Chlorophyll fluorescence measures of seagrasses Halophila ovalis and Zostera capricorni reveal differences in response to experimental shading

Transcription

1 Mr Biol (7) 152: DOI 1.17/s RESEARCH ARTICLE Chlorophyll fluoresene mesures of segrsses Hlophil ovlis nd Zoster priorni revel differenes in response to experimentl shding Junit S. Bité Æ Sturt J. Cmpell Æ Len. J. MKenzie Æ Roert G. Coles Reeived: 5 My 6 / Aepted: 3 April 7 / Pulished online: 28 April 7 Ó Springer-Verlg 7 Astrt In ostl wters nd esturies, segrss medows re often sujet to light deprivtion over short time sles (dys to weeks) in response to inresed turidity from nthropogeni disturnes. Segrsses my exhiit negtive physiologil responses to light deprivtion nd suffer stress, or tolerte suh stresses through photo-dpttion of physiologil proesses llowing more effiient use of low light. Pulse Amplitude Modulted (PAM) fluorometery hs een used to rpidly ssess hnges in photosyntheti responses long in situ grdients in light. In this study, however, light is experimentlly mnipulted in the field to exmine the photosynthesis of Hlophil ovlis nd Zoster priorni. We imed to evlute the tolerne of these segrsses to short-term light redutions. The segrsses were sujet to four light tretments,, 5, 6, nd 9% shding, for period of 14 dys. In oth speies, s shding inresed the photosyntheti vriles signifintly (P <.5) deresed y up to 4% for mximum eletron trnsport rtes (ETR mx ) nd 7% for sturting irrdines (E k ). Photosyntheti effiienies () nd effetive quntum yields (DF/Fm ) inresed signifintly (P <.5), in oth speies, for 9% shded plnts ompred with % shded plnts. H. ovlis ws more sensitive to 9% shding thn Z. priorni, showing greter redutions in ETR mx, inditive of redued photosyntheti pity. An inrese in E k,fm nd DF/Fm for H. ovlis nd Z. priorni under 9% shding suggested n inrese in photohemil effiieny nd more effiient use of low-photon flux, onsistent with photo-limtion to shding. Similr responses were found long depth grdient from to1 m, where depth relted hnges in ETR mx nd E k in H. ovlis implied strong differene of irrdine history etween depths of nd 5 1 m. The results suggest tht H. ovlis is more vulnerle to light deprivtion thn Z. priorni nd tht H. ovlis, t depths of 5 1 m, would e more vulnerle to light deprivtion thn intertidl popultions. Both speies showed strong degree of photo-dpttion to light mnipultion tht my enle them to tolerte nd dpt to short-term redutions in light. These onsistent responses to hnges in light suggest tht photosyntheti vriles n e used to rpidly ssess the sttus of segrsses when sujeted to sudden nd prolonged periods of redued light. Communited y G.F. Humphrey. J. S. Bité (&) Shool of Tropil Environment Studies nd Geogrphy, Jmes Cook University, Townsville, QLD 481, Austrli e-mil: Junit.Bite@ju.edu.u S. J. Cmpell Len. J. MKenzie R. G. Coles Queenslnd Fisheries Servie, Deprtment of Primry Industries nd Fisheries, Northern Fisheries Centre, PO Box 5396, Cirns, QLD 487, Austrli S. J. Cmpell Len. J. MKenzie CRC Reef Reserh Centre, P.O. Box 772, Townsville, QLD 481, Austrli Introdution Costl wters nd esturies re highly produtive nd eologilly vlule eosystems. These systems re under inresing stress from nthropogeni disturnes due to sediment dredging, thment runoff nd urnistion. These disturnes nd the inresing frequenies of nturl disturnes (e.g. flooding nd ylones) (Preen et l. 1995; Cmpell nd MKenzie 4) diretly redue the distriution of eologilly importnt primry produers

