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Assessing the impacts of solar ultraviolet radiation on the early life stages of crustacean zooplankton and ichthyoplankton in Marine coastal systems

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Assessing the impacts of solar ultraviolet radiation on the early life stages of crustacean zooplankton and ichthyoplankton in Marine coastal systems
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  -- --Friday Jan 10 2003 01:02 PMAllen Press  •  DTProN System  G A L L E Y estu 26_104 Mp_30File # 04em 30   2003 Estuarine Research FederationEstuaries Vol. 26, No. 1, p. 30–39 February 2003 Assessing the Impacts of Solar Ultraviolet Radiation on the EarlyLife Stages of Crustacean Zooplankton and Ichthyoplankton inMarine Coastal Systems 1 H OWARD  I. B ROWMAN * Institute of Marine Research, Austevoll Aquaculture Research Station, N-5392 Storebø, Norway   ABSTRACT: Over the past 10–15 years, solar ultraviolet B (UV-B, 290–320 nm) levels have increased significantly at mid-latitude areas of the Northern and Southern Hemispheres. These increases in UV-B are linked to reductions of stratospheric ozone. Although the variables that affect UV-B penetration into water columns are still under active inves-tigation, there are typically strong correlations between dissolved organic carbon (DOC), chlorophyll  a   (chl  a  ), and UV attenuation. This is particularly significant in the context of possible UV-B impacts on marine coastal systems, since DOCand chl  a   are usually much more highly concentrated in these waters than in the open ocean. Observations indicate that the early life stages of crustacean zooplankton and ichthyoplankton present in the first meter of coastal water columns(likely only a small percentage of the total population) are susceptible to UV-B radiation. Variability in cloud cover, watertransparency (and the variables that affect it), and vertical distribution and displacement of organisms within the mixedlayer have a greater effect on the flux of UV-B radiation to which plankton are exposed than will ozone layer depletion. Although exposure to UV-B can negatively affect planktonic organisms, such direct effects are likely minimal in coastalzones, and within the context of all the other environmental factors that produce the very high levels of mortalitytypically observed in their early life stages. Indirect effects (e.g., UV-B-induced reduction in the nutritional quality of the foodbase) have not as yet been adequately evaluated. Introduction Long-term data series on solar ultraviolet-B ra-diation (290–320 nm, UV-B) incident at the Earth’ssurface indicate that, over the past 10–15 years, UV-B levels have increased significantly at mid-latitudeand high-latitude areas of the Northern and South-ern Hemispheres (Crutzen 1992; Kerr and Mc-Elroy 1993; Madronich et al. 1995; Wardle et al.1997). These increases in UV-B are linked to re-ductions of stratospheric ozone (Kerr and McElroy 1993; Madronich 1994; Madronich et al. 1995). Se- vere seasonal reductions in ozone layer thicknessare not restricted to the Antarctic; dramatic deple-tions have also been recorded over the Arctic (Fer-gusson and Wardle 1998; Goutail et al. 1999; Dahl-back 2001). As a result of air mass mixing, deepozone depletion at these high latitudes tends todraw ozone north and southward, resulting inozone thinning at middle latitudes (Bjo¨rn et al.1998; Fergusson and Wardle 1998; Goutail et al.1999). Ozone layer depletion, and concomitant in- 1 Presented within two symposia dealing with essential fish-eries habitat and implications of global climatic change at the American Fisheries Society’s 24th Annual Larval Fish Confer-ence at Gulf Shores, Alabama, November 6–10, 2000. Sympo-sium and meeting conveners were Richard F. Shaw and JamesH. Cowan, Jr.