Primary and bacterial production compared to growth and food requirements of Daphnia longispina in Lake Kvernavatnet, west Norway

Primary production, and bacterial production as measured byincorporation of [³H-methyl]thymidineinto ice cold TCA insolublematerial were investigated during 1984 in Lake Kvernavatnet,west Norway. Primary production averaged 222 mg C m^–2day^–1 and bacterial production averaged 163 mg C m^–2day^–1. The bacterial production in the euphotic pelagiczonecontributed -60% of the total pelagic bacterial production.The zooplankton was dominated byDaphnia longispina. From growthexperiments with animals fed only natural food in coarse filteredlake water, the population daily growth increments were calculated.The average production of D.longispina was 151 mg C m^–2day^–1 during the period investigated. The estimated primaryproduction was too low to sustain both the bacterial productionand the zooplankton food requirements. These results imply thatthe carbon cycle of the lake is dependent on the supply of allochtonousmaterial, or that the current methods for measuring productionrates in aquatic environments are systematical erratic.     

A morphological study of two populations of Bosmina longispina exposed to different predation

Two populations of Bosmina longispina existing in two similarand closely situated Norwegian mountain lakes were investigatedwith respect to morphological variations. The two lakes areinhabited by different predators, Lake Gopollen is dominatedby whitefish and Lake Djupen by a sparse population of browntrout and invertebrate predators. The B. longispina in LakeDjupen were both larger and also longer at maturation than theB. longispina of Lake Gopollen. In Lake Djupen the mucronesof B. longispina were twice as long and their relative antennulalength was also larger than in Lake Gopollen. However, the relativeeye size did not differ between the two populations. The observationsfit the present hypothesis on morphological predator avoidanceadaptations in B. longispina, fairly well. Invertebrate predatorsfavour large B. longispina with long protuberances, body sizeand length of mucro being the most important features.     

Somatic polyploidy examined by flow cytometry in Daphnia

Flow cytometry was used to examine the patterns of somatic polyploidyand to determine the haploid genome sizes in Daphnia. The averageproportions of polyploid nuclei among adult animals of D.pulex,D.longispina, D.pulex x D.longispina and D.magna equalled 27,24, 24 and 24%, respectively. Both interclonal and developmentallyregulated variation were observed in the level of somatic polyploidy.Adult animals expressed more extensive polyploidy than did juveniles.The estimates of the haploid genome sizes (C values) of D.pulex,D.longispina, D.pulex x D.longispina and D.magna equalled 0.32,0.27, 0.26 and 0.37 pg, respectively. These are the smallestgenomes reported in crustaceans. Apparently, somatic polyploidycompensates for the loss of genetic material and gives riseto genetic plasticity which may contribute to the considerablelevel of phenotypic plasticity observed in Daphina.     

The late Pleistocene/Holocene and recent Bosmina (Eubosmina) fauna (Crustacea: Cladocera) of the pre-alpine Starnberger See (FRG)

An analysis of the remains of Bosmina (Eubosmina) in a sedimentcore from the pre-alpine, mesotrophic Starnberger See showedthat the lake was inhabited by B. longispina since the OldestDryas. In the Subatlantic- -1900 years ago — the kesslerimorph of B. coregoni invaded the lake and both taxa have sinceco-existed. Although there was considerable morphological variation,B. longispina clearly differed from Circumbaltic populationsthrough most of the late Pleistocene and Holocene periods. Inthe recent Bosmina fauna of the lake two morphologically wellseparated taxa (B. longispina ruehei and a morph of B. coregoni)occur which differ clearly from the Bosmina remains found inthe core up to the uppermost sample from 7 cm sediment depth.B. coregoni f. kessleri appeared in the lake in a period whenit was supposedly still oligotrophic. The Bosmina successionobserved in the Starnberger See resembles the pattern foundin deep north German lakes.     

