Unlike terrestrial systems, the penetration of UV-B as well as photorepairing wavelengths of solar radiation into water is predominantly controlled by non-living organic matter, mainly the colored (chromophoric) component of dissolved organic matter (CDOM) (Figure 5.4). Enhanced UV-B can affect the balance between the biological processes that produce the organic matter and the chemical and microbial processes that degrade it. Observations in lakes and the sea have shown that, in stratified waters, exposure to solar UV radiation helps cause net decreases in CDOM concentrations and increases in UV-B penetration. These changes, which are reinforced by changes in climate and acidification, potentially have broad impacts on the effects of enhanced UV-B on biogeochemical cycles. See following discussion on decomposition and UV-B penetration and related considerations in Chapter 4.
Much recent research has focused on
the effects of UV-B radiation on the interactions of UV-B and dissolved
organic carbon (DOC). In particular, evidence is accumulating that microbial
activity is enhanced by the UV-induced decomposition of polymeric components
of DOC to biologically-available organic compounds and mineral nutrients.
Direct photodegradation of DOC to carbon dioxide has be shown to be an
important decomposition pathway in freshwater and coastal waters. UV-B
effects on aquatic biogeochemical cycles, coupled with the influences of
other global changes that are occurring in climate and other factors, result
in changes in UV penetration and trace gas exchange.
|Fig 5.4 Conceptual model of the interactions of UV-B radiation with biogeochemical cycles in aquatic ecosystems.The colored dissolved organic matter (CDOM) controls the penetration of UV-B into the water. UV-B can affect the concentrations of CDOM by inducing photobleaching that is reinforced by changes in climate and acidification. Photoreactions driven by UV-B can enhance the biological availability of the CDOM and can produce various greenhouse and chemically-reactive trace gases. Adapted from Pomeroy and Wiebe (1988).|
Photosynthesis and growth of phytoplankton can be enhanced by reducing exposure to current levels of UV-B and inhibited by enhanced UV-B such as the levels observed under the ozone hole in Antarctica (Smith et al., 1992; Karentz et al., 1994; Cullen and Neale, 1994). Ozone depletion may influence the ability of the ocean to take up atmospheric CO2 (Peng, 1992), but the net impact of a reduction in primary production on the ocean sink for atmospheric CO2 is uncertain. In addition to photoinhibition and repair mechanisms, diurnal and depth-related changes in solar spectral irradiance, vertical mixing dynamics, microbial cycling (see below) and other factors affect net carbon storage. The net impact clearly is not a linear function of increased UV-B exposure. Recent papers have discussed these factors in more detail (e.g. Smith and Cullen 1995; Moran and Zepp 1997; PrJ zelin et al. 1998, Neale et al 1998).
Although it is well-established that the chemical composition of phytoplankton, and presumably of submerged vegetation, is altered on exposure to UV-B radiation (Rozema et al., 1997b), e.g. through formation of UV-protecting compounds such as mycosporin-like amino acids, and that similar changes reduce the decomposability of the litter from terrestrial plants (see Section 5.2.2), there are no reports on the effects of such changes on the decomposability of the detritus derived from exposed primary producers.
Carbon storage in aquatic ecosystems reflects the balance between biomass production and decomposition processes. Research on interactions between UV-B radiation and decomposition, which has been particularly active since our last report, is considered in the following section.
Decomposition of the DOC and particulate organic carbon (POC) in the euphotic zone (the layer of the sea in which most photosynthesis occurs) can be affected by UV-B through inhibition of microbial activity, by direct photodegradation of the CDOM and POC to CO2 and other gases, and by UV-induced photodegradation of the persistent, polymeric components of the DOC to readily decomposable compounds.
