LETTERS. Climate-driven trends in contemporary ocean productivity

Transcription

1 Vol Decemer 26 doi:1.138/nture5317 LETTERS Climte-driven trends in contemporry ocen productivity Michel J. Behrenfeld 1, Roert T. O Mlley 1, Dvid A. Siegel 3, Chrles R. McClin 4, Jorge L. Srmiento 5, Gene C. Feldmn 4, Allen J. Millign 1, Pul G. Flkowski 6, Ricrdo M. Letelier 2 & Emmnuel S. Boss 7 Contriuting roughly hlf of the iosphere s net primry production (NPP) 1,2, photosynthesis y ocenic phytoplnkton is vitl link in the cycling of cron etween living nd inorgnic stocks. Ech dy, more thn hundred million tons of cron in the form of CO 2 re fixed into orgnic mteril y these uiquitous, microscopic plnts of the upper ocen, nd ech dy similr mount of orgnic cron is trnsferred into mrine ecosystems y sinking nd grzing. The distriution of phytoplnkton iomss nd NPP is defined y the vilility of light nd nutrients (nitrogen, phosphte, iron). These growth-limiting fctors re in turn regulted y physicl processes of ocen circultion, mixed-lyer dynmics, upwelling, tmospheric dust deposition, nd the solr cycle. Stellite mesurements of ocen colour provide mens of quntifying ocen productivity on glol scle nd linking its vriility to environmentl fctors. Here we descrie glol ocen NPP chnges detected from spce over the pst decde. The period is dominted y n initil increse in NPP of 1,93 tergrms of cron yer (Tg C yr 21 ), followed y prolonged decrese verging 19 Tg C yr 21. These trends re driven y chnges occurring in the expnsive strtified low-ltitude ocens nd re tightly coupled to coincident climte vriility. This link etween the physicl environment nd ocen iology functions through chnges in upper-ocen temperture nd strtifiction, which influence the vilility of nutrients for phytoplnkton growth. The oserved reductions in ocen productivity during the recent post-1999 wrming period provide insight on how future climte chnge cn lter mrine food wes. Internnul chnges in phytoplnkton undnce re smll reltive to totl stnding stocks nd require ccurte, long-term mesurements to detect. The first stellite sensor to hve this cpility is the Se-viewing Wide Field-of-View Sensor (SeWiFS) nd so fr it is the only ocen colour sensor to hve lunr-viewing clirtion cpilities 3. These mesurements llow SeWiFS to chieve wter-leving rdince ises cross the SeWiFS visile nds ( nm) of less thn 4% when compred to field open-ocen oservtions 4. The centrl iologicl vrile derived from these mesurements is upper-ocen chlorophyll concentrtion, which cn e used to estimte phytoplnkton stnding stocks nd productivity (Fig. 1) throughout the photic zone 5. Glol, depth-integrted chlorophyll iomss (SChl) for the 1997 to 26 SeWiFS record vried from 4.2 to 5. Tg (Tg g). Monthly nomlies in glol SChl for this period revel two highly significnt (P,.1) trends (green line in Fig. 1). The initil.3 Tg rise in SChl corresponds to the 1997 to 1999 El Niño tol Niñ trnsition 2. Since 1999, glol chlorophyll concentrtions dropped on verge.1 Tg per yer, with severl secondry fetures tht ended in 25 to 26 rise (Fig. 1). The two primry trends in glol SChl re driven y chnges occurring in the permnently strtified regions of the ocen (grey circles in Fig. 1). These regions hve nnul verge se surfce tempertures (SSTs) over 15 uc (lck lines in Fig. 1) nd represent 74% of the glol ice-free ocen. Cooler, sesonl ses lso experienced locl vriility during the SeWiFS period, ut when integrted sptilly they exhiit no significnt overll trend (P..5) (Supplementry Fig. 1). The entire phytoplnkton iomss of the glol ocens is consumed every two to six dys 6. This rpid turnover implies tht vriility in SChl (Fig. 1) corresponds to fr more sustntil chnges in NPP. One of the most commonly used models for estimting NPP from stellite chlorophyll dt is the Verticlly Generlized Production Model (VGPM) 6. The VGPM gives glol NPP estimtes for the SeWiFS record tht vry strongly with seson nd rnge from 3.8 to g of C per month. Monthly nomlies in this importnt cron flux gin revel cler (P,.1) temporl trends eginning with n El Niño/L Niñ-driven 262 Tg increse in NPP etween 1997 nd 