2 46 Mr Biol (7) 152: nd inhiits the mintenne of helthy mrine eosystems (Dennison 1987; Durte 1991; Aloverro et l. 1). Segrsses re often the dominnt primry produers in ostl eosystems nd ontriute to mintining wter qulity y inresing the stility of sediments, iogeohemil yling nd trophi dynmis. The distriution of segrsses is widely epted s rometer of ostl wter qulity (Durte 1991; Gllegos nd Kenworthy 1996; Bier et l. 5). However, loss of segrss iomss nd suoptiml segrss growth re usully deteted when ostl eosystems re lredy degrded nd their poor ondition is unle to e reversed y mngement tions. Consequently, for segrss eosystems, the development of set of rpid nd preditle sulethl stress inditors tht respond to low-light fluxes hve een the fous of intense reserh efforts in reent times (Kremer nd Hnisk ; Bier et l. 5). In response to the need for mesures of sulethl stress thresholds for segrsses, there hve een mny efforts to quntify the physiologil responses of segrsses in response to light deprivtion over short time sles (dys to weeks) ommensurte with timesles of turidity events in ostl wters (Longstff et l. 1999; Irr-Ondo et l. 4; Bier et l. 5). In situ nd rpid ssessments of sulethl stress hve generlly een restrited to morphologil nd struturl photo-dptive responses of segrsses to light redution, inluding mesures of nopy height, shoot densities nd lef re indies (Longstff nd Dennison 1999). Chlorophyll fluoresene tehniques, suh s Pulse Amplitude Modulted (PAM) fluorometery, offer rpid in situ tool to develop sulethl physiologil inditors of stress, deteting response in segrsses to light redution within dys. Chlorophyll fluoresene (fluoresene yield) n e mesured nd used to lulte the proportion of photons sored y the photosyntheti PSII retion entres (i.e. the quntum yield), sine fluoresene is inversely orrelted to photosyntheti effiieny. In lightdpted plnts, these photons re used for photosyntheti eletron trnsport, desried s the effetive quntum yield (DF/Fm ). The effetive quntum yield of plnt s photosystem II (PSII) is DF=Fm ¼ F Fm =Fm ; where the minimum fluoresene (F), s mesured immeditely efore the sturting pulse, nd sutrted from the mximl fluoresene (Fm ), mesured immeditely fter sturting pulse of light nd then divided y Fm. F Fm is desried s the vrile fluoresene (DF) (Beer et l. 1; Rlph nd Gdemnn 5). Reserh hs foused on mximum quntum yield in drk dpted plnts, ut this hs not een found to e sensitive inditor of plnt stress (Longstff nd Dennison 1999; Bier et l. 5). In nturlly flututing light limtes there re preditle hnges in mximl eletron trnsport rtes (ETRs), photosyntheti effiienies nd sturting irrdines (Beer et l. 1998; Rlph et l. 1998; Cmpell et l. 3). Suh mesures n e simply derived from rpid light urves (RLCs), using PAM fluoresene, nd offer more effetive mens of evluting the photosyntheti performne of segrsses s ffeted y their previous light history (Beer et l. 1; Rlph nd Gdemnn 5). Nonetheless, there is n sene of mnipultive studies tht hve investigted photosyntheti derivtives of hlorophyll fluoresene (e.g.etr mx, photosyntheti effiieny nd sturting irrdine), s physiologil inditors, tht respond to hnges in light vilility over reltively short timesles of dys to weeks, efore segrss mortlity. In reent times, segrsses in Hervey By hve flututed onsiderly in response to nthropogeni disturne (Preen et l. 1995; Cmpell nd MKenzie 4). Hervey By is sutropil emyment ontining ~2, km 2 of fst growing segrss speies inluding Hlophil ovlis, Hlodule uninervis nd Zoster priorni. These speies hve reltively smll rhizomes nd roots with low rohydrte storge pity (Al et l. 1994), mking them suseptile to light redution over short timesles (i.e. dys to weeks) (Longstff nd Dennison 1999). Currently there re no rpid methods to detet stress, in segrsses, in response to light redution efore morphologil hnges or mortlity ours. In this study, we exmined differenes in the photosyntheti responses of two o-existing speies of segrss to 2-week light redution, period tht segrss medows re often sujet to severe depletion of light used y nthropogeni nd nturl disturnes. Photo-dpttion ws lso investigted s mehnism potentilly employed y oth speies to survive severe light depletion. Chlorophyll fluoresene nd, in prtiulr, RLCs were used to rpidly ssess if photo-dpttion ould e deteted under different short-term shding mnipultions nd efore segrss mortlity. Methods Shding experiment Site lotion Two, m depth, intertidl sites were hosen in Hervey By. The sites were loted t Urngn ( ), nd Burrum Heds ( ) (Fig. 1). Both sites onsisted of mixed medows of H. ovlis nd Z. priorni nd were exposed t low tide for 2 3 h during tidl yle. The mximum tidl height rnges from 2 to 3 m.

