Microbial food web in a large shallow lake (Lake Balaton, Hungary)

Seasonal variations of phyto-, bacterio- and colourless flagellate plankton were followed across a year in the large shallow Lake Balaton (Hungary). Yearly average chlorophyll-a concentration was 11 µg 1^–1, while the corresponding values of bacterioplankton and heterotrophic nanoflagellate (HNF) plankton biomass (fresh weight) were 0.24 mg 1^–1 and 0.35 mg 1^–1, respectively. About half of planktonic primary production was channelled through bacterioplankton on the yearly basis. However, there was no significant correlation between phytoplankton biomass and bacterial abundance. Bacterial specific growth rates were in the range of 0.009 and 0.09 h^–1, and ended to follow the seasonal changes in water temperature. In some periods of the year, predator-prey relationships between the HNF and bacterial abundance were obvious. The estimated HNF grazing on bacteria varied between 3% and 227% of the daily bacterial production. On an annual basis, 87% of bacterial cell production was grazed by HNF plankton.     

Turbidity enhances feeding abilities of larval Pacific herring,Clupea harengus pallasi

Fishes inhabiting estuaries, rivers, and embayments are subject to turbid conditions. Larvae of many fishes utilize estuaries as nursery areas. For visual plankton feeders such as larval fishes, turbidity may reduce search and reaction distances, resulting in lowered feeding abilities. In this study feeding Pacific herring larvae, Clupea harengus pallasi, were exposed to suspensions of estuarine sediment and Mount Saint Helens volcanic ash at concentrations ranging from 0 mg · l^–1 to 8 000 mg · l^–1. In all experiments, maximum feeding incidence and intensity occurred at levels of suspension of either 500 mg · l^–1 or 1000 mg · l^–1 with values significantly greater than controls (0 mg · l^–1). Feeding decreased at greater concentrations. The suspensions may enhance feeding by providing visual contrast of prey items on the small perceptive scale used by the larvae. Larval residence in turbid environments such as estuaries may serve to reduce predation from larger, visual planktivores, while searching ability in the small larval perceptive field is not decreased.     

The use of dialysis culture in a study of chlorophyll budget in a brackish lagoon,Japan

Growth rates of the entire phytoplankton community of a brackish lagoon in northeastern Japan were estimated by measuring increasing chlorophyll a content in dialysis bags during the summer and early autumn of 1986. The chlorophyll a contents of lagoon water fluctuated between 20 and 200 mg m^–3. At lower densities of phytoplankton (20–50 mg chl. a m^–3), growth rates (the rate of increase of chlorophyll a) exceeded 1 turnover per day, while at higher densities (more than 50 mg chl. a m^–3), the growth rate decreased rapidly. Tidal exchanges of chlorophyll a showed net exports of chlorophyll a from the lagoon to adjacent waters. The exchange rate of chlorophyll a was estimated to be 0.65 d^–1. At about 140 mg m^–3 of chlorophyll a concentration, the increase of chlorophyll in the lagoon water compensated for tidal export. Only a small proportion of primary production was consumed by zooplankton in the lagoon. There were also net exports of ammonium and phosphate from the lagoon. Nutrient flux from sediment exceeded the phytoplankton requirement and was the major source of the ammonium and phosphate exports from the lagoon. The low inorganic N/P atom supply ratio in the lagoon suggests that nitrogen is a major nutrient limiting phytoplankton growth.     

Effect of natural zooplankton feeding on the tissue and skeletal growth of the scleractinian coral<Emphasis Type="Italic"> Stylophora pistillata</Emphasis>

Laboratory experiments were designed to estimate the ingestion rates of the scleractinian coral Stylophora pistillata under varying prey concentrations and feeding regimes and to assess the effect of feeding on the tissue and skeletal growth. Six sets of corals were incubated under two light (80 and 300 µmol photons m^–2 s^–1) and three feeding levels (none, fed twice, and fed six times per week) using freshly collected zooplankton. Results showed that the number of prey ingested was proportional to prey density, and no saturation of feeding capability was reached. Capture rates varied between 0.5 and 8 prey items 200 polyp^–1 h^–1. Corals starved for several days ingested more plankton than did fed corals. Fed colonies exhibited significantly higher levels of protein, chlorophyll a, and chlorophyll c₂ per unit surface area than starved colonies. Feeding had a strong effect on tissue growth, increasing it by two to eight times. Calcification rates were also 30% higher in fed than in starved corals. Even moderate levels of feeding enhanced both tissue and skeletal growth, although the processes involved in this enhancement remain to be determined.     