The influence of autotrophic and heterotrophic foods on the demography of Daphnia longispina under starved, semi-natural and enriched conditions

We tested the influence of different types of food on the fitnessof Daphnia longispina during a period of poor resources by comparingthe growth and reproduction of individuals fed on differentdiets. When Daphnia were under starvation conditions, both dissolvedorganic carbon and metabolic products enhanced bacterial growth,and the cladoceran produced 15 neonates and survived for the55 days of experiment. In the natural water, the edible foodconsisted essentially of bacteria and heterotrophic flagellates,which enabled the cladoceran to grow and reproduce. In waterenriched with Cryptomonas (an alga) or Chilomonas (a heterotrophicflagellate), the daphnid growth rates increased considerablyfrom generation to generation, but later in the time courseof the experiment with Chilomonas than with Cryptomonas. Thisalga enhanced the phosphorus:carbon ratio. The primiparous daphnidsfed with Cryptomonas were smaller than those fed with the heterotrophicflagellate and enrichment with the heterotrophic flagellatealso enhanced reproduction. In Lake Tazenat, when the algalbiomass was insufficient to support the Daphnia life cycle,the heterotrophic organisms were an essential food source. Whenfeeding essentially on bacteria, Daphnia has the capacity togrow and reproduce at very low food concentrations, and thisis important in determining its fitness under competitive conditions.     

Resource quality effects on Daphnia longispina offspring fitness

The present study addressed the question: is the fitness ofDaphnia longispina neonates related to the quality (species)of the resource their mothers consumed? A 2 2 factorial designwith two maternal and neonate resource types was employed, withRhodomonas and Microcystis used as high- and low-quality resources,respectively. Neonate fitness was assessed using demographicparameters obtained from a life-table study. The resource typeneonates consumed always had a stronger effect on the neonatesthan did the maternal type. Neonate consumption of Rhodomonas.relative to Microcystis. increased fitness, as estimated bythe instantaneous population growth rate, by 25–28%. whilematernal consumption of Rhodomonas increased fitness by 6-8%.Neonate Rhodomonas consumption also reduced the age at reachingthe primiparous instar; increased brood size at the primiparousinstar and body length immediately after the primiparous instar;increases average dutch size, number of clutches produced, totalegg production. and mean body length up to and on age 30 days.Regardless of resource type, maternal Rhodomonas consumptionresulted in neonates which reached maturity at an earlier ageand had a larger individual size immediately after reachingmaturity. The mean clutch size, total egg production, and lengthup to and at age 30 days were all significantly affected bymaternal diet when neonates consumed Microcystis; however, thesevariables were not influenced by maternal type when neonatesconsumed Rhodomonas. This is the first study to document maternallymediated effects of resource quality on the fitness of crustaceanzooplankton neonates.     

In situ bacterial production and growth yield measured by thymidine, leucine and fractionated dark oxygen uptake

In situ bacterial net production and growth yield were measuredusing thymidine, leucine incorporation and dark oxygen consumptiontechniques in marine enclosures and in the Bay of Aarhus, Denmark.Bacterial respiration was significantly correlated with thymidine(r² = 0.42, P < 0.01, y = 0.l2x + 0.054) and leucine (r²= 0.45, P < 0.01, y = 0.09x + 0.043). The range of bacterialgrowth yield, calculated from the relationship net production/netproduction + respiration, was 0.07–0.77 with 74% of theobservations lying in the 0.15–0.45 growth yield interval.Substrate was an important determinant of growth yields. A significantdifference was found between growth yields obtained from anenclosure with added glycine (mean 0.32±0.096) and onewith added inorganic nutrients (mean 0.16±0.051) (P <0.01, t-test). Growth yield showed a weak but significant negativecorrelation with temperature (r² = 0.0.35, P < 0.001, y =–0.017 + 0.52). No correlation between chlorophyll a andgrowth yield was found (r² = 0.25, P > 0.05). The resultssuggest that thymidine and leucine techniques reflect the levelsof bacterial production to better than an order of magnitude.The variations found in the growth yield support the notionthat relying on fixed growth yields reduces the accuracy ofestimating gross bacterial production.     