3 Mr Biol (7) 152: Fig. 1 Lotion of experimentl shding experiments in Hervey By, Austrli At oth sites three 2 2 m 2 replite plots were exposed to three different shde tretments. Three replite ontrol plots with no shde tretment were lso mrked, giving totl of 12 plots. The shde tretments onsisted of 1 1 m 2 mesh loths suspended on n luminium struture 15 m ove the sediment. The Z. priorni nopy rehed mximum of 1 m t Urngn nd 6 m t Burrum Heds, while H. ovlis plnts hd mximum length of 3 m. The shde loths were positioned so tht plnts reeived ttenuted light during dylight. A 25 m 25 m squre ws ut into the entre of eh shded plot, 15 m into the sediment, ensuring ll rhizomes were ut. Cutting prevented the shded segrss reeiving photosynthtes trnsloted, vi the rhizomes, from nonshded prts of the plnt (Longstff nd Dennison 1999). The uts were repeted severl times during the durtion of shding. The three tretments onsisted of 9, 6 nd shding. plots of segrss, without shde loth, were lso mrked. The shde loths were deployed for totl of 14 dys, eing hnged every 3 dys to redue the effet of lgl fouling. At dy, etween 11: nd 13: h during low tide nd under sunny onditions, mesures of RLCs, using diving PAM, were mde on light dpted leves of H. ovlis nd Z. priorni from the entre of nine replite plots to e treted nd eh of three ontrol plots. Helthy green leves were pled in plsti lef lip in situ (Wlz, Effeltrih, Germny) to reord RLCs. The PAM light proe ws tthed to the lef lip t fixed distne from the lef. The diving PAM utomtilly generted RLCs using n inrementl sequene of 1 s tini illumintion periods, with light intensities inresing in the eight steps, 5, 15, 34, 58, 85, 1,18 nd 1,76 lmol qunt m 2 s 1 photosynthetilly tive rdition (PAR). For eh illumintion period initil fluoresene (F) nd following sturting pulse of white light (8 ms of 8, lmol qunt m 2 s 1 PAR), mximum fluoresene (Fm ), were mesured. The sme proedure ws followed fter 14 dys (14 dys) of ontinuous shding. Depth grdient experiment Site lotion Two sites were hosen t eh depth of interest (, 5 nd 1 m) for nlyses of photosyntheti vriles of H. ovlis long depth grdient. Using the diving PAM nd the proedure desried ove, the photosyntheti performne of H. ovlis ws exmined t three depths (, 5 nd 1 m), with two replite sites for eh depth (totl of six sites). At eh site, RLCs were mesured, s desried ove, for ten replite light dpted H. ovlis plnts, etween 11: nd 13: h, under sunny onditions. In situ underwter light mesurements Amient underwter light ws mesured s photon flux using the diving PAM, whih ws lirted underwter with Li-189 light meter (LiCoR, Linoln, NE, USA). Five

4 48 Mr Biol (7) 152: photon flux mesurements were mde under every shde loth t dys nd 14 during PAM mesurements (n = 75). Three to five mesurements were mde t eh depth during PAM mesures long the depth grdient. All light mesurements were mde under sunny onditions, voiding loudy periods. Men vlues of light mesures re presented. Seson The shde experiment nd depth grdient photosyntheti mesures were ompleted within the month of Novemer 3, immeditely prior to the onset of the wet seson. The wter temperture ws 23 C for the durtion of the shde experiment nd the depth grdient photosyntheti mesures. Clultion of photosyntheti vriles Apprent photosyntheti ETRs were lulted s the produt of effetive quntum yield (F Fm =Fm ¼ DF=Fm ; where F is initil fluoresene, Fm, is mximum fluoresene nd DF is vrile fluoresene), the inution irrdine (I) nd the sorne ftor (AF), i.e. the frtion of light sored y the lef. This produt ws further multiplied y.5 euse it ws ssumed tht hlf the photons required for the movement of eletrons long the photosystem pthwys re sored y PSII (Shreiers et l. 1995). Effetive quntum yield (DF/Fm ) ws lulted from the RLC for the first of the eight tini illumintions. AF vlues for H. ovlis nd Z. priorni were derived y mesuring the proportion of light sored y single leves ording to the method desried y Beer et l. (1). The mximum ETR (ETR mx ) nd photosyntheti effiieny () were lulted y fitting the RLC dt to n exponentil funtion; ETR ¼ ETR mx ð1 exp½ ðiþ=etršþ (Jssy nd Pltt 1976) where ETR = eletron trnsport rte nd I = irrdine. The onset of light sturtion (E k ) ws lulted s ETR mx /. In the few ses where downregultion drop in ETR ws reorded, mesures from high photon flux (e.g. 1,76 lmol qunt m 2 s 1 PAR) were removed efore fitting dt to the exponentil model. Dt nlysis Shding experiments One-wy ANOVAs were used to determine the effets of plots prior to shding tretments on