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.     

Size-fractionated primary production studies in the vicinity of the Subtropical Front and an adjacent warm-core eddy south of Africa in austral winter

Results are presented of size-fractionated primary productionstudies conducted in the vicinity of the Subtropical Front (STF),an adjacent warm-core eddy, and in Sub-antarctic waters duringthe third South African Antarctic Marine Ecosystem Study (SAAMESIII) in austral winter (June/July) 1993. Throughout the investigation,total chlorophyll (Chl a) biomass and production were dominatedby small nano- and picophytoplankton. No distinct patterns intotal Chl a were evident. At stations (n = 7) occupied in thevicinity of the STF, total integrated biomass values rangedfrom 31 to 53 mg Chl a m^–2. In the vicinity of the eddy,integrated biomass at the eddy edge (n = 3) ranged from 24 to54 mg Chl a m^–2 and from 32 to 43 mg Chl a m^–2 inthe eddy (n = 2). At the station occupied in the Sub-antarcticwaters, total integrated biomass was 43 mg Chl a m^–2.Total daily integrated production was highest at stations occupiedin the vicinity of the STF and at the eddy edge. Here, totalintegrated production ranged from 150 to 423 mg C m^–2day^–1 and from 244 to 326mg C m^–2 day^–1, respectively.In the eddy centre, total integrated production varied between134 and 156 mg C m^–2 day^–1. At the station occupiedin the Sub-antarctic waters, the lowest integrated production(141 mg C m^–2 day^–1) during the entire survey wasrecorded. Availability of macronutrients did not appear to limittotal production. However, the low silicate concentrations duringthe survey may account for the predominance of small nano- andpicophytoplankton. Differences in production rates between theeddy edge and eddy core were related to water column stability.In contrast, at stations occupied in the vicinity of the STF,the control of phytoplankton production appears to be relatedto several processes, including water column stability and,possibly, iron availability.     

Phytoplankton photosynthetic efficiency and primary production rates estimated from fast repetition rate fluorometry at coastal embayments affected by upwelling (Rias Baixas, NW of Spain)

Previous studies have not always found a significant relationshipbetween fast repetition rate fluorometry (FRRF)-derived and¹⁴C-based primary production rates. This apparent discrepancymight be related to environmental control of the coupling betweenphotosynthetic electron transport through photosystem II (PSII)and carbon uptake in phytoplankton. In this study, we lookedat this relationship under upwelling conditions favourable forphytoplankton growth. The combination of both techniques allowedthe calculation of the quantum efficiency of carbon fixation,which averaged 0.12 mol C (mol quanta)^–1 indicating thatphytoplankton populations were very efficient in convertingabsorbed photons into photosynthetically produced organic carbon.The tight coupling observed between phytoplankton photosyntheticelectron transport through PSII and carbon uptake resulted ina statistically significant linear relationship between FRRF-derivedand ¹⁴C-based carbon incorporation rates, with a slope of 1.43.In conclusion, the results of the this study indicate that FRRFcan be a useful tool to derive high spatial and temporal resolutionprimary production estimates under environmental conditionsfavouring the close coupling between PSII electron transportand carbon uptake, such as those characteristic of this coastalupwelling system.     

Fatty acid composition in photosynthetic products of natural phytoplankton population in Lake Biwa, Japan

Fatty acid composition of phytoplankton photosynthetic productswas determined by a ¹³C tracer and gas chromatography-mass spectrometry(¹³C-GC-MS) method from August 1985 to June 1986 in Lake Biwa,Japan. The total fatty acid production rate varied from 2.8to 10.9 µg C l^–1 day^–1 at the water surfaceand accounted for 9.1–30% of photosynthetic productionof particulate organic carbon. A high contribution of fattyacid to the particulate organic carbon production rate was noticedduring winter time, and an increase in the fatty acid contributionresulted in an increase in the C/N value in the photosyntheticproducts. The fatty acid composition varied throughout the year,mainly depending on the seasonal change in the dominant phytoplanktonspecies. The contribution of polyunsaturated fatty acids tototal fatty acids was low during the summer period, probablydue to nitrogen limitation of phytoplankton growth.     