Vertical distributions of bacteria and algae in a steeply stratified humic lake under high grazing pressure from Daphnia longispina

The vertical distributions of bacteria and algae in a steeply stratified, highly humic lake were studied during three 24 h periods in summer. The highest bacterial and algal densities and biomasses were recorded in the anoxic hypolimnion. The bacterial biomass in the hypolimnion was composed mainly of photosynthetic green sulphur bacteria (Chlorobium) which occurred at very low light intensity (< 1.5 μmol m⁻² s⁻¹). The numbers and biomasses of bacteria, both in the epilimnion and hypolimnion, were mostly higher at night than during the day, indicating possible asynchrony between the production and loss of bacteria. Because of vertical migration, the diurnal vertical distribution of algae was more variable than that of bacteria. Particularly in July and August, when cryptomonads were abundant, the biomass of algae was much higher in the epilimnion during the day than at night. The flagellated chlorophytes, Chlamydomonas spp. and Scourfieldia cordiformis, stayed mainly in the upper hypolimnion close to the oxic-anoxic boundary zone where only a small proportion of Daphnia longispina was continuously present. Unpalatable Mallomonas chrysophytes with silicified plates and bristles, and small, presumably heterotrophic, flagellates stayed in the oxic epilimnion together with a dense (up to 300 ind l⁻¹) population of D. longispina. The results indicated that, besides the physical and chemical properties of the water column, grazing pressure by Daphnia longispina strongly affected the vertical distribution of microorganisms in this polyhumic lake.     

Littoral-pelagial interchange and the decomposition of dissolved organic matter in a polyhumic lake

The small, polyhumic lake, Mekkojärvi (southern Finland), is bordered by a moss vegetation zone (Warnstorfia and Sphagnum species) which provides a habitat-rich and productive environment for many planktonic and periphytic animals. Impacts of moss on the metabolism of bacterioplankton, phytoplankton and zooplankton in polyhumic water were investigated in laboratory throughflow systems. Growing Warnstorfia (together with epiphytic algae and bacteria) suppressed the production of planktonic algae but had no clear effect on leucine uptake, and hence bacterial production, or on the decomposition of humic substances. Phenol uptake and mineralization rates, however, were lower in the littoral water than in the pelagial water. Excretion of organic carbon by Warnstorfia algae or Daphnia longispina (the predominant crustacean in the pelagial water) provided only a minor contribution to bacterial production; therefore, a major contribution had to be from humic substances. A bacterial production efficiency of 31–38% could account for the microbial respiration in the water. The results indicated that bacterial, or detrital matter (originating largely from the littoral zone), could not obviate the need for algal food, and that a great deal of particulate matter in the water was poor or useless food for Daphnia. In all, the bulk of dissolved organic matter in Lake Mekkojärvi was biochemically highly recalcitrant. Our results indicate that humic substances (from watershed or littoral area) which, through bacterial degradation, enter the planktonic food web of the lake are mainly lost through respiration by microorganisms.     

Vulnerability of melanic Daphnia to brown trout predation

The co-existence of melanic Daphnia cf longispina and facultativelyplanktivorous brown trout is reported from a clear-water, alpinelake. This cooccurrence is uncommon, presumably due to the vulnerabilityof pigmented Daphnia to fish predation. Lake Bjornesfjorden(Norway) provided an opportunity to test this assumption. About20% of the fish caught in gill nets had fed on Daphnia. Thetrout exerted a marked selection for large-sized Daphnia prey,and a very strong selection for pigmented individuals relativeto transparent ones The persistence of a pigmented Daphnia populationprobably relies on limited recruitment and a low stock of thepredator, and the availability of more favourable benthic preyorganisms. ⁴Present address: Rådgivende Biologer A/S, Bredsgården,Bryggen, N-5003 Bergen, Norway     