ETR mx, photosyntheti effiieny (), minimum irrdine t whih the plnt is photosynthetilly sturted (E k ), initil fluoresene yield (F), mximum fluoresene yield (Fm ), effetive quntum yield (DF/Fm ) nd AFs. Effetive quntum yield nd AF dt from Urngn nd Burrum Heds were rsin squre-root trnsformed prior to nlysis. Three-wy ANOVAs were used to determine the effets of speies, shde tretment nd site on ETR mx,, E k, F, Fm, DF/Fm nd AF. Photosyntheti effiieny, DF/Fm nd AF dt from Urngn nd Burrum Heds were rsin squre-root trnsformed prior to nlysis. ETR mx nd Fm dt from Burrum Heds were log e trnsformed. Depth grdient nd shding ANOVAs were used to test for signifint differenes in men photosyntheti vriles in Hlophil ovlis (ETR mx,, E k, F, Fm, DF/Fm nd AF) (n = 3 1) mong oth sites (Urngn nd Burrum Heds) nd depths (, 5 nd 1 m), nd mong ll shde tretments (ontrol, 5, 6 nd 9%) t the two sites, t 14 dys (n = 14). Post ho Bonferroni tests were used to test for signifint differenes mong mens. ETR mx nd E k,dtwerelog e (x + 1) trnsformed prior to nlysis. All nlyses were performed using SYSTAT (Version 1.2). Results Before shding Immeditely prior to the shding mnipultions t dy, there were no signifint differenes (P <.5) for photosyntheti vriles mong eh of the tretment plots t either site, Urngn nd Burrum Heds. Shding mnipultions Three-wy ANOVA reveled signifint intertion etween site, speies nd shde tretment for ETR mx nd sturting light intensity (E k )(Tle1). Post ho, Bonferroni, nlysis showed the intertion ws due to signifintly higher men (±SE) ETR mx ( ± 17.53) for nonshded (ontrol) Z. priorni t Burrum Heds ompred with the mens of non-shded Z. priorni (96.87 ± 2.61) nd Hlophil ovlis plnts (81.21 ± 5.32) t the Urngn site (Fig. 2). At Burrum Heds the men ETR mx for Z. priorni non-shded plnts ( ± 17.53) ws lso signifintly higher thn 6% (94.62 ± 23.69) nd 9% (83.78 ± 7.64) shded plnts (Fig. 2). At Urngn, mens of ETR mx for (13.95 ± 12.16) nd 6% (91.26 ± 1.) shded Z. priorni were higher thn the men of 9% shded Z. priorni (74.49 ± 2.29) plnts (Fig. 2). Men ETR mx of 9% shded H. ovlis t Burrum Heds

5 Mr Biol (7) 152: Tle 1 Three-wy ANOVA of the effet of site (Urngn nd Burrum Heds), speies (Hlophil ovlis nd Zoster priorni) nd shde tretment (, 5, 6 nd 9% light redution) on photosyntheti vriles in Hervey By, n = 3 Photosyntheti vrile df MS F P ETR mx Site Speies Shding Site speies Site shde Speies shde Site speies shde Error A Site Speies Shding Site speies Site shde Speies shde Site speies shde Error 32.2 E k Site 1 63, Speies 1 84, Shding 3 274, Site speies 1 1, Site shde 3 38, Speies shde 3 66, Site speies shde 3 5, Error 32 14,7.54 F Site 1 155, Speies 1 59, Shding 3 8, Site speies Site shde Speies shde 3 3, Site speies shde 3 6, Error 32 3, Fm Site 1 63, Speies 1 84, Shding 3 274, Site speies 1 1, Site shde 3 38, Speies shde 3 66, Site speies shde 3 5, Error 32 14,7.54 Tle 1 ontinued Photosyntheti vrile df MS F P DF/Fm Site Speies Shding Site speies Site shde Speies shde Site speies shde Error 32.5 Asorne ftor Site Speies Shding Site speies Site shde Speies shde Site speies shde Error 32.2 (78.2 ± 14.86) nd Urngn (48.61 ± 1.92) ws signifintly lower ompred with non-shded nd shded H. ovlis t oth sites. For E k the signifint three-wy intertion (Tle 1) ws est explined y signifintly higher men E k of nonshded Z. priorni (733.7 ± 92.4) nd H. ovlis ( ± 51.51) t Burrum Heds ompred with nonshded Z. priorni ( ± ) nd H. ovlis plnts (365.6 ± 5.58) t Urngn (Fig. 2). Speies intertions were est explined y higher men E k for 6% ( ± 54.93) nd 9% (29.7 ± 9.7) shded Z. priorni ompred with men E k for 6% (29.7 ± 21.32) nd 9% (12.7 ± 3.88) shded H. ovlis, respetively,t Urngn, ut no suh differenes were found t Burrum Heds (Fig. 2). At Burrum Heds there ws no differene etween men E k vlues in 6% shded ( ) nd 9% shded Z. priorni (331.3 ± 72.68), ut t Urngn men E k of 6% shded Z. priorni ( ± 54.93) ws signifintly higher thn the men E k of 9% shded plnts (29.7 ± 9.7) (Fig. 2). At Burrum Heds the men E k of shded Z. priorni ( ± 69.62) ws signifintly higher thn 6% shded ( ) Z. priorni, wheres t Urngn no differene ws found etween men E k of shded ( ± 1.49) nd 6% shded ( ± 54.93) Z. priorni (Fig. 2). A shding effet ws evident for ll prmeters mesured, exept F. At oth sites, 9% shde treted H. ovlis (1,1.67 ± 69.2 t Urngn; ± t Burrum Heds) showed higher men Fm ompred

