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Freshwater Biology (2003) 48, 2141–2148Photosynthetic and growth responses of three freshwateralgae to phosphorus limitation and daylengthELENA LITCHMAN, DANIEL STEINER AND PETER BOSSARDLimnological Research Centre, EAWAG, Kastanienbaum, SwitzerlandSUMMARY1. Three common species of freshwater phytoplankton, the diatom Nitzschia sp., greenalga Sphaerocystis schroeteri and cyanobacterium Phormidium luridum, were grown undercontrasting daylengths [18 : 6 h light : dark cycles (LD) versus 6 : 18 h LD] andphosphorus (P) regimes (P-sufficient versus 1 lM P). The rates of growth and photosynthesis,as well as growth efficiencies and pigment concentrations, were compared amongtreatments.2. The growth and photosynthetic parameters of the three species depended on both Pstatus and daylength in a species-specific way. The responses to P limitation depended ondaylength and, conversely, the responses to daylength depended on P status.3. Growth rates and the maximum rates of photosynthesis (P max ) of all species decreasedunder P limitation under both light regimes. However, the decrease of P max because of Plimitation was greater under long daylength. The P max of the green alga S. schroeteridecreased the most (ca. sixfold) under P limitation compared with the other two species.The photosynthesis saturation parameter I k also decreased under P limitation; the declinewas significant in Nitzschia and Sphaerocystis. P-limitation significantly increased photoinhibition(b) inNitzschia and Sphaerocystis, but not in Phormidium. The excessphotochemical capacity (the ratio of the maximum photosynthesis rate to the photosynthesisrate at the growth irradiance), characterising the ability to utilise fluctuating light,was significantly lower under P limitation.4. The growth efficiency (growth rate normalised to daylength) declined with increasingdaylength in all species. Under short daylength the cyanobacterium Phormidium had thelowest growth efficiency of the three species.5. The cellular chlorophyll a concentration in both Nitzschia and Sphaerocystis wassignificantly higher under short daylength, but only under P-sufficient conditions. InNitzschia, under short daylength, P-limitation significantly decreased cellular chlorophyllconcentration. In contrast, P-limitation increased cellular chlorophyll concentration inSphaerocystis, but under long daylength only. The ratio of chlorophyll a to b in the greenalga also declined under short daylength and under P-limited conditions.Keywords: daylength, growth rate, phosphorus limitation, photosynthesis, phytoplanktonIntroductionPhytoplankton in most natural environmentsexperience significant seasonal changes in daylength.Correspondence: Elena Litchman, School of Biology, 310 FerstDr., Georgia Institute of Technology, Atlanta, GA 30332, U.S.A.E-mail: elena.litchman@biology.gatech.eduExperimental studies show that daylength affectsgrowth and photosynthesis of many groups of algaeand these effects may be species-specific. Shorter daylengthdecreased growth rates of cyanobacteria anddiatoms, and the decrease was greater in cyanobacteria(Foy & Gibson, 1993). The cyanobacterium Oscillatoriaagardhii had higher maximum rates of photosynthesisunder shorter daylengths and its growth rate was aÓ 2003 Blackwell Publishing Ltd 2141

