Seasonal photosynthetic activity of autotrophic picoplankton in Lake Kinneret, Israel

Autotrophic picoplankton populations in Lake Kinneret are composedof picocyanobacteria and picoeukaryotes. Overall, the ratesof photosynthetic carbon fixed by autotrophic picoplankton duringthis study were low (0.01–1.5 mg Cm^–3 h^–1).The highest chlorophyll photosynthetic activity of the <3µm cell-size fraction was found in spring, when picoeukaryotespredominated and in addition small nanoplankton passed throughthe filters. The maximum cell-specific photosynthetic rate ofcarbon fixation by picocyanobacteria and picoeukaryotes was2.5 and 63 fg C cell^–1 h^–1, respectively. The highestspecific carbon fixation rate of autotrophic picoplankton was11 µg C µg^–1 Chl h^–1 The proportionalcontribution of autotrophic picoplankton to total photosynthesisusually increased with depth. Picocyanobacteria collected fromthe dark, anaerobic hypolimnion were viable and capable of activephotosynthesis when incubated at water depths within the euphoticzone. Maximum rates of photosynthesis (P_max) for picocyanobacteriaranged from 5.4 to 31.4 fg C cell^–1 h^–1 with thehighest values in hypolimnetic samples exposed to irradiance.Photosynthetic efficiency (     

Dinoflagellate bloom development and collapse in Lake Kinneret: a sediment trap study

Warm monomictic Lake Kinneret, Israel, is characterized by awinter–spring water bloom of the large (~50 µm diameter)dinoflagellate Peridinium gatunense Nygaard. Usually the P.gatunense bloom declines in May–June and a less prominentbloom of smaller dinoflagellates (mostly Peridiniopsis spp.of ~20–30 µm diameter) develops. Water column abundancesand sedimentation losses to those dinoflagellates were followedthroughout 1994 and 1995. The objective was to quantify thevariables that describe population dynamics, that in turn willshed more light on the seasonal patterns of bloom dynamics.Sedimentation losses were measured by means of sediment trapswith and without a preservative (formaldehyde) that were exposedfor 24 h once every 2–3 weeks. Annual sedimentation lossesof Peridinium (hypolimnetic trap catches) were 209 g wet wtm^-2 year^-1 in 1994 and 187 g wet wt m^-2 year^-1 in 1995, whichconstituted 16 and 23% of Peridinium production in those years,respectively. This study revealed that increased death ratespreceded a mass sedimentation flux of Peridinium and causedthe decline of the bloom in Lake Kinneret. Annual sedimentationlosses of Peridiniopsis were 55 g wet wt m^-2 year^-1 in 1994and 34 g wet wt m^-2 year^-1 in 1995. In contrast to live Peridiniumcells, Peridiniopsis cells continued to swim to the lower trapafter the onset of thermal stratification, possibly taking advantageof the higher nutrient concentrations below the thermocline,at a time when the lake is already stratified and the epilimnionis nutrient depleted. This could be an important factor allowingPeridiniopsis spp. to peak after the decline of Peridinium.     

Seasonal dynamics of picophytoplankton in Lake Kinneret, Israel

1. Picophytoplankton (picocyanobacteria and picoeukaryotes) communities in Lake Kinneret were studied from 1988 to 1992. No prochlorophytes were observed in the lake. 2. Picocyanobacteria were a prominent and ubiquitous component of the phytoplankton, being present at all depths throughout the year, with concentrations ranging from 2 ± 10–8 ± 10⁵ cells ml^?1. Low cell numbers in winter and spring were followed at the end of the annual dinoflagellate bloom by maximal abundances in summer-autumn in the epilimnion. High cell numbers (> 10⁴ cells ml^?1) were sometimes also found in the anaerobic hypolimnion. Net growth rates for picocyanobacteria ranged from 0.29 to 0.60 divisions day^?1. 3. Picoeukaryotes were a very minor constituent of the picoplankton, mostly present in winter and spring, and sometimes at the end of autumn, with concentrations ranging from 44 to 5700 cells ml^?1. Higher cell numbers tended to occur in the near surface water layers. In August-September, picoeukaryotes were found only in the hypolimnion. In December, the occurrence of picoeukaryotes in the deep water layers probably resulted from advection with cold water currents from the Jordan river. Net growth rates for picoeukaryotes ranged from 0.26 to 0.43 divisions day^?1. 4. Overall, the contribution of picophytoplankton to the phytoplankton standing crop in Lake Kinneret was limited; picocyanobacteria and picoeukaryotes accounted for no more than 7.0 and 0.1% of total algal biomass (semiannual average), respectively. 5. Picophytoplankton cell numbers in pelagic waters were usually similar to those in shallower lake stations. 6. Picocyanobacteria appear to be an autochthonous population, whereas picoeukaryotes are probably brought annually by the Jordan River and do not maintain themselves in the lake.     

A five-year study of autotrophic winter picoplankton in Lake Balaton,Hungary

Picoeukaryotes dominate the phytoplankton of Lake Balaton—the largest shallow lake in Central Europe—in the winter period. We examined the annual dynamics of picoplankton abundance and composition in the lake in order to establish if the picoeukaryotes merely survive the harsher winter conditions or they are able to grow in the ice-covered lake when the entire phytoplankton is limited by low light and temperature. Lake Balaton has an annual temperature range of 1–29°C, and it is usually frozen between December and February for 30–60 days. In the spring-autumn period phycocyanin and phycoerythrin rich Cyanobacteria are the dominant picoplankters, and picoeukaryotes are negligible. Our five-year study shows the presence of three types of picophytoplankton assemblages in Lake Balaton: (1) Phycoerythrin-rich Cyanobacteria—the dominant summer picoplankters in the mesotrophic lake area; (2) Phycocyanin-rich Cyanobacteria—the most abundant summer picoplankters in the eutrophic lake area and; (3) Picoeukaryotes—the dominant winter picoplankters in the whole lake. The observed winter abundance of picoeukaryotes was high (up to 3 × 10⁵ cells ml⁻¹), their highest biomass (520 μg l⁻¹) exceeded the maximum summer biomass of picocyanobacteria (500 μg l⁻¹). Our results indicate that the winter predominance of picoeukaryotes is a regular phenomenon in Lake Balaton, irrespective of the absence or presence of the ice cover. Picoeukaryotes are able to grow at as low as 1–2°C water temperature, while the total phytoplankton biomass show the lowest annual values in the winter period. In agreement with earlier findings, the contribution of picocyanobacteria to the total phytoplankton biomass in Lake Balaton is inversely related to the total phytoplankton biomass, whereas no such relationship was observable in the case of picoeukaryotes.     

Food selection and growth of the planktonic ciliate Coleps hirtus isolated from a monomictic subtropical lake

A strain of Coleps hirtus (Ciliophora, Prorodontida) was isolatedfrom the epilimnion of monomictic Lake Kinneret. Growth of thisciliate was tested in response to 12 species of planktonic algaeand seven species of cultured bacteria from lake isolates whichwere offered as food. Eight species of algae (one Cryptophyceaeand seven Chlorophyceae) and four bacteria supported good toexcellent growth of C.hirtus. Growth rates (µ) and doublingtimes (DT) ranged from 0.008 to 0.029 h^–1 and from 23.9to 90.8 h respectively. C.hirtus was able to grow on bacteriaat concentration levels as low as 2–8 x 10⁵ cells ml^–1.No correlation was observed between growth rate of C.hirtusand cell volume of the prey. ^aPresent address: Istituto di Ecologia, Universita di Parma,43100 Parma, Italy     

Effects of Blue Light Pretreatments on Internode Extension Growth in Mustard Seedlings after the Transition to Darkness: Analysis of the Interaction with Phytochrome