6 41 Mr Biol (7) 152: ETR mx (µmol e - m -2 s -1 ) d 6% 9% α (µmol e - /µmol photons) d 6% 9% ETR mx (µmol e - m -2 s -1 ) d 6% 9% α (µmol e - /µmol photons) d 6% 9% E k (µmol photons m -2 s -1 ) Initil Fluoresene (F ) 4 E k (µmol photons m -2 s -1 ) Initil Fluoresene (F) 4 d 6% 9% d 6% 9% d 6% 9% d 6% 9% Mximum Fluorene (Fm') d 6% 9% Effetive Quntum Yield (DF/Fm') d 6% 9% Mximum Fluoresene (Fm') d 6% 9% Effetive Quntum Yield (DF/Fm' ) d 6% 9% Asorne ftor (AF) Asorne ftor (AF) d 6% 9%.2 d 6% 9% Fig. 2 Effet of shding on photosyntheti vriles ETR mx,, E k, F, Fm, DF/Fm nd AF, t Urngn () nd Burrum Heds () sites. Vlues re shown t dy nd fter 14 dys shding for ontrol, 5, 6 nd 9% shde treted plots for H. ovlis (white rs) nd Z. priorni (shded rs) (men ± SE, n =3) with ontrol H. ovlis (47.33 ± t Urngn; ± t Burrum Heds) plnts (Fig. 2, Tle 1). At oth sites, 9% shded H. ovlis hd signifintly higher men (.2813 ±.3 t Burrum Heds;.447 ±.45 t Urngn) nd men DF/Fm (.6943 ±.68 t Burrum Heds;.7553 ±.18 t Urngn) ompred with nonshded (.212 ±.189 t Burrum Heds nd.2267 ±.156 t Urngn; DF/Fm.5143 ±.88 t Burrum Heds nd.67 ±.255 t Urngn) nd shded (.21 ±.492 t Burrum Heds nd.265 ±.42 t Urngn; DF/Fm.4647 ±.799 t Burrum Heds nd.7423 ±.234 t Urngn) H. ovlis plnts, t respetive sites. At Urngn, 9% shded Z. priorni hd signifintly higher men (.3557 ±.62) ompred with non-shded (.222 ±.14), (.3147 ±.278) nd 6% (.327 ±.296) shded Z. priorni. At Urngn, DF/Fm ws signifintly higher for 9% shded Z. priorni (.7683 ±.143) ompred with non-shded Z. priorni (.694 ±.381). For ETR mx nd sturting light intensity (E k ), the opposite ws generlly oserved for oth speies t oth sites, where 9% shded plnts hd signifintly lower vlues thn ontrol nd shded plnts, espeilly in H. ovlis (Fig. 2). At Urngn 9% shded Z. priorni (.6368 ±.154) nd H. ovlis (.6632 ±.23) hd higher AFs thn respetive shded Z. priorni (.612 ±.297) nd H. ovlis (.56 ±.481) plnts (Fig. 2). Hlophil ovlis: depth grdient nd shding experiment Plots of ETR mx, photosyntheti effiieny (), sturting irrdine (E k ), effetive quntum yield (DF/Fm ) nd AF ginst depth nd shding tretments reveled deline in

7 - m - 2 s - 1 ) E TR m x (µmol e s - 1 ) m m m 1m 1m 6% 9% 6% 9% m m 1m 1m 6% 9% 6% 9% - Mr Biol (7) 152: photons /µmol α (µmol e d -2 s - 1 ) E k (µmol photons m m m 1m 1m 6% 9% 6% 9% E ffetive Quntum Yield ( D F/Fm ' ) m m 1m 1m 6% 9% 6% d 9%.8 1 Asorne ftor s - 1 ) m photons PFD (µmol m m 1m 1m 6% 9% 6% 9% m m 1m 1m 6% 9% 6% 9% Fig. 3 Effet of depth (, 5 nd 1 m) on photosyntheti vriles (ETR mx,, E k, F, Fm, DF/Fm nd AF) for H. ovlis. Eh grph inludes the effet of shding tretments t 14 dys (ontrol, 5, 6 nd 9% shde plots) t Urngn (grey rs) nd Burrum Heds (white rs). Photon flux density t eh depth nd shde tretment re lso shown (men ± SE, n = 3). Different letters ove rs indite signifint differenes (Bonferoni) etween tretment mens t P <.5 ETR mx nd E k nd generl inrese in nd DF/Fm with inresing depth nd shde (Fig. 3). Signifint differenes mong mens of eh vrile were found (Tle 2). Men ETR mx of H. ovlis in non-shded nd shded plots t Urngn were signifintly lower thn plnts t m ut not signifintly different thn ETR mx of plnts t 5 m (Fig. 3). In ontrst ETR mx nd E k of nonshded plnts t Burrum Heds were no different to plnts t m, while 9% shded H. ovlis hd men ETR mx nd E k vlues tht were signifintly lower thn plnts t m ut not signifintly different ompred with plnts growing t 5 nd 1 m (Fig. 3). Men nd DF/Fm vlues in ontrol nd shded plots were not signifintly different ompred with men vlues t m. Similrly men nd DF/Fm reorded for 6% shded H. ovlis were not signifintly different when ompred with men nd DF/ Fm of H. ovlis t 5 m (Fig. 3). Men nd DF/Fm vlues in 9% shded H. ovlis were signifintly lower thn plnts t m ut not signifintly different ompred with H. ovlis t 5 nd 1 m. Disussion nd onlusions Light is key ftor ontrolling the photosyntheti performne of segrsses. Both long-term nd short-term light deprivtion diretly influene segrss photosynthesis nd survivl (Longstff et l. 1999; Ruiz nd Romero 1;