Functional response and food selection of the water flea, Bosmina longispina

The functional response and the feeding behaviour of Bosminalongispina in relation to varying food concentrations were investigatedunder laboratory conditions using radioactively labelled Scenedcsmusacutus and different sized fluorescent latex beads to measureclearance rates and size selectivity. Bosmina longispina reducedfeeding activity at low concentrations of food, having a sigmoidfunctional response. Particles of 5.18 µm were filteredmuch more efficiently than both 1.04- and 0.25-µm particles,and the selectivity of large particles increased with increasingfiltering rates. Scenedesmus acutus was filtered at approximatelythe same rate as 5.18-µm latex beads. The increased selectivityat high filtering rates is explained as an adjustment in thefiltering apparatus to allow for greater water flow. Both thesefeeding responses would tend to reduce energy expenditure insituations when energy intake is low.     

Variation in life history responses of Daphnia to toxic Microcystis aeruginosa

Three clones of Daphnia pulex and two clones of Daphnia longispinawere exposed to toxic Microcystis aeruginosa for 21 days ina lifetable experiment. The growth and reproduction of individualdaphnids were followed daily to study the long-term effectsof toxic Microcystis. Exposure to Microcystis increased mortality,decreased growth, delayed maturation and decreased offspringproduction, indicating nutritional deficiency and toxic effects.We found variation in life history responses between speciesand among clones. Our results suggest that toxic cyanobacteriamay act as a modifying agent in zooplankton communities at boththe species and clonal level.     

Filtering rates on natural bacteria by Daphnia longispina and Eodiaptomus japonicus in Lake Biwa

Filtering rates on [³H]thymidine-labelled natural unattachedbacteria and that on [¹⁴C]bicarbonate-labelled natural planktonwhich pass through the 25 µm-mesh-size screen were measuredfor Daphnia longispina and Eodiaptomus japonicus in Lake Biwa.Errors associated with the radioisotope technique, i.e the lossof labels after feeding trials and the self-absorption of thebeta emittance of ³H, were checked and corrected for the calculationof the filtering rates. It was suggested that Daphnia collectsbacteria efficiently, although the efficiency is somewhat variabledepending on food particle composition (i.e. presence and absenceof larger particles) and feeding condition (i.e. animal densityand physical disturbance). By contrast, copepodites of Eodiaptomuswere suggested to be less efficient bacteria feeders. Food resourceexploitation strategies of these two co-existing zooplanktersare discussed.     

Cladoceran and rotifer grazing on bacteria and phytoplankton in two shallow eutrophic lakes: in situ measurement with fluorescent microspheres

Metazooplankton grazing on bacteria and on the phytoplanktonof various sizes was estimated in shallow eutrophic lakes Kaiavereand Võrtsjärv (Estonia) by in situ feeding experimentswith fluorescent microspheres (diameters 0.5 µm for bacteriaand 3, 6 and 24 µm for phytoplankton). Zooplankton communitycomposition, abundance and food density were important factorsdetermining grazing rates in these lakes. Cladocerans and rotifersfiltering rates (FR) and ingestion rates (IR) on bacteria andphytoplankton were several times higher in Lake Kaiavere wherebacterivorous rotifers and Daphnia contributed more to zooplanktonassemblage. While cladocerans were generally the main phytoplanktonconsumers, both lakes differed with respect to the groups ofbacterivores. Based on consumption of fluorescent microspheres,the metazooplankton grazing rates were relatively low and hadlow impact on production and standing stock of bacteria andingestible phytoplankton (<30 µm). On average, 0.5and 0.1% of standing stock of bacteria and 2.6 and 1.0% of standingstock of ingestible phytoplankton was grazed daily by metazooplanktonin lakes Kaiavere and Võrtsjärv, respectively. Thatcorresponded to daily grazing of 4.1% of the bacterial productionand 0.43% of the total primary production (PP) by metazooplanktonin Lake Kaiavere compared with 4.3 and 0.06% in Lake Võrtsjärv,respectively. The results suggest that the majority of consumptionof the bacterial and phytoplankton PP is most likely channelledthrough the microbial loop.     