Response of Sargasso Sea phytoplankton biomass, growth rates and primary production to seasonally varying physical forcing

The response of phytoplankton biomass, growth rates and primaryproduction to seasonally varying physical forcing was studiedat a station southeast of Bermuda over an 18 month period. Phytoplanktongrowth rates and primary production were measured using thepigment-labeling method, and phytoplankton biomass was calculatedfrom these measurements. Phytoplankton carbon biomass variedsystematically over the year. Highest values were observed duringthe winter and spring. Seasonal variations of chlorophyll (Chi)a in the surface layer could primarily be attributed to variationsin phytoplankton biomass and secondarily to photoacclimation.During the summer period, average values of carbon (C)/Chl ratios(g C g^–1 Chi) ranged from 160 at the surface to 33 atthe 1.6% light level, changes attributed to photoacclimationof the phytoplankton, consistent with the observation that phytoplanktonbiomass did not vary as a function of depth. Phytoplankton growthrates in the surface layer did not vary systematically overthe year, ranging from 0.15 to 0.45 day^–1, in spite ofseasonally varying concentrations of nitrate. Growth rates variedas a function of depth from average values of 0.3 day^–1in the surface layer to <0.1 day¹ at the 1.6% light level.Thus, the primary response of the phytoplankton community tonutrient enrichment during the winter period was an increasein phytoplankton biomass rather than an increase in growth rates.A simple nutrient-phyto-plankton-zooplankton model was usedto explore this phenomenon. The model demonstrated that theobserved response of the phytoplankton to nutrient enrichmentis only possible when phytoplankton growth is not severely limitedby nutrients.     

The effects of light and temperature on photosynthate partitioning in Antarctic freshwater phytoplankton

The effects of temperature and radiation flux on the partitioningof photosynthetically fixed carbon into four intracellulai metabolicpools was investigated for natural phytoplankton assemblagesfrom an Antarctic freshwater lake. At ambient temperature, proteinsynthesis was saturated at low photon flux densities (30–40µmol m^–2 s^–1) and above this flux fixed carbonwas increasingly stored as lipid and polysaccharide. Increasingtemperature raised both the saturated rate of protein synthesisand the photon flux at which saturation occurred. There wasa corresponding decline in the accumulation of reserve products,particularly at low radiation fluxes. The consequences of thispattern of uptake for the phytoplankton is discussed.     

'St Martin's Summer': the case of an autumn phytoplankton bloom in the Gulf of Naples (Mediterranean Sea)

Phytopiankton abundance. species composition and primary productionof the Gulf of Naples were investigated during an autumn bloomin November 1985. Hydrographic data and surface phytoplanktonsamples were collected during a 3 day cruise, whereas investigationson in situ primary production and phytoplankton vertical distributionwere conducted from a second boat on three different dates.A coast-offshore gradient was recorded for most of the chemicaland biological parameters analysed. At stations closer to thecoast, which were affected by land run-off, phytoplankton populationsattained concentrations of 2.5 10⁶ cells l^–1 with amarked dominance of diatoms belonging to the genera Thalassiosiraand Chaetoceros. The most striking character of the system wasa remarkably high carbon assimilation rate (up to 1.04 g C m^–2day^–2) at stations closer to the coast. The causativemechanism for this bloom appeared to be land-derived nutrientenrichment, possibly enhanced by autumn rains, followed by aperiod of favourable meteorological conditions, which occursrecurrently in the region for a brief period around November,known locally as ‘St Martin's Summer’. We hypothesizethat a similar mechanism can stimulate phytoplankton growthmore than once every year. since high-stability penods followingmeteoro logical perturbations can occur several times in temperateregions of the northern hemisphere in autumn.     