1999, followed y 197 Tg decrese to 24, nd finlly modest increse during 25 (green line in Fig. 1c). These prominent NPP trends re gin trcele to chnges in the permnently strtified ocens (grey circles in Fig. 1c), re highly correlted (r ) with chnges in SChl (Fig. 1), nd remin when NPP is ressessed using lterntive productivity models (Supplementry Fig. 2). Perhps the most chllenging spect of understnding vriility in iologicl processes is ssociting detected chnges with the environmentl forcings responsile. Importnt climte vriles linked to ocen circultion nd productivity re se-level pressure, surfce winds, SST, surfce ir temperture, nd cloudiness. These physicl fctors hve een comined into Multivrite ENSO Index (MEI) for evluting the strength of El Niño/Southern Oscilltion cycles 7,8. Strong El Niño/Southern Oscilltion events hve mjor impcts on phytoplnkton, fisheries, mrine irds nd mmmls 2,9 12 nd re striking exmples of climtic influences on ocen iology. Importntly, the MEI does not distinguish etween nturl nd nthropogenic chnges in climte forcing, ut insted provides n integrted index of climte conditions for comprison with oserved chnges in ocen productivity. We find cler, strong correspondence etween MEI vriility nd SeWiFS-sed nomlies in NPP (r , P,.5) (Fig. 2) nd SChl (Supplementry Fig. 3). An increse in the MEI (tht is, wrmer conditions) results in decrese in NPP nd SChl, nd vice vers. This reltionship emphsizes the pre-eminent role of climte 1 Deprtment of Botny nd Plnt Pthology, 282 Cordley Hll, nd 2 College of Ocenogrphic nd Atmospheric Sciences, Oregon Stte University, Corvllis, Oregon 97331, USA. 3 Institute for Computtionl Erth System Science nd Deprtment of Geogrphy, University of Cliforni, Snt Brr, Snt Brr, Cliforni , USA. 4 NASA Goddrd Spce Flight Center, Greenelt, Mrylnd 2771, USA. 5 Atmospheric nd Ocenic Sciences Progrm, Princeton University, PO Box CN71, Princeton, New Jersey 8544, USA. 6 Environmentl Biophysics nd Moleculr Ecology Progrm, Institute of Mrine nd Costl Sciences nd Deprtment of Geologicl Sciences, Rutgers University 71 Dudley Rd, New Brunswick, New Jersey 891, USA. 7 School of Mrine Sciences, 29 Liy Hll, University of Mine, Orono, Mine , USA Nture Pulishing Group

2 NATURE Vol Decemer 26 LETTERS vriility on ocen productivity trends. The oserved physicl iologicl coupling etween NPP nd the MEI (Fig. 2) functions through n effect of climte on wter column strtifiction. Climtic chnges tht llow surfce wrming cuse n increse in the density contrst etween the surfce lyer nd underlying nutrient-rich wters. For the glol strtified ocens, the density difference etween the surfce nd depth of 2 m provides useful mesure of strtifiction. Monthly nomlies in this strtifiction index exhiit similr trends s the MEI for the SeWiFS period (Fig. 2). Importntly, enhnced strtifiction suppresses nutrient exchnge through verticl mixing. Conversely, surfce cooling fvours elevted verticl exchnge. Phytoplnkton in the ocen s upper lyer (tht is, the popultions oserved from spce) rely on verticl nutrient trnsport to sustin productivity, so intensified strtifiction during rising MEI period (Fig. 2) is ccompnied closely y decresing NPP (Fig. 2) (r , P,.5). The MEI cptures glol-scle trends in climte forcings on surfce ocen growth conditions for phytoplnkton (Fig. 2). However, while n incresing MEI corresponds to n overll wrming period, t the.15 1, NPP (mg C m 2 d 1 ) regionl scle oth wrming nd cooling events re found. These regionl chnges re registered in stellite SST dt nd re well illustrted y the 1999 to 24 period of incresing MEI. Comprison of SST chnges (Fig. 3) nd modelled NPP chnges (Fig. 3) for this period revels the nticipted inverse reltionship of incresing tempertures coupled to decresing production (Fig. 3c). Quntittively, over 74% of the glol ocens with SST chnges exceeding 6.15 uc etween 1999 nd 24 exhiited n inverse reltionship etween temperture nd NPP chnges (Fig. 3c). A consistent reltionship is likewise oserved etween SST nd SChl (Supplementry Fig. 4). Our results clerly link SChl nd NPP chnges to SST, strtifiction nd climte vritions nd provide n