Seasonal variations in copepod size: effects of temperature, food abundance, and vertebrate predation

Seasonal dry weights of female and male Cyclops bicuspidatusthomasi, Diaptomus ashlandi and Diaptomus minutus were studiedin southeastern Lake Michigan during 1975–1981. Smallestanimals occurred during summer and early fall, and largest animalsin winter and spring, a pattern reported for other copepods.The range of weights for the species and sexes decreased fromwinter to summer, and converged in summer to a value of approximately2 µg/animal. Surface water temperature and abundancesof young-of-the-year (YOY) fish were inversely correlated withweights of males and females of the three copepod species. Standingstocks of important phytoplankton groups, especially pennatediatoms and flagellates, were positively correlated with copepodweight. Seasonal change of copepod body size appears to be morethan simply a function of temperature; seasonal predation byYOY fish may be a factor influencing sizes of adult copepods.     

Photosynthesis and primary production of Microcystis aeruginosa Kutz. in Lake Kasumigaura

Seasonal changes in the photosynthesis and primary productionof Microcystis aeruginosa Kütz. were investigated in LakeKasumigaura during 1981–1982. Microcystis always showeda light-saturated photosynthesis-light curve. Both P_max andthe initial slope of the photosynthesis-light curve of Microcystisin early summer were very high, so it was concluded that Microcystisutilized both low and high light intensities efficiently. TheP_max of Microcystis was found to be a function of the watertemperature except in August and September. The linear regressionon the temperature-P_max relationship discontinued at 11°C,where the P_max value dropped; Microcystis did not photosynthesizebelow 4°C. The initial slope of the curve was also descendingbelow 11°C. It is suggested that Microcystis changes itsphysiological properties below 11°C. The highest value ofgross production calculated for M. aeruginosa was 5.4 gC m^–2d^–1 in July; the annual gross production was estimatedto be 300 gC m^–2year^–1 (i.e., 40% of the total primaryproduction in this lake).     

Egg production and hatching success in the calanoid copepods Calanus helgolandicus and Calanus finmarchicus in the North Sea from March to September 2001

Spatial and seasonal egg production rates (E_r) and egg hatchingsuccess in the copepods Calanus finmarchicus and Calanus helgolandicuswere measured in the North Sea from March to September. Foodavailability was monitored by chlorophyll and protist concentrationsand three size fractions of seston fatty acids. Seasonal andspatial distribution and production differed between the species.Calanus finmarchicus was found only offshore of the 50-m isobath,with decreasing E_r (37–28 eggs female^–1 day^–1)from March to July. Calanus helgolandicus had two abundancepeaks, in spring and autumn, with a low in May during whichtime the highest E_r were observed (38 eggs female^–1 day^–1).At other times, E_r in C. helgolandicus remained lower than inC. finmarchicus (     

Morphological, Histological and Lipid Changes during Adventitious Budding inPinus pinaster Cultured Cotyledons

Tranvan, H., Troton, D. and Calvayrac, R. 1988. Morphological,histological and lipid changes during adventitious budding inPinus pinaster cultured cotyledons.—J. exp. Bot. 39: 907–915. Isolated cotyledons of Pinus pinaster were cultured under bud-formingconditions. The morphology, histology, lipid content and thechlorophyll pools of the cotyledons were followed during buddevelopment. Comparison with cotyledons cultured continuouslyon hormone-free or hormone-supplemented medium was performed.Three periods were distinguished during bud formation: (i) theinitiation period, characterized by a drastic decrease of triacylglycerols,enhanced by the presence of the hormones; (ii) the differentiationperiod, during which the intense cell divisions and the increaseof the polar lipid and chlorophyll contents appeared stronglycorrelated and (iii) bud development, characterized by stablelevels of both chlorophylls and neutral lipids. Moreover, itappeared that the continuous presence of hormones in the culturemedium inhibited the synthesis of molecular components, in particularchlorophylls and membrane lipids, required for normal cell differentiationand physiology. Key words: Adventitious bud formation, chlorophyll, neutral lipids, Pinus pinaster, cotyledons.     