Seasonal fluctuations in the biomass and metabolic activity of bacterioplankton and phytoplankton in a well-mixed estuary: the Ems-Dollard (Wadden Sea)

The biomass and metabolic activity of bacterioplankton weremeasured over 1 year in the Ems Dollard Estuary, a part of theDutch-German Wadden Sea. Very productive phyzoplankton blooms,composed mainly of diatoms and the haptophycean alga Phaeocystispouchetii, are a feature of the estuarine section studied. Theproduction of bacterial populations, as measured using the [³H]thymidinemethod broadly followed the phytoplankton density during bloomsin spring and late summer. There was no indication of a disproportionateincrease in bacterial production during the bloom or declineof particular algal species. The rate at which ¹⁴C-labelledglucose, glutamate and leucine were incorporated by bacterialpopulations, measured as a metabolic potential, varied seasonally,but did not precisely follow the rate of [³H]thymidine incorporation.The utilization of the absorbed ¹⁴C substrates for respirationor for cellular synthesis was constant over a prolonged periodof bacterial growth; apparent yields of 0.7, 0.4 and 0.8 weremeasured for glucose, glutamate and leucine, respectively. Inthe colder season most of the absorbed substrate was respired.The production by pelagic bacteria was 60 gC m^–2 y^–1,a value that amounted to 12% of the pelagic primary production.A preliminary experiment with a mixed culture of the abundantlyoccurring diatom species Thalassiosira excentrica and a marinespirillum, indicated that part of the algal exudates were rapidlyconverted by the bacteria, but another part resisted degradation.The incomplete bacterial degradation of algal exudates togetherwith the short residence time of the estuarine phytoplanktonmay contribute to the apparently incomplete mineralisation ofthe primary production in the water.     

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.     

Growth of Daphnia longispina L. in a polyhumic lake under various availabilities of algal,bacterial and detrital food

The availability and importance of food sources for growth of Daphnia longispina L. from a highly coloured fishless lake with anoxic hypolimnion were assessed by combining in situ and laboratory experiments. In in situ experiments populations were enclosed in tubes with natural temperature stratification and with or without anoxic hypolimnion. In the laboratory experiments the importance of food source (littoral zone vs pelagic epilimnion) was assessed by enclosing moss thalli and a natural zooplankton population in a large-scale flow-through system supplying food for experimental Daphnia. Growth of juveniles of Daphnia in epilimnetic water was determined in batch culture experiments and the importance of increasing concentrations of bacteria and algae for their growth and development was investigated with a small-scale flow-through system. Access to the anoxic hypolimnion enhanced the growth of Daphnia by 23–24%. Growth rates in the tubes with anoxic hypolimnion were 0.36 and 0.16 d^–1 in July and August respectively. In tubes without anoxia the corresponding values were 0.29 and 0.13. In batch-cultures the highest growth rate determined was 0.16 and the overall rates were lower than in in situ experiments. In batch culture Daphnia was able to grow in darkness for 10 days with a rate of 0.16. In the large-scale flow-through system Daphnia population fed with littoral water reproduced well despite the low concentration of algae and increased its number by a factor of c. 32 in 10 days. However, the animals were small and net production of Daphnia population thus lower under the littoral influence than in the control treatment. Population could survive and grew slowly on pelagial water processed by a natural zooplankton community and with very little algae left. It is thus possible that bacteria serve as a life-support system enabling the population survival over periods of algal shortage. Small-scale flow-through experiments revealed that Daphnia longispina is able to mature and reproduce on a bacterial diet if the food concentration is high enough and Daphnia on bacterial food could achieve growth rates similar to those on an algal diet. The threshold food level for Daphnia longispina was estimated to be c. 18.5 g C 1^–1. Detrital material is of limited value in nutrition of Daphnia even in a lake where more than 75% of carbon is bound in particulate detritus.     