Microbial Decomposition. Research has continued on the effects of enhanced UV radiation on bacterial activity in the sea and in freshwaters. UV-B radiation inhibits the activity of bacteria (Karentz et al., 1994; Jeffrey et al., 1996a, 1996b; Herndl et al., 1993, 1997; Reitner et al., 1997) and direct DNA damage (pyrimidine dimerization) has been demonstrated in field studies (Jeffrey et al., 1996a; 1996b). The greatest damage is observed in poorly-mixed, stratified waters. However, several recent studies in lakes, coastal waters, and the Gulf of Mexico showed that the reduction in microbial activity is attenuated with increased winds and surface layer mixing (Jeffrey et al., 1996a, 1996b) and the activity is rapidly restored in the dark (within a few hours) via repair and regrowth (Mullerniklas et al., 1995; Jeffrey et al., 1996a, 1996b; Herndl et al., 1997; Kaiser and Herndl, 1997). Thus, adverse effects of UV light on microbial activity can change the timing and location of microbial decomposition of labile organic matter in the upper ocean. Very recent studies have shown that the amount and distribution of marine viruses also are affected by UV-B radiation in the sea (Weinbauer et al. 1997; Wilhelm et al., 1998). Viruses can influence microbial diversity and activity, including decomposition. Research on light-induced repair of sunlight-damaged viruses in the presence of bacteria, probably by photoreactivation, has recently appeared (Wilhelm et al. 1998).
Photochemical Degradation. In addition to its effects on microbial activity, UV-B has direct effects on decomposition. Research on these effects has expanded since the last report in 1994. There is substantial evidence now that UV-B plays an important role in the photodegradation of CDOM by solar radiation, based on a number of action spectra that have been reported (De Haan 1993; Frimmel 1994; Hongve 1994; Moran and Zepp 1997). The radiation amplification factors for photodegradation of the DOM from a variety of marine and freshwater environments fall in the same range as those calculated for most environmental effects considered in this report (see Chapter 1). When sunlight is absorbed by the polymeric CDOM of either freshwater or marine origin, it is cleaved to a variety of photoproducts and its average molecular weight is reduced (for review see Moran and Zepp 1997). Major processes include the direct photochemical mineralization of the CDOM and POC to inorganic compounds, e.g., carbon monoxide, carbon dioxide, and other forms of dissolved inorganic carbon (DIC) (Salonen and Vähätalo 1994; Miller and Zepp 1995; Granéli et al. 1996; Amon and Benner 1996). Direct photodegradation can account for a large fraction of the mineralization of DOC in certain freshwaters and coastal regions (Miller and Zepp 1995; Granéli et al. 1996; Amon and Benner 1996).
|Fig. 5.5 Photodegradation rates of DOM to DIC versus depth in two Swedish lakes. The samples were filtered to remove algae and most bacteria prior to their exposure in quartz containers placed at the depths indicated. Depth-integrated production of DIC by photodegradation was about the same in both lakes (Granéli et al., 1996).|
The photodegradation processes involve, in part, reactive oxygen species such as superoxide ions and hydroxyl radicals, that are produced on absorption of UV radiation (Blough and Zepp, 1995; Zafiriou et al., 1998; Vaughan and Blough 1998). Complexation of freshwater DOC with iron affects its reaction with these reactive transients and stimulates its photodegradation (Voelker and Sulzberger, 1996; Voelker et al. 1997; Gao and Zepp, 1998). These recent studies have important implications for marine biogeochemistry. Marine scientists have long puzzled over the fate of riverine organic matter on entry to the ocean. Isotopic studies indicate that the DOC in the open ocean is primarily of marine origin although some terrestrial character would have been expected. The recent studies of DOC oxidation induced by sunlight support the hypothesis that DOC photodegradation likely accounts for the observed loss of riverine DOC in the coastal ocean (Miller and Zepp 1995). This hypothesis is further supported by observations that the lignocellulose component of DOC is substantially reduced in coastal regions (Opsahl and Benner 1997).
Photochemically-mediated Bacterial Decomposition. Other organic photoproducts are formed that can be significantly more biologically active than the parent DOM from which they were formed (Wetzel et al. 1995; Lindell et al. 1995, 1996; Mullerniklas et al., 1995; Corin et al., 1996; Dahlen et al., 1996; Miller and Moran, 1997; Moran and Zepp 1997; Kaiser and Herndl, 1997; Jørgensen et al. 1998). Photochemically-mediated bacterial degradation (via photochemical modification of otherwise refractory CDOM into biologically labile forms) also can be an important pathway for decomposition of CDOM, although the relative importance of this pathway compared to direct photodegradation seems to strongly depend on the DOC source. In the few previous studies that have quantitatively compared rates of photochemical carbon gas formation to rates of photochemical formation of identifiable, low molecular weight biologically labile photoproducts, carbon gases have been found to be produced at rates many-fold higher (see Moran and Zepp 1997). A recent study, however, indicated that alternating exposure of CDOM to bacterial degradation and sunlight results in the production of a large pool of unidentified substrates that are readily assimilated by microorganisms. These substrates are believed to be comprised of low molecular weight carbonyl compounds that have not yet been identified and higher molecular weight compounds and humic substances that have been modified by exposure to sunlight although not transformed into small, chemically-identifiable compounds (Miller and Moran 1997).