Temporal variability of diversity and biomass of tintinnids (Ciliophora) in a southwestern Atlantic temperate estuary

Seasonal variations in diversity and biomass of tintinnids (Ciliophora:Tintinnida) were investigated at two fixed stations in the innerpart of the Bahía Blanca Estuary (38°42' S, 61°50'W) during an annual cycle. The variations were analysed in relationto surface temperature, salinity, transparency, solar radiationand chlorophyll a (Chl a)concentration. Biomass was calculatedin terms of biovolume and carbon units. Diversity was estimatedas the number of species and the Shannon Index (H', ln based).Density of tintinnids ranged from 100 to 7800 individuals L^–1H' ranged from 0 to 1.81. The biomass varied from 0.3 to 127.78x 10⁶ µm³ L^–1 (0.02–39.4 µg C L^–1).Density was significantly related to temperature, solar radiationand Secchi distance (P < 0.01); diversity was significantlyrelated to temperature (P < 0.01) and solar radiation (P< 0.05). Biomass was significantly related only to temperature(P < 0.01) in one of the stations. According to principalcomponents analysis (PCA) tintinnids exhibited a segregationof three groups: winter, spring–summer and autumn forthe most internal station and winter, spring and summer–autumnfor the most external station. H' values were lower than thoseobserved in other coastal systems found at about the same latitudein the northern hemisphere.     

Periodicity of composition, abundance, and vertical distribution of summer zooplankton (1977/1978) in Ezcurra Inlet, Admiralty Bay (King George Island, South Shetland)

Seasonal succession and variation in species composition, density,biomass, age distribution and frequency of zooplankton (mainlythe Copepoda) were analysed during the austral summer of of1977/1978 in Ezcurra Inlet, a part of the Antarctic coastalecosystem. Small zooplankters (i.e., cyclopoids of the generaOncaea and Oithona, and calanoids Drepanopus pectinatus andScolocithricella glacialis) were found to predominate in termsof abundance and percentage contribution. The zooplankton biomasswas dominated by larger organisms of the Metrididae and Calanidae(Calanoida). The maximum abundance and maximum biomass of copepodswere recorded in February; two small peaks in copepod biomassbeing observed in late December and late January, and a lesserbiomass peak in late December. The vertical distribution ofcopepods in terms of their diel and seasonal (December, January,February, March) changes showed a day-time maximum to have occurredin the near-bottom layer, the nocturnal distribution being bimodalwith peaks within 0–10 m and 25–50 m. The summerzooplankton community in Ezcurra Inlet is controlled by trophic(phytoplankton composition and density) and hydrological (waterexchange with Bransfield Strait) conditions prevailing in thearea.     

The spring bloom of the diatom Melosira italica subsp. subarctica in Lake Pavin: biochemical, energetic and metabolic aspects during sedimentation

The overall biochemical composition, the adenylate energy chargeand the metabolic activity of the diatom Melosira italica subsp.subarctica were studied during its spring development in LakePavin. The phytoplankton community of this lake was largelydominated by this diatom, which formed 80–95% of the phytoplanktonbiomass in the meta- and hypolimnion during the study period.Sedimentation of living Melosira was observed; 40–60%of the cells reaching the hypolimnion were viable. The spatialand temporal changes of the biochemical composition of thiscommunity suggest that Melosira utilized reserve products duringsedimentation, essentially carbohydrates and secondarily alsolipids. The increase of the protein/carbohydrate ratio withdepth and a stable adenylate energy charge level clearly showedthe prevalence of catabolic over anabolic reactions for thecells leaving the euphotic zone. Assays of inorganic carbonincorporation into macromolecules suggest that the settlingMelosira adapt to the changing environmental conditions by increasingthe synthesis of rapidly catabolisable polymers (polysaccharidesand lipids) and by decreasing syntheses of proteins and incorporationinto low mol. wt compounds.     