2142 E. Litchman et al.saturating function of daylength (Post, Loogman & Mur,1986). Growth rates normalised to daylength decreasedwith increasing daylength (Tang & Vincent, 2000).The responses of phytoplankton to daylength mayalso depend on various environmental factors. Forexample, Tang & Vincent (2000) showed that growthand photosynthetic responses of an Arctic cyanobacteriumto daylength depended on temperature. Inmany temperate lakes phytoplankton are phosphoruslimitedduring at least part of the year (Hecky &Kilham, 1988). P limitation may modify growth andphotosynthetic responses to daylength. Such influenceof P limitation may be species-specific. At the sametime, daylength may influence algal responses to Plimitation. The differential physiological responses ofspecies to P limitation under different daylengths maythen affect community composition. Our modelinvestigations indicate that daylength may affectcompetition for P among phytoplankton (Litchman& Klausmeier, 2001; Litchman, Klausmeier & Bossard,in press), in part because P uptake is light-dependent.Sommer (1994) found that the dynamics and outcomeof competition for nutrients among marine phytoplanktondepended on daylength. Moreover, theinteractions between light regime and nutrient statusdetermine the nutritional quality of phytoplankton forzooplankton (Sterner & Elser, 2002). Therefore it isimportant to assess simultaneous influence of differentdaylengths and P status on phytoplankton.Here we report an investigation of how photosynthesisand growth of three species of freshwaterphytoplankton respond to contrasting daylength andP conditions.Materials and methodsAlgae and growth conditionsThree species of freshwater phytoplankton from threemajor taxa were used in the experiments. The diatomNitzschia sp. and the green alga Sphaerocystis schroeteriChodat were obtained from the Plant Biology Department,University of Minnesota and the cyanobacteriumPhormidium luridum var. olivace Boresch (UTEX 426) wasobtained from the University of Texas Culture Collection.All these species can be quite common in temperatelakes. The genus Phormidium is similar to the genusOscillatoria in its ecological niche and can be eitherplanktonic or benthic (Whitford & Schumacher, 1984).Cultures were grown in a temperature-controlledroom at 20 °C on shaker tables (100 r.p.m.). Twodaylengths were used: 18 : 6 and 6 : 18 h LD. Lightwas provided by cool-white fluorescent lamps(Osram, Munich, Germany). In both light regimesthe irradiance (400–700 nm) was 100 lmol quantam )2 s )1 measured with a quantum scalar sensor(Biospherical Instruments, QSL-100, San Diego, CA,U.S.A.) inside the bottles with the medium. Cultureswere grown in the WC medium (Guillard, 1975) insemicontinuous regime (daily dilutions of 0.2 day )1 ).The WC medium for P-limited cultures had 1 lMphosphate concentration. The cultures were grownunder experimental conditions for at least 4 days(nutrient-replete conditions) or 8 days (P-limited conditions)before photosynthetic and other parameterswere measured.Growth rates were determined as the slope of thelinear regression of the natural logarithm of cell densityversus time adjusted for dilution rate. To compare growthefficiency (light utilisation efficiency) between daylengths,growth rates were normalised to daylength.Photosynthesis–irradiance curvesPhotosynthesis–irradiance (P–I) curves were determinedby 14 C incorporation in the incubator which isa custom modification of the small volume photosynthetron(Lewis & Smith, 1983). Samples (7 mL) wereincubated in 20-mL scintillation vials placed in metalholders through which water circulated to maintainthe desired temperature. Light was provided from thebottom by four 250 or 400 W high intensity dischargelamps (Osram HQI-D) with the spectrum close tonatural sunlight. The UV portion of the spectrum wasblocked by a sheet of UV-opaque plexiglas (395 nmcutoff). The platform holding the metal racks wasshaken constantly so that the samples were shaken aswell. Irradiance was adjusted with layers of neutraldensity screen. The irradiance was determined as theaverage of several measurements made over 1–2 minand recorded with a LICOR Inc. data logger (Lincoln,NE, U.S.A.) to take into account changes because ofshaking.The samples were incubated for 2 h. After theincubation, 50 lL of 1 N HNO 3 was added andsamples were bubbled with air for 1 h (Gächter &Mares, 1979). Dark uptake measured in triplicate wassubtracted from the light uptake.Ó 2003 Blackwell Publishing Ltd, Freshwater Biology, 48, 2141–2148