Implications of element deficits for zooplankton growth

Daily measurements of mass and phosphorus (P) were made for Daphnia magna feeding on a high concentration (1 mg C l^–1) of P-sufficient (molar C:P = 70–80, `P+') or P-deficient (C:P = 1000, `P–') green algae (Scenedesmus obliquus or Ankistrodesmus falcatus). Animals feeding on P+ algae for 12 d showed a sharp decline in growth during the last juvenile instar (mean ± SE from 0.57 ± 0.02 to 0.25 ± 0.03 d^–1) and a modest decline in P-content at the first adult instar (from 1.55 ± 0.01 to 1.46 ± 0.02% mass). Animals feeding on P– algae showed declines in P-content to as low as 0.84% after 5 d, with the sharpest decline in the first day. Growth of animals feeding on P– algae declined sharply over time and showed a linear relationship with the grazer's P-content. Growth during the first day of exposure to P– algae was relatively high (mean 0.39 ± 0.03 d^–1) but approached zero as P-content declined to <0.90% mass. For animals feeding on P+ algae, the ontogenetic decline in P-content resulted in only a small difference between growth calculated for mass and growth calculated for Daphnia P. In contrast, for animals feeding for 4 d on P– algae, growth calculated for mass was 2–5 times higher than growth based on P. Thus, this study shows that short-term growth assays may substantially underestimate the negative impact of P-deficient resources, unless the decline in the grazer's P-content is taken into account. In this situation, measurement of growth in terms of nutrient content (e.g. grazer's P-content) should provide a better indication of the potential for sustained growth than calculations based on change in mass. This study also supports the `integration of growth' hypothesis (Sterner & Schwalbach, 2001), in which animals undergoing diel migration between water layers with P-deficient and P-sufficient resources experience a reduced growth penalty.     

Life cycle and growth ofTrichostegia minor (Curtis) in temporary woodland pools (Trichoptera: Phryganeidae)

Larvae of the caddisTrichostegia minor (Curtis) were collected from four woodland pools in The Netherlands, three of which are temporary, from August 1986 till June 1987. Eggs and larvae of this species proved to be very well adapted to drought, freezing, strongly fluctuating pH and alkalinity levels and prolonged oxygen deficit. The life cycle ofT.minor in a small woodland marsh overgrown byCalla palustris took one year. Adult flight period started at the end of May. Oviposition took place independent of water. Hatching of the eggs started in September and was probably induced by immersion. During the larval stage from September until May, 5 instars could be distinguished by the size of the head capsule. Growth of instars I, II and III during autumn was moderate. Most larvae overwintered as instar III or IV. Possibly there was a larval diapause during winter. In spring rapid growth to instar V took place prior to pupation. Growth rate, expressed as the increase of mean individual dry weight was highest from March to April (2.05±0.75% DW.m^–2.d^–1). In extremely shallow water growth in spring was initially more rapid compared to growth in deeper water. During winter the growth rate decreased to 0.038±0.071% DW.m^–2.d^–1. Net annual production based on the changes of momentary biomass was 183.2±31.7 mg DW.m^–2.y^–1 or 177.2±31.3 mg AFDW.m^–2.y^–1. Production loss during the winter season was 75.1±10.8 mg DW.m^–2.y^–1 or 72.3±10.6 mg AFDW.m^–2.y^–1.     

Seasonal cycle of the microbial plankton in Crooked Lake,Antarctica

Summary Changes in the abundance of the components of the microbial plankton between July 1990 and March 1991 in Crooked Lake, one of the largest and deepest freshwater lakes in Antarctica, are described. Chlorophyll a concentration is low (0.2–0.4g·1^–1) and there is no discernable spring increase. The phytoplankton is largely dominated by flagellates. Bacterioplankton exhibits a seasonal pattern of abundance ranging from 1.0 × 10⁸·1^–1 in July to 3.25 × 10⁸·1^–1 in September. Changes in bacterial abundance probably relate to temperature and grazing by heterotrophic and mixotrophic flagellates. Total flagellated protozoan concentrations ranged between 25–136 × 10²·l^–1. Autotrophic and heterotrophic flagellate abundances were coupled and peaks in their abundance oscillated with peaks in bacterioplankton concentration. Four species of ciliated protozoa, dominated by oligotrichs, particularly the plastidic Strombidium, inhabit the lake. The plankton is characterised by the presence of floes which act as loci for bacteria, flagellates and amoebae and feeding sites for the ciliates and the two sparce metazoan components of the plankton. Crooked Lake is extremely oligotrophic but nonetheless supports a plankton community with a low species diversity and simple trophodynamics.     