opportunity for comprison with simulted ocen responses to wrming climte. Atmosphere ocen generl circultion models coupled to ocen ecosystem models re essentil tools for understnding glol cron cycle climte feedcks. When forced y rising tmospheric CO 2 concentrtions, these prognostic models consistently yield incresed SSTs nd net decreses in strtified ocen NPP 13 16, consistent with oserved chnges in NPP during the 1999 to 24 period of rising MEI (Fig. 2). Also consistent with our results, model-simulted NPP chnges re lrgely due to intensified wter-column strtifiction nd re not uniformly distriuted cross the strtified ocens A wrming climte in tmosphere ocen generl circultion models is lso generlly ssocited with incresed NPP t higher ltitudes, owing to improved mixed-lyer light conditions nd extended growing sesons For our reltively rief oservtionl record, this nticipted net increse in high-ltitude NPP ws not oserved, lthough NPP chnges were found regionlly (Fig. 3). NPP nomly (Tg C per month) Chlorophyll nomly (Tg) c Yer Figure 1 Distriution nd trends in glol ocen phytoplnkton productivity (NPP) nd chlorophyll stnding stocks., Annul verge NPP showing high vlues where surfce nutrients re elevted. Low-ltitude, permnently strtified wters with nnul verge surfce tempertures over 15 uc re delineted y lck contour lines., Anomlies in glolly integrted wter-column chlorophyll concentrtions (green line) re dominted y chnges occurring in permnently strtified ocen regions (grey circles nd lck line). c, Anomlies in glol NPP (green line) re likewise driven y chnges in the permnently strtified ocens (grey circles nd lck line). Trend lines in nd c re lest-squres fits to pre-1999 nd post-1999 dt. NPP nomly (Tg C per month) NPP nomly (Tg C per month) 26 Nture Pulishing Group Yer MEI Strtifiction nomly (kg m 3 ) Figure 2 Ocen productivity is closely coupled to climte vriility., NPP nomlies in the permnently strtified ocens (grey symols, left xis) re highly correlted (r ) with the MEI of climte vriility (red symols, right xis). NPP dt re from Fig. 1c., Chnges in ocen strtifiction (red symols, right xis) link climte vriility to ocen iology, nd re well correlted (r ) with NPP nomlies (grey symols, left xis) in ocen regions with nnul verge surfce tempertures over 15 uc. Strtifiction strength ws ssessed s the density differences etween the surfce nd depth of 2 m using SODA dt (see Methods). Pulicly ccessile SODA dt re ville only to the end of 24. Note tht the MEI nd strtifiction xes (right) increse from top to ottom. 753

3 LETTERS NATURE Vol Decemer 26 SST chnges (ºC) NPP chnges (%) c NPP NPP SST SST Figure 3 Climte controls on ocen productivity cuse NPP to vry inversely with chnges in SST. Glol chnges in nnul verge SST () nd NPP () for the 1999 to 24 wrming period (Fig. 2). c, For 74% of the permnently strtified ocens (tht is, regions etween lck contour lines), NPP nd SST chnges were inversely relted. Yellow, increse in SST, decrese in NPP. Light lue, decrese in SST, increse in NPP. Drk lue, decreses in SST nd NPP. Drk red, increses in SST nd NPP. A similr inverse reltionship is oserved etween SST nd chlorophyll chnges. (See Supplementry Fig. 4 for dditionl informtion.) Nture Pulishing Group Estlishing reltionships etween ocen iology nd climte relies hevily on the ccurcy to which chnges in ocen ecosystems cn e detected 3. The unique opertionl chrcteristics of SeWiFS hve enled detection of the glol ocen trends reported here. Unfortuntely, the SeWiFS sensor is well eyond its design lifetime nd no currently scheduled ocen-colour missions will hve equl cpilities. SeWiFS lso represents n immture stte of ocencolour remote sensing. Modelling studies suggest tht shifts in ecosystem structure from climte vritions my e s or more importnt thn the ltertions in ulk integrted properties reported here 13. Susceptile ecosystem chrcteristics include shifts in txonomic composition 13, physiologicl sttus 17, nd light sorption y coloured dissolved orgnic mteril 18. Resolving these properties nd their chnges over time will require improved ccurcies nd significnt expnsions in the spectrl rnge nd resolution