Phytoplankton responses to phosphorus and daylength 2143Dissolved inorganic carbon concentration wasdetermined from pH and alkalinity measurements.Alkalinity was determined by titration with HCl.Chlorophyll a (and b for S. schroeteri) was determinedafter filtering a sample onto a GF/F filter (WhatmanInternational Ltd, Kent, UK) and extracting chlorophyllin 90% methanol 10 min at 75 °C. Chlorophyll cin the diatom was not determined because the highperformance liquid chromatography (HPLC) systemwas not calibrated for chlorophyll c. Pigment concentrationswere determined using an HPLC system witha LichroSpher 100 RP-18 column (Merck & Co. Inc.,White House Station, NJ, USA) connected to a JascoAS-950 autosampler and PU-980 liquid chromatographysystem. The system was calibrated with thechlorophyll a and b standards (Fluka).We modelled the P–I data using the hyperbolictangent equation (Jassby & Platt, 1976) modified toinclude photoinhibition: 1P ¼ P max tanhðaI=P max Þð1Þ1 þ bIusing two-way ANOVA. In some cases, to meet theassumptions of ANOVA, the data were log-transformed.Student–Neuman–Keuls test was used formultiple pairwise comparisons. Within each factor(daylength or P status) treatments were comparedusing a t-test.ResultsPhotosynthetic parametersBoth daylength and P status influenced photosyntheticparameters of all three species (Fig. 1; Table 1).The maximum rates of photosynthesis (P max ) normalisedto chlorophyll a (P max /Chl a ¼ Assimilationwhere P is the rate of photosynthesis at irradiance I,P max is the maximum potential light-saturated photosyntheticrate (with no photoinhibition), a is the initialslope of the P–I curve and b characterises inhibition ofphotosynthesis at high irradiance. We also determinedI k , the saturation parameter, as the ratio of P max to a.The hyperbolic tangent equation (eqn 1) gaveslightly higher R 2 for most P–I curves than theequation by Platt, Gallegos & Harrison (1980); therefore,here we report and compare parameters for thisequation only. Previous studies noted that photosyntheticparameters such as a can only be comparedwhen the same equation is used (Frenette et al., 1993).At least two, and in most cases three or four,replicate cultures were grown under each combinationof daylength and P concentration and the P–I curveswere determined for each culture. We also determinedthe excess photochemical capacity as the ratio of themaximum photosynthetic rate, P max to the photosyntheticrate under growth irradiance, P g (Cullen &MacIntyre, 1998). High excess photochemical capacityindicates good capability to utilise fluctuating light.Statistical analysesThe effects of daylength, P concentration and theirinteraction on various parameters were analysedFig. 1. Representative P–I curves for the three species. (a)Nitzschia; (b) Sphaerocystis; (c) Phormidium. LDL, long daylength;SDL, short daylength; P-lim, P-limited; P-suff, P-sufficientcultures.Ó 2003 Blackwell Publishing Ltd, Freshwater Biology, 48, 2141–2148