8 412 Mr Biol (7) 152: Tle 2 Hlophil ovlis: ANOVA of the effet of shde plots (ontrol, 5, 6 nd 9% t two sites) nd depths (, 5 nd 1 m) in Hervey By Photosyntheti vrile df MS F P F Tretment 13 26, Error 69 2,956.8 Fm Tretment 13 81, Error 69 41,911.4 ETR mx Tretment Error Tretment Error 69.1 E k Tretment Error DF/Fm Tretment Error 69.2 Asorne ftor Tretment Error 41.3 ETR mx nd E k vlues were log e trnsformed prior to nlysis (n = 3 1) Perlt et l. 2). We found short-term hnges in the physiology of segrsses y mnipulting light during experimentl shding. Of ll the photosyntheti vriles tested (ETR mx,e k,, Fm, DF/Fm nd AF) ETR mx nd E k showed the most onsistent responses mong shde tretments nd depth grdients, hrterised y redutions in ETR mx nd inreses in E k, with light depletion. Both Hlophil ovlis nd Z. priorni exhiited typil responses to shding yet H. ovlis hd more onspiuous response, suggesting tht this speies is more sensitive to redued trnsient light thn Z. priorni. The results suggest tolerne nd limtion to light deprivtion in oth speies of segrss etween 5 nd 6% of surfe irrdine nd re onsistent with findings tht H. ovlis hs limited tolerne to light deprivtion when ompred with morphologilly lrger speies of segrss (Longstff et l. 1999). There ws onsistent trend mong speies nd sites, s shown y the deresing ETR mx nd E k nd inresing nd DF/Fm with light depletion, nd strong seprtion of photosyntheti responses etween high nd low light tretments. These photosyntheti responses re evidene tht photosyntheti prmeters derived from RLCs (Beer et l. 1) re useful tool tht n e used to evlute short-term responses of segrsses etween highly differentited light limtes. The prmeters sensitive to 9% shding in H. ovlis were redued ETR mx, inresed, redued sturting irrdine (E k ) nd inresed effetive quntum yield (DF/Fm ). While redution in oth ETR mx nd E k suggests limited photosyntheti pity suh redution indites photo-limtion nd improved photon pture for onversion to hemil energy. Similrly, the inrese in oth nd DF/Fm in response to shding indites greter proportion of photons were used in photosynthesis nd more effiient use of light (Beer et l. 1). These photo-dptive responses to irrdine or photon flux deprivtion re nlogous with other photodptive responses suh s hlorophyll inreses under low light onditions (Dennison nd Alerte 1986; Longstff nd Dennison 1999). These findings re onsistent with reports of redutions in photosyntheti performne nd mortlity in segrsses in response to shding (Longstff nd Dennison 1999; Longstff et l. 1999) nd depth (Shwrz nd Helllom 2; Durko et l. 3). The differenes in photosyntheti performne of oth segrss speies mong shde tretments t different sites re esily explined y site fetures influening light fluxes nd the speies responses to these differenes. At low tide there ws little ovious differene in light limtes etween sites, yet during tidl inundtion high mounts of suspended fine silty sediments t Urngn were evident more so thn t Burrum Heds, where the sediments hve high snd omposition nd re less inlined to e re-suspended during tidl flux. These high silt lods t Urngn resulted in lower light vilility nd more pronouned shde-type responses (e.g. lower E k ) in oth segrss speies t Urngn, ompred with segrsses t Burrum Heds. In ddition, the fouling of shde sreens ppered hevier t the Urngn site, despite regulr lening, nd my lso hve ontriuted to lower photon fluxes under the shde sreens. At oth sites Hlophil ovlis ws more sensitive to hnges in light vilility thn Zoster priorni nd thus exhiited the gretest redution in ETR mx nd E k nd the highest inreses in photosyntheti effiienies in response to shding nd in situ light depletion t Urngn. A likely explntion is tht H. ovlis is less struturlly omplex nd fster growing thn Z. priorni, with low storge pity for rohydrtes tht n e used for growth during periods of low light (Longstff et l. 1999). The inrese in effetive quntum yield (DF/Fm ) of oth segrss speies under 9% shding, reltive to ontrols, ws due to elevted Fm vlues. Elevtion of Fm suggests n inrese in photohemil effiieny, response tht is onsistent with photo-limtion of plnts to shding. This ontrsts with Longstff et l. (1999) who found tht quntum yield in drk dpted

9 Mr Biol (7) 152: H. ovlis remined unhnged during omplete light deprivtion for 24 dys. However, experimentl shding units deployed y Longstff nd Dennison (1999) nd Longstff et l. (1999) showed photo-dptive responses of redued hlorophyll nd sugr onentrtion in H. ovlis leves fter 15 dys of shding tht re onsistent with findings of redued ETR mx, inresed nd inresed E k in the present study. In the present study no hnge in DF/Fm ws found for Z. priorni. Bier et l. (5) onluded tht the mximum quntum yield of Z. mrin in response to light deprivtion ws not sensitive inditor of hroni stress s Z. mrin ws le to limte to mient light onditions. A lose exmintion of Bier et l. (5) shows ler stress response in helthy leves, with quntum yield vlues of.5 t 4 weeks, 1 week prior to n oserved deline in shoot numers nd lef re. Similrly, they show tht quntum yield vlues in Hlodule wrightii, in drk onditions, delined to.6 fter 3 weeks nd.2 fter 4 weeks, t lest week