecosystems. The same underpinnings of that stability prevail in terrestrial ecosystems and likely in coastal as well as much if not most of the marine ecosystem.
Detritus includes non-living particulate, colloidal, and dissolved organic matter, and metabolically size only affects rates of hydrolytic attack . Inland aquatic ecosystems collect organic matter, particularly in dissolved forms, from terrestrial, wetland, and littoral sources in quantities that supplement if not exceed those produced autochthonously. Rates of utilization of that organic matter are slowed by a combination of chemical recalcitrance as well as displacement to anoxic environments. As a result, inland aquatic ecosystems are heterotrophic and functionally detrital bowls, not algal bowls.
The high organic matter production of terrestrial and particularly land-water interface regions (wetlands, littoral areas) commonly results in loading of excessive organic carbon, usually primarily in the form of dissolved organic compounds, to inland waters. A significant portion of that DOM is metabolized, sorbed and sedimented, or photolyzed while moving through lakes and rivers, but nonetheless a portion does reach coastal oceanic regions. The extent of this allochthonous loading to oceanic waters is unclear, although estimates are as high as 20% of the total oceanic DOC [63,64]. Because of long periods of exposure of much of this allochthonous DOM to photolytic degradative processes en route to the open ocean, its metabolic regulatory functions are clearly less than is the case in inland water ecosystems. More labile DOM products of algal photosynthesis dominate in the marine pelagic, and as a result undergo rapid utilization and exploitation until limiting conditions for sustained growth prevail. It is hypothesized that these conditions are appreciably less stable that those containing high concentrations of chromophoric and non-chromophoric DOM emanating largely from higher plant tissues. As a result, effects of altered rates of fluence of UVR in the oceanic pelagic impact the ecosystem by more direct means, such as direct damage to genetic constituents of the biota, rather than the major roles in altering the chemistry of organic macromolecules.
In long-term evolutionary scales, humans now have the abilities to intervene rapidly in this interdependent relationship and alter the stability of the rates of metabolism of organic matter. For example, reduction of ozone in the stratosphere and associated increased UV-B could lead to accelerated photolytic degradation of macromolecules of DOM to C02 by both abiotic and biotic pathways. In addition, the photolytic enhancement of substrates for bacterial metabolism by UV photolysis can result in accelerated rates of biogeochemical cycling of nutrients and stimulated productivity of the ecosystems. In addition to decreasing the metabolic stability of the lakes and streams, the enhanced microbial respiration will certainly lead to increased generation of C02 and evasion to the atmosphere.
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• UVR climatology
• Water column optics and penetration of UVR
• Modulation of UVR exposure and effects by vertical mixing and advection
Mario Blumthaler and Ann R. Webb
Table of contents
2.1 Introduction 23
2.2 Theory 24
2.2.1 Energy from the sun 24
2.2.2 Planetary motion and geometry 25
2.2.3 The atmosphere 27
2.2.4 Absorption and scattering 29
2.2.5 Determining the UV spectrum at the ground 33
184.108.40.206 Ozone absorption 33
220.127.116.11 Stratospheric ozone chemistry 34
18.104.22.168 Changes in stratospheric ozone 35
22.214.171.124 Tropospheric ozone chemistry 35
126.96.36.199 Other attenuators 36
188.8.131.52 Final result at the surface 37
2.3 Measurements 39
2.3.1 Ground-based measurements 39
184.108.40.206 Instrumentation 39
220.127.116.11 Results 42
2.3.2 Space-born measurements 50
2.4 Trends in solar UVR 52
2.4.1 Long-term ozone changes 52
2.4.2 Long-term UVR changes 52
2.4.3 Future levels of UVR 54
18.104.22.168 Forecasting UVR 54
22.214.171.124 Future UV scenarios 54
Ultraviolet radiation at the Earth's surface is determined by the emission of the sun, and subsequent modification when passing through the atmosphere. The Earth-sun distance and the position of the observer on the Earth determine the power of the incoming radiation while the UV spectrum at the ground varies with time and place, because of the wavelength dependent attenuation processes within the atmosphere. The most important determinants are solar elevation, ozone and aerosol content, altitude and albedo, and cloudiness. Measurements with high spectral resolution allow detailed investigation of the effects of these parameters. Simpler broadband measurements provide information from a great number of locations. Model calculations can estimate irradiance levels well if the input parameters (state of atmosphere) are well known. Thus, estimates based on space-born measurements provide for world-wide distribution and temporal variation of UVR, but incomplete knowledge of some atmospheric parameters still limits the absolute accuracy.
As total ozone amount decreases, especially in mid- and high-latitudes, UV-B tends to increase. The interaction of all potential climate change influences on ozone makes predictions of future UV difficult, but best estimates do not expect recovery on a global scale earlier than within 10 to 20 years.
The UVR reaching aquatic organisms in their natural habitat comes from the sun. Extra-terrestrial radiation is modified as it passes through the Earth's atmosphere and there are many factors that influence the radiation arriving at the surface of the Earth. These include the state of the atmosphere (clear, clean, cloudy, polluted), position on the Earth (latitude and altitude) and season (relative position of the sun to location on Earth). Further attenuation then occurs as the radiation passes through the water environment to reach aquatic organisms. These latter complications are dealt with in Chapter 3; here we deal only with the UVR incident at the ground, or water surface.
UVR covers the part of the electromagnetic spectrum at wavelengths below 400 nm, between X-rays and visible radiation. The UV is split, somewhat arbitrarily, into narrower wavebands with designations (from different branches of science) such as vacuum-, far-, near-UV. In considering UV at the surface of the Earth we are concerned with the longest wavelengths in the UV part of the spectrum, those between 280-400 nm, designated as UV-B and UV-A radiation (the more harmful UV-C (200-280 nm) and shorter wavelengths are completely attenuated by the atmosphere). The Commission Internationale d'Eclairage (CIE) define UV-B as 280-315 nm, and UV-A as 315-400 nm. However, UV-B can frequently be found described as 280-320 nm, for the pragmatic reason that 320 nm is about where the solar spectrum "flattens out", and where biological action spectra approach a region of zero or very small response. In reality these waveband distinctions are arbitrary bound aries in the continuous spectra of both solar radiation and biological reactions.
The first section of this chapter discusses the basic physics of radiation and radiative transfer in general. In the following sections, measurements of UVR are discussed, subdivided in ground-based and space-born methods. The instrumentation and the results with respect to the different parameters affecting UVR at the Earth's surface are presented. Finally, trends in solar UVR are analyzed. Starting with the observed long-term ozone changes, the resulting changes in UVR are discussed. Future levels of UVR refer to forecasting for a short time scale (days) as well as possible scenarios in the next decades.
Natural UVR originates with nuclear reactions in the interior of the sun. The energy generated in this way travels outwards through the gaseous body of the sun to a layer called the photosphere. The photosphere is the layer that emits the radiation we receive on Earth. It emits approximately like a blackbody at 5800 K, that is its emission is continuous across the electromagnetic spectrum and the spectral shape is determined by Planck's law. The temperature of the photosphere is such that the emission covers the spectral region from gamma rays to the near infra-red (about 4000 nm) (Figure 1). The wavelength of maximum emission is given by Wien's law
Amax = 2897IT
and for the temperature of the sun this is 0.5 /mi (500 nm), in the blue-green visible part of the spectrum. However, the shape of the Planck curve, the relative sensitivity of the human eye, and the spectrally dependent interactions of the radiation with the atmosphere (discussed below) lead to the yellow sun that we observe.
The total amount of solar radiation emitted by the sun is determined by the Stefan-Boltzmann law
where a is the Stefan-Boltzmann constant of 5.67 x 10~8 W m~2 K-4. However, incident energy at a distance from an emitting object is proportional to the square root of distance, hence the energy reaching the top of the Earth's atmosphere becomes
where So is the solar constant, rs is the solar radius, and r0 is the average Earth-sun distance. The solar "constant", best estimated as 1370 W m~2, varies on several time scales. Over the lifetime of the sun its temperature, and therefore both its total emission and spectral properties, have changed (it is estimated that emission has increased by 20-40% in 4.5 x 109 years). On a time scale that we can
Figure 1. Spectra of a black body at 6000 K, the sun outside the atmosphere, and the sun at the Earth's surface.
Figure 1. Spectra of a black body at 6000 K, the sun outside the atmosphere, and the sun at the Earth's surface.
comprehend, the activity of the sun, associated with the observable sunspots, varies in a broadly cyclic manner of 22 years duration. However, this includes a reversal of the sun's magnetic field and the cycle in sunspot numbers (our main concern) is 11 years. Active sunspots appear as dark patches on the face of the sun and their magnetic activity leads to solar flares - great eruptions of energy with enhanced ultraviolet and X-ray emission. These rather unpredictable emissions affect the solar constant, but in a wavelength dependent manner: the peak-to-peak change for a wavelength of 160 nm is about 10%, while for wavelengths greater than 300 nm it is less than 1 %, and for the solar constant as a whole it is of order 0.1%. Finally, the 27-day rotation cycle of the sun leads to variation of several percent in the solar UV output, although again at longer wavelength (>250 nm) this variation is less than 1%.