2144 E. Litchman et al.Table 1. Various physiological parameters of the three species under different light regimes and P conditionsLight regimePconditionsNitzschia Sphaerocystis PhormidiumLDL SDL LDL SDL LDL SDLa, ·10 )2 , mg C mg Chlorophyll h )1 P-sufficient 4.2 ± 1.2 4.9 ± 0.8 5.4 ± 1.0 4.6 ± 0.5 7.4 ± 2.0 6.8 ± 0.4P-limited 2.7 ± 0.2 5.2 ± 0.2 3.0 ± 0.8 3.8 ± 0.3 2.6 ± 0.1 6.4 ± 0.7I k , lmol quanta m )2 s )1 P-sufficient 91.5 ± 15.3 106.7 ± 12.24 127.8 ± 36.5 132.4 ± 10.8 138.5 ± 15.3 104.7 ± 15.7P-limited 52.8 ± 4.2 65.8 ± 13.0 59.6 ± 15.6 71.4 ± 4.6 93.5 ± 13.9 70.8 ± 10.8b, ·10 )4 P-sufficient 1.0 ± 3.5 )4.5 ± 1.7 )3.1 ± 0.6 )1.4 ± 0.3 )8.7 ± 0.3 )8.1 ± 0.5P-limited )7.6 ± 1.4 )4.8 ± 0.7 )6.1 ± 1.4 )3.2 ± 1.1 )8.4 ± 1.5 )7.9 ± 1.0Chlorophyll a, pg cell )1 P-sufficient 4.4 ± 1.6 13.4 ± 0.7 12.7 ± 3.6 28.6 ± 2.6 nd ndP-limited 3.4 ± 1.0 8.9 ± 3.3 35.7 ± 7.0 34.6 ± 10.0 nd ndChlorophyll a:b P-sufficient na na 3.31 ± 0.01 2.9 ± 0.08 nd ndP-limited na na 2.61 ± 0.07 2.76 ± 0.12 nd ndGrowth rate, day )1 P-sufficient 0.87 ± 0.09 0.32 ± 0.04 0.62 ± 0.07 0.32 ± 0.08 0.54 ± 0.10 0.23 ± 0.07P-limited 0.16 ± 0.05 0.09 ± 0.04 0.16 ± 0.05 0.10 ± 0.03 0.13 ± n.d. 0.07 ± 0.04Excess photochemical capacity P-sufficient 1.26 ± 0.06 1.31 ± 0.08 1.52 ± 0.16 1.55 ± 0.09 1.48 ± 0.12 1.25 ± 0.11P-limited 1.00 ± 0.03 1.06 ± 0.06 1.03 ± 0.03 1.10 ± 0.02 1.34 ± 0.19 1.05 ± 0.06LDL, long daylength (18 : 6 h LD); SDL, short daylength (6 : 18 h LD); na, not applicable; nd, not determined.Excess photochemical capacity was determined as the ratio of P max , maximum rate of photosynthesis to the P g , the photosynthetic rateat the growth irradiance (Cullen & MacIntyre, 1998).number) in all species were significantly lower underP-limited conditions (Fig. 2). The decrease of P max dueto P limitation (DP max ¼ P suff ) P lim ) was greaterunder long daylength (LDL) than under short daylength(SDL). The decline of P max under P-limitationwas the most pronounced in Sphaerocystis under LDL(more than sixfold, p < 0.01), in Nitzschia it wasthreefold (p < 0.05) and in Phormidium it was2.6-fold (p < 0.05) (Fig. 2). Also, the difference in theP max decrease because of P-limitation betweenLDL and SDL {difference ¼ [(P-suff LDL – P-lim LDL )–(P-suff SDL – P-lim SDL )]} was the greatest in Sphaerocystis.Under P limitation the daylength had a strongeffect on P max : it was significantly higher under SDLthan under LDL (Fig. 2). The saturation parameter I kalso decreased under P limitation under both SDL andLDL (see Table 1). The decrease was significant inNitzschia (p ¼ 0.018) and Sphaerocystis (p < 0.001).The initial slope of the P–I curves a appeared to behigher in P-sufficient cultures, the difference was not,however, significant in Nitzschia and Sphaerocystis. InPhormidium, both P status and the interaction term(daylength by P status) significantly influenced a(p ¼ 0.016 and 0.033, respectively, two-way ANOVA).P-sufficient Phormidium cultures had significantlyhigher a compared with P-limited cultures, but onlyunder LDL. Under P-limited conditions, the LDLcultures had significantly lower a than the SDLcultures of Phormidium.Under P-sufficient conditions, the cyanobacteriumPhormidium was significantly more inhibited (greaterb) than the diatom Nitzschia and the chlorophyteSphaerocystis (one-way ANOVA, p¼ 0.005). P-limitationsignificantly increased photoinhibition inNitzschia (two-way ANOVA, p¼ 0.034) and Sphaerocystis(p ¼ 0.048), but not in Phormidium. InNitzschia,photoinhibition was different between P-limited andP-sufficient cultures only under LDL.The excess photochemical capacity (EPC) decreasedunder P limitation; the difference was highly significantfor both Nitzschia and Sphaerocystis (p ¼ 0.002and 0.05).Growth ratesGrowth rates of all species decreased significantlyunder P limitation under both light regimes. Thedecline due to P limitation was more pronouncedunder LDL (Table 1). The growth rate of Nitzschiadeclined more than fivefold under this light regimeversus 3.6-fold under the short daylength.Under P-sufficient conditions, the growth rateswere the lowest under the shortest daylength andincreased linearly with daylength in Sphaerocystis[growth rate ¼ (0.0217 · daylength) + 0.2; R 2 ¼ 0.98]and Phormidium [growth rate ¼ (0.0308 · daylength) +0.03; R 2 ¼ 0.98] (Fig. 3a). The growth rates underÓ 2003 Blackwell Publishing Ltd, Freshwater Biology, 48, 2141–2148