* Tele: 47 56 18 22 64; fax: 47 56 18 22 22; e-mail:howard.browman@imr.no. creases in UV-B, are world-wide phenomena that are inextricably linked to global climatic change(sensu Staehelin et al. 2001).In contrast to the meteorological database, highspectral resolution measurements of UV-B under- water are rare (reviewed in Kirk 1994; Booth andMorrow 1997; Franklin and Forster 1997). It isclear that UV-B penetrates to depths greater thanis widely accepted, in some water columns dramat-ically so (Booth and Morrow 1997). Although the variables that affect UV-B attenuation are still un-der active investigation, dissolved organic carbon(DOC) and chlorophyll  a   (chl  a  ) are key correlates(Scully and Lean 1994; Morris et al. 1995; Laurionet al. 1997; Kuhn et al. 1999). A rapidly growing number of studies indicatethat UV-B radiation, at current levels, is harmful toaquatic organisms and may reduce the productivity of marine ecosystems (e.g., Siebeck et al. 1994;Ha¨der 1997; deMora et al. 2000; Hessen 2001).Such UV-B-induced reductions in productivityhavebeen reported for phytoplankton, heterotrophs,and zooplankton, the key intermediary levels of marine food chains (Damkaer 1982; Thomson1986; Cullen and Neale 1994; Chalker-Scott 1995;Smith and Cullen 1995; Booth et al. 1997; Ha¨der1997; Browman et al. 2000; Browman and Vetter2001). Analogous studies on planktonic fish eggsand larvae indicate that exposure to levels of UV-  Impacts of Ultraviolet Radiation  31 -- --Friday Jan 10 2003 01:02 PMAllen Press  •  DTProN System  G A L L E Y estu 26_104 Mp_31File # 04em Fig. 1. Ten percent depth penetrations (the depth to which10% of irradiance just below the surface penetrates) at selectedlocations in the estuary and Gulf of St. Lawrence, Canada. All values fall within the range reported by Scully and Lean (1994)for the highly UV-opaque Lake Cromwell, Que´bec, and Smithand Baker (1979) for extremely clear marine waters. (Modifiedfrom Browman et al. 2000). B currently incident at the Earth’s surface resultsin higher mortality that may lead to poorer re-cruitment to adult populations (Pommeranz 1974;Hunter et al. 1981, 1982; Williamson et al. 1997; Walters and Ward 1998; Zagarese and Williamson2000, 2001; Lesser et al. 2001).Since ozone layer depletion is expected to con-tinue for many more years (albeit at a lower rate;Staehelin et al. 2001), the possible impacts of solarUV-B radiation on marine organisms and ecosys-tems are under active investigation (recently re- viewed in Ha¨der 1997; Browman et al. 2000;deMora et al. 2000; Hessen 2001). This paper as-sesses the potential impacts of ozone depletion-re-lated increases in UV-B radiation on the early lifestages of fishes in coastal ecosystems. Crustaceanzooplankton are included in this analysis becauseof their key role as forage for fish larvae. Overview of UV-B Optics in Marine Coastal Water Columns Because the effects of UV radiation are strongly  wavelength dependent (e.g., Williamson et al.2001), accurate measurement of spectral irradi-ance is fundamental to any study on its biologicalimpact. High resolution UV measurements are es-sential for the application of biological weightingfunctions (BWFs), especially for the shortest andmost damaging wavelengths: 280–312 nm (Mad-ronich 1993; Williamson et al. 2001). In order tomake an assessment of the biological impacts of UV radiation on crustacean zooplankton and ich-thyoplankton in coastal ecosystems, measurementsof UV irradiance spectra are required. A general optical characterization of water col-umns is obtained from diffuse attenuation coeffi-cients (K  d  ), which are calculated from empiricalmeasurements of spectral irradiance at variousdepths. For comparative purposes, wavelength-spe-cific 10% depths (the depth to which 10% of thebelow-surface irradiance penetrates) are often de-rived from the K  d   values. It is important to notethat the choice of 10% depth is arbitrary—it is not based upon any correlation with biological impact.Measurements of K  d   for UV-B radiation in coastalzones indicate 10% depths of 1–4 m at a wave-length of 310 nm (Booth and Morrow 1997; Kuhnet al. 1999; Fig. 1). Ultraviolet-A radiation (320–400 nm, UV-A) reaches even greater depths. 