Feeding and reproduction of Calanus finmarchicus during non-bloom conditions in the Irminger Sea

Simultaneous ingestion and egg production experiments were conductedwith female Calanus finmarchicus in April/May and July/August2002 in the Irminger Sea. Experimental animals were providedwith natural microplankton food assemblages and incubated underin situ conditions for 24 h. The quantity of food consumed wassignificantly related to the concentration of prey cells, withtotal daily ingestion rates ranging from 0.6 to 8.1 µgof carbon female^–1 day^–1, corresponding to carbon-specificrates of 0.6–4.7% day^–1. Egg production rates (EPRs)remained relatively low (0.3–11 eggs female^–1 day^–1)during both periods of investigation and were not influencedby food availability. The data were used to construct energeticbudgets in which the microplankton carbon ingested, includingciliates, was compared with the carbon utilized for egg productionand respiration. These budgets showed that ingestion alone couldnot provide the necessary carbon to sustain the observed demandsfor growth and metabolism. Although ciliates constituted >80%of the total material ingested at times, they were not sufficientto provide the metabolic shortfall. Indeed, the females weretypically lacking 5 µg of carbon each day, 5% of theircarbon biomass. Our study results highlight the possible importanceof internal reserves in sustaining reproduction in C. finmarchicusduring periods of food scarcity.     

Microzooplankton herbivory and bacterivory in Newfoundland coastal waters during spring, summer and winter

Grazing by microzooplankton on autotrophic and heterotrophicpicoplankton as well as >0.7 µm phytoplankton (as measuredby chlorophyll a) was quantified during July, August, October,January and April in the surface layer of Logy Bay, Newfoundland(47°38'14'N, 52°39'36'W). Rates of growth and grazingmortality of bacteria, Synechococcus and >0.7 µm phytoplanktonwere measured using the sea water dilution technique. Microzooplanktoningested 83–184, 96–366 and 64–118% of bacterial,Synechococcus and >0.7 µm phytoplankton daily potentialproduction, respectively and 34–111, 25–30 and 16–131%of bacterial, Synechococcus and >0.7 µm phytoplanktonstanding stocks, respectively. The trends in prey net growthrates followed the seasonal cycles of prey biomass, suggestingthat microzooplankton are important grazers in Newfoundlandcoastal waters. Ingestion was lowest during January and October(~2 µg C l^–1 day^–1) and highest in August(~20 µg C l^–1 day^–1). Aside from April when>0.7 µm phytoplankton represented the majority (~80%)of carbon ingested, bacterioplankton and <1 µm phytoplanktonrepresented most of the carbon ingested (~40–100%). Althoughmicrozooplankton have here-to-fore been unrecognized as an importantgrazer population in Newfoundland coastal waters, these resultssuggest that they play an important role in carbon flow withinthe pelagic food web, even at low temperatures in Logy Bay.     

Carbon dynamics in the 'grazing food chain' of a subtropical lake

Studies were conducted over a 13 month period at four pelagicsites in eutrophic Lake Okeechobee, Florida (USA), in orderto quantify carbon (C) uptake rates by size-fractionated phytoplankton,and subsequent transfers of C to zooplankton. This was accomplishedusing laboratory ¹⁴C tracer methods and natural plankton assemblages.The annual biomass of picoplankton (<2 µm), nanoplankton(2–20 µm) and microplankton (<20 µm averaged60, 389 and 100 µg C 1^–1 respectively, while correspondingrates of C uptake averaged 7, 51 and 13 µg C1^–1h^–1. The biomass of microzooplankton (40–200 µm)and macrozooplankton (<200 µm averaged 18 and 60 µgC 1^–1, respectively, while C uptake rates by these herbivoregroups averaged 2 and 3 µg C 1^–1 h^–1. Therewere no strong seasonal patterns in any of the plankton metrics.The ratio of zooplankton to phytoplankton C uptake averaged7% over the course of the study. This low value is typical ofthat observed in eutrophic temperate lakes with small zooplanktonand large inedible phytoplankton, and indicates ineffectiveC transfer in the grazing food chain. On a single occasion,there was a high density (<40 1^–1) of Daphnia lumholrzii,a large-bodied exotic cladoceran. At that time, zooplanktoncommunity C uptake was <20 µg C 1^–1 h^–1and the ratio of zooplankton to phytoplankton C uptake was near30%. If D.lumholrzii proliferates in Lake Okeechobee and theother Florida lakes where it has recently been observed, itmay substantially alter planktonic C dynamics.     

Diurnal and seasonal variation in the contributions of autotrophic pico-, nano- and microplankton to the primary production of an upland lake

An investigation of the diurnal variation in productivity andcontribution to production of populations of autotrophic picoplankton(0.2–2.0 µm), nanoplankton (>2 <20 µm)and microplankton (>20 µm) was carried out at monthlyintervals, from May to October 1989, in Llyn Padarn a mesotrophicupland lake in North Wales. Maximum rates and contributionsto production of the lake by autotrophic picoplankton occurredduring mid-late summer, with the highest average daily contributionfrom picoplankton (64%) recorded in September at 4 m depth.Diurnal variation in contributions from picoplankton was pronounced,with greatest input, recorded at the end of the day, duringthe period of picoplankton dominance in mid-late summer. Maximumcontribution from picoplantkon (86% of total, 9.2 mg C m^–3h^–1) was recorded in September. Nanoplankton primary productionwas of greatest significance in June and July, although levelswere lower than for picoplankton in subsequent months. Contributionsvia nanoplankton increased with depth in the lake at this time,reaching a maximum of 78% of the total at the end of the dayat 9 m depth in early July. At this time, diurnal variationin contributions via nanoplankton was considerable, with maximumphotosynthesis generally at the end of the photoperiod at depthsof 4 and 9 m. Microplankton made the greatest impact on primaryproduction during the mixed water conditions of spring and autumn,and at these times did variation in production was less thanthose of both pico and nanoplankton during summer thermal stratification.Photosynthetic capacity was lower for picoplankton than fornanoplankton and microplankton; the highest values were 5, 33and 51 mg C (mg chl a)^–1) h^–1) for pico-, nano-and microplankton, respectively. The photosynthetic efficiencyof all three size categories of phytoplankton increased withdepth. Maximum values were similar for all phytoplankton groups,between 75 and 131 mg C (mg chl a)^–1) E^–1) m² butmean levels of photosynthetic efficiency for the 6 months werelower for picoplankton than for nano- or microplankton. Ratesof carbon fixation per cell for picoplankton spanned three ordersof magnitude, varied considerably diurnally and reached maximumvalues of 484 fg C(cell)^–1) h^–1) in the afternoonin near-surface waters in the early stages of exponential populationgrowth in July. During the population maximum of picoplanktonin August and September, maximum daily values of carbon fixationper cell, assimilation number and photosynthetic efficiencywere all recorded at the end of the day. The seasonal and diurnalpatterns of production of the three size categories of planktonicalgae in Llyn Padarn were distinct. During spring, microplankton(mainly diatoms) were the dominant primary producers. As thermalstratification developed, nanoplankton were the major contributorsto phytoplanktonic production, particularly in the deeper regionsof the euphotic zone. Picoplankton made the greatest contributionto production in August and September, exhibiting maximum inputtowards the end of the light cycle. Diatoms became the majorphotosynthetic plankton in the mixed water conditions prevalentin Uyn Padarn in October.     

The Carbon Economy of Rubus chamaemorus L. II. Respiration

Respiratory activity and seasonal changes in carbohydrate contentof the storage organs of Rubus chamaemorus L. have been investigated.Leaf dark respiration rate increases in a non-linear mannerfrom 0·7 mg CO₂ evolved dm^–2 h^–1 at 0 °Cto 4·6 rng CO₂ evolved dm^–2 hh^–1 at 30 °C.Root and rhizome respiration rates increase from 1 µ1O₂ uptake g^–1 fresh weight h^–1 at 0.7 ° C to10 µ10, uptake g^–1 f. wt h^–1 at 20 °C.Rhizome carbohydrate reserves decline from a September peakof 33 per cent alcohol insoluble d. wt to 16 per cent in May. The circumpolar distribution of R. chamaemorus is discussedin relation to the evidence presented here and in the precedingpaper of the series.     

Summer nutrient dynamics of the Middle Atlantic Bight: nitrogen uptake and regeneration

Nitrate and ammonium uptake and ammonium regeneration rates(by zooplankton, microplankton and benthos) were measured onthe Atlantic continental shelf (Middle Atlantic Bight) duringsummer, 1980. Euphotic zone profiles of NO₃^– and NH₄⁺uptake rates were similar in magnitude and vertical structureover a large geographical area. Microplankton NH₄⁺ regenerationrates, although measured less frequently, also showed a relativelyconsistent vertical structure; rates were positively correlatedwith uptake rates. Nitrate assimilation (‘new’ production)was used to estimate vertical eddy diffusivity and paniculatesinking rates. Eddy diffusion estimates ranged from <0.1to >2.0 cm² s^–1 and were positively related to arealprimary production. Estimated particulate sinking rates averaged5 mg at Nm^–2d^–1 and compared favorably with sedimentationrates measured from free-floating and moored sediment traps.Benthic nitrogen regeneration rates represented <10% of thispaniculate nitrogen flux. Within the mixed layer, NH₄⁺ assimilation(‘regenerated’ production) represented 50–80%of the total (NO₃^– + NH₄⁺ ) nitrogen productivity and33% for the euphotic zone. Of this, 30% was attributed to zooplankton,63% to microplankton (<100 µm) and 7% to benthos. Onthe average, 74% of the microplankton NH₄⁺ regeneration wasassociated with organisms passing 1 µm filters.     

Growth and feeding rates of the newly hatched larval ctenophore Mnemiopsis leidyi A. Agassiz (Ctenophora, Lobata)

Mnemiopsis leidyi: larvae depend on microplankton (<200 µm) prey duringthe first few days following hatching until larvae are >0.5mm in length and can successfully capture and consume mesozooplanktonprey. Feeding and growth rates of newly hatched M. leidyi larvaewere measured in controlled laboratory experiments. When fednatural microplankton assemblages, newly hatched larvae consumedsignificant quantities of both autotrophic and heterotrophicprey, including diatoms, phototrophic, heterotrophic and mixotrophicdinoflagellates, euglenoid flagellates, aloricate and tintinnidciliates, and rotifers. Average per capita clearance rates were1.99–7.59 mL individual^–1 h^–1 ( = 4.01 mL individual^–1 h^–1; SD = 1.95)and total per capita ingestion was 0.01–4.70 µgC individual^–1 day^–1 x 10² ( = 0.83 µg C individual^–1 day^–1 x 10²; SD =1.89). Larval growth rates were –0.13 to 0.56 mm individual^–1day^–1 (equivalent to –1.72 to 4.33 µg C individual^–1day^–1) over a range of larval sizes from 0.5 (<0.5µg C) to 5 mm (85 µg C). A diet consisting entirelyof microplankton prey supported larval growth for >2 weeks,and growth rate decreased when larvae reached 4–5 mm inlength, corresponding to the beginning of their morphologicaltransition from tentaculate to lobate feeding mode. The grossgrowth efficiency of larvae fed natural microplankton assemblageswas 3%.     

Seasonal and spatial distribution of bacterial biomass and the percentage of viable cells in a reservoir of Alabama

Spatial community dynamics of bacterioplankton were evaluatedalong the length of the former stream channel of Elledge Lake,a small reservoir in western Alabama. The reservoir was stronglystratified from April to October with up to a 10°C temperaturedifference across the 1 m deep metalimnion. Bacterial biomasswas highest during late summer, with a general pattern of increasingabundance from the inflowing river (10 µg C l^-1) to thedam (20–30 µg C l^-1). Bacterial numbers also increasedfollowing a >10-fold increase in turbidity associated witha major precipitation event, although only 10% of these cellswere viable. The percentage of viable cells generally increasedthrough the stratified period with 50–70% viable cellsin late summer. Overall, an average of 38% of bacterial cellswere viable, with a range from <20 to 70%. Although thesevalues were similar to those found by others, additional patternswere identified that have not been previously observed: a markeddecline in viable cells was found following turbid storm inflowsand increases in the percentage of viable cells occurred duringspring warming and following autumnal mixing events. Althougha modest increase in abundance occurred along the gradient frominflow down-reservoir to the dam, bacterial abundance did notincrease near the dam in a pattern coincident with the commonlyobserved increased algal biomass in the lacustrine portion ofreservoir ecosystems. The increases observed in bacterial viabilitymoving from the inflowing rivers towards the dam and later instratified periods stress the importance of differences in environmentalconditions in time and space in regulating bacterial biomassand development, as well as of shifts that would be anticipatedaccompanying altered hydrological regimes under climatic change.     

Temperature and Antarctic plankton community respiration

Antarctic plankton community respiration rates were determinedfrom in vitro changes in dissolved oxygen. Oxygen consumptionrates, measured at in situ temperatures between 0 and 6°C,were found to lie in the range 0.3–3.7 µmol O₂ l^–1per 24 h. Water samples were collected between East FalklandIsland and South Georgia, South Atlantic Ocean, and incubatedshipboard in the dark at up to 36 temperatures between –2and 14–C. A respiration rate at each temperature was thendetermined and used to calculate the temperature coefficient(Q₁₀) of Antarctic planktonic community respiration from theArrhenius equation. Fourteen Q₀ values lay in the range 1–3,with four further values >5. This range of temperature coefficientvalues for community respiration is comparable to the publishedrange of values for plankton photosynthesis. Frequency distributionsof temperature coefficients for the two processes show similarmodal Q₁₀5 of 2–3. Thus, this study does not lend supportto the hypothesis of a differential response of photosynthesisand community respiration to low temperature.     

Influence of Elevated CO2 and Temperature on the Photosynthesis and Respiration of White Clover Dependent on N2 Fixation

Single clonal plants of white clover (Trifolium repens L) grownfrom explants in a Perlite rooting medium, and dependent fornitrogen on N₂ fixation in root nodules, were grown for severalweeks in controlled environments which provided two regimesof CO₂, and temperature 23/18 °C day/night temperaturesat 680 µmol mol^–1 CO₂, (C680), and 20/15 °Cday/night temperatures at 340 µmol mol^–1 CO₂ (C340)After 3–4 weeks of growth, when the plants were acclimatedto the environmental regimes, leaf and whole-plant photosynthesisand respiration were measured using conventional infra-red gasanalysis techniques Elevated CO₂ and temperature increased ratesof photosynthesis of young, fully expanded leaves at the growthirradiance by 17–29%, despite decreased stomatal conductancesand transpiration rates Water use efficiency (mol CO₂ mol H₂O^–1)was also significantly increased Plants acclimated to elevatedCO₂, and temperature exhibited rates of leaf photosynthesisvery similar to those of C340 leaves ‘instantaneously’exposed to the C680 regime However, leaves developed in theC680 regime photosynthesised less rapidly than C340 leaves whenboth were exposed to a normal CO₂, and temperature environmentIn measurements where irradiance was varied, the enhancementof photosynthesis in elevated CO₂ at 23 °C increased graduallyfrom approx 10 % at 100 µmol m^–1 s^–1 to >27 % at 1170 µmol m^–2 s^–1 In parallel, wateruse efficiency increased by 20–40 % at 315 µmolm^–2 s^–1 In parallel, water use efficiency increasedby 20–40 % at 315 µmol m^–2 s^–1 In parallel,water use efficiency increased by 20–40 % at 315 µmolm^–2 s^–1 In parallel, water use efficiency increasedby 20–40 % at 315 µmol m^–2 s^–1 to approx100 % at the highest irradiance Elevated CO₂, and temperatureincreased whole-plant photosynthesis by > 40 %, when expressedin terms of shoot surface area or shoot weight No effects ofelevated CO₂ and temperature on rate of tissue respiration,either during growth or measurement, were established for singleleaves or for whole plants Dependence on N₂, fixation in rootnodules appeared to have no detrimental effect on photosyntheticperformance in elevated CO₂, and temperature Trifolium repens, white clover, photosynthesis, respiration, elevated CO₂, elevated temperature, water use efficiency, N₂ fixation     

Predation and respiration by the small cyclopoid copepod Oithona similisr: How important is feeding on ciliates and heterotrophic flagellates?

Feeding on natural plankton populations and respiration of thesmall cyclopoid copepod Oithona similis were measured duringthe warm season in Buzzards Bay, Massachusetts, USA. AlthoughO.similis did not significantly ingest small autotrophic andheterotrophic flagellates (2–8 µn), this copepodactively fed on >10 µm particles, including autotrophic/heterotrophic(dino)flagel-lates and ciliates, with clearance rates of 0.03–0.38ml animal^–1 h^–1. The clearance rates increased withthe prey size. O.similis also fed on copepod nauplii (mainlycomposed of the N1 stage of Acartia tonsa with a clearance rateof 0.16 ml animal^–1 h^–1. Daily carbon ration fromthe combination of these food items averaged 148 ng C animal^–1day^–1 (41% of body C day^–1), with ciliates and heterotrophicdino-flagellates being the main food source ({small tilde}69%of total carbon ration). Respiration rates were 020–0.23µl O₂ animal^–1 day^–1. Assuming a respiratoryquotient of 0.8 and digestion efficiency of 0.7, the carbonrequirement for respiration was calculated to be 125–143ng C animal^–1 day^–1, close to the daily carbon rationestimated above. We conclude that predation on ciliates andheterotrophic dinoflagellates was important for O.similis tosustain its population in our study area during the warm season.     

Size-selective grazing of bacteria by Bosmina longirostris--an image-analysis study

Image analyses on the filtering apparatus of Bosmina longirostricshowed that the filter mesh is finer on the gnathobasic filterplates of the second and third trunk limbs (ranges from 0.43to 0.97 µm) and coarser for the outer ones of the thirdlimb (ranges from 0.5 to 1.36 µm), and the intersetulardistances increase with body length. Grazing experiments combinedwith image analysis confirmed the efficient grazing of B.longirostrison natural bacteria with cell lengths equal to or larger thanthe intersetular distances of the gnathobasic filter plates.During the experiments, the animals minimized the average celllength of the bacterioplankton assemblages from 0.77–0.96µm to 0.55–0.68 µm, corresponding to the meanof the filter mesh size on the fine gnathobasic filter platesof the experimental populations. The clearance rates for large,elongated or dividing cells with maximal lengths of 0.88–8.40µm were 2.3–17.7 times higher than those for smallsingle coccoids with a diameter of <0.45 µm. The resultsprovide evidence of a significant differential impact of B.longirostrison the bacterial community structure with respect to the shapeand size of the cells, and demonstrate that the species is amore effective bacterial feeder than considered previously.     

Dynamics of inorganic phosphorus in pelagic communities of the Sea of Okhotsk

Inorganic phosphorus uptake and regeneration in the OkhotskSea waters were investigated in July–August 1994 withthe use of radioisotopic techniques. The rates of PO₄-P uptakeby microplankton in the upper mixed layer were between 1.5 and6.6 µg P l^-1 day^-1 (average 2.75) in areas of diatom dominance,and between 0.68 and 1.68 µg P l^-1 day^-1 (average 1.16)in areas of intense warming and summer phytoplankton minimum.The residence time of PO₄-P standing stock in water at differentstations varied between 1.5 and 24 days (mean 9 days). The shareof bacterioplankton contributing to total PO₄-P uptake was 50%in areas of the summer phytoplankton minimum and 20–30%in areas of diatom dominance. The PO₄-P regeneration rate wasmeasured first time experimentally in the temperate sea. Itsrates varied from 0.30 to 1.65 µg P l^-1 day^-1. In areasof diatom dominance, it compensated with 30–60% of PO₄-Puptake. In zones of summer phytoplankton minimum and in thelayers of deep chlorophyll maxima at 10–25 m depths, thePO₄-P regeneration rate often exceeded its uptake. Primary phytoplanktonproduction correlated well with PO₄-P uptake values in the uppermixed layer, while no correlation was found between primaryproduction and the ambient PO₄-P content in water.     

A comparison of net and gross rates of oxygen production as a function of light intensity in some natural plankton populations and in a Synechococcus culture

In vitrorates of gross and net oxygen production were measuredas a function of light intensity in some plankton communitiescollected from Bedford Basin, Nova Scotia, and in a monoclonalculture of Synechococcus. The rate of gross oxygen productionwas measured by a technique in which the stable oxygen isotope,¹⁸O, serves as a photosynthetic tracer Net oxygen productionwas measured by automated Winkler technique. The rate of communityrespiration in the light was then determined by the differencebetween gross and net rates of oxygen production. In the naturalpopulations examined, neither gross nor net oxygen productionrates were significantly inhibited at the highest light intensitymeasured (500–800 µE m^–2 s^–1) In a samplein which the dark respiration rate was small relative to themaximal rate of production [P_max;sensu Platt et al (1980) JMar. Res., 38, 687–701] the rates of ‘light’respiration were 3 times greater. In two other communities,with high rates of dark respiration relative to P_maxthe ratesof ‘light’ respiration were closer to rates of darkrespiration. In the Synechococcus clone, both gross and netoxygen production rates were inhibited at high light intensities.Rates of ‘light’ respiration were found to varyas a function of light intensity. The greatest rates of respirationwere measured in samples incubated at light intensities thatwere just saturating (100 µE m^–2 s^–1). Therates of ¹⁴C production were also measured as a function oflight intensity The photosynthetic quotients, based on ¹⁴C productionrates and gross oxygen production rates, average 1 9     

Resolution of Net Dark Fixation of Carbon Dioxide into Its Respiration and Gross Fixation Components in Bryophyllum daigremontianum

Dark respiration rate increased with temperature between 10and 24°C (Q₁₀ =2.3–2.7). The rate of gross dark CO₂fixation (GDF) was affected by temperature, but irregularly.Cumulative GDF was not affected by temperature in this range.Cumulative respiration increased from 17 per cent of cumulativeGDF at 10°C, to 72 per cent at 24°C and was thus responsiblefor the 65 per cent drop in net dark fixation between thesetwo temperatures. and respiration rates were functions of the light intensityin the preceding light period. The function for cumulativeGDFwas of the saturation form, maximum accumulation being obtainedat 12 mW cm^–2. It is concluded that both GDF and respirationrates depend on levels of substrates formed during the lightperiod. However, the rate of GDF did not appear to be directlyrelated to the rate of respiration.     

Respiration in eutrophic lakes: the contribution of bacterioplankton and bacterial growth yield

The contribution of bacterioplankton to total plankton respirationwas measured in two eutrophic Danish lakes and in experimentalenclosures treated with planktivorous fish and nutrients. Bacterialrespiration was calculated from measured oxygen uptake ratesin particles passing a 1.0-µm pore size filter, the rateswere then corrected for the size distribution of glucose uptake.During summer the respiration of the planktonic bacteria contributed{small tilde}50% of the community respiration in the two lakes.Prolific phytoplankton growth induced by biomanipulation andnutrient addition created situations where the contributionof the bacteria decreased to 20%. High bacterial contributionsto community respiration were found when the phytoplankton biomassdecreased. Simultaneous measurements of bacterial respirationand production (by means of [³H]thymidine incorporation) allowedan estimation of bacterial growth yield, which ranged from 9to 66%. In the two lakes the growth yield was constant witha mean of 29 ± 5% (±SD, RQ = 1). The variabilityof the growth yield was larger in the enclosures. The wide range(9–66%) was mainly caused by changes in bacterial netproduction without concomitant changes in respiration. The discussionincludes an evaluation of the oxygen uptake method in size fractionatedsamples and the availability of labile organic substrates asa factor controlling bacterial growth yield. Present address: Institute of Biology and Chemistry, Universityof Roskilde, P. Box 260, DK-4000 Roskilde, Denmark     

The composition of particulate organic matter and biomass in the Peruvian upwelling region during ICANE 1977 (Nov. 14 - Dec. 2)

This paper presents the results of plankton studies in the Peruvianupwelling region off Chimbote from November - December, 1977,during the Investigación Cooperativa de la Anchovetay su Ecosistema (ICANE). Primary productivity, respiratory ETSactivity, composition of particulate organic matter, and microplanktoncell numbers were determined. Phytoplankton growth, and bacterialand ciliate carbon-uptake rates were computed from cell counts. Inshore waters were dominated by diatoms and were more productivethan offshore waters which were dominated by dinoflagellatesand ciliates. Particulate primary production averaged 5.26 ±5.24 g C m^–2d^–1, and the POC standing stock was7.75 ± 2.74 g C m^–2 for the euphotic layer of 7shelf stations. On the shelf, microplankton respiration rateswere higher in plankton populations dominated by dinoflagellatesand ciliates (47% of particulate primary production per 24 h)than those in diatom dominated populations (11%, respectively).The diatom populations, which were dominated by Chaetocerosspecies, varied in their ecophysiological properties (assimilationnumbers, proportion of water soluble carbohydrate, and protein/nitrogenratios). The relationships between these variations and growthconditions were investigated. A 40 h time-series station revealedpatchiness which was superimposed on physiological changes ofthe plankters. Bacterial numbers of 2 x 10⁶ cells/ml were foundin the euphotic layer corresponding to approximately 17 µgC/l bacterial biomass (or 6% of the POC standing stock). Ciliatebiomass (Lohmanniella oviformis was the dominating species)ranging from 2 to 9% of the POC standing stock were found evenin diatom dominated populations. From a rough carbon balancefor the euphotic layer it was deduced that in diatom dominatedpopulations, 36–77% of particulate primary productionwas potentially available to adult anchoveta grazing.     

Photosynthetic production and exoenzymatic degradation of organic matter in the euphotic zone of a eutrophic lake

Photosynthetic production of organic matter, and its exoenzymaticdecomposition were studied in the euphotic zone of a naturallyeutrophic lake during early spring phytoplankton bloom, andafter its breakdown. Phytoplankton were the major biomass producerswhen algae were actively growing, and the algal fraction (>3.0µm) contributed on average 75–80% to the total biomassof microplankton. When the phytoplankton bloom began to declinebacterial biomass increased rapidly and, at the end of the bloom,bacteria contributed 48.7–69.98% to the total biomassof microplankton. The high bacterial abundance during phytoplanktonbloom breakdown followed the highest rates of glucose uptake,and the highest rates of alkaline phosphatase, leucine-amino-peptidase,ß-galactosidase and ß-glucosidase activities.The majority of enzyme activity was associated with the bacterialsize fraction of seston. The activities of free (dissolved inwater) exoenzymes were negligible. The synthesis of bacterialexoenzymes was under control of an induction/derepression mechanism,and depended on the amount of easily assimilable substrates,and/or the presence of polymeric organic compounds in the water,which served as substrates for exoenzymatic hydrolysis. Thetight metabolic coupling between bacterial exoenzymatic hydrolysisand uptake of low molecular weight substrates, and its ecologicalsignificance is discussed.     

Unusual allometry between respiration rate and body size in Chaoborus species

The respiration rates of all four instars of Chaoborus flavicanswere measured with a flow-through respirometer at an experimentaltemperature of 20°C. The respiration rates (µg O₂larva^-1 h^-1) increased parallel to the larval stages accordingto R = 0.027 x W^0.416 (W = µg dry weight), reaching arespiration rate eight times higher for instar IV than for instarI. The slope of the increase with body weight was as low asin two tropical Chaoborus species and was considerably lowerthan usually found for other aquatic animals. Instar IV larvaecollected in the spring showed a significantly higher respirationrate than those collected in the fall. The respiration rateof the fourth instar approximately doubled with a Q₁₀ of 2.1when the experimental temperature was increased from 10 to 20°C.     

Relationship between Bacterioplankton Richness, Respiration, and Production in the Southern North Sea

We investigated the relationship between bacterioplankton production (BP), respiration (BR), and community composition measured by terminal restriction fragment length polymorphism in the southern North Sea over a seasonal cycle. Major changes in bacterioplankton richness were apparent from April to December. While cell-specific BP decreased highly significantly with increasing bacterioplankton richness, cell-specific BR was found to be variable along the richness gradient, suggesting that bacterioplankton respiration is rather independent from shifts in the bacterial community composition. As a consequence, the bacterial growth efficiency [BGE = BP/(BP + BR)] was negatively related to bacterioplankton richness, explaining ~43% of the variation in BGE. Our results indicate that despite the observed shifts in the community composition, the main function of the bacterioplankton, the remineralization of dissolved organic carbon to CO₂, is rather stable.