Seasonal variation of reproduction rates and body size of Calanus sinicus in the Southern Yellow Sea, China

Annual variations of egg production rate (EPR) and clutch sizeof Calanus sinicus, as well as body size of females (prosomelength and dry weight), were investigated at a series of stationsin the Southern Yellow Sea by onboard incubation. Calanus sinicuswas spawning in all the 11 cruises investigated, and the annualvariation of EPR was bimodal. Monthly average EPR was highestfrom May to July, respectively, 5.97, 5.36 and 6.30 eggs female^–1d^–1, then decreased dramatically to only 1.37 eggs female^–1d^–1 in August and attained the lowest 1.07 eggs female^–1d^–1 in October. In November, average EPR increased againto 4.31 eggs female^–1 d^–1. Seasonal variation ofclutch size was similar to EPR, except that it decreased graduallyafter August rather than dramatically as did EPR. Prosome lengthof females was maximum in May and minimum in October, but dryweight was highest in November. Monthly average EPR correlatedbetter with prosome length than dry weight, while clutch sizewas rather determined by dry weight of females. It is suggestedthat egg production of C. sinicus was active during two discontinuousperiods when both surface and bottom temperature fell into itsfavorite range (i.e. 10–23°C), and different reproductivestrategies were adopted in these two reproductive peaks: otherthan the highest EPR, longer prosome length was also achievedby C. sinicus from May to July, while females in November developedshorter bodies but accumulated more energy for reproduction.     

Biomass, feeding and production of Noctiluca scintillans in the Seto Inland Sea, Japan

The abundance and biomass of the large heterotrophic dinoflagellateNoctiluca scintillans, together with the changes in its potentialprey items, were monitored in the Seto Inland Sea, Japan, duringsummer 1997 (17 July-11 August). Growth and grazing rates ofNscintillans fed natural plankton populations were also measuredeight and seven times, respectively, during the survey period.The abundance and biomass of N scintillans averaged over thewater column (19 m) were in the range 1–345 cells 1^–1(temporalaverage = 93 cell1^–1) and 0.1–49.6 µg C l^–1(temporalaverage = 13.8 µg C l^–1; three times higher thanthat of calanoid copepods during the same period). Noctilucascintillans populations followed the changes in phytoplankton:N.scintillans biomass was increasing during the period of diatomblooms and was at a plateau or decreasing during periods oflow chlorophyll a. The growth rates of N.scintillans (µ)were also consistent with the wax and wane of the N.scintillanspopulation: N.scintillans showed highest growth rates duringdiatom blooms. A simple relationship between µ and chlorophylla concentration was established, and the production of N.scintillanswas estimated using this relationship and the measured biomass.The estimated production averaged over the water column wasin the range >0.1–5.2 µg C l^–1 day^–1(temporalaverage = 1.4 µg C l^–1 day^–1; 64% of the productionof calanoid copepods during the same period). Diatom clearancerates by N.scintillans were in the range 0.10–0.35 mlcell^–1 day^–1, and the phytoplankton population clearanceby N.scintillans was >12% day^–1. Thus, although thefeeding pressure of N.scintillans on phytoplankton standingstock was low, N.scintillans was an important member of themesozooplank-ton in terms of biomass and production in the SetoInland Sea during summer.     

The seasonal abundance of the phototrophic ciliate Mesodinium rubrum in Southampton Water, England