Seasonal variations of bacterial abundance and biomass and their relation to phytoplankton in the hypertrophic tropical lagoon Cienaga Grande de Santa Marta, Colombia

The seasonal development of bacteria was studied in the hypertrophiccoastal lagoon Ciénaga Grande de Santa Marta (Caribbeancoast of Colombia). This large but only 1.5 m deep lagoon issubject to strong seasonal variations of salinity from almostfully marine (April/May) to brackish conditions in October/November.Chlorophyll ranged from 6 to 182 µg L^–1, and grossprimary production amounted to 1690 g C m^–2 per year.Total bacterial number (TBN) ranged from 6.5 to 90.5 x 10⁹ cellsL^–1 and bacterial biomass (BBM) from 77 to 1542 µgC L^–1, which are among the highest ever reported for naturalcoastal waters. Neither TBN nor BBM varied significantly withsalinity, phytoplankton or seston concentrations. Only the bacterialmean cell volume showed a significant relation to salinity,being highest (0.066 µm³) during the period of increasingand lowest (0.032 µm³) during decreasing salinity. Bacterialprotein accounted for 24% (19–26%) and phytoplankton proteinfor 57% (53–71%) of total seston protein. The ratio (annualmean) of bacterial carbon to phytoplankton carbon was 0.44 (range0.04–1.43). At low phytoplankton abundance [chlorophylla (Chl a) < 25 µg L^–1], bacterial carbon wasalmost equal to phytoplankton biomass (i.e. the mean ratio was1.04). In contrast, at Chl a > 100 µg L^–1, BBMwas low compared to phytoplankton biomass (the mean ratio was0.16). In general, BBM varied less than phytoplankton biomass.Most probably, the missing correlation between bacterial andphytoplankton variables was due to (i) organic material partlyderived from allochthonous sources serving as food resourcefor bacteria and (ii) a strong resuspension of bacteria fromthe sediment caused by frequent wind-induced mixing of the veryshallow lagoon.     

Photosynthetic oxygen production and community respiration in the Indian sector of the Antarctic Ocean during austral summer

Photosynthetic oxygen production by phytoplankton and community respiration in the Indian sector of the Antarctic Ocean were estimated from changes in oxygen concentrations in light and dark bottles. Gross production varied between 0.1 and 5.1 µmol O₂ l^-1 day^-1. In the same water, community respiration (the sum of oxygen consumption by heterotrophs and phytoplankton) was 0.4-3.6 µmol O₂ l^-1 day^-1, which accounted for 47-343% of the gross production. Algal and heterotrophic respirations were distinguished using some assumptions. These estimates showed that heterotrophic respiration accounted for most of the community respiration (70-91% depending upon the assumptions), indicating that heterotrophic respiration plays an important role in the mineralization of phytoplankton production in the surveyed sea area. Gross production rate correlated with chlorophyll a concentration, showing that the photosynthetic production rate of oxygen depends on the abundance of phytoplankton. Moreover, there was a significant relationship between gross production and community respiration rates. These regression equations suggested that negative net production occurred under the usually low concentration of chlorophyll observed in the Indian sector of the Antarctic Ocean. Hence, the net exchange of carbon dioxide due to biological processes through the sea surface seemed to be not as large as expected in the Antarctic Ocean, although the number of data were limited at this stage.     

Nitrate Nutrition and Temperature Effects on Wheat: a Synthesis of Plant Growth and Nitrogen Uptake in Relation to Metabolic and Physiological Processes

Lawlor, D. W., Boyle, F. A., Keys, A. J., Kendall, A. C. andYoung, A. T. 1988. Nitrate nutrition and temperature effectson wheat: a synthesis of plant growth and nitrogen uptake inrelation to metabolic and physiological processes.—J.exp. Bot. 39: 329-343. Growth of spring wheat was measured in cool (13°C day/10°Cnight) or warm (23°C/18°C) temperatures, combined withlarge and small amounts of nitrate fertilizer. The rate of growthof dry matter was less at cool temperatures but total growthover the same period of development was slightly greater inthe cool than in the warm. Main-shoot and tiller leaves grewslower and, despite growing for a longer period, were shorterin the cool than in the warm. They had greater fresh and drymass and content of starch and fructosans per unit area. Coolconditions increased root dry mass, root to shoot ratio andnitrogen content in dry matter. Additional nitrate increasedleaf area of main shoots slightly but of tillers greatly; itincreased leaf and tiller dry matter and total plant dry mass.Additional nitrate decreased the proportion of dry matter inroots and in stems and the N content of dry matter in all plantparts. Regulation of growth by temperature, nitrate supply andthe rôle of photosynthesis and nitrogen uptake, is consideredin relation to the mechanisms of incorporation of carbon andnitrogen into biochemical constituents. It is concluded thattemperature regulates the rate of protein synthesis, which determinesplant growth rate. Nitrogen flux into the plant is not directlylinked to protein synthesis so that the content of NO^–₃and of amino acids is related both to growth and to conditionsgoverning NO^–₃ uptake and its reduction. When nitrogensupply is large, growth is limited by temperature, not NO^–₃.Inadequate nitrate supply decreases protein synthesis (and thereforegrowth) more than it decreases carbon assimilation, so thatorgans such as roots and stems increase in dry matter relativeto shoots and all tissues have smaller proportions of nitrogenin dry matter. Cool conditions, although decreasing the rateof protein synthesis, increase its duration and decrease thesize of leaves, so that the content of protein per unit leafarea is greater in cool than in warm grown leaves. Consequencesof changes in the balance of N and C supply and growth ratefor dry matter distribution in plants are discussed. Key words: Wheat, nitrate nutrition, temperature     

Energetics and feeding dynamics of Euphausia superba in the South Georgia region during the summer of 1994

Measurements of adult Antarctic krill (Euphausia superba) gutcontents, evacuation and egestion rates, as well as digestiveefficiency, were carried out during February-March 1994 in thevicin ity of South Georgia to estimate in situ daily ration.These were combined with acoustically derived biomass data tocalculate the grazing impact of Antarctic krill and its contributionto the carbon flux in the region. Individual levels of gut pigmentconcentrations and evacuation rates ranged from 27 to 1831 ngchlorophyll a-eq. ind.^–1 and from 0.133 to 0.424 h^–1,respectively. Losses of pigment fluor escence during digestionwere very high, ranging from 58 to 98% of the total pigmentdigested. Daily carbon consumption estimated using the gut fluorescencemethod varied from 0.234 to 0.931 mg C ind.^–1 day^–1(or 0.4–1.7% of body carbon), compared to {small tilde}2.73mg C ind.^–1 day^–1 (or {small tilde}5% of body carbon)using the faecal pellet production data. The 3-fold higher dailyration estimated using egestion rate data may be explained bypredation on micro-and mesozooplankton. Maximum krill grazingimpact ranged from 0.4 to 1.9% of the total phytoplankton stockor from 10 to 59% of the total daily primary production. However,grazing impact on the microphytoplankton (>20 µm) wassubstantially higher, at times exceeding 100% of the daily microphytoplanktonproduction. It is suggested that to meet its energetic demands,kriil must consume a substantial proportion of heterotrophiccarbon. ³Present address Zoology Department, University of Fort Hare,P/Bag X1314, Alice, 5700, South Africa     

Phytoplankton rate processes in the oligotrophic waters of the central North Pacific Ocean

The central North Pacific is one of the more oligotrophic regionsof the world oceans. There the particulate organic nitrogen:cabonratio of surface waters is variable and less than the Redfieldratio of 16N:106C by atoms. The phytoplankton P/B ratio basedupon both C and N assimilation rate varied directly with theparticulate matter PON:POC ratio as did the productivity index[mg C (mg chl a)^–1h^–1]. At steady state the doublingtime of the phytoplankton, the turnover time of the limitingnutrient supplied via herbivore grazing, and the time for herbivoresto filter a unit volume of water would be equivalent. They appearto be of the order of 5–9 days based on present methodologyand straightforward interpretation of its results. The rate measurements involved incubation of water samples forseveral hours in bottles. In the central N. Pacific the valueswere similar using bottles of different sizes. Addition of chelatorsdid not enhance the rates implying no poisoning of the planktonby heavy metal contaminants. The observed specific activitiesof ¹⁴C and ¹⁵N of the particulate matter in the rate measurementsare inconsistent with the notion of an active, rapidly growingand recycling microplankton food web within the incubation bottlesand support the idea that phytoplankton are growing slowly.     

Photosynthetic organic carbon production and respiratory organic carbon consumption in the trophogenic layer of Lake Biwa

Measurement of the photosynthetic production rate in Lake Biwawas camed out from May 1985 to September 1987. In the light-saturatedlayer, the seasonal variation in the photosynthesis rate perchlorophyll a was regulated by water temperature. The depth-integratedphotosynthetic production rate was 0.21-1.48 g C m^–2 day^–1and the maximum value was observed in midsummer when the watertemperature of the mixed surface layer was highesL The criticalnutrient for photosynthesis may be dissolved reactive phosphorus,which was generally <1 µg P 1^–1 throughout theobservation period. In the trophogenic layer, respiratory organiccarbon consumption, estimated from measurement of respiratoiyelectron transport system activity, was 0.35-1.07 (mean 0.66)g C m^–1 day^–1 and corresponded, on average, to 79%of the photosynthetic carbon production rate. This implies thatthe major part of photosynthetic fixed organic matter mightbe recycled in the trophogenic layer. The estimated settlingorganic carbon flux at 20 m depth, from calculation of theseparameters and changes in the particulate organic carbon concentration,was 0.01 (-0.09 to 0.13) g C m^–1 day^–1 The meansettling organic carbon flux measured by sediment trap at 20m was 0.19 (0.09-0.31) g C m^–1 day^–1 higher thanthe estimated value. It seemed that organic matter collectedby sediment trap may contain allochthonous matter and resuspendedepilimnetic sediment matter.     

Physiology of diatom Skeletonema costatum (Grev.) Cleve photosynthetic extracellular release: evidence for a novel coupling between marine bacteria and phytoplankton

The photosynthesis of cellular materials by phytoplankton isaccompanied by release of organic molecules from the algal cellsinto the water. The patterns of carbon fixation in particulateand dissolved pools were investigated in Skeletonema costatumcultured under 12 h light/12 h dark cycles. The short-term production(1–15 min) of particulate organic carbon (POC) and extracellularorganic carbon (EOC) compounds was studied by measuring theuptake of ¹⁴C-labelled sodium bicarbonate and its subsequentincorporation and release into organic compounds. Slightly modifiedtraditional ¹⁴C radiotracer protocols were used, including separationby electrophoresis and thin-layer chromatography and detectionby autoradiography. Results indicated that there was a distinctdifference between radiolabelled compounds in the POC and EOCpools. Several metabolites found in the EOC pool were not presentin the POC pool, indicating the active release of these productsfrom the cells into the ambient water during short-term incubations,and indicating that inorganic carbon fixation pathways in marineautotrophs might be partly extracellular.     

中国近海生物固碳强度与潜力

通过现场调查和数据分析,首次探讨了中国近海浮游植物的固碳强度与潜力以及近海人工养殖大型藻类的固碳能力.结果表明,渤、黄、东海的浮游植物固碳强度约为2.22亿t a-1,固碳量的季节变化从大到小的顺序依次为春季、夏季、秋季、冬季,其中春夏季的固碳量占全年的65.3%.南黄海1999～2005年10～11月间浮游植物固碳强度有较大的年际变化,10～11月份7a间其浮游植物最低固碳量为3.54万t d-1,最高为16.58万t d-1,平均为10.50万t d-1,没有明显的年际变化趋势,磷对浮游植物固碳强度的影响最为明显,次之的影响因素是Chl a和亚硝氮 (NO2-N)的含量.南海的固碳能力约为渤黄东海的2倍,为4.16亿t a-1,其季节变化和渤黄东海不同,南海浮游植物在冬季的固碳能力最强,在夏季最弱.整个中国近海浮游植物年固碳量达6.38亿t,可占全球近海区域浮游植物年固碳量的5.77%.实际外海龙须菜的养殖发现,龙须菜每年固定的碳为8.18 t,养殖密度与方式对其产量和固碳量影响巨大.近几年,我国大型经济藻类养殖产量每年在120～150万t左右,换算为固碳量为36～45万t,平均每年40万t,如果海藻养殖产量每年增加5%,到2010年,我国大型经济藻类养殖的固碳量可达57万t a-1,海藻养殖是海洋增加碳汇有多重价值的重要措施.     

Particulate and dissolved phytoplankton production of the Patos Lagoon estuary, southern Brazil: comparison of methods and influencing factors

Uptake rates of ¹⁴C (filtration and the acidification-bubblingmethod—ABM) were measured weekly in a shallow region ofthe Patos Lagoon estuary (3207'S, 5206'W) between March 1989and March 1990. Phytoplankton production varied seasonally,the lowest values occurring in the austral winter (June–August1989) and the highest rates during spring and summer (March1989; September 1989–March 1990). Particulate carbon productionvaried between 0.65 and 70.6 mg C m^–3 h^–1 and wasmostly associated with organisms <20 µm (mean = 73.4%).Dissolved organic carbon (DOC) released by phytoplankton variedbetween 0.1 and 89.3 mg C m^–3 h^–1 representing     

Estimation of phytoplankton loss rates from daily photosynthetic rates and observed blomass changes in Lake Constance

Potential growth rates of phytoplankton biomass were estimatedyear-round from production rates and biomass and were comparedwith observed changes in euphotic phytoplankton biomass. Potentialgrowth was always greater than observed growth. The discrepancybetween both is attributed to losses. Relative loss rates showedwide seasonal fluctuations with highest values during the springbloom and autumnal phytoplankton maximum, respectively. Lossrates of photoassimilated carbon showed one peak in late Maywhich lead to a clear-water phase. Relative loss rates werehighly correlated with potential growth rates whereas observedgrowth rates were not. This suggests that most losses occurimmediately after the production process and do not lead toincreases in biomass. During the spring bloom grazing by zooplanktonis the single most important factor leading to losses from thephytoplankton community. During that time, 80–98% of overalllosses can be accounted for by grazing, sedimentation and wash-outcombined. During brief periods in summer and autumn, sedimentationrates comprised >50% of overall losses. In autumn only 30–40%of overall losses were due to the above-mentioned processes.Residual losses can be attributed to respiration, lysis andbacterial remineralization. Grazing, respiration and lysis leadto recycling of carbon and nutrients. Sedimentation rate measurementssuggest an average euphotic carbon regeneration rate of 85%.For the transfer efficiency of carbon along the food chain therelative significance of respiratory losses in overall lossesis of fundamental importance. ¹Dedicated to Professor Elster on his 75th birthday. ^*This paper is the result of a study made at the Group for AquaticPrimary Productivity (GAP) First International Workshop heldat the Limnological Institute, University of Konstanz, in April1982.     

Pelagic carbon metabolism in a eutrophic lake during a clear-water phase

Dissolved and paniculate organic carbon (DOC and POC, respectively),primary production, bacterial production, bacterial carbon demandand community grazing were measured for 9 weeks in eutrophicFrederiksborg Slotssø. The period covered the declineof the spring bloom, a clear-water phase and a summer phasewith increasing phytoplankton biomass. The process rates andchanges in pools of organic carbon were combined in a carbonbudget for the epilimnion. The POC budget showed a close balancefor both the post-spring bloom and the clear-water phase, whilea surplus was found in the summer phase. Production of POC wasdominated by phytoplankton (2/3) compared to bacteria (1/3)during all phases, and there was a significant correlation betweenphytoplankton and bacterial production rates (r² = 0.48, P <0.039). Bacterial demand for DOC was balanced by productionand changes in the pool of DOC during the decline of the springbloom, but the calculated demand exceeded the supply by 81 and167%, respectively, during the other two periods. The discrepancywas most probably due to an underestimation of bacterial growthefficiency and an overestimation of in situ bacterial productionin carbon units. Production of bacterial substrate by zooplanktonactivity was estimated to be higher than the direct excretionof organic carbon from phytoplankton. The biological successionwas regulated by the balance between area primary productionand community grazing. The clear-water phase was initiated bya combination of low primary production due to low surface irradianceand high community grazing (100 mmol C m^–2 day^–1),which caused a decrease in phytoplankton biomass. However, dueto the high initial phytoplankton biomass, community grazingwas not high enough to cause a significant decrease in areaprimary production. The summer phase was initiated by a decreasein community grazing followed by an increase in phytoplanktonbiomass. Based on these observations and calculations of areaprimary production as a function of chlorophyll concentrations,we suggest that the possibility for zooplankton to regulatephytoplankton biomass in temperate lakes decreases with increasingnutrient level.     

Bacterial response to seasonal changes in primary production and phytoplankton biomass in Lake Constance

Parameters characterizing bacterial biomass and metabolic activityare compared with phytoplankton biomass and daily primary productionrates throughout the year. Between late March (before the onsetof the phytoplankton spring bloom) and mid-July (diatom maximum),bacterial degradation of organic matter was more closely relatedto phytoplankton productivity than during the rest of the year.Bacterial production (as estimated by amino acid net uptake)was significantly correlated with concentrations of chlorophyll and pheopigments. However, bacterial production was correlatedless closely with primary production and only weakly with bacterialbiomass. Bacterial biomass was also only weakly correlated withprimary production but significantly with pheopigments. Numbersof active bacteria as estimated by autoradiography covariedclosely with bacterial production and cell numbers. Wheneverbacterial production was low, enhanced proportions of aminoacids were respired. Oxygen consumption measurements showedthat the size fraction <3 µm contributed 25–75%to total respiration. On average, bacterial biomass comprised11 % of paniculate organic matter and roughly equalled phytoplanktonbiomass. During the growing season, bacterial production inthe uppermost 20 m comprised about 20% of phytoplankton productionwith large seasonal fluctuations. A tentative carbon budgetof the euphotic zone including primary production, zooplanktongrazing, bacterial production and sedimentation is presented. ¹Present address: Institute of Marine Resources A-018, ScrippsInstitution of Oceanography, University of California, San Diego,La Jolla, CA 92093, USA     

Effects of chronic, low levels of UV radiation on carbon allocation in Cryptomonas erosa and competition between C.erosa and bacteria in continuous cultures

We conducted a long term (4 week) continuous culture studyto measure the chronic effects of UV radiation on the alga,Cryptomonas erosa, using three different fluence rates of UVradiation. We measured carbon allocation into carbohydrate,protein and lipid pools, as well as chlorophyll a concentrationsand algal and bacterial density. After 21 days, algal densityin the control and lowest UV treatment (treatment 1 = 3.4 Wm^–2 UVR unweighted) was significantly lower than in thetwo highest UV treatments (treatment 2 = 14.9 W m^–2 andtreatment 3 = 16.2 W m^–2 UVR unweighted), and did notrecover in the following week of no UV exposure. Chlorophylla and carbohydrate content (ng algal cell^–1) for the controland treatment 1 were clearly lower than treatments 2 and 3 byday 15, and did not recover by day 28. Percentage total lipidfor the control and treatment 1 also decreased compared withtreatments 2 and 3 by the end of the exposure period. However,by day 21, protein content for the control and treatment 1 wassignificantly higher than treatments 2 and 3, and demonstrateda further increase by day 28. The results were largely attributedto competition effects between C.erosa and bacteria in thesenon-axenic cultures. Bacterial density was significantly (x4)higher in the control and lowest UV treatment compared withthe two highest UV treatments. Our findings suggest a competitiveadvantage of phytoplankton over bacteria under these conditions.If UV radiation, in general, affects bacteria to a greater extentthan algae, there are likely to be changes in (i) bacterialutilization of dissolved organic matter produced by phytoplankton,(ii) competition between phytoplankton and bacteria for nutrientminerals and (iii) predation rates on bacteria by micro-flagellates.