10%penetration depths are generally smaller in estua-rine waters than in clearer offshore waters. All of the values recorded from coastal waters fall nearthe middle of the rather broad range of 10% depthpenetrations previously reported; at 310 nm, a 10%depth of 0.1 m was recorded by Scully and Lean(1994) in Lake Cromwell, Que´bec, and values ashigh as 20 m were observed for clear ocean watersby Smith and Baker (1979). Organisms residing inthe near-surface layer could be exposed to biolog-ically damaging levels of UV radiation.In the estuary and Gulf of St. Lawrence, Canada,DOC was more highly correlated with K  d  than waschl  a   (Kuhn et al. 1999). While this is perhaps themost common situation, in extreme cases chl  a   ishighly correlated with UV attenuation (Stambler et al. 1997). In freshwater, DOC is the dominant fac-tor in UV attenuation and chl  a   is most often un-important (Scully and Lean 1994; Morris et al.1995; Laurion et al. 1997). For most marine watertypes, there is a significant autocorrelation be-tween DOC and chl  a,  making it difficult to deter-mine their respective contributions to the diffuseattenuation coefficients. The slopes of these rela-tionships imply that yellow substance, and there-fore DOC, is important (Kuhn et al. 1999). Be-cause of the mixed influence chl  a   and DOC haveon UV attenuation in marine waters, seasonalchanges in the relative concentrations of these pa-rameters will significantly affect UV penetration.Inthis same sense, other factors that affect DOC andchl  a   over the long and short term—climatechange, habitat destruction, acid rain, eutrophi-cation of coastal zone waters—will greatly influ-ence the level of UV to which marine organismsare exposed (Schindler et al. 1996; Schindler and  32  H. I. Browman -- --Friday Jan 10 2003 01:02 PMAllen Press  •  DTProN System  G A L L E Y estu 26_104 Mp_32File # 04em Fig. 2. A) Biological weighting function (BWF) for egg mor-tality in  Calanus finmarchicus   (solid line). B) BWF for egg mor-tality in Atlantic cod,  Gadus morhua   (solid line). In both panels,the wavelength-dependence of damage to the naked DNA mol-ecule (data drawn from Setlow 1974) is superimposed as a dot-ted line. The Setlow curve was normalized against the BWF’s value at 300 nm for ease of comparison. (Modified from Brow-man et al. 2000.) Curtis 1997; Gibson et al. 2000; Pienitz and Vin-cent 2000). Overview of UV Impacts on CrustaceanZooplankton and Ichthyoplankton D IRECT  E FFECTS Some marine copepods are negatively affectedby current levels of UV-B radiation (Thomson1986). UV-B-induced naupliar mortality, reducedsurvival and fecundity in females, and sex ratioshifts have all been reported (Karanas et al. 1979,1981; Chalker-Scott 1995; Naganuma et al. 1997;Tartarotti et al. 2000; Lacuna and Uye 2001). UV-B-induced damage to the DNA of crustacean zoo-plankton has been detected in samples collectedfrom depths of up to 20 m (Malloy et al. 1997).Eggs of   Calanus finmarchicus,  a prominent memberof the mesozooplankton community throughout the north Atlantic, incubated under UV radiationexhibited low percent hatching compared to thoseprotected from UV (Alonso Rodriguez et al. 2000).Percent hatching in UV-B-exposed eggs was not sig-nificantly lower than that in eggs exposed to UV-A only; under natural sunlight, UV-A radiation ap-peared to be more detrimental to  C. finmarchicus  embryos than UV-B. In analogous experiments with Atlantic cod ( Gadus morhua  ) eggs, exposureto UV-B produced a significant negative effect (Be´-land et al. 1999); UV-A had no negative effect oncod eggs. Additional experiments using a solar simulatorrevealed high wavelength-dependent mortality inboth  C. finmarchicus   and cod embryos exposed toUV (Kouwenberg et al. 1999a,b). The strongest ef-fects occurred under exposures to wavelengths be-low 312 nm. At the shorter wavelengths (   305nm) UV-B-induced mortality was strongly dose-de-pendent, but (for both  C. finmarchicus   and cod)not significantly influenced by dose rate. TheBWFs derived for UV-B-induced mortality in  C. fin- marchicus   and cod eggs were similar in shape to theaction spectrum for UV-B effects on naked DNA (Fig. 2). The wavelength-dependence of DNA dam-age was similar to that for the mortality effect (Browman et al. 2000; Browman and Vetter 2001).These observations suggest that UV-induced mor-tality in  C. finmarchicus   and cod eggs is a direct result of DNA damage.These results indicate that   C. finmarchicus   may besensitive to variation in incident UV radiation insubarctic regions of the northwest Atlantic Ocean.In these regions, and also in coastal zones,  C. fin- marchicus   eggs, probably spawned near the surfaceat night or in the early morning (Runge and Plour-de 1996), are constrained to develop in the warmsurface waters above the thermocline. Observa-tions of   C. finmarchicus   egg distribution in the Lau-rentian channel (Quebec, Canada) show the ma- jority of eggs residing in the surface layer (above5–10 m) during daytime, where they hatch into thefirst naupliar stage 1–2 d after maternal release. Although  C. finmarchicus   appears susceptible toUV radiation, results from the few other speciesthat have been studied are highly variable; somespecies suffer strong negative impacts, while othersare resistant (Damkaer 1982; Thomson 1986; Dey et al. 1988; reviewed in Zagarese and Williamson2000). The factors determining this susceptibility are many and complex, but include seasonality andlocation of spawning, vertical distribution, pres-ence of UV screening compounds, and the ability   Impacts of Ultraviolet Radiation  33 -- --Friday Jan 10 2003 01:02 PMAllen Press  •  DTProN System  G A L L E Y estu 26_104 Mp_33File # 04em to repair UV-induced damage to tissues and theDNA molecule (e.g., Williamson et al. 2001).The work of Marinaro and Bernard (1966),Pommeranz (1974), and Hunter et al. (1979, 1981,1982) provided clear evidence of the detrimentaleffect of UV-B on the planktonic early life stagesof marine fishes. Hunter et al. (1979), working with northern anchovy (  Engraulis mordax  ) and Pa-cific mackerel ( Scomber japonicus  ) embryos and lar- vae, reported that exposure to surface levels of UV-B could be lethal. Significant sub-lethal effects were also reported: lesions in the brain and retina,and reduced growth rate. The study concludedthat, under some conditions, 13% of the annualproduction of northern anchovy larvae could belost as a result of UV-B related mortality (Hunteret al. 1981, 1982). With the exception of a small(but rapidly growing) number of recent studies,little additional information has been generatedfor the effects of UV-B on ichthyoplankton. Theresults presented above for Atlantic cod substanti-ate earlier results on the lethal effects of UV-B onplanktonic fish eggs. As is the case for copepods,the early life stages of fishes will be differentially susceptible to UV radiation and for the same rea-sons. Some studies conclude that UV effects will besignificant (e.g., Williamson et al. 1997; Battini et al. 2000; Lesser et al. 2001), while others concludethat they will not (e.g., Kuhn et al. 2000; Dethlef-sen et al. 2001; Steeger et al. 2001).I NDIRECT  E FFECTS The great majority of UV-B radiation researchexamines direct effects on specific organisms. Thefew studies that have investigated indirect effectsillustrate how UV-B-induced changes in food-chaininteractions can be far more significant than direct effects on individual organisms at any single tro-phic level (e.g., Bothwell et al. 1994; Williamson et al. 1999; and see discussion in Hessen et al. 1997).Recent investigations point to the possibility of such a food-chain effect in both marine and fresh- waters; UV-B exposure (even at low dose rates) re-duces the total lipid content of some microalgae(Arts and Rai 1997; Plante and Arts 1998; Arts et al. 2000) and this effect includes the polyunsatu-rated fatty acids (PUFA; Goes et al. 1994; Wang andChai 1994; Hessen et al. 1997). For zooplanktonand fish larvae, the only source of these fatty acidsis dietary—since they cannot synthesize them denovo, they must be obtained through prey organ-isms (e.g., Goulden and Place 1990; Rainuzzo et al.1997; Reitan et al. 1997; Sargent et al. 1997). Di-etary deficiencies of these fatty acids are manifest-ed in many ways. For example, in the freshwaterCladoceran  Daphnia   spp., growth rates are corre-lated with the sestonic content of eicosapentaenoicacid (Mu¨ller-Navarra 1995a,b; De Lange and VanDonk 1997; Scott et al. 1999). In Atlantic herring( Clupea harengus  ), dietary deficits of essential fatty acids, in particular docosahexaenoic acid, reducesthe number of rods in the eyes (Bell and Dick1993) and also negatively affects the feeding of these fish under low light intensities (Bell et al.1995; Masuda et al. 1998). Other negative conse-quences of essential fatty acid deficits have alsobeen reported (e.g., Kanazawa 1997; Rainuzzo et al. 1997; Bell et al. 1998). A UV-B-induced reduc-tion in the PUFA content of microalgae will bepassed on to the herbivorous zooplankton that graze upon them, also decreasing the levels of thisessential nutrient that are available to be taken upby fish larvae. Since fish larvae (and their prey)require these essential fatty acids for proper devel-opment and growth, such a reduction in the nu-tritional quality of the food base has potentially  widespread and significant implications for theoverall productivity and health of aquatic ecosys-tems.Exposure to UV radiation, especially UV-B, hasmany harmful effects on animal health. These may result in poorer performance, or death, eventhough they are not directly induced by the UV exposure. UV-B suppresses both systemic and localimmune responses to a variety of antigens, includ-ing micro-organisms (Hurks et al. 1994; Garssen et al. 1998). In addition to suppressing T-cell-medi-ated immune reactions, UV-B also affects nonspe-cific cellular immune defences. Recent studiesdemonstrate disturbed immunological responsesin UV-B-irradiated roach ( Rutilus rutilus   L.); thefunction of isolated head kidney neutrophils andmacrophages (immuno-responsive cells) were sig-nificantly altered after a single dose of UV-B (Saloet al. 1998). Natural cytotoxicity, assumed to be animportant defence mechanism in viral, neoplastic,and parasitic diseases, was reduced. A single UV-Bexposure decreased the ability of fish lymphocytesto respond to activators, and the reduction was still visible 14 d after the single exposure (Jokinen et al. 2001). This indicates altered regulation of lym-phocyte-dependent immune functions. Exposureto UV-B induces a strong systemic stress response which is manifested in the fish’s blood by an in-creased number of circulating phagocytes and el-evated plasma cortisol levels (Salo et al. 2000a).Exposure to UV-A radiation induced some of thesame negative effects on the immune system (Saloet al. 2000b). Since high cortisol levels induce im-munosuppression in fishes (Bonga 1997) it is now clear that the effect of UV-B exposure on the im-mune system has both direct and indirect compo-nents. These findings strongly suggest that the im-mune system of fishes is significantly impacted by   34  H. I. Browman -- --Friday Jan 10 2003 01:02 PMAllen Press  •  DTProN System  G A L L E Y estu 26_104 Mp_34File # 04em exposure to a single, moderate-level dose of UV-Bradiation. At the population level, such a reductionin immune response might be manifested as low-ered resistance to pathogens and in increased sus-ceptibility to diseases. The ability of the fish im-mune system to accommodate increases in solarUV-B radiation are unknown. It is possible that theimmune system of young fishes is highly vulnerableto UV-B radiation because lymphoid organs arerapidly developing and critical phases of cell pro-liferation, differentiation, and maturation are oc-curring (Grace and Manning 1980; Botham andManning 1981; Chilmonczyk 1992). It is also pos-sible that exposure to ambient UV-B radiation im-pedes the development of the thymus or other lym-phoid organs resulting in compromised immunedefense later in life. The effect of UV radiation onthe immune function of fish embryos and larvae,and on the development of the immune system, isunknown.Other indirect effects of UV radiation are alsopossible. For species that spawn in the surface lay-er, UV-B may affect sperm quality (sensu Don and Avtalion 1993; Valcarcel et al. 1994) and affect fer-tilization rate and genome transfer. If UV reducesthe productivity of protozoans and crustacean zoo-plankton there will be less prey available for fishlarvae and other organisms that feed upon them.Existing studies of UV-B impacts have almost allexamined the effects of short-term exposure on bi-ological end points such as skin injury (sunburn),DNA damage, development and growth rates, im-mune function, or outright mortality. Few studieshave examined the potential effects of longer-term(low-level) UV-B exposures (but see Fidhiany and Winckler 1999). All of these indirect (and longer-term) effects of UV radiation have yet to be inves-tigated.  An Overview of the Ecological Context   A quantitative assessment of direct UV-B effectson the population dynamics of the planktonicearly life stages of marine organisms requires detailedinformation on vertical distributions in the mixedlayer of the water column (with high resolution inthe upper 10 m), surface UV-B irradiance duringthe reproductive season and subsurface spectral ir-radiance for waters in nursery areas (see Kuhn et al. 1999), biological weighting functions—whichexplicitly consider the possibility of photorepair—for the effect of UV-B radiation on mortality (seeKouwenberg et al. 1999a,b; Browman et al. 2000;Tartarotti et al. 2000; Grad et al. 2001), and amathematical simulation model to predict the ver-tical position of passive particles (such as eggs) inthe mixed layer, and particularly their daily resi-dence time near the surface under various mete-orological and hydrographic conditions. All of these components can be incorporated into abroader simulation model to provide an assess-ment of UV-B effects on a population of early lifestages distributed (and circulating) throughoutthemixed layer (see Neale et al. 1998, 2001; Kuhn et al. 2000; Neale 2001).The model developed by Kuhn et al. (2000) in-corporates all of the physical and biological infor-mation listed above and generates an absolute es-timate of mortality under different meteorologicaland hydrographic conditions. As a result, the rel-ative impacts of differing combinations of environ-mental conditions—for example, a typical clear versus a typical overcast sky; a typical clear versusa typical opaque coastal water column; current am-bient versus realistically thinned ozone layer—canbe evaluated (Kuhn et al. 2000).For  C. finmarchicus   eggs in the estuary and Gulf of St. Lawrence, UV-B-induced mortality under allmodel scenarios ranged between    1% and 51%, with a mean  SD of 10.05  11.9% (n  48 mod-elled scenarios). For cod, none of the model sce-narios produced a UV-B-induced mortality greaterthan 1.2%, with a mean    SD of 1.0    0.63% (n  72 modelled scenarios). The most important de-terminant of survivorship (for both species) was water column transparency (Fig. 3). Even whenozone layer depletions of 50% were modelled, theeffect on mortality remained far lower than that resulting from either thick cloud cover or opacity of the water column. Analogous simulation modelsconstructed for phytoplankton (primary produc-tion) yielded similar interpretations of the relativeeffects of mixed layer depth, cloud cover, andozone layer thickness on UV-induced impacts(Neale 2001; Neale et al. 2001). The simulationmodel of Huot et al. (2000) indicated that ozonethickness could in some instances be the singlemost important determinant of DNA damage inbacterioplankton. Although these simulation model based predic-tions are instructive, data to parameterize suchmodels are scarce, and it will be some time before we are in a position to make similar predictions formany species inhabiting a range of geographic lo-cations. These models assume that the BWFs usedto parameterize them do not seriously violate theprinciple of reciprocity (the Bunsen-Roscoe prin-ciple, De Gruijl et al. 1986; Coohill 1991; Buma et al. 1997; Cullen and Neale 1997; Browman et al.2000; Zagarese and Williamson 2000; Browmanand Vetter 2001; Grad et al. 2001). In the context of a UV exposure experiment, reciprocity holds if the effect of cumulative dose is the same regardlessof the dose rate at which it was delivered. If reci-procity fails, a short intense exposure would result 
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