The abundance of the marine phototrophic planktonic ciliateMesodinium rubrum was monitored throughout an annual cycle attwo stations in the Southampton Water estuary. Seasonal changesin the concentration of nitrate, ammonia and phosphate weremonitored both at the inner estuary station (NW Netley) andouter estuary station (Calshot). Nutrient levels in the winterwere similar at both stations, and were diminished during sequentialdiatom blooms dominated initially by Sketetonema costatum andthen by Rhizosolenia dclicatula. Nitrate was reduced to a seasonalminimum in the outer estuary following these spring diatom blooms,but in the inner estuary was sustained >500 µg I^–1until the onset of the M.rubrum bloom. During the developmentof a visible red tide of M.rubrum in June/July at NW Netley,nutrient concentrations were considerably reduced. Cell numbersof M.rubrum varied between 2 and 3 cells ml^– during winterto >400 cells ml^–1 during the bloom at NW Netley, whereasat Calshot cell abundance did not increase above 25 cells ml^–1at any time of the year. At NW Netley, dense accumulations ofthe ciliate occurred over restricted depth intervals duringthe bloom and possible factors influencing the observed verticaldistribution of cells are considered. ¹Present address: Biology Department, Faculty of Science, AddisAbaba University, PO Box 1176, Addis Ababa, Ethiopia     

Decadal change in zooplankton seasonal succession in the Bahia Blanca estuary, Argentina, following introduction of two zooplankton species

Seasonal zooplankton succession (mainly of the mesozooplankton)in the inner zone of Bahía Blanca estuary during theperiod 1990–1991 was compared to that from the period1979–1980 in order to detect changes having taken placein 10 years. Taxonomic composition, zooplankton abundance, speciesoccurrence and temperature–salinity conditions were studiedwithin the two annual cycles. This estuarine ecosystem has beendeeply affected by a population increase and by the rapid growthof industrial activity and maritime traffic over the last twodecades. Several changes in the zooplankton assemblage compositionand structure were detected, probably due to temperature–salinitygradient patterns, which showed discernible differences. Dominance,during winter–spring, of two new invading species, namely,the cirriped Balanus glandula and the copepod Eurytemora americana,causing a decrease in the autochthonous key copepod Acartiatonsa, was one of the principal features of the period 1990–1991.Seasonal changes in diversity and species richness, as wellas differences in the multivariate general pattern of the zooplanktoncommunity structure with a consequent decrease in the communitystability during the second period, were also observed.     

Nutrients and primary production in a shallow stratified ecosystem in the Iroise Sea

Seasonal changes of nutrients and primary production were studiedin a shallow stratified ecosystem in the Iroise Sea (Atlanticcoast of France) in 1983–84. Changes of nutrients andprimary production followed the pattern known for the temperatecoastal waters that become seasonally stratified. Continuednitrification in the bottom waters, a rapid regeneration ofsilicon, and ammonium regeneration by microzooplankton are possiblymore important to sustain production than the winter stocksof nitrate and silicon, and ammonium released by macrozooplankton.Light is the major environmental factor controlling primaryproduction. Nutrient limitation of productivity occurs onlyin May–July. Production rates predicted by empirical modelsagreed well with measured rates. Dissolved nitrogen-containingorganic substances trigger the Gyrodmium aureolum bloom. Prolongedstratification and an abundant nitrate supply maintain it anda total depletion of nitrate terminates it.     

Accumulation of Pigments during the Greening of Etiolated Seedlings of Hordeum vulgare L.

The pigment changes that occur during transformation of etioplaststo fully developed chloroplasts have been studied in seedlingsof barley (Hordeum vulgare L.) by greening with white lightof low (15–25 µmol m^–2 s^–1) and medium(150 µmol m^–2 s^–1) intensity. At least 24h longer was required in the low light regime for the same concentrationof pigment to be accumulated in the seedlings. The increasein pigment content was mainly due to the synthesis of chlorophyllsa and b. Of the carotenoids present, the increases in the levelsof neoxanthin and, especially, ß-carotene were muchgreater than those observed for the other carotenoids. Levelsof lutein also increased but this change was small by comparisonto those observed for neoxanthin and ß-carotene. Inthe long-term the concentration of violaxanthin remained unalteredalthough significant transient changes were recorded. The levelsof antheraxanthin and zeaxanthin were markedly reduced duringgreening. The rate of pigment synthesis decreased with increasingcell age, i.e. from the base to the tip of the primary leaf.Overall, carotenoid levels increased by approximately 100% atthe base of the seedling but hardly at all at the tip. Key words: Hordeum vulgare, carotenoids, violaxanthin-cycle, etiolation     