A few recent studies have indicated that photoreactions driven by UV-B can reduce the microbial availability of certain organic substrates such as peptone and algal exudates (Thomas and Lara, 1995; Naganuma et al. 1996; Tranvik and Kokalj 1998). In at least one study, it was demonstrated that this phenomenon involves light-induced cross-linking between the CDOM and algal exudates (Tranvik and Kokalj 1998).
Decomposition and UV-B Penetration. Recent research has shown that CDOM controls the penetration and spectral distribution of solar radiation in freshwater (Williamson et al., 1996; Morris and Hargreaves, 1997) and marine systems, especially in regions close to the coast (Blough and Green, 1994; Kirk, 1994; Nelson and Guarda, 1995; Siegel and Michaels, 1996; Siegel and Dickey, 1987; Degrandpre et al., 1996; Vodacek et al, 1997). The presence of CDOM can reduce UV exposure, but CDOM also attenuates visible radiation required for photosynthesis. Efforts to model these competing spectral effects of CDOM on primary production in the sea have recently appeared (Arrigo et al. 1996). Spectral changes accompanying the decomposition of CDOM can influence the extent of UV-B damage and photorepair. The photochemical degradation and/or photochemically-mediated bacterial decomposition of CDOM are accompanied by loss of absorbance, i.e. photobleaching, in the UV and visible spectral regions. The spectral changes observed on irradiation of freshwater CDOM involve fractional decreases in the UV region that exceed those observed in the visible region (Morris and Hargreaves 1997; Miller and Zepp unpublished results). Seasonal and depth changes in UV attenuation have been attributed to the photochemical degradation of CDOM, coupled with climatic factors such as stratification of the water caused by temperature gradients (Siegel and Dickey, 1987; Morris and Hargreaves, 1997; Vodacek et al., 1997). Such spectral changes, which affect the penetration of damaging UV-B as well as that of photorepairing radiation, are discussed in more detail in Section 5.3.4 and in Chapter 4. Photochemical changes in CDOM absorbance can affect both UV-B penetration as well as photorepair of UV-B damage (see Chapter 4 for more details).
|Fig. 5.6. Photoproduction of ammonium from dissolved organic nitrogen on exposure of water from a coastal Florida river to simulated solar radiation with and without filters in place to block various wavelengths of UV radiation. The results show that exclusion of UV-B radiation (<320 nm) reduces the photo-ammonification rate (Bushaw et al., 1996).|
Interactions of UV-B radiation and CDOM provide a pathway for the release of N-containing nutrients. The photodegradation of DOM by sunlight is accompanied by the formation of readily assimilable nitrogen compounds such as ammonium (Bushaw et al. 1996) and amino acids (Bushaw et al. 1996, Jørgensen et al. 1998) that stimulate microbial activity. Under N-limiting conditions, the release of nitrogenous photoproducts from DOM photodegradation was found to significantly increase rates of bacterial growth (Bushaw et al. 1996). This photodegradation process occurs most efficiently with UV-B radiation (Figure 5). Nitrogen-rich photoproducts are likely to be of greatest biological interest in coastal regions and other ecosystems where plankton are nitrogen limited and concentrations of light-absorbing DOM are high (Bushaw et al. 1996).
Enhanced UV-B can indirectly affect phosphorus cycling as well. Dissolved organic matter forms complexes with extracellular phosphatase enzymes that inhibit their activity. Exposure to UV-B degrades these complexes thus restoring their activity (Wetzel 1992).
Increased pollution of aquatic environments by substances derived from the usage of fossil fuels, such as polycyclic aromatic hydrocarbons (PAH), also may interact with increased UV-B to affect biogeochemical cycles. Studies conducted since the 1994 report have provided extensive evidence that ultraviolet light greatly enhances the toxicity of PAH to aquatic organisms (Ankley et al. 1994; Ankley et al. 1995).
Emissions of other VOC, however, did not correlate well with chlorophyll concentrations, indicating that other natural processes must account for VOC production as well. VOC emissions via these other processes are stimulated by enhanced UV-B (Riemer et al., submitted). Factors influencing the photoproduction of VOC from DOM have been investigated (Riemer et al., submitted). The photoproduction rate of DOM to form VOC in filtered seawater exposed to sunlight was approximately proportional to the absorbance of the water. Action spectra showed that solar UV-B radiation is predominantly responsible for VOC photoproduction in sunlight. The formation rates of these compounds were decreased with increasing oxygen concentration. Other results indicated that high concentrations of alkenes are present in surface regions of Antarctic waters under the ozone hole (Atlas et al, 1994). These studies of VOC production provide additional evidence of linkage between ozone depletion, changes in surface UV-B irradiance, chemical exchange at the air-sea interface and the oxidative capacity of the marine boundary layer.
Carbon Monoxide. CDOM photoreactions are believed to be the main source of CO in seawater; CO loss has been ascribed primarily to exchange to the atmosphere and microbial metabolism. As a result of these processes, CO emissions from the sea follow a diurnal pattern with maximum near surface ocean concentrations greatly exceeding saturation during daylight. Strong gradients of CO were observed in the lowest 10 meters of the atmosphere over the Atlantic Ocean (Springer-Young et al. 1996) in the boundary layer near the Azores during the Marine Aerosol and Gas Exchange (MAGE) program. The samples nearest the ocean surface were some 50 ppb higher than at the 10 meter altitude sampling inlet. These data showed that that the ocean is a source of CO to the atmosphere and that this source influences the marine boundary layer concentration of CO.
New research results have shown that the photoproduction of CO in the ocean is induced mainly by the UV component of solar radiation (Atlas et al., 1994; Zafiriou, unpublished results). Quantum yields (the quantum yield is the fraction of absorbed radiation that results in photoreaction) for CO production at wavelengths >297 nm were highest in the UV-B region. (Zafiriou, unpublished results). These results are similar to those previously reported for CO photoproduction in freshwaters.
Recent estimates of the global oceanic CO source range from 13 Tg/yr up to 1200 Tg/yr (Erickson 1989; Zuo and Jones 1995; Bates et al. 1995). The estimate of 1200 Tg/yr is based on the assumption that most of the CO that is produced in the upper ocean escapes to the atmosphere. This assumption is supported by observations that CO uptake by chemoautotrophic bacteria are quite slow under environmental conditions and by observations of CO oxidation in the Sargasso Sea, a region of the ocean that is very oligotrophic with low microbial biomass. On the other hand, the much-lower estimate of 13 Tg/yr was derived using extensive data on CO concentrations in near-surface seawater and the air above the Pacific Ocean. This flux estimate was computed by air-sea exchange equations that use measured CO concentrations and exchange coefficients that are related to wind speed. The large difference between CO flux estimates derived from these two approaches is not understood at this point. One possibility is that there must be a major oceanic CO sink that has not been previously identified. If so, the magnitude of that sink must be approximately as large as the atmospheric sink for CO that involves reaction with the hydroxyl radical. Alternatively, the CO concentrations determined on the cruises may not have been measured at depths and locations optimal for computing realistic global fluxes.
Sulfur Gases. Atmospheric sulfur plays an important role in the radiative balance of the atmosphere. Dimethyl sulfide (DMS) is strongly related to the amount, activity and species assemblage of surface ocean primary producers. DMS is related to certain species of phytoplankton (Bates et al., 1987, 1992; Keller et al., 1989). The transfer of DMS from the ocean to the atmosphere and the subsequent transformation into sulfate particles may influence the atmospheric radiative balance (Twomey, 1977; Charlson et al., 1987; Andreae et al., 1988; Atkinson, 1989; Ayers and Gras, 1991; Boers et al., 1994). To a first approximation, one may expect that since DMS is closely related to primary productivity of certain phytoplankton species, the DMS concentration may decrease during ozone depletion episodes when primary productivity decreases. Stimulation of microbial activity by enhanced UV-B (discussed above) possibly may reinforce the decrease in DMS concentration, since bacterial decomposition of DMS is its major fate in the upper ocean (Kieber et al., 1996). This simplistic view must, however, be tempered by the observation that DMS concentrations increase when the phytoplankton are stressed by zooplankton grazing (Dacey and Wakeham, 1986). Should increased UV-B at the ocean surface stress the phytoplankton in similar ways as zooplankton grazing, then the DMS input may actually increase, at least for a short period of time.