Phytoplankton responses to phosphorus and daylength 2145Fig. 3. (a) Growth rates of the three species and (b) daylengthnormalisedgrowth rates (growth efficiencies) under differentlight regimes and nutrient-sufficient conditions for Nitzschia(cross), Sphaerocystis (circle) and Phormidium (diamond). Growthrates at the 24 h daylight were taken from Litchman (2000).Fig. 2. Chlorophyll-normalised maximum rates of photosynthesisof the three species under P-limited and nutrient-sufficientconditions. (a) Nitzschia; (b) Sphaerocystis; (c) Phormidium. Openbars are long daylength treatments and hatched bars are shortdaylength treatments.continuous irradiance of 100 lmol quanta m )2 s )1were taken from Litchman (2000). In Nitzschia, thegrowth rate dependence on daylength was betterdescribed by a saturating curve (Fig. 3a) {growthrate ¼ [)0.0036 · (daylength) 2 ] + (0.1314 · daylength) )0.34; R 2 ¼ 1} (Fig. 3a).Under P-sufficient conditions growth rates normalisedto light dose (growth efficiency) were the greatestunder the shortest daylength (Fig. 3b). The growth(light utilisation) efficiency increased with decreasingdaylength in all species. The pattern of increase was,however, species-specific: the increase was less pronouncedin Phormidium compared with Nitzschia andSphaerocystis (Fig. 3b).Cellular chlorophyll concentrationsChlorophyll concentration (measured in Sphaerocystisand Nitzschia, no data for Phormidium) tended to behigher under SDL (Table 1). Under P-sufficient conditions,the effect of daylength on cellular chlorophyllÓ 2003 Blackwell Publishing Ltd, Freshwater Biology, 48, 2141–2148

2146 E. Litchman et al.concentration was much more pronounced thanunder P-limited conditions. In Nitzschia, the effect ofboth P-status and daylength on cellular chlorophyllconcentration were highly significant, as well as theirinteraction term (p ¼ 0.003, 0.002 and 0.01). In bothNitzschia and Sphaerocystis, the LDL treatments hadsignificantly lower chlorophyll a concentrations thanthe SDL treatments, but only under P-sufficientconditions (Table 1). P-limitation significantly decreasedchlorophyll concentration in Nitzschia, butunder SDL only. In Sphaerocystis, under LDL only,P-limited cultures had higher cellular chlorophyllconcentration than P-sufficient cultures (p ¼ 0.035).The ratio of chlorophyll a to b in Sphaerocystis underLDL was significantly higher (p < 0.001) in nutrientsufficientcultures compared with its P-limited cultures(3.3 versus 2.6, respectively). The interactionterm of daylength and P limitation (daylength · Pstatus) was also significant (p ¼ 0.038). UnderP-sufficient conditions, the chlorophyll a : b ratio alsosignificantly depended on daylength: it was 3.3 underLDL versus 2.9 under SDL, p < 0.01).DiscussionBoth P limitation and daylength had significant effectson major physiological parameters of the threespecies. The growth rates of all three species declinedunder short daylength and the decline was speciesspecific.Nicklisch (1998) also observed a speciesspecificdecrease in growth rates under shortdaylength. The ratio of the growth rate undercontinuous light to the growth rate under the shortestphotoperiod was the greatest in the cyanobacterium,indicating the greatest inhibition of growth fromshortening the daylength. Foy & Gibson (1993) alsofound greater decline in growth with decreasingphotoperiod in cyanobacteria compared with diatoms.A decline in growth rate efficiency (growth rate perlight hour) with increasing daylength has also beenobserved for many other species in both freshwaterand marine environments (Gilstad & Sakshaug, 1990;Foy & Gibson, 1993; Tang & Vincent, 2000). Thecyanobacterium had a relatively low growth efficiencyunder short daylength compared with the diatom andthe green alga. However, the growth efficiencies weresimilar for the three species under continuous light.This may contribute to the dominance of cyanobacteriain the summer, under long daylengths and to theirdisadvantage in the spring and fall when the daylengthis shorter.The responses of photosynthetic parameters toshortening of daylength were similar to the lowirradiance acclimation. The increase in cellular chlorophylla concentration under shorter light periodsappears to be similar to the increase under lowerirradiance and has been observed in other species,such as the cyanobacterium O. agardhii (Post et al.,1986). The ratio of chlorophyll a to b is also known todecline under low light (Maxwell et al., 1994; Falkowski& Raven, 1997). In the present study undernutrient-sufficient conditions the chlorophyll a:bratio was significantly lower under short daylength.P limitation decreased the maximum rates ofphotosynthesis in all three species, but the decreasewas species-specific: the green alga was the mostsensitive to P limitation and the diatom was the leastsensitive. Many other studies have reported lowerrates of light-saturated photosynthesis under P limitation(Senft, 1978; Geider et al., 1998). The susceptibilityto photoinhibition also increased due toP-limitation in Nitzschia and Sphaerocystis. A greaterphotoinhibition of photosynthesis in nutrient-limitedphytoplankton has been reported previously (Litchman,Neale & Banaszak, 2002). A decrease in cellularchlorophyll concentration under P-limitation has beenobserved for both marine and freshwater microalgae(Geider et al., 1998; Wykoff et al., 1998). An increase incellular chlorophyll concentration under P-limitationin Sphaerocystis may possibly be explained by slowercellular division rates resulting in larger cells withhigher chlorophyll content. The effect of nutrientlimitation on photosynthetic apparatus in somerespects is similar to the effect of low irradiance, withdecreased maximum rate of photosynthesis and lowersaturation parameter values. Lower chlorophyll-specificmaximum rates of photosynthesis as well aslower chlorophyll a:b ratios are characteristic forlow-light acclimated phytoplankton. In the presentstudy we observed significantly lower chlorophylla:bratio under P-limited conditions. P limitation alsodecreased excess photochemical capacity (Cullen &MacIntyre, 1998), suggesting that nutrient-limitedphytoplankton are less capable of utilising lightfluctuations.The effect of P limitation on the rates of photosynthesisand the degree of photoinhibition was morepronounced under long daylength. It is possible thatÓ 2003 Blackwell Publishing Ltd, Freshwater Biology, 48, 2141–2148

Phytoplankton responses to phosphorus and daylength 2147by allowing faster growth, longer daylength leads to astronger P limitation and subsequently to a greaterdecrease in the light-saturated rates of photosynthesisand greater susceptibility to photoinhibition. LowerP max have been reported for algae grown under longerdaylength under nutrient sufficient conditions (Nielsen,1997). In our study this effect of daylength wassignificant only under P-limited conditions.Under nutrient-sufficient conditions daylength hada significant effect on growth rates and chlorophyllconcentrations. The results show that the growth andphotosynthetic responses to P limitation also dependon daylength, with stronger effects of limitation underlonger daylength. Both photosynthetic and growthrates declined more under LDL due to P limitation. Itis thus important when in the season P limitationoccurs: the effects of P limitation on photosynthesiswould be less pronounced in the early spring andautumn.AcknowledgementsEL was supported by the Swiss NSF (SNF) grant 31-50803.97. We thank Professor R.I. Jones and twoanonymous reviewers for their helpful comments andsuggestions.ReferencesCullen J.J. & MacIntyre J.G. (1998) Behavior, physiologyand the niche of depth-regulating phytoplankton. In:Physiological Ecology of Harmful Algal Blooms, vol. 41(ed. D.M. Anderson, A.D. Cembella & G.M. Hallegraeff), pp. 559–579. Springer, Berlin.Falkowski P.G. & Raven J.A. (1997) Aquatic Photosynthesis.Blackwell Science, Malden.Foy R.H. & Gibson C.E. (1993) The influence ofirradiance, photoperiod and temperature on thegrowth kinetics of three planktonic diatoms. EuropeanJournal of Phycology, 28, 203–212.Frenette J.-J., Demers S., Legendre L. & Dodson J. (1993)Lack of agreement among models for estimating thephotosynthetic parameters. Limnology and Oceanography,38, 679–687.Gächter R. & Mares A. (1979) Comments on theacidification and bubbling methods for determiningphytoplankton production. Oikos, 33, 69–73.Geider R.J., MacIntyre H.L., Graziano L.M. & McKayR.M.L. (1998) Responses of the photosynthetic apparatusof Dunaliella tertiolecta (Chlorophyceae) tonitrogen and phosphorus limitation. European Journalof Phycology, 33, 315–332.Gilstad M. & Sakshaug E. (1990) Growth rates of ten diatomspecies from Barents Sea at diferent irradiances and daylengths. Marine Ecology Progress Series, 64, 169–173.Guillard R.R.L. (1975) Culture of phytoplankton forfeeding marine invertebrates. In: Culture of marineinvertebrate animals (ed. W.L. Smith & M.H. Chanley),pp. 29–60. Plenum Press, New York.Hecky R.E. & Kilham P. (1988) Nutrient limitation ofphytoplankton in freshwater and marine environments.Limnology and Oceanography, 33, 786–822.Jassby A.D. & Platt T. (1976) Mathematical formulation ofthe relationship between photosynthesis and light forphytoplankton. Limnology and Oceanography, 21, 540–547.Lewis M.R. & Smith J.C. (1983) A small volume, shortincubation-timemethod for measurement of photosynthesisas a function of incident irradiance. MarineEcology Progress Series, 13, 99–102.Litchman E. (2000) Growth rates of phytoplankton underfluctuating light. Freshwater Biology, 44, 223–235.Litchman E. & Klausmeier C.A. (2001) Competition ofphytoplankton under fluctuating light. American Naturalist,157, 170–187.Litchman, E., Neale, P.J. & Banaszak, A.T. (2002)Increased sensitivity to ultraviolet radiation in nitrogen-limiteddinoflagellates: photoprotection andrepair. Limnology and Oceanography, 47, 86–94.Litchman E., Klausmeier C.A. & Bossard P. Phytoplanktonnutrient competition under dynamic light regimes.Limnology and Oceanography, in press.Maxwell D.P., Falk S., Trick C.G. & Huner N.P.A. (1994)Growth at low temperature mimics high light acclimationin Chlorella vulgaris. Plant Physiology, 105, 535–543.Nicklisch A. (1998) Growth and light absorption of someplanktonic cyanobacteria, diatoms and Chlorophyceaeunder simulated natural light fluctuations. Journal ofPlankton Research, 20, 105–119.Nielsen M.V. (1997) Growth, dark respiration andphotosynthetic parameters of the coccolithophoridEmiliania huxleyi (Prymnesiophyceae) acclimated todifferent day length-irradiance combinations. Journal ofPhycology, 33, 818–822.Platt T., Gallegos C.L. & Harrison W.G. (1980) Photoinhibitionof photosynthesis in natural assemblages ofmarine phytoplankton. Journal of Marine Research, 38,687–701.Post A.F., Loogman J.G. & Mur L.R. (1986) Photosynthesis,carbon flows and growth of Oscillatoria agardhiiGomont in environments with a periodic supply oflight. Journal of General Microbiology, 132, 2129–2136.Ó 2003 Blackwell Publishing Ltd, Freshwater Biology, 48, 2141–2148

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