Ageing of Cucumber and Onion Seeds: Phospholipase D, Lipoxygenase Activity and Changes in Phospholipid Content

Cucumber (Cucumis sativus L. cv. Mesa) and onion (Allium cepaL. cv. Rijnsburger Heldis) seeds were rapidly aged at 40 °Cand 74% relative humidity. Onion seeds were also slowly agedat 40 °C with 15% relative humidity for 11 months and onemore month at 28% relative humidity. Significant loss of totaland individual phospholipids was an early event during bothstorage treatments. With slow ageing of onion, loss of phosphatidylcholineoccurred several months before loss of viability and vigourwas detected. Phosphatidic acid, the lipid product of phospholipaseD action, increased during rapid ageing of both cucumber andonion. Phosphatidic acid was present in onion seeds before theageing treatments and its content remained unchanged in theslowly aged seeds. There was 1600 (cucumber) and 2000 (onion)times more phospholipase D activity (6 x 10⁵ and 2·9x 10⁵ nmol g^–1 d^–1 in cucumber and onion, respectively)in crude extracts from non-aged seeds than was required to accountfor the fastest fall in phospholipids (72, 372 and 144 nmolg^–1 d^–1 for cotyledons and radicles of cucumberand onion, respectively, over the first 9 d [cucumber] or 1d [onion] of ageing) and fastest increase in phosphatidic acid(7, 162 and 37 nmol g^–1 d^–1). How accurate a guidethe in vitro activity of phospholipase D was to the in vivoactivity was unclear. However, the considerable excess activityseen with the formation of phosphatidic acid supports the proposalthat hydrolysis of phospholipids by phospholipase D is a firststep in deterioration during ageing. Substantial lipoxygenaseactivity was also detected (58 x 10³ and 54 x 10³ nmol g^–1d^–1 respectively, for non-aged cucumber and onion seeds).However, the increase in conjugated dienes (an early productof peroxidation) in ageing cucumber seeds was comparativelysmall (90 nmol g^–1 d^–1 over 21 d ageing), and increasein malondialdehyde could not be detected, indicating that peroxidationmay not have been a major factor in cucumber. The increase inconjugated dienes during rapid ageing of onion seeds was larger(1·5 x 10³ nmol g^–1 d^–1 over days 0–2of rapid ageing), much greater than the decrease in phospholipidacyl groups (260 nmol g^–1 d^–1 over days 0–2of rapid ageing) indicating the occurrence of peroxidation offatty acids released from reserve as well as from membrane lipids.This higher level of conjugated dienes during onion ageing wasthe main difference between cucumber and onion, indicating thatthe level of peroxidation could be an important difference betweenfast and slow ageing seeds. However, peroxidation is not theonly possible deleterious process since hydrolysis of the normalmembrane phospholipids to phosphatidic acid increased the contentof non-bilayer-forming lipids and this too could be a directmembrane-destabilizing consequence of phospholipase D actionduring ageing. Key words: Cucumis sativus L. (cucumber), Allium cepa L. (onion), seed ageing, phospholipase D, lipoxygenase, phospholipids     

Alterations in Membrane Lipids and Nutrient Uptake during Root Development of Rape-Seed (Brassica napus L.)

Diepenbrock, W. 1988. Alterations in membrane lipids and nutrientuptake during root development of rape-seed (Brassica napusL.). J. exp. Bot. 39: 193–198. Oil-seed rape was grown hydroponically in two experiments. Inthe first one, the effects of root development on phosphorusbound in lipids (lipid-P) and on nutrient uptake were tested.The content of lipid-P in root tissue decreased markedly withtime. This decline was accompanied by reduced rates of nitrogenaccumulation in the shoot. In the second experiment, plantswere grown in a split-root system with two chambers per pot.A part of the root system was harvested at 28 d after seedlingtransfer (DAS) referring to ‘continuous growth’.The other part was cut at 14 DAS and roots from the same chamberwere gathered at 14 d after root cutting (DAR) referring to‘re-growth’ Not only at 14 DAS but also at 14 DARhigh concentrations of lipid-P were detected whereas at 28 DASthe lipid-P content decreased dramatically. Changes in lipid-Pwere reflected in the fluctuation of fatty acid compositionof phosphatidyl choline, a major phospholipid in root tissue.It was clearly demonstrated that during re-growth, nutrientuptake was intensified as compared to continuous growth. Generally,membrane lipids and nutrient uptake were closely related. Key words: Root, lipids, nutrient uptake     