prior to notiele delines in shoot numers. In the ontext of the ler photosyntheti responses to light redution found in the present study, this result demonstrtes the importne of using suite of photosyntheti vriles, suh s ETR mx,, E k nd DF/Fm, derived from RLCs to ssess short-term responses of segrsses to light deprivtion, not mesures of mximum quntum yield lone. The influene of depth on ETR mx nd E k implies strong seprtion of irrdine history etween depths of nd 5 1 m. After 2 weeks of exposure to 9% redued light vilility, H. ovlis exhiited shde-type physiologil responses, i.e. lower ETR mx nd E k, similr to plnts growing t 5 1 m. The similrity in the light limtes etween 9% shded plnts nd mient light t 5 1 m is the likely explntion. The 4% lower ETR mx nd 67% lower E k vlues in 9% shded H. ovlis ompred with ontrols, ws generlly lower thn depth relted redutions of ETR mx nd E k from to 5 m nd from to 1 m. Intertidl plnts my e more suseptile to light redution thn sutidl plnts nd exhiit greter photodptive responses in order to optimise light quisition nd mximise the use of low-light limtes. In ddition, fouling of shde loths in etween lening my hve lowered ville photon flux density (PFD) elow reorded levels nd ontriuted to heightened photodptive response. Comprtive lowering of ETR mx nd E k vlues hs een reorded for Hlophil stipule (Shwrz nd Helllom 2) nd H. ovlis (Rlph 1996). In order to optimise light quisition nd mximise their use of low-light limtes, these plnts n diurnlly regulte their sturting irrdine s deteted in numer of segrss speies (Rlph et l. 1998; Cmpell et l. 3). We hve lso mesured three to fourfold inrese in light sturting irrdine, etween 6 nd 1 hours, in deepwter H. ovlis (unpulished dt). Asorne ftors hve een shown to vry onsiderly with geogrphi lotion (Silv nd Sntos 3), depth (Shwrz nd Helllom 2), speies (Beer et l. 1; Cmpell et l. 3; Ln et l. 5) nd lef ge (Enríquez et l. 2). In Hervey By there ws no hnge in AF with depth, ut the high AFs of drk shded (9%) H. ovlis t Urngn were inditive of photo-dptive response ut were unlikely lone to e relile inditor of light redution. The generl lk of hnge in AFs with depth nd shding suggests tht ftors (e.g. time of yer, nutrient sttus) other thn light ttenution my e influening AFs over time sles of weeks to months. The stronger responses to shding suggest tht H. ovlis ws more vulnerle to light deprivtion thn Z. priorni nd tht H. ovlis t depths of 5 1 m would e more vulnerle to light deprivtion thn intertidl popultions. Intertidl H. ovlis ppers to e more vulnerle to redutions in irrdine thn intertidl Z. priorni, impliting lol light onditions s importnt influenes on speies speifi photosyntheti performne. The lower photosyntheti responses of H. ovlis t depths greter thn 5 m ompred with intertidl plnts, lso suggest greter vulnerility to light deprivtion with inresed depth. Compred with lrge segrsses, the more pronouned responses of H. ovlis to light redution my e due to differenes in the pity of smll segrsses to store rohydrtes nd mintin photosyntheti performne (Czerny nd Dunton 1995; Kremer nd Alerte 1995; Longstff et l. 1999). The short-term response of segrsses to in situ redutions in light vilility is likely to result in redued growth rtes nd therefore n provide rpid nd preise inditor of su-lethl stress. The photodptive responses from experimentl light redution nd the mesures of light ttenution nd sturting irrdines long depth grdients, prtiulrly t lower depth limits of survivl, ould lso e used to predit redutions in photosyntheti pity efore mortlity eventutes. Both speies showed photo-dptive response to light depletion tht my help them tolerte short-term yet severe redutions in light ommonly enountered in Hervey By. Responses of ETR mx, sturting irrdine (E k ) nd photosyntheti effiieny () to light redution were the most onsistent nd re therefore likely to provide the est inditors of photo-dpttion nd possile segrss stress. Aknowledgements This work ws onduted s prt of Ph.D. degree in the Shool of Tropil Environment Studies nd Geogrphy, Jmes Cook University. The work ws supported y n Austrlin Postgrdute Awrd (Industry) from the Austrlin Reserh Counil nd Industry prtners, Queenslnd Deprtment of Primry Industries nd Fisheries, Queenslnd, Prks nd Wildlife Servie (Environment Protetion Authority) nd World Wide Fund for Nture

10 414 Mr Biol (7) 152: (WWF). The pproprite permit, required to remove segrsses in Queenslnd, ws otined from the Queenslnd Deprtment of Industries nd Fisheries. The uthors would like to knowledge S. Kerville nd D. Foster for their tehnil nd field ssistne. Referenes Al EG, Lonergn NR, Bowen P, Perry CJ, Udy JW, Dennison WC (1994) Physiologil nd morphologil responses of the segrss Zoster priorni Ashers. to light intensity. J Exp Mr Biol Eol 178: Aloverro T, Cerin E, Bllesteros E (1) The photosyntheti pity of the segrss Posidoni oeni: influene of nitrogen nd light. J Exp Mr Biol Eol 261:17 12 Beer S, Vilenkin B, Weil A, Veste M, Susel L, Eshel A (1998) Mesuring photosyntheti rtes in segrsses y pulse mplitude modulted (PAM) fluorometry. Mr Eol Prog Ser 174:293 3 Bier PD, Perl HW, Gllegos CL, Kenworthy WJ (5) Evluting inditors of segrss stress to light. In: Bortone SA (ed) Esturine inditors. CRC, Bo Rton, FL, pp Cmpell SJ, MKenzie LJ (4) Flood relted loss nd reovery of intertidl segrss medows in southern Queenslnd, Austrli. Esturine Cost Shelf Si 6: Cmpell SJ, Miller C, Steven A, Stephens A (3) Photosyntheti responses of two temperte segrsses ross wter qulity grdient using hlorophyll flouresene. J Exp Mr Biol Eol 291:57 78 Czerny AB, Dunton KH (1995) The effets of in situ light redution on the growth of two sutropil segrsses, Thlssi testudinum nd Hlodule wrightii. Esturties 18: Dennison WC (1987) Effets of light on segrss photosynthesis, growth nd depth distriution. Aqut Bot 27:15 26 Dennison WC, Alerte RS (1986) Photodpttion nd growth of Zoster mrin L. (eelgrss) trnsplnts long depth grdient. J Exp Mr Biol Eol 98: Durte CM (1991) Segrss depth limits. Aqut Bot 4: Durko MJ, Kunzelmn JI, Kenworthy J, Hmmerstrom KK (3) Depth-relted vriility in the photoiology of two popultions of Hlophil johnsonii nd Hlophil deipiens. Mr Biol 142: Enríquez S, Merino M, Iglesis-Prieto R (2) Vritions in the photosyntheti performne long the leves of the tropil segrss Thlssi testudinum. Mr Biol 14:891 9 Gllegos CL, Kenworthy WJ (1996) Segrss Depth limits in the Indin River Lgoon (Florid, USA): pplition of n optil wter qulity model. Esturine, Cost Shelf Si 42: Irr-Ondo SE, Hek KL, Spitzer PM (4) Effets of simultneous hnges in light, nutrients, nd herivory levels, on the struture nd funtion of sutropil turtlegrss medow. J Exp Mr Biol Eol 31: Jssy AT, Pltt T (1976) Mthemtil formultion of the reltionship etween photosynthesis nd light for phytoplnkton. Oenogrphy 21: Kremer GP, Alerte RS (1995) Impt of dily photosyntheti period on protein synthesis nd rohydrte stores in Zoster mrin L. (eelgrss) roots: implitions for survivl in lightlimited environments. J Exp Mr Biol Eol 185: Kremer GP, Hnisk MD () Physiologil nd growth responses of Thlssi testudinum to environmentlly-relevnt periods of low irrdine. Aqut Bot 67:287 3 Ln C-Y, Ko W-Y, Lin H-J, Sho K-T (5) Mesurement of hlorophyll fluoresene revels mehnisms for hitt nihe seprtion of the intertidl segrsses Thlssi hemprihii nd Hlodule uninervis. Mr Biol 148:25 34 Longstff BJ, Dennison WC (1999) Segrss survivl during pulsed turidity events: the effets of light deprivtion on the segrsses Hlodule pinifoli nd Hlophil ovlis. Aqut Bot 65: Longstff BJ, Lonergn NR, O Donohue MJ, Dennison WC (1999) Effets of light deprivtion on the survivl nd reovery of the segrss Hlophil ovlis (R.Br.) Hook. J Exp Mr Biol Eol 234:1 27 Perlt G, Pérez-Lloréns JL, Hernández I, Vergr JJ (2) Effets of light vilility on growth, rhiteture nd nutrient ontent of the segrss Zoster noltii Hornem. J Exp Mr Biol Eol 269:9 26 Preen AR, Long WJL, Coles RG (1995) Flood nd ylone relted loss, nd prtil reovery, of more thn 1 km 2 of segrss in Hervey By, Queenslnd, Austrli. Aqut Bot 52:3 17 Rlph PJ (1996) Diurnl photosyntheti ptterns of Hlophil ovlis (R.Br.) Hook f. In: Kuo J, Phillips RC, Wlker DI, Kirkmn H (eds) Segrss iology: proeedings of n interntionl workshop, Rottnest Islnd, Western Austrli, pp Rlph PJ, Gdemnn R (5) Rpid light urves: powerful tool to ssess photosyntheti tivity. Aqut Bot 82: Rlph PJ, Gdemnn R, Dennison WC (1998) In situ segrss photosynthesis mesured using sumersile, pulse-mplitude modulted fluorometer. Mr Biol 132: Ruiz JM, Romero J (1) Effets of in situ experimentl shding on the Mediterrnen segrss Posidoni oeni. Mr Eol Prog Ser 215:17 12 Shreiers U, Hormnn H, Neuuer C, Klughmmer C (1995) Assessment of photosystem II photohemil quntum yield y hlorophyll fluorosene quenhing nlysis. Aust J Plnt Physiol 22:29 22 Shwrz A-M, Helllom F (2) The photosyntheti light response of Hlophil stipule growing long depth grdient in the Gulf of Aq, the Red Se. Aqut Bot 74: Silv J, Sntos R (3) Dily vrition ptterns in segrss photosynthesis long vertil grdient. Mr Eol Prog Ser 257:37 44