In addition to possible effects of solar UV radiation on the biological production of DMS, DMS emissions to the atmosphere also are affected by competing marine biological and photochemical decomposition of DMS in the upper ocean. DMS photoreactions accounted for 7% to 40% of the total turnover of DMS in the surface mixed layer of the equatorial Pacific Ocean (Kieber et al., 1996). The photoreaction involved conversion to dimethylsulfoxide, but the yield of the conversion was only 14%.
One of the best examples that demonstrates the link between the natural atmospheric sulfur cycle and the physical climate system is the observational evidence that links the satellite derived stratus cloud optical depth and observed DMS derived cloud condensation nuclei (CCN) concentrations at Cape Grim, Australia (Boers et al. 1994). Statistical evidence indicates that the optical depth of the clouds is correlated with the number of CCN in the atmosphere. Thus, any UV-B related changes at the surface of the ocean that results in the alteration in DMS flux to the atmosphere and the subsequent formation of particles would also alter the atmospheric radiation budget for the affected region.
Carbonyl sulfide (OCS) is produced in surface seawater by the photochemical degradation of dissolved organic matter (Uher and Andreae 1997; Fl` ck and Andreae 1996) and the ocean is a net source of OCS to the atmosphere. The photoproduction of OCS and CO in the sea appear to be linked by competitive reactions involving free radical species (Pos et al., 1998). Recent research has confirmed that estuaries are a major source of marine OCS with sea-to-air fluxes over 50 times more than those from the open ocean (Zhang et al. 1998); sedimentary processes also contribute to estuarine production of OCS. Observed seasonal variations of OCS at a coastal site in Australia were consistent with an ocean source in the summer and a land-based sink during the winter (Griffith et al. 1998). The action spectra for the production of OCS in seawater indicate that this gas is produced most efficiently by the UV-B component of solar radiation (Erickson and Eaton, 1993; Zepp and Andreae 1994, Weiss et al. 1995a, Weiss et al. 1995b ). There is a competition between UV-B induced OCS production, OCS loss by hydrolysis and vertical mixing which leads to temporal and spatial changes in OCS air-sea fluxes (Najjar et al. 1995). The strong influence of vertical mixing in the upper ocean may also influence the near surface pool of OCS available for air-sea exchange (Najjar et al. 1995). For example, recent data indicate that certain regions of the ocean can act as a net sink for OCS, especially during the winter when UV-B irradiance is low or in low productivity, warm regions of the sea where CDOM and reactive organosulfur concentrations are low and hydrolysis rates are high (Ulshofer et al., 1995; Weiss et al, 1995b). The hydrolysis of OCS, although it reduces the efficiency of its escape to the atmosphere, results in the production of sulfide in the upper ocean (Elliot et al., 1989) where it can affect various metals cycles.
OCS is very resistant to oxidation and/or removal by processes in the troposphere, although uptake by terrestrial plants and soils can reduce its concentration (see earlier discussion in Section 5.2.6). On transport of OCS to the stratosphere it is converted to background stratospheric sulfate. Until recently it was believed that OCS was the major source of background sulfate in the stratosphere, but recent studies indicates that other sources are more significant (Chin and Davis 1995; Kjellstrom 1998). Much of the OCS that enters the stratosphere returns to the troposphere and there is increasing evidence that other tropospheric sulfur gases, such as SO2, may be contributing to the background stratospheric sulfate layer.
We wish to thank Dr. D. Gwynn-Jones for helpful input
and we are grateful to him, Dr. S. Moody, Dr. N. Paul, Dr. U. Johanson,
Dr. Solheim, Dr. C. Gehrke, Dr. C. Suttle, Dr. D. Riemer, Dr. O. Zafiriou,
and Dr. R. Zika for making as yet unpublished material available. The U.K.
Department of the Environment kindly supported the participation of T.V.C.