Feeding and growth of larval herring,Clupea harengus,in relation to density of copepod nauplii

Synopsis Feeding and growth rates of 1–3 wk old herring larvae from four different stocks were compared in laboratory experiments (8°C). For most of the larval groups, feeding rate was saturated at nauplii (Acartia tonsa, nauplii stages 3–5) densities over 301^–1 (5 g d.w. 1^–1). Specific growth rate increased asymptotically with nauplii density, and reached about 6% d^–1 at densities over 120 nauplii 1^–1. The growth rates attained in the laboratory were similar to field measured growth rates of similarly aged herring larvae at comparable food densities. Since food particles were homogenously distributed in the laboratory tanks, patches of dense plankton concentrations are, thus, apparently not necessary for larval growth and survival in the sea. Growth efficiency differed between larval groups, with large sized larvae being the most efficient in transforming ingested matter into growth. The difference probably relates to different sizes rather than to the different geographical origins of the larvae.     

Growth and nutrient uptake by two species of Elodea in experimental conditions and their role in nutrient accumulation in a macrophyte-dominated lake

The capacity of Elodea nuttallii (Planch.) St. John and Elodea canadensis Michx. to remove nitrogen from water was evaluated in laboratory experiment. The growth rate of plants and their effect on the nitrogen level of hypertrophic Lake Zwemlust (the Netherlands) as well as on lake water enriched with nitrogen were investigated. The plants grew best in water enriched with up to 2 mg NH₄-Nl^–1 and 2 mg NH₄-Nl^–1 plus 2 mg NO₃ Nl^–1. During a 14 day experiment, plants absorbed from 75% to 90% of nitrogen. Higher nitrogen concentration than 4 mg l^–1 had a negative effect on growth of both species. Elodea nuttallii and E. canadensis prefer NOinf4/^p+ over NOinf4/^p– when both ions were present in water in equal concentrations.     

Effect of zinc sulphate on the growth of mono- and multispecies cultures of some marine plankton algae

The effect of zinc sulphate has been investigated in mono- and multispecies cultures of the dinoflagellatesScrippsiella faeroense, Prorocentrum micans andGymnodinium splendens and of the diatomsSchroederella schroederi andThalassiosira rotula. Multiplication rate, in vivo chlorophyll fluorescence, maximum cell densities and Zn-conditioned disturbance of the species equilibrium of the multispecies cultures were used as criteria of sublethal toxic inhibition. In monocultures, the first effect became manifest after addition of 0.01 to 0.1 mg Zn⁺⁺ · l^–1. Diatoms proved to be more sensitive than dinoflagellates. In multispecies cultures, the growth of each species depended on the ratio of the inocula. Interrelation between interspecific competition and Zn-caused decrease in the number of algal cells regulated further growth of the cultures. Algal sensitivity to zinc increased with the number of species combined in the test medium: in a 5-species culture sublethal changes appeared already after addition of 0.005 to 0.01 mg Zn⁺⁺ · l^–1. In a few cases, interspecific competition depressed the growth of some species to an appreciable extent, even in the control cultures. At the resulting low cell numbers, the effect of zinc became apparent only in higher concentrations from 5 to 10 mg Zn⁺⁺ · l^–1. Morphological aberrations became manifest inScrippsiella faeroense and in the diatoms in concentrations from 1 and 0.01 mg Zn⁺⁺ · l^–1 respectively. The results show that multispecies experiments are a more sensitive test method for investigating the influence of zinc on plankton algae than are monoculture experiments. In natural plankton communities, however, the toxicity of heavy metals may become effective at considerably lower limit concentrations; this is suggested by the simplified model investigations in the laboratory.     

The relationship between food density and short term assimilation rates in Potamopyrgus antipodarum and Deleatidiumsp.

Much of the variation in individual growth rates can be attributed to differences in individual feeding rates. Therefore, in order to build predictive models of individual, or population growth, the factors influencing an individual's feeding rate must be described. An important determinant of the feeding rate is the relationship between the local abundance of food and the individual's ingestion rate – otherwise known as the functional response. We determined functional responses for two species of invertebrate grazers: the snail Potamopyrgus antipodarum and the mayfly Deleatidium sp., by measuring their assimilation rate with increasing densities of radiolabelled periphyton. The assimilation rates were consistent with the Holling Type II or Michelis Menten functional response curve. The parameters of the functional response yielded estimates of the search area and handling time for the stream invertebrates. Our functional response data indicate that the half-saturation food density for P. antipodarum and Deleatidium sp. were 980 mg and 3200 mg AFDM m^–2, respectively, suggesting that Deleatidium growth may be subject to food limitation more often than is P. antipodarum – despite the lower assimilation efficiency of the latter species.     