The effects of blue light (B) pretreatments on internode extensiongrowth and their possible interaction with phytochrome mediatedresponses were examined in Sinapis alba seedlings grown for11 d under 280 µmol m^–2 s^–1 of continuousblue-deficient light from low pressure sodium lamps (SOX). SupplementaryB (16 µmol m^–2 s^–1) caused no detectable inhibitionof the first internode growth rate under continuous SOX, butgrowth rate was inhibited after transfer to darkness. This effect,and the growth promotion caused by far-red bend-of-day' lightpulses were additive. The addition of B at 16 µmol m^–2s^–1 during 11 d, or only during the first 9 or 10 d orthe latest 0.75, 1 or 2 d of the SOX pretreatment caused approximatelythe same extent of inhibition after the transition to darkness.A single hour of supplementary B before darkness caused morethan 50% of the maximum inhibition. However, 24 h of lower fluencerates of B (4 or 7 µmol m^–2 s^–1) were ineffective.Covering the internode during the supplementary B period didnot prevent the response to B after the transition to darkness.Far-red light given simultaneously with B (instead of the SOXbackground) reduced the inhibitory effect of B. Above a given threshold fluence rate, B perceived mainly inthe leaves inhibits extension growth in subsequent darkness,provided that high phytochrome photo-equilibria are presentduring the irradiation with B. Once triggered, this effect doesnot interact significantly with the ‘end-of-day’phytochrome effect. Key words: Blue light, extension growth, phytochrome     

The annual patterns of photosynthesis in two large, freshwater, ultra-oligotrophic Antarctic lakes

The phytoplankton productivity and biomass of two large, freshwater Antarctic lakes (Vestfold Hills, eastern Antarctica) were investigated over a 12-month period. Crooked Lake was sampled at one site, while Lake Druzhby, a complex lake with two shallow and one deep basin, was subject to a more detailed investigation. Concentrations of chlorophyll a were usually below 1 µg l^-1, indicating ultra-oligotrophic conditions. Despite periodic low nutrient levels, low temperatures (range 0.4-2.8°C) and periodic poor light climate, some degree of photosynthesis was measurable throughout the year, including the dark winter phase. Snow cover had a pronounced impact on the light climate of the water column and inhibited photosynthesis. Mean rates of carbon fixation in the 0- to 15-m water column varied between 0 and 38.47 µg C l^-1 day^-1 in Crooked Lake and 0.24 and 37.68 µg C l^-1 day^-1 in Lake Druzhby. There were significant differences in the seasonal patterns of primary production between the basins of Lake Druzhby. The shallow basins had highest productivity in August, whereas the deep basin had highest rates in summer. Chlorophyll specific rates of photosynthesis or assimilation numbers [µg C (chl. a)^-1 h^-1] varied between 0.05 and 44.9, and photosynthetic efficiency [µg C (chl. a)^-1 h^-1 µmol m^-2 s^-1] between 0.02 and 5.19. The data suggest that the phytoplankton of these lakes is adapted to low irradiance levels, low temperatures and nutrient limitation.     

Phytoplankton primary productivity characteristics in response to photosynthetically active radiation in three Kenyan Rift Valley saline alkaline lakes

Phytoplankton photosynthetic characteristics in the Kenyan RiftValley lakes Bogoria, Nakuru and Elmentaita were studied betweenNovember 2003 and February 2005. In these world-famous saline–alkalinelake systems, long-term continuous monitoring and photoautotrophicprimary productivity modelling have been done for the firsttime. High light attenuation coefficients were observed withlakes means around 13 m^–1 reflecting the huge phytoplanktonbiomass. No photoinhibition was observed in the primary productivityfield measurements. High values of the photosynthesis–irradiancecurve initial slope () up to 0.85 (mg O₂ mg Chl a^–1 h^–1)(µmolphotons m^–2 s^–1)^–1 and a low onset of productivitysaturation (E_k) down to 11.4 µmol photons m^–2 s^–1as an acclimation to poor light supply were found. For the trophogeniczone, high mean net primary production (NPP) rates of 6.8, 10.7and 8.5 g O₂ m^–2 day^–1 were recorded for Bogoria,Nakuru and Elmentaita. For the whole water column, NPP decreasedto –1.4, 1.6 and 7.2 g O₂ m^–2 day^–1 becauseof high community respiration. Modelling of the gross primaryproduction (GPP, Chlorophyll a, light supply, initial slope, maximum production rates considered) gave annual values of4.9, 6.8 and 4.2 kg O₂ m^–2 year^–1, respectively,for Bogoria, Nakuru and Elmentaita, annual NPP values down tothe compensation depth were 70, 65 and 55% of the GPP.     