Environment of planktic crustaceans as changed by hornwort: Effect on abundance of Daphnia

The effect of submersed hydrophyte Ceratophyllum demersum L. on the abundance of Daphnia longispina O.F. Müller has been studied. The experiments demonstrated that the hydrochemical characteristics in overgrowths of Ceratophyllum are within the limits of tolerance for D. longispina. The concentrations of waterborne phosphates, chlorophyll a, and bacterial biomass increase in the presence of hornwort. In plant overgrowths, Daphnia is suppressed due to the restriction of its movement. In a medium containing only metabolites of Ceratophyllum, the abundance of Daphnia longispina increases due to the enrichment of food resources.     

Empty loricas and the dynamics of Kellicottia longispina in a subalpine,oligotrophic lake

The seasonal dynamics of Kellicottia longispina in Lake Tahoe and an isolated embayment of Lake Tahoe, Emerald Bay, were investigated for an 18-month period in 1977–79. Population birth and death rates were similar in the two systems, although productivity and Mysis relicta densities were higher in Emerald Bay. The timing of population changes was also similar. A major population increase in late winter 1978 was preceded by an increase in egg ratio; the subsequent spring decline of K. longispina was concurrent with decreased birth rates and increased death rates. Empty loricas of K. longispina were occasionally abundant in the plankton samples and seemed to result from K. longispina deaths when densities were high and when egg ratios were declining; it is possible that population senescence was responsible for the high densities of empty loricas observed. A potentially important predator, M. relicta, defecates K. longispina remains in compact fecal pellets; however, it is unlikely that the observed empty loricas resulted from Mysis-related deaths.     

Biological productivity of Lake Towada,a north temperate,oligotrophic, kokanee fishery lake

During the Lake Towada survey from April through October 1998, we measured primary production at shore and offshore stations, and calculated crustacean zooplankton production from samples collected at the offshore station. We then analyzed these data and compared them with commercial fishery data in order to discuss the energy flow in this lake where kokanee (Oncorhynchus nerka) fisheries are one of the main businesses. At all stations, primary production was relatively high: 150–300mgCm^–2day^–1 in April–mid-June and lower at ca. 100mgCm^–2day^–1 thereafter. The variation in primary productivity could largely be explained by multiple regression models that included phytoplankton biomass and ambient nutrient conditions as independent variables. Among zooplankton, rotifers had their peak abundance in May, before the crustacean zooplankton (Daphnia longispina, Bosmina longirostris, and Acanthodiaptomus pacificus) population was well established. D. longispina dominated the crustacean zooplankton community in terms of biomass and production; their production during the study period made up 80% of crustacean community production (19.6gdry-wtm^–2), which was 40% of primary production during the survey. In July, when the abundance of D. longispina was particularly high, their daily production slightly exceeded daily primary production, which resulted in ca. 30% and 75% reduction in the amount of particulate organic carbon and chlorophyll a, respectively, during this period. The community ingestion rate of crustacean zooplankton, calculated from their daily production (D. longispina accounted for 90%) and the assumption that their gross production efficiency (K₁) was 60%, could roughly explain this reduction of particulate matter, corroborating previous studies that the grazing of D. longispina can significantly improve the water transparency of this lake. The catch of kokanee and pond smelt (Hypomesus transpacificus nipponensis, another important fish in the lake) during the survey corresponded to 1.1% of crustacean community production, and corresponded to 0.45% of the primary production, which is one of the highest recorded values. Bearing in mind that D. longispina was the major food item of planktivorous fish such as kokanee and pond smelt, the present study suggests that the energy transfer from phytoplankton to zooplankton to fish is outstandingly efficient, compared with other aquatic ecosystems, when D. longispina dominates in the lake.