Vectorial transport of toxins from the dinoflagellate Gymnodinium breve through copepods to fish

Toxins from the dinoflagellate Gymnodinium breve were tracedthrough experimental food chains from dinoflagellates, throughcopepod grazers, to juvenile fish. The generality of this foodweb transfer was demonstrated using three different combinationsof copepods and juvenile fish during different seasons. Fishwere not exposed directly to the toxic dinoflagellates but werefed toxin-laden copepods in order to examine sublethal vectorialintoxication. Toxins were shown to move from fish viscera tomuscle tissue within periods of 2–6 h to 25 h. A new toxindetection method was used in this first stepwise demonstrationof multi-trophic-level intoxication of a planktonic food chainby G.breve. Micellar electrokinetic capillary electrophoresiswith laser-induced fluorescence allowed measurements of toxinsat trace levels and nanoliter-sized volumes critical for planktonicfood web transfer studies.     

Modification of the root plasma membrane lipid composition of cadmium-treated Pisum sativum

The effects of in vivo Cd treatments on pea root plasma membrane(PM) lipid composition were studied. In the long-term experiment,plants were supplied with Cd: moderate stress (10 µM)or strong stress (50 µM) for 10 d. Growth of root andshoot was severely affected in 50 µM Cd-treated plants,as evidenced by the approximately 7-fold reduction in theirRelative Growth Increment (RGI). Treatment with Cd (10 µM)resulted in changes to the lipid composition of the pea rootPM, including increases in the degree of unsaturation of phospholipid-associatedfatty acids and in the relative amount of stigmasterol (30–42%).This change was accompanied by a reduction in sitosterol content(26.8 to 17.4 µg mg^–1 protein). However, the sterolcomposition was not altered in plants treated with 50 µMCd for 10 d. The content of phosphatidylethanolamine and phosphatidylcholine(major phospholipids present in pea root PM) decreased as Cdlevel increased, but the ratio between them remained unaffected.In the short-term experiment, plants exposed to Cd (50 µM)accumulated less sitosterol (from 27.7 to 14.0 µg g mg^–1protein) over 72 h, but no significant effect on other measuredlipids was observed. The physiological repercussions of changesin plasma membrane lipid composition, as a result of Cd exposureare discussed. Key words: Cadmium, lipids, pea, Pisum sativum, plasma membranes     

Zooplankton in the Vasikkalampi pond, a warm water effluent recipient in Central Finland

Seasonal and vertical fluctuations of zooplankton species composition,biomass, and production were monitored by weekly sampling duringa two year period in one eutrophic pond in Central Finland.The study was one part of a more comprehensive study programto investigate the effects of warm water effluents from onesmall thermal power plant (35 MW) on the pond ecosystem. Becauseof the circulation of the pond water through the pumps in thepower plant the crustacean populations were very sparse in planktonduring the seasons the power plant was in operation (late Augustto May). During that time rotifers were dominant and some speciesreached very high densities (e.g., Keratella cochlearis s.l.ca. 15 000 ind. l^–1 in sping). In summer months Asplanchnapriodonta, Ceriodaphnia quadrangula, Bosmina longirostris, Mesocyclopsleuckarti and Thermocyclops oithonoides were dominant. A totalof 96 planktonic and meroplanktonic taxa were identified (26ciliates, 46 rotifers, 21 cladocerans and 3 copepods). The dryweight biomass of total zooplankton was 10 mg m^–3 in wintermonths, 10–100 mg m^–3 in spring and 300–1000mg m^–3 in summer. The total yearly production of zooplanktonwas 8552 mg dry wt m^–3 a^–1 in 1979 and 8440 mg drywt m^–3 a^–1 in 1980, from which the proportion ofrotifers was 33–39%, cladocerans 52–58% and copepods8.6 –9.4%. The winter production was 0.2–0.5% ofthe total yearly production, that of spring and autumn togetherwas 8.1–10.4% and the remainder (89–91%) was summerproduction.     