Food consumption of Sarotherodon mossambicus (Trewaves) exposed to sublethal concentration of diammonium phosphate

S. mossambicus was exposed to toxic and sublethal concentrations of the fertilizer diammonium phosphate (0.2 to 1.0 g l^–1). Mortality, food utilization and growth were studied. At a concentration of 0.6 g l^–1 DAP, 100% mortality was observed within 96 h; no mortality occurred at 0.5 g l^–1; LC50 was 0.55 g l^–1. Rearing the fish in increasing sublethal concentrations of DAP, it was found that the feeding rate decreased from 25.4 mg g^–1 fish^–1 d^–1 (fish reared in DAP-free water) to 10.1 mg g^–1 d^–1 at the highest sublethal concentration (0.5 g l^–1). Growth rate was drastically reduced. At high sublethal concentrations of DAP, the fish lost reserve energy, in addition to the energy obtained from food intake for survival, as a result of increased swimming activity and opercular beats.     

A model for growth of Artemia franciscana cultures based on food ration-dependent gross growth efficiencies

Laboratory cultures of Artemia franciscana grown under batch regimes at constant temperatures (28 °C) and salinity (35 g l^–1), three initial food concentrations (0.1, 0.4 and 1 M cells ml^–1), various daily food rations (0.1–9M Dunaliella tertiolecta cells Artemia ^–1), and different population densities (1–16 ind ml^–1) were used to develop a model of population growth. Growth rates and gross growth efficiencies (K ₁) were largely independent of population densities and initial food concentrations but determined by age and daily amount of food ingested. While maximum growth rates were found with the highest rations, K _{1 max} peaked at rations of 0.5 million cells d^–1 and decreased at feeding levels above this. A contour plot showing the trend relating K ₁ to Artemia size and ingestion rate in combination and was used to model growth in analogous controlled feeding conditions. Computer simulations using this model paralleled published results of final 15-day average individual sizes of Artemia. Optimal results for near constant food utilization are predicted for high initial population densities (100 Artemia nauplii ml ^–1) and daily culls of enough animals to equilibrate food demand with food availability. This strategy could permit a range of Artemia sizes harvested, maximize final individual sizes and retain high total yields (> 1.2 kg dry wt 1^–1). Effects of different culture strategies are discussed.     

Distribution of Gammarus lacustris Sars (Amphipoda, Gammaridae) in Lake Shira (Khakasia, Siberia) and laboratory study of its growth characteristics

Spatial distribution of Gammarus lacustris in Lake Shira and growth of young specimens that feed on lake biota were studied. The amphipods have been shown to inhabit the littoral, sublittoral and the upper aphytal zones of the lake on stony-sandy soil and silted sand. The young and adult individuals stay apart. The young live in submerged or semi-submerged vegetation in the littoral, the adults in the sublittoral and upper aphytal zones. Maximum density of amphipods was encountered in the areas influenced by human activity.The feeding experiments revealed that the lake plankton is a more important food source for G. lacustris than any other food species. The specific growth rate measured was 0.039 d^–1, with a length increment 0.095 mm d^–1.     

Differentiation during asexual reproduction and regeneration in a microturbellarian

Affordable biological technology for the reclamation of wastes and water of the waste streams from intensive livestock units is important in a country short of water. This study tested the concept of reclamation of waste by Streptocephalus macrourus (Crustacea: Anostraca) from the effluent of a high rate algal pond processing livestock wastes. S. macrourus showed a growth efficiency of 39% to 74% when fed optimal rations and cultured at densities between 10 and 400 1^–1. The maximum daily growth rates (0.15–0.21) approximate the growth rates of cladoceran or rotifer cultures managed for maximal biomass production. S. macrourus' ability to withstand crowding enabled the production from S. macrourus cultures (up to 91.8 mg dry mass l^–1 d^–1, or 1241 mg wet mass l^–1 d^–1) to exceed production recorded from cladoceran or rotifer cultures. Temperature influenced growth rate, with the highest growth rate occurring at 24 °C. The dilution rate of continuously fed cultures influenced growth rate, with the optimum dilution rate tested being 10 ml organism ^–1 d^–1. Mass mortality occurred when organisms were held at a density of 4000 l^–1. S. macrourus is able to convert algae grown on livestock waste efficiently into anostracan biomass, and is able to give a very high daily production.     

Predator–prey interactions between a planktonic ciliate Strombidium sp. (Ciliophora, Oligotrichida) and the dinoflagellate Pfiesteria piscicida (Dinamoebiales, Pyrrophyta)

The functional response of a planktonic ciliate, Strombidium sp. feeding on the dinoflagellate Pfiesteria piscicida non-toxic zoospores (NTZ) was experimentally studied with four different prey concentrations (43–3153 cells ml⁻¹). Data from direct observations (NTZ inside individual Strombidium sp.) was used to calculate predator–prey specