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Impacts of ultraviolet radiation on crustacean zooplankton and ichthyoplankton: case studies from subarctic marine ecosystems

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Impacts of ultraviolet radiation on crustacean zooplankton and ichthyoplankton: case studies from subarctic marine ecosystems
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  13Impacts ofUltraviolet Radiation on Crustacean Zooplankton and Ichthyoplankton:Case Studies from Subarctic Marine Ecosystems H.I.Browman and  R.D.Vetter 13.1Introduction A rapidly growing number ofstudies indicate that solar ultraviolet B radiation(280–320nm,UV-B),at current levels,is harmful to aquatic organisms and may reduce the productivity ofmarine ecosystems (e.g.Siebeck et al.1994;Häder1997;DeMora et al.2000).Such UV-B-induced decreases in productivity havebeen reported for bacterioplankton,phytoplankton,heterotrophs andzooplankton,the key intermediary levels ofmarine food chains (Damkaer1982;Thomson 1986;Cullen and Neale 1994;Chalker-Scott 1995;Smith andCullen 1995;Häder 1997).Analogous studies on the planktonic (often neus-tonic) early life history stages ofcrustacean zooplankton and ichthyoplankton,although much rarer,indicate that exposure to levels ofUV-B currently incident at the earth’s surface could result in higher mortality that may lead topoorer recruitment to the adult populations ofmarine and freshwater fishes(Pommeranz 1974;Hunter et al.1981,1982;Williamson et al.1997;Walters andWard 1998;Zagarese and Williamson 2000).This chapter focuses on the effectsofUV (280–400nm) radiation on crustacean zooplankton and ichthyo-plankton in subarctic marine ecosystems.The effects ofUV radiation on these two trophic levels have been thorough-ly reviewed in several recent primary publications and book chapters (e.g.Siebeck et al.1994;Browman et al.2000;Zagarese and Williamson 2000).Thus,we will not cover this same ground here.Rather,our material is presented asindependent case studies.The first case study – conducted in Norway – appliedmolecular techniques to assess UV-induced DNA damage to the eggs andlarvae ofArcto-Norwegian cod ( Gadus morhua ),incubated in situ.The secondcase study – conducted in Canada – used a combination ofapproaches toaddress the issue ofUV-induced effects on the calanoid copepod  Calanus finmarchicus ,and on the eggs ofAtlantic cod ( Gadus morhua ).Among otheradvantages,this format provides readers insight into the development and Ecological Studies,Vol.153D.Hessen (ed.) UV Radiation and ArcticEcosystems© Springer-Verlag Berlin Heidelberg 2002  implementation oftwo research programs,in two parts ofthe world,whichapplied different approaches to address similar questions. 13.2Case Study I – Lofoten,Norway  R.D.Vetter and colleagues from the Southwest Fisheries Science Center,LaJolla,California,USA,together with Osmond Holm-Hansen and associates atthe Scripps Institution ofOceanography,La Jolla,have been collaboratingwith Professor Hans Christian Eilertsen and others at the Norwegian CollegeofFisheries,University ofTromsø,Tromsø,Norway,on a series ofongoingstudies into the effects ofUV radiation on phytoplankton,zooplankton andichthyoplankton in high latitude environments.In this chapter,results on theichthyoplankton portion ofthese studies will be summarized.Unlike the Antarctic,the Arctic has significant areas ofhuman settlementand a high dependence ofindigenous peoples on marine resources.This isparticularly true in northern Europe where the ocean currents and localclimatology provide relatively mild ice-free conditions.Tromsø,Norway,is acity ofmore than 40,000 located at about 70°N latitude (roughly the equi-valent latitude to the edge ofthe permanent ice pack in Antarctica).The city is a hub for year-round fisheries and aquaculture activities.Our investigationshave combined experimental studies conducted at the University ofTromsø’s H.I.Browman and R.D.Vetter262 Fig.13.1. Map ofthe case study I site near Lofoten,Norway   Center for Aquaculture (Havbruksstasjonen) on Ringvassøy in northernNorway,and field studies conducted in the Lofoten Islands (Fig.13.1).Ourstudies have taken advantage ofthe aquaculture facilities at Havbruk-stasjonen as a source ofeggs and larvae ofknown parentage and age withwhich to carry out experiments.Although solar simulators are improving andprovide a means ofadministering a precise and repeatable dose (e.g.Kouwen-berg et al.1999a,b),it is difficult to precisely match the vertical spectralattenuation,and diel changes in intensity and spectral character ofnaturallight.This is particularly true at high latitudes where fish can experience 24hofdaylight during the summer months.Since we initially knew nothing aboutthe nature ofdamage and the process and timing ofphotorepair in highlatitude fishes,we elected to use natural solar radiation for all experiments.Thus,our studies focus on the effects ofnatural solar ultraviolet radiation onthe early life history stages ofthe Arcto-Norwegian stock ofcod ( Gadusmorhua ). 13.2.1Hydrographics ofthe Lofoten Area The high latitude environment ofnorthern Norway is the site ofsome ofthemost productive fisheries in the world.In Norwegian waters,cod leave thecontinental shelfand migrate to known spawning locations within coastalfjords.In these locations,cod congregate near the bottom and spawnrepeatedly over the course ofseveral weeks.In the Lofoten Islands,thesespawning banks have been the location oftraditional cod fisheries for the past1000years (Hjort 1914).Cod eggs are spawned at depth and the positively buoyant eggs gradually rise to the surface.The surface waters ofthe spawningbanks contain vast quantities ofeggs and newly hatched larvae.It is theseearly life stages,and the optical conditions,on the spawning banks,that havebeen the focus ofour investigations into the effects ofsolar ultravioletradiation on cod.Our observations have included a range ofresponses to UVexposure including behaviour,developmental delay,cell cycle changes,mortality,and DNA damage.All experimental studies have employed cod eggsand larvae spawned in captivity.All field and laboratory exposures have usednatural sunlight. 13.2.2The UV Environment ofthe Norwegian Coast Frequent cloud cover punctuated by bright sunny days characterizes themaritime climate ofnorthern Norway.Ozone thickness is clearly importantwith respect to long-term charges in the northern high latitude UV environ-ment.Ozone thickness has been monitored by the University ofTromsø since Case Studies from Subarctic Marine Ecosystems263  1935 and continues to be monitored to this day (Henriksen et al.1992a,b,1993,1994).Atmospheric changes do seem to be occurring (IASC Report1995) but have not been detected on the ground in Tromsø (Henriksen et al.1992b;Stolarski et al.1992).This may be due to the high incidence ofcloudcover which is the primary determinant ofdaily changes in surface UVirradiance in these maritime environments (Lubin and Jensen 1995).Inaddition to surface irradiance,the UV environment ofeggs and larvae is alsodetermined by the UV-absorbing properties ofseawater and the vertical H.I.Browman and R.D.Vetter264 Fig.13.2. Vertical profiles oftemperature and the attenuation ofUV irradiance at aspawning location in Austnesfjorden,Lofoten Islands,Norway,7 June 1995.Measure-ments were made with a Biospherical Instruments PUV 500.UV-B at 305nm was rapidly attenuated and was not detectable below 10m  distribution ofthe eggs and larvae.Here,the unusual properties offjordsprovide an environment different from the open waters ofthe GulfofSt.Lawrence study site ofBrowman and colleagues (Browman et al.2000;thisChap.).UV attenuation in surface waters is dependent upon living and deadparticulate matter (generally in the form ofsediments and phytoplankton),and the presence ofdissolved organic compounds that absorb in the UV(Smith and Baker 1979;Kirk 1994;Kuhn et al.1999).UV attenuation istypically exponential with depth in well-mixed surface waters.Unlike typicaloceanic waters,the fjord environment often contains high amounts of freshwater runoffofrecent terrestrial origin.This melt water can containlarge amounts ofsurface sediments,which ofcourse block and scatter allwavelengths.However,they also contain terrestrial plant organic matter(Gelbstoff) that has high specific absorbance in the UV.As a result,apparently clear fjord waters may have a high capacity to block UV.At our study sites,including the Lofoten Islands,UV-B was generally not detectable below 10m(Helbling et al.1996;Fig.13.2).In the protected fjord environment wind sheer is often reduced relative tonearby open water.When wind is low,mixing is weak,and buoyant eggs arefree to float at-or-near the air-sea interface where UV dose is highest(Solemdal and Sundby 1981;Sundby 1983).Plankton tows taken near thesurface in the Lofoten Islands often contain massive numbers ofcod eggs andlarvae. 13.2.3UV Responses in Arcto-Norwegian Cod Eggs 13.2.3.1Behavioural Responses Behavioural responses to UV are not generally considered with respect toeggs,but to the extent that depth and the time ofday at which eggs arereleased are regulated,they should be considered as potential responses of ecological importance.Large adult cod in sea pens will submerge on sunny days and remain near the surface on cloudy days,so it is not unreasonable toassume that they can sense and respond to the UV environment.There issome evidence that buoyancy in cod eggs can be regulated,and this wouldaffect the potential for exposure to high doses near the sea surface.Buoyancy changes in response to environmental salinity have been observed (Thorsenet al.1996) but no studies have yet been done that examine changes inspawning behaviour or egg properties in response to UV. Case Studies from Subarctic Marine Ecosystems265
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