Intraspecific variation in the light/dark modulation of ribulose 1,5-bisphosphate carboxylase-oxygenase activity in soybean

A comparative study was made of the inhibition of ribulose-1,5-bisphosphatecarboxylase-oxygenase (Rubisco) amongst six cultivars of Glycinemax L. Merr., associated with synthesis of 2-carboxyarabinitol1-phosphate (CA1P) during darkness. Significantly lower meanvalues of dark inhibition of Rubisco were observed in soybeancv. Davis than in cvs Bragg, Cobb, Hardee, Gordon, and Kirby.The CA1P synthesis/degradation cycle during dark/light transitionsremained operational in cv. Bragg plants grown at low irradiance(40 µmol photons m–2 s–1). However, CA1P synthesisand degradation rates were slower in the dark (t_0.5 = 240 versus25 min), and light (t_0.5 = 20 versus 3.8 min) respectively,as compared to plants grown at higher irradiance (550 µmolphotons m^–2 s^–1). In addition, the activation stateof Rubisco in low-light-grown plants showed only a small declineafter a transition to darkness. We conclude that (a) cultivar-dependentvariation occurs amongst soybeans with respect to CAlP regulationof Rubisco, and (b) soybeans acclimated to low irradiance maydepend more on CA1P synthesis/degradation to regulate Rubisco,and less on changes in the enzyme activation state. Key words: Activation state, Glycine max, photosynthesis, Rubisco, 2-carboxyarabinitol 1-phosphate     

Age, Fluence Rate, and Anthocyanin Production in Zea mays Seedlings

Increase in fluence rates of white light over the range of 5to 80 µmol m^–2 s^–1 brought about a correspondingincrease in amounts of anthocyanin production in shoots of Zeamays L. seedlings. Roots also exhibited a similar relationshipbetween increased fluence rate and increased anthocyanin productionover the range of 5 to 40 µmol m^–2 s^–1 whereasfluence rates above 40 µmol m^–2 s^–1 broughtabout decreases in anthocyanin production. Rates of productionand amounts of accumulation of anthocyanin in both shoots androots were found to vary with the age of the seedlings at thetime of exposure to light. Age, fluence rates, anthocyanin, seedlings, Zea mays     

The Effects of Temperature and Light Intensity on Embryo Numbers in Wheat Doubled Haploid Production through WheatxMaize Crosses

Triticum aestivumxZea mayscrosses are now widely used in theproduction of wheat doubled haploids to produce homozygous lines.Seasonal effects are known to influence the number of haploidembryos produced through wheatxmaize crosses, but the effectsof temperature and light have not been quantified. This studyinvestigated the effect of temperature and light intensity onhaploid embryo production. New Zealand wheat cultivars weregrown in a glasshouse until booting when they were transferredto growth cabinets at three temperatures (day/night; 17/12,22/17 or 27/22 °C at an irradiance of 250 µmol m^-2s^-1PAR).In another experiment, wheat lines were transferred to a growthcabinet at one of three light intensities (300, 500 or 1000µmol m^-2s^-1PAR at 22/17 °C day/night, with a photoperiodof 16 h). The temperature and light intensity at which pollinationswere made and subsequent fertilisation and embryo developmentoccurred, significantly (P<0.01) influenced the frequencyof haploid embryo production. The optimal temperature for embryorecovery was 22/17 °C. The greatest number of embryos wasproduced at a light intensity of 1000 µmol m^-2s^-1. Thesefindings will result in improvements in the overall efficiencyof the wheatxmaize system for wheat doubled haploid production.Copyright1998 Annals of Botany Company Intergeneric crossing, temperature, light intensity,Triticum aestivum,wheat,Zea mays,maize.     

The Diurnal Pattern of Nitrate Uptake and Reduction by Spinach (Spinacia oleracea L.)

Spinach plants were grown in bowls of aerated nutrient solutionin a controlled environment chamber for 24 h, and harvestedevery 3·5-5 h to record their growth, nitrate and wateruptake, and plant nitrate concentration. Twelve such experimentsare described, either with a 14/10 h dark/light regime, or continuouslight or darkness. The irradiance was either 110, 320, or 510µmol m^-2 s^-1 (PPFD). All these regimes began at the endof the light period of a 14/10 h dark/light regime (510 µmolm^-2 s^-1) lasting approximately 2 weeks. Nitrate uptake rate per g of dry weight of plant continued almostunabated at about 17 µmol h^-1 through the initial 14-hdark period, and then fell away sharply if the light was notrestored, but increased slightly when it was. With continuouslight at 510 µmol m^-2 s^-1, uptake rate rose steadily forthe first 24 h of light, and then fell sharply for about 6 h.Shoot nitrate concentration increased about three-fold in thedark phase, and declined in the light at a rate which was positivelyrelated to the irradiance. Root nitrate concentration was severaltimes higher than that of the shoot: its diurnal change wassmaller (relative to the mean) than that of the shoot. Nitratereduction occurred to a small extent in the dark, and increasedrapidly as soon as the lights came on, to remain at a roughlyconstant rate (related to the irradiance) throughout the lightphase. Dry matter increase in the light was related to irradiance,but with little increase above 320 µmol m^-2 s^-1. Respiratoryweight loss in the dark was not detectable. Rate of fresh weightincrease was approximately constant throughout light and darkperiods. The results compare quite well with the predictions of a simplesimulation model, based on the pump/leak principle.Copyright1994, 1999 Academic Press Spinacia oleracea, nitrate, uptake, reduction, influx, efflux, diurnal, regulation, model, simulation     

Light environment within dense algal populations: cell size influences on self-shading

The influence of algal cell size on the light environment withindense phytoplankton populations was analysed in cultures of11 freshwater algae (mostly Chlorophyta) varying widely in cellsize and growing at saturated (220 µM m² s^–1) andlimited (11 µM m^–2 s^–1) irradiances Chlorophyll-a-specificabsorbance coefficients did not bear any obvious relationshipto cell size and, as expected from previous analyses, were stronglyinfluenced by the light intensity at which the algae were grown.However, biomass (fresh weight)-specific absorbance coefficientsdecreased as algal cell volume increased regardless of the incomingirradiance Intracellular chlorophyll a concentration (c₁), whichdecreased as algal cell size increased, explained a substantialfraction of the variance in biomass (fresh weight)-specifjcabsorbance coefficients. Thus, this study supports the notionthat reduced serf-shading allows large algae to support greatermaximal biomass than smaller algae.     

Control of the Acclimation of Photosynthesis to Light and Temperature in Relation to Partitioning of Photosynthate in Developing Soybean Leaves

Photosynthetic acclimation was examined by exposing third trifoliolateleaves of soybeans to air temperatures of 20 to 30°C andphotosynthetic photon flux densities (PPFD) of 150 to 950µmolphotons m^–2 s^–1 for the last 3 d before they reachedmaximum area. In some cases the environment of the third leafwas controlled separately from that of the rest of the plant.Photosynthesis, respiration and dry mass accumulation were determinedunder the treatment conditions, and photosynthetic capacity,and dry mass and protein content were determined at full expansion.Photosynthetic capacity, the light-saturated rate of net carbondioxide exchange at 25°C and 34 Pa external partial pressureof carbon dioxide, could be modified between 21 and 35 µmolCO₂ m^–2 s^–1 by environmental changes after leaveshad become exporters of photosynthate. Protein per unit leafmass did not differ between treatments, and photosynthetic capacityincreased with leaf mass per unit area. Photosynthetic capacityof third leaves was affected by the PPFD incident on those leaves,but not by the PPFD on other leaves on the plant. Photosyntheticcapacity of third leaves was affected by the temperature ofthe rest of the plant, but not by the temperature of the thirdleaves. Photosynthetic capacity was linearly related to carbondioxide exchange rate in the growth regimes, but not to daytimePPFD. At high PPFD, and at 25 and 30°C, mass accumulationwas about 28% of the mass of photosynthate produced. At lowerPPFD, and at 20°C, larger percentages of the photosynthateproduced accumulated as dry mass. The results suggest that photosynthatesupply is an important factor controlling leaf structural growthand, consequently, photosynthetic acclimation to light and temperature. Key words: Glycine max (L.) Merr., photosynthesis, temperature acclimation, light acclimation, photosynthate partitioning     

Nitrate and ammonium uptake by plankton in an Amazon River floodplain lake

Uptake of ammonium and nitrate by plankton was measured in tropicalLake Calado, Brazil. Nitrate uptake was strongly influencedby light and was light saturated at {small tilde}340 µEm^–2 s^–1. In contrast, uptake of ammonium was lessinfluenced by light, and saturated at {small tilde}250 µEm^–2 s^–1. Uptake rates of both forms of nitrogenwere inhibited by up to 80% at light intensities higher thanthose required for saturation. Concentrations of ammonium andnitrate also had a strong influence on uptake rates. Half-saturationconstants (0.3–5 µM) were usually greater than ambientconcentrations (0.1–0.6 µM), indicating that uptakerates at ambient concentrations were less than one-half of thesaturated rates. Ammonium is the more important type of inorganicnitrogen for plankton of Lake Calado because nitrate concentrationsremain low to undetectable except during periodic inputs ofnitrate-rich water from the Amazon River. Using the observeddependence of uptake on concentration and light, maximum uptakerates per unit chlorophyll were computed to be in reasonableagreement with rates derived from P^B_m values for carbon uptake. ¹ Present address: Florida Department of Natural Resources,Marine Research Laboratory, St Petersburg, FL 33701, USA     

Photoacclimation in the marine alga Nannochloropsis sp. (Eustigmatophyte): a kinetic study

Exponentially growing cultures of Nannochloropsis sp. were transferredfrom high light (650 µ.mol quanta m^–2 s^–1)to low light (30 (µrnol quanta m^–2 s^–1). Thekinetics of changes in the various parameters of the photoacclimationprocess were followed until steady state was reached. In thisorganism, the observed 4.25 increase in cellular chlorophylla in the low-light-acclimated cells was linearly correlatedwith an increase in the number of photosynthetic units (PSU),while the PSU size remained constant. The new steady state wasreached after a transition period of 87 h and following an initialovershoot in the chlorophyll a concentration. During the processof photoacclimation, harvested and utilized light was increaseddue to a combination of increased cellular chlorophyll and anincrease of the quantum yield. These improvements in efficiencywere coupled with the reduction of respiration. The overalleffect of photoacclimation, therefore, amounted to reducingthe impact of low light on the growth rate of the algae. Ourstudy demonstrates the importance of the photoacclimation processin the context of the planktonic way of life. An index of the'effectiveness of photoacclimation' is proposed.     

Temporal and vertical variation of chlorophyll a concentration, phytoplankton photosynthetic activity and light attenuation in Lake Kinneret: possibilities and limitations for simulation by remote sensing

The relationship between chlorophyll a (Chl a) and primary productivity(PP) in the uppermost water layer and the water column-based(0–15 m) integral values of those variables were examinedusing measurements taken in Lake Kinneret (Israel) from 1990to 2003. In 81% of all Chl a profiles examined, the distributionwas fairly uniform within the entire 0–15 m water column,and 12.3% of instances showed a prominent subsurface maximum,when the lake phytoplankton was dominated by the dinoflagellatePeridinium gatunense. Chl a can be reliably estimated by remotesensing techniques in the productive and turbid water of LakeKinneret, since Chl a concentration at surface layers can beextrapolated to the entire water column. Light vertical attenuationcoefficient average for wavelengths from 400 to 700 nm, K_d,ranged from 0.203 to 1.954 m^–1 and showed high degreeof temporal variation. The maximal rate of photosynthetic efficiency,P_B^opt [average 3.16 (±1.50)], ranged from 0.25 to 8.85mg C m^–3 h^–1 mg Chl a^–1. Using measured dataof Chl a, P_B^opt, and light as an input, a simple depth-integratedPP model allowed plausible simulation of PP. However, a lackof correlation between photosynthetic activity and temperature(or other variable with remotely sensed potential) renders theuse of models that require input of photosynthetic efficiencyto calculate integrated PP of little value in the case of productiveand turbid Lake Kinneret.     