Shorter time-scale changes, and more immediately relevant, are due to the astronomical motions of the Earth and sun. The Earth's annual orbit around the sun is slightly elliptical and the Earth-sun distance varies, leading to small changes in the available energy throughout the year. The current eccentricity of the orbit means that the Earth is closest to the sun (perihelion) in the January, the Southern Hemisphere summer (Northern Hemisphere winter) and furthest from the sun (aphelion) in July, the Northern Hemisphere summer. The difference in Earth-sun distance between the two extremes is about 3.4%, giving a difference in extraterrestrial radiation of about 6.9%. The eccentricity itself varies on a 110000 year cycle (becoming more and then less elliptical), with extreme positions that would give no more than a 0.17% change in Earth's incident flux. The position of perihelion also changes as a result of gravitational interactions (mainly with the planet Jupiter) that cause the elliptical orbit of the Earth to precess, which in turn leads to a precession of the timing of the equinoxes. Such changes occur over time periods of 18 800 years and 23 000 years. While they do not affect the total energy received by the earth they do affect the way that the energy is distributed over the surface of the planet.
The most noticeable change in solar energy received at a given location, the seasonal effect, is caused by the tilt of the Earth's axis. This obliquity, the angle between the earth's axis and the plane of the ecliptic, is currently 23.5° (it varies between 22 and 24.5° over a period of about 40000 years). It affects both the length of daylight and the height of the sun in the sky, which change with time and location on the Earth's surface. In June the sun is overhead at the Tropic of Cancer (23.5°N), at the equinoxes (March and September) it is overhead the equator, and in December it has reached its other extreme position overhead at the Tropic of Capricorn (23.5°S). Since the Earth's axis is tilted there is a differential shading of latitudinal bands that changes with the position of the overhead sun (Figure 2) and day-length is approximately equal to the fraction of a latitude circle that is unshaded. At the equinoxes this is 12 hours everywhere, while the polar circles go from 24 hours darkness in their winter to 24 hours daylight in their respective summers. The sun's height in the sky is usually expressed in terms of the solar zenith angle, z. This is the angle between the local vertical and the position of the sun. The solar zenith angle is given by cos z = sin 9 sin 5 -f cos 6 cos S cos h where 6 is the latitude, 5 is the solar declination (latitude where the sun is where 6 is the latitude, 5 is the solar declination (latitude where the sun is
perihelion (December, RHS). Note that the sun is overhead at 23.50°N and S respectively, resulting in the polar regions experiencing 24 hours of either light or dark because of the tilt of the Earth's axis.
perihelion (December, RHS). Note that the sun is overhead at 23.50°N and S respectively, resulting in the polar regions experiencing 24 hours of either light or dark because of the tilt of the Earth's axis.
overhead at noon), and h is the hour angle. The hour angle is zero at local solar noon and increases by 15° (n /12) for every hour from noon. Note that local solar noon is a function of longitude and is not necessarily coincident with the local time zone (clock time). Local solar noon is further modified by the "equation of time", which gives a variation within the year by about ±15 minutes, as a consequence of the elliptical Earth's orbit around the sun and of the tilt of the Earth's axis relative to the plane of the orbit. The sun rises and sets when cos z = 0, leading to an expression for the half daylength H of cos H = — tan 0 tan d
The amount of incoming energy on a horizontal surface at the top of the atmosphere above a given location is then
Eo = S0(r0/r)2 cos z where r is the instantaneous Earth-sun distance, and r0 its mean value.
As it enters the atmosphere this radiation becomes subject to interactions with the atmospheric constituents. The atmosphere changes in density, composition and temperature as a function of height so the types of interaction and the wavelengths of radiation affected are also a function of height. At the surface we observe the net effect of attenuation throughout the depth of the atmosphere.
The atmosphere is not a homogenous medium. At best it can be considered as a series of uniform horizontal layers, the simplification that is most often made when calculating radiative transfer through the atmosphere. In reality many of the atmospheric properties can change on a range of space and time scales. However, the physics can be discussed in terms of a 1-dimensional atmosphere of horizontal layers.
The vertical temperature and density structure of the atmosphere are shown in Figure 3, while the composition of the lower atmosphere is shown in Table 1. Number density, n (the number of gas molecules in unit volume) can be determined from the ideal gas law n = P/kT
where P is pressure, T is absolute temperature and k is Boltzmann's constant (1.381 x 10-23 J K_1). In the atmosphere pressure and number density both decrease with altitude (h) in an approximately exponential way. Under the hypothesis of constant temperature, to the pressure applies
where H is a scale height and is about 8 km in the lower regions of the atmosphere. Note that n also depends on temperature (see above) which is neither a constant nor a simple function of height for the whole atmosphere (Figure 3), so n does not have a purely exponential decrease with altitude.
TEMPERATURE (K) 160 200 240 260
TEMPERATURE (K) 160 200 240 260
i miiiil i i mal hihi i nii.nl t mini i iliwil i im
Figure 3. The vertical pressure, density and temperature of a standard atmosphere. Table 1. Composition of the lower part of the atmosphere (without water vapor)
i miiiil i i mal hihi i nii.nl t mini i iliwil i im
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