Size structures of microplankton biomass and production in Jiaozhou Bay, China

Seasonal investigations of size-fractionated biomass and productionwere carried out from February 1992 to May 1993 in JiaozhouBay, China. Microplankton assemblages were separated into threefractions: pico- (0.7–2 µm), nano- (2–20 µm)and netplankton (20–200 µm). The biomass was measuredas chlorophyll a (Chi a), paniculate organic carbon (POC) andparticipate organic nitrogen (PON). The production was determinedby ¹⁴C and ¹⁵N tracer techniques. The seasonal patterns in biomass,though variable, were characterized by higher values in springand lower values in autumn and summer (for Chi a only). Theseasonal patterns in production, on the other hand, were moreclear with higher values occurring in summer and spring, andlower values occurring in autumn and winter. Averaged over thewhole study period, the respective proportions of total biomassaccounted for by net-, nano- and picoplankton were 26, 45 and29% for Chi a, 32, 33 and 35% for POC, and 26, 32 and 42% forPON. The contributions to total primary production by net-,nano- and picoplankton were 31, 35 and 34%, respectively. Therespective proportions of total NH₄⁺–N uptake accountedfor by net-, nano- and picoplankton were 28, 33 and 39% in thedaytime, and 10, 29 and 61% at night. The respective contributionsto total NO₃^–-N uptake by net-, nano- and picoplanktonwere 37, 40 and 23% in the daytime, and 13, 23 and 64% at night.Some comprehensive ratios, including C/N biomass ratio, Chla/C ratio, C uptake/Chl a ratio, C:N uptake ratio and the f-ratio,were also calculated size separately, and their biological andecological meanings are discussed.     

Light rather than iron controls photosynthate production and allocation in Southern Ocean phytoplankton populations during austral autumn

The role of iron and light in controlling photosynthate productionand allocation in phytoplankton populations of the Atlanticsector of the Southern Ocean was investigated in April–May1999. The ¹⁴C incorporation into five biochemical pools (glucan,amino acids, proteins, lipids and polysaccharides) was measuredduring iron/light perturbation experiments. The diurnal Chla-specific rates of carbon incorporation into these pools didnot change in response to iron addition, yet were decreasedat 20 µmol photons m^–2 s^–1, an irradiancecomparable with the one at 20–45 m in situ depth. Thissuggests that the low phytoplankton biomass encountered (0.1–0.6µg Chl a L^–1) was mainly caused by light limitationin the deep wind mixed layer (>40 m). Regional differencesin Chl a-specific carbon incorporation rates were not foundin spite of differences in phytoplankton species composition:at the Antarctic Polar Front, biomass was dominated by a diatompopulation of Fragilariopsis kerguelensis, whereas smaller cells,including chrysophytes, were relatively more abundant in theAntarctic Circumpolar Current beyond the influence of frontalsystems. Because mixing was often in excess of 100 m in thelatter region, diatom cells may have been unable to fulfil theircharacteristically high Fe demand at low average light conditions,and thus became co-limited by both resources. Using a modelthat describes the ¹⁴C incorporation, the consistency was shownbetween the dynamics in the glucan pool in the field experimentsand in laboratory experiments with an Antarctic diatom, Chaetocerosbrevis. The glucan respiration rate was almost twice as highduring the dark phase as during the light phase, which is consistentwith the role of glucan as a reserve supplying energy and carbonskeletons for continued protein synthesis during the night.