Uptake of [32P]orthophosphate by algae adn bacteria in Lake Kinneret

Differential filtration was used to apportion [³²p]orthophosphate(P₁) uptake to predominantly bacterial (<3 µm) or algal(>3 µm) components of Lake Kinneret microplankton.Bacteria generally showed preferential ³²P_i uptake in comparisonwith algae. Nevertheless, in most cases, the relative proportionof ³²P counts retained on 3 µm filters was greater thanthe proportion of ¹⁴C counts from heterotrophic bacterial incorporationof [¹⁴Clglucose, indicating that algae were competing for P_iwith bacteria with some measure of success. Most time courseexperiments did not show any consistent transfer of ³²P frombacteria to algae. The addition of a bacterial inhibitor (garamycin)caused a relative increase in the proportion of algal to bacterial³²P_i uptake. Added organic P substrates lowered the amount of³²P_i uptake and appeared to be preferentially utilized by bacteria.Apparent residence times for P_i in Lake Kinneret ranged from0.4 h (prior to overturn) to 17.4 h during bomothermy. Despitelow ambient P_i concentrations, P limitation in Lake Kinneretis not as extreme as in many other aquatic environments.     

The influence of irradiance on growth, photosynthesis and respiration of Gyrodinium cf. aureolum

The bloom-forming marine dinoflagellate Gyrodinium cf. aureolumwas grown in batch cultures over a range of irradiances (35–380µmolm^–2 s^–1 and growth, photosynthesis and respirationrates determined. Saturation of growth occurred at irradiancesof 100µmol m^–2 s^–1 Below this light level,decreases in growth rates and cell size, and a relative increasein carbon specific respiration rates, were observed. On theother hand, photosynthesis-irradiance relationships determinedfrom dissolved oxygen incubations showed that on a cellularand carbon basis, cultures grown at low irradiances had higherrates of light-limited and light-saturated photosynthesis, mainlyas a result of large increases in cell chlorophyll content.This adaptation strategy enables low-light-grown organisms toexploit available high irradiance through a relatively highphotosynthetic capacity. In cells grown at higher light levels(>100µmol m^–2 s^–1), excess photosynthatemay be diverted to storage rather than used for growth.     

Phytoplankton seasonal dynamics in a Mediterranean coastal lagoon: emphasis on the picoeukaryote community

The dynamics of the phytoplankton community were investigatedin a marine coastal lagoon (Thau, NW Mediterranean) from February1999 to January 2000. Dilution experiments, chlorophyll a (Chla) size-fractionation and primary production measurements wereconducted monthly. Maximum growth and microzooplankton grazingrates were estimated from Chl a biomass fractions to separatepico- from nano- and microphytoplankton and by flow cytometryto distinguish between picoeukaryotes and picocyanobacteria.In spring, the phytoplankton community was dominated by Chaetocerossp. and Skeletonema costatum, which represented most of biomass(B) and primary production (P). Nano- and microphytoplanktongrowth was controlled by nutrient availability and exceededlosses due to microzooplankton grazing (g). Picoeukaryote andcyanobacteria growth was positively correlated with water temperatureand/or irradiance, reaching maximum values in the summer (2.38and 1.44 day^–1 for picoeukaryotes and cyanobacteria, respectively).Picophytoplankton accounted for 57% of the biomass-specificprimary productivity (P/B). Picophytoplankton was strongly controlledby protist grazers (g = 0.09–1.66 day^–1 for picoeukaryotes,g = 0.25–1.17 day^–1 for cyanobacteria), and microzooplanktonconsumption removed 71% of the daily picoplanktonic growth.Picoeukaryotes, which numerically dominate the picoplanktoncommunity, are an important source of organic carbon for theprotistan community and contribute to the carbon flow to highertrophic levels.