of future remote-sensing mesurements. Climte effects on ocen iology re documented here for nerly decde of stellite ocen colour mesurements. The compelling correspondence found etween NPP chnges nd the MEI (Fig. 2) suggests tht this integrted index of climte vriility, which extends ck to 195 (refs 6, 7), my provide first-order proxy for vritions in photosynthetic cron fixtion in the ocens for the period predting stellite ocen-colour mesurements. It is lso cler from the current nlysis tht surfce wrming in the permnently strtified ocen regions is ccompnied y reductions in productivity. The index used here (MEI) to relte climte vriility to NPP trends does not distinguish nturl from nthropogenic contriutions, ut oservtionl nd modelling efforts indicte tht recent chnges in SST re strongly influenced y nthropogenic forcing These oservtions imply tht the potentil trnsition to permnent El Niño conditions in wrmer climte stte 23 would led to lower nd redistriuted ocen cron fixtion reltive to typicl contemporry conditions. Such chnges will inevitly lter the mgnitude nd distriution of glol ocen net ir se CO 2 exchnge 15,24,25, fishery yields 9,12,26, nd dominnt sin-scle iologicl regimes 12. METHODS Ocen NPP ws clculted with the VGPM using monthly 1,8 3 2,16 pixel resolution (tht is, 18 km spcing t the Equtor) OC4-v4 chlorophyll lgorithm products from SeWiFS reprocessed version 5.1 dt ( ns.gov). Comprison of this dt with,1,4 in situ mtch-up surfce chlorophyll dt yields medin difference of 33%, which is comprle to mesurement uncertinties in the field. SChl ws clculted from the stellite chlorophyll dt using reltionships descried in ref. 5, which re sed on 3,86 in situ chlorophyll profiles nd ccount for susurfce chlorophyll fetures. The VGPM descries light-dependent chnges in wter-column NPP using reltionship sed on 1,698 field productivity mesurements 6 nd SeWiFS cloudcorrected photosyntheticlly ctive rdition dt ( gov). The stndrd VGPM descries photosynthetic efficiencies using polynomil function tht increses with temperture elow 2 uc nd decreses ove 2 uc. Ocen NPP ws dditionlly clculted using two lterntive models, with results demonstrting the roust nture of the primry temporl trends descried here (Supplementry Fig. 2). MEI dt were from no.gov/people/klus.wolter/mei. Strtifiction nomlies were clculted from Simple Ocen Dt Assimiltion (SODA) ( monthly glol density dt, which ssimilte ville field oservtions. AVHRR SST dt were from Desesonlized chlorophyll, NPP, nd strtifiction nomlies were clculted y sutrcting from ech monthly vlue the corresponding monthly verge for the entire time series. Reported sttistics for reltionships etween NPP nomlies nd MEI nd strtifiction nomlies ccount for uto-correltion effects inherent in comprisons etween time-series dt sets. Received 18 August; ccepted 6 Octoer Field, C. B., Behrenfeld, M. J., Rnderson, J. T. & Flkowski, P. G. Primry production of the iosphere: Integrting terrestril nd ocenic components. Science 281, (1998). 2. Behrenfeld, M. J. et l. Biospheric primry production during n ENSO trnsition. Science 291, (21). 3. McClin, C. R., Feldmn, G. C. & Hooker, S. B. An overview of the SeWiFS project nd strtegies for producing climte reserch qulity glol ocen io-opticl time series. Deep-Se Res. II 51, 5 42 (24). 4. Biley, S. W. & Werdell, P. J. A multi-sensor pproch for the on-orit vlidtion of ocen color stellite dt products. Remote Sens. Environ. 12, (26). 5. Morel, A. & Berthon, J-F. Surfce pigments, lgl iomss profiles, nd potentil production of the euphotic lyer: reltionships reinvestigted in view of remotesensing pplictions. Limnol. Ocenogr. 34, (1989). 6. Behrenfeld, M. J. & Flkowski, P. G. Photosynthetic rtes derived from stellitesed chlorophyll concentrtion. Limnol. Ocenogr. 42, 1 2 (1997). 7. Wolter, K. The Southern Oscilltion in surfce circultion nd climte over the tropicl Atlntic, Estern Pcific, nd Indin Ocens s cptured y cluster nlysis. J. Clim. Appl. Meteorol. 26, (1987). 8. Wolter, K. & Timlin, M. S. Monitoring ENSO in COADS with sesonlly djusted principl component index. In Proc. 17th Climte Dignostics Workshop (Normn, Oklhom) (NOAA/N MC/CAC, NSSL, Oklhom Climte Survey, CIMMS nd the School of Meteorology, Univ. Oklhom, 1993). 9. Brer, R. T. & Chvez, F. P. Biologicl consequences of El Niño. Science 222, (1983). 1. Chvez, F. P. et l. Biologicl nd chemicl response of the equtoril Pcific ocen to the El Niño. Science 286, (1999). 11. Turk, D., McPhden, M. J., Lewis, M. R. & Buslcchi, A. J. Remotely-sensed iologicl production in the Tropicl Pcific during Science 293, (21). 12. Chvez, F. P., Ryn, J., Lluch-Cot, S. E. & Niquen, M. C. From nchovies to srdines nd ck: Multidecdl chnge in the Pcific ocen. Science 299, (23). 13. Boyd, P. W. & Doney, S. C. Modelling regionl responses y mrine pelgic ecosystems to glol climte chnge. Geophys. Res. Lett. 29, 186, doi:1.129/21gl1413 (22). 14. Bopp, L. et l. Potentil impct of climte chnge on mrine export production. Glo. Biogeochem. Cycles 15, (21). 15. Le Quéré, C., Aumont, O., Monfry, P. & Orr, J. Propgtion of climtic events on ocen strtifiction, mrine iology, nd CO2: Cse studies over the

4 NATURE Vol Decemer 26 LETTERS period. J. Geophys. Res. 18, 3375, doi:1.129/21jc92 (23). 16. Srmiento, J. L. et l. Response of ocen ecosystems to climte wrming. Glo. Biogeochem. Cycles 18, GB33, doi:1.129/23gb2134 (24). 17. Behrenfeld, M. J., Boss, E., Siegel, D. A. & She, D. M. Cron-sed ocen productivity nd phytoplnkton physiology from spce. Glo. Biogeochem. Cycles 19, GB16, doi:1.129/24gb2299 (25). 18. Siegel, D. A., Mritoren, S., Nelson, N. B. & Behrenfeld, M. J. Independence nd interdependences of glol ocen opticl properties viewed using stellite color imgery. J. Geophys. Res. 11, C711, doi:1.129/24jc2527 (25). 19. Levitus, S., Antonov, J. I., Boyer, T. P. & Stephens, C. Wrming of the world ocen. Science 287, (2). 2. Brnett, T. P., Pierce, D. W. & Schnur, R. Detection of nthropogenic climte chnge in the world s ocens. Science 292, (21). 21. Levitus, S. et l. Anthropogenic wrming of Erth s climte system. Science 292, (21). 22. Brnett, T. P. et l. Penetrtion of humn-induced wrming into the world s ocens. Science 39, (25). 23. Wr, M. W., Rvelo, A. C. & DeLney, M. L. Permnent El Niño-like conditions during the Pliocene wrm period. Science 39, (25). 24. Srmiento, J. L., Hughes, T. M. C., Stouffer, R. J. & Mne, S. Simulted response of the ocen cron cycle to nthropogenic climte wrming. Nture 393, (1998). 25. Lws, E. A., Flkowski, P. G., Smith, W. O. Jr & McCrthy, J. J. Temperture effects on export production in the open ocen. Glo. Biogeochem. Cycles 14, (2). 26. Wre, D. M. & Thomson, R. E. Bottom-up ecosystem trophic dynmics determine fish production in the Northest Pcific. Science 38, (25). Supplementry Informtion is linked to the online version of the pper t Acknowledgements We thnk the Ocen Biology Processing Group t the Goddrd Spce Flight Center, Greenelt, Mrylnd for their diligence in providing the highest qulity ocen colour products possile nd the NASA Ocen Biology nd Biogeochemistry Progrm for support. Author Informtion Reprints nd permissions informtion is ville t The uthors declre no competing finncil interests. Correspondence nd requests for mterils should e ddressed to M.J.B. (mj@science.oregonstte.edu). 26 Nture Pulishing Group 755

5 NATURE Vol Decemer 26 NEWS & VIEWS Superfluid frction dt 1 26 dt Pressure (r) Figure 1 Supersolid component. The superfluid frction in crystlline 4 He in the low-temperture limit, s function of pressure, from Kim nd Chn s dt 1,3. Crystls grown t constnt pressure nd temperture (26 dt) 3 show somewht less vrition in the supersolid frction thn do those grown t constnt volume (24 dt) 1. The frction lso does not decrese shrply with incresing pressure, s would e expected if the phenomenon were the result of quntum-mechnicl exchnge processes in perfect crystl. The solid line is guide to the eye. component should hve flowed immeditely through the fine pores, ut no superflow ws oserved. This yer, however, Sski nd collegues 6 hve oserved ulk superflow. Agin, it is not seen in ll smples, ut only in those tht hve lrge, oservle oundry etween two grins (regions of distinct crystllogrphic orienttions) extending cross the smple. Superflow seems to occur long, or close to, these grin oundries. To understnd the significnce of these results 3 6, one must first know tht there re two prevlent pictures of superfluidity. The first of these focuses on the quntum-mechnicl exchnge, or tunnelling, of toms etween lttice sites. At low temperture, the de Broglie wvelength of 4 He toms quntummechnicl mesure of prticle s extent in spce is long nd covers mny sites. A long wvelength enles long-rnge exchnge of toms etween lttice sites. If the exchnges extend right cross the smple, superfluidity occurs. The second picture is sed on the phenomenon of Bose Einstein condenstion. Any numer of the prticles known s osons, which possess integer spin, my occupy single quntum stte. If, t low temperture, mcroscopic frction of osons condenses into single stte, long-rnge coherence is rought to the system tht mkes superfluidity possile. In liquid 4 He, the clssic superfluid, Bose Einstein condenstion is known to occur elow the criticl temperture for superfluidity in this system, 2.17 K. As the temperture pproches solute zero where ρ s is 1% the condensed frction is round 7%. Erly theoreticl discussions 8 1 of possile superflow in solid 4 He involved quntum tunnelling through ground-stte vcncies (these re lttice sites tht re vcnt t solute zero), s well s Bose Einstein condenstion nd quntum exchnges within the lttice. The predicted 1 superfluid frction within the lttice ws of the order.1%. Unfortuntely, however, ground-stte vcncies hve not een oserved in solid 4 He. Thermlly ctivted vcncies re found ut t tempertures ove T C tht re irrelevnt to the estlishment of superfluidity. Recent clcultions 11 lso find tht individul vcncies re not stle in the ground stte of crystlline 4 He. The vcncies insted colesce or migrte to surfce; this is the mechnism y which therml vcncies leve clssicl crystl when it is cooled. Clcultions hve lso shown tht there is no long-rnge coherence, nd so no Bose Einstein condenstion, in perfect crystl of 4 He, essentilly ecuse condenstion requires doule occupncy of lttice site 12,13. In rel crystls, the condenste frction is oserved in neutron-scttering experiments 14 to e.2.6% tht is, comptile with zero t temperture of.8 K. Similrly, long-rnge quntum exchnges within crystlline 4 He re too infrequent to explin the high ρ s oserved 15. As seen in solid 3 He, quntum exchnge rtes decrese drmticlly s solid is compressed under pressure. If the superfluid frction rises from exchnges of prticles etween lttice sites, ρ s should decrese y orders of mgnitude under incresing pressure. But Kim nd Chn 3 find ρ s to e lrgely independent of pressure (Fig. 1). Superflow in crystlline 4 He thus does not seem to e phenomenon of the perfect ulk solid 11 13,15, or to involve individul point defects (vcncies) tht re in equilirium t solute zero. Given the recently discovered dependence of ρ s on smple nneling 3 5, nd the correltion of superflow with grin oundries 6, supersolidity seems to hinge on mcroscopic, long-rnge defects such s grin oundries or morphous chnnels in the helium crystl. But t the sme time, superfluid density of the sme mgnitude is oserved 1 in helium confined in nnoporous medium such s Vycor. It is difficult to imgine tht extended defects could e the sme in helium thus confined nd in ulk helium. The sitution is fr from cler: reveling the secrets of this ltest superfluid is very much work in progress. Henry R. Glyde is in the Deprtment of Physics nd Astronomy, University of Delwre, Newrk, Delwre 19716, USA. e-mil: glyde@udel.edu 1. Kim, E. & Chn, M. H. W. Nture 427, (24). 2. Kim, E. & Chn, M. H. W. Science 35, (24). 3. Kim, E. & Chn, M. H. W. Phys. Rev. Lett. 97, (26). 4. Rittner, A. S. C. & Reppy, J. D. Phys. Rev. Lett. 97, (26). 5. Shirhm, K., Kondo, M., Tkd, S. & Shiym, Y. Am. Phys. Soc. Mrch Meet. Astr. G41.7 (26). 6. Sski, S., Ishiguro, R., Cupin, F., Mris, H. J. & Blir, S. Science 313, (26). 7. Dy, J. & Bemish, J. Phys. Rev. Lett. 95, 1534 (26). 8. Andreev, A. F. & Lifshitz, I. M. Sov. Phys. JETP 29, (1969). 9. Chester, G. V. Phys. Rev. A 2, (197). 1. Leggett, A. J. Phys. Rev. Lett. 25, (197). 11. Boninsegni, M. Phys. Rev. Lett. 97, 841 (26). 12. Boninsegni, M., Prokof ev, N. & Svistunov, B. Phys. Rev. Lett. 96, 1531 (26). 13. Clrk, B. K. & Ceperley, D. M. Phys. Rev. Lett. 96, 1532 (26). 14. Dillo, S. O., Perce, J. V., Tylor, J. W., Kirichek, O. & Glyde, H. R. Quntum Fluids nd Solids Symp. Kyoto, August 26 (in the press). 15. Ceperley, D. M. & Bernu, B. Phys. Rev. Lett. 93, (24). OCEANOGRAPHY Plnkton in wrmer world Scott C. Doney Stellite dt show tht phytoplnkton iomss nd growth generlly decline s the ocens surfce wters wrm up. Is this trend, seen over the pst decde, hringer of the future for mrine ecosystems? Ornges in Florid, wildfires in Indonesi, plnkton in the North Pcific wht links these seemingly disprte items is tht they re ll ffected y yer-to-yer fluctutions in glol-scle climte. On pge 752 of this issue, Behrenfeld et l. 1 descrie how such fluctutions, especilly in temperture, re connected to the productivity of phytoplnkton in the world s ocens. Their nlyses re sed on nerly decde of stellite dt, nd for much of the ocens they find tht recent wrmer surfce tempertures correspond to lower ocenic iomss nd productivity. Behrenfeld et l. rgue tht these ptterns rise ecuse climte-induced chnges in ocen circultion reduce the supply of nutrients needed for photosynthesis. Smll photosynthetic phytoplnkton grow in the well-illuminted upper ocen, forming the se of the mrine food we nd supporting the fish stocks we hrvest. They lso form the sis of the iogeochemicl cycling of cron nd mny other elements in the se. Phytoplnkton growth depends on temperture nd the vilility of light nd nutrients, including nitrogen, phosphorus, silicon nd iron. Most of this nutrient supply to the surfce ocen comes from the mixing nd upwelling of cold, nutrient-rich 26 Nture Pulishing Group 695

6 NEWS & VIEWS NATURE Vol Decemer 26 Mixing Tropics nd mid-ltitudes (nutrient-limited) Nutrient flux Climte wrming Higher ltitudes (light-limited) Climte wrming, freshening wter from elow, with n dditionl source of iron from minerl dust swept off the continentl deserts. Phytoplnkton iomss cn vry y fctor of 1 in surfce wters; its geogrphicl distriution is determined lrgely y ocen circultion nd upwelling, with the highest levels eing found long the Equtor, in temperte nd polr ltitudes, nd long the western oundries of continents. Although the rod sptil ptterns of phytoplnkton iomss nd productivity re well documented 2, lrge-scle temporl vritions hve only recently ecome quntifile with the dvent of stellite ocen-colour sensors 3. The ocen is vst, nd the limited numer of reserch ships move t out the speed of icycle, too slow to mp the ocen routinely on ocen-sin to glol scles. By contrst, stellite cn oserve the entire gloe, t lest the cloud-free res, in few dys. Phytoplnkton iomss nd growth rtes cn e estimted remotely from spce ecuse chlorophyll, the min photosynthetic pigment in phytoplnkton, sors lue nd red sunlight more redily thn green sunlight. Ocen-colour sensors mesure, y wvelength nd, the smll frction of sunlight scttered ck to spce from elow the surfce. The resulting surfcechlorophyll dt cn e comined with empiricl reltionships to estimte phytoplnkton growth rtes or net primry production 4. Not tht this procedure is strightforwrd: other constituents of se wter sor light; mny photons reching the stellite sensor come from tmospheric erosols or reflection t the wter surfce; nd opticl detectors on Reduced mixing, lower nutrient supply, decrese in plnkton ner the surfce Reduced mixing, increse in plnkton in the illuminted surfce wters Figure 1 Predicted phytoplnkton response to incresed temperture in ocen surfce wters 1., In the tropics nd t mid-ltitudes, phytoplnkton re typiclly nutrient-limited, nd stellite dt tie reduced iologicl productivity to upper-ocen wrming nd reduced nutrient supply., At higher ltitudes, the opposite iologicl response to future wrming, nd extr freshwter input, my occur. In these regions, phytoplnkton re often light-limited; reduced mixing would keep plnkton close to the surfce where light levels re higher. stellites degrde with time. But with creful clirtion, high-qulity, long-term records of ocen-colour dt cn e constructed for detecting climte-driven trends. The est such record t present is from GeoEye nd from NASA s Se-viewing Wide Field-of-view Sensor (SeWiFS) 3, lunched in In the SeWiFS time series, glol chlorophyll nd productivity increse shrply during nd then decline grdully to 25. Behrenfeld et l. 1 show tht these trends closely follow chnges in climte. The phytoplnkton increse in mtches negtive (cold) phse of the El Niño Southern Oscilltion (ENSO), nd the susequent slow drop occurred s the plnet moved into n extended wrm ENSO period. ENSO is dominnt mode of internnul climte vriility, nd involves lrge-scle reorgniztions of tmosphere nd ocen circultion tht originte in the equtoril Pcific nd extend cross the gloe. Decreses in productivity re eqully well relted to increses in se surfce tempertures nd verticl temperture grdients in the upper ocen. The climte plnkton link is found primrily in the tropics nd mid-ltitudes, where there is limited verticl mixing ecuse the wter column is stilized y therml strtifiction (tht is, when light, wrm wters overlie dense, cold wters). In these res, the typiclly low levels of surfce nutrients limit phytoplnkton growth. Climte wrming further inhiits mixing, reducing the upwrd nutrient supply nd lowering productivity (Fig. 1). At higher ltitudes, phytoplnkton re often light-limited ecuse intense verticl mixing crries them hundreds of metres down into drkness where sunlight does not penetrte. In these regions, future wrming nd greter influx of fresh wter, mostly from incresed precipittion nd melting seice, will contriute to reduced mixing tht my ctully increse productivity 5 (Fig. 1). The recent oserved glol surfce wrming (out.2 C per decde) is expected to ccelerte in coming decdes s we continue to relese excess cron dioxide into the tmosphere. In fct, the plnet my soon e wrmer thn t ny time in the pst million yers 6. Extrpolting the stellite oservtions into the future suggests tht mrine iologicl productivity in the tropics nd mid-ltitudes will decline sustntilly, in greement with climtemodel simultions 7 9. In those simultions, the geogrphicl oundries tht seprte specific mrine ecosystems (the ocen equivlents of forests, grsslnds nd so on) migrte towrds the poles, nd productivity increses t high ltitudes ecuse of surfce wrming, enhnced freshwter input nd reduced deep mixing. Ecosystem dynmics re complex nd nonliner, however, nd unexpected phenomen my rise s we push the plnet into this unknown climte stte. For exmple, ocenic fixtion of tmospheric nitrogen into iologiclly ville forms is concentrted in wrm, nutrient-poor surfce wters; under more strtified conditions, fixtion might increse nd enhnce overll productivity 8,1. Detecting the impct of climte chnge requires incresed nd more sophisticted monitoring of ocen iology nd environmentl chnge, oth from spce nd with instruments in the wter. Idelly, we need mesures not only of such prmeters s chlorophyll concentrtion nd productivity, ut lso of plnkton txonomy (wht is there?) nd physiology (how helthy re they?). New remotesensing technologies my help meet these chllenges 11. Scott C. Doney is in the Mrine Chemistry nd Geochemistry Deprtment, Woods Hole Ocenogrphic Institution, Woods Hole, Msschusetts 2543, USA. e-mil: sdoney@whoi.edu 1. Behrenfeld, M. J. et l. Nture 444, (26). 2. Longhurst, A. Ecologicl Geogrphy of the Se 2nd edn (Acdemic, New York, 26). 3. McClin, C. R., Feldmn, G. C. & Hooker, S. B. Deep-Se Res. II 51, 5 42 (24). 4. Crr, M.-E. et l. Deep-Se Res. II 53, (26). 5. Polovin, J. J., Mitchum, G. T. & Evns, G. T. Deep-Se Res. I 42, (1995). 6. Hnsen, J. et l. Proc. Ntl Acd. Sci. USA 13, (26). 7. Bopp, L. et l. Glo. Biogeochem. Cycles 15, (21). 8. Boyd, P. W. & Doney, S. C. Geophys. Res. Lett. 29, 186 (22). 9. Srmiento, J. L. et l. Glo. Biogeochem. Cycles 18, GB33; doi:1.129/23gb2134 (24). 1. Krl, D. et l. Nture 388, (1997). 11. NASA OBB Advnced Plnning Tem Pln for Erth s Living Ocen: The Unseen World to Set NASA s Ocen Biology nd Biogeochemistry Agend (NASA, in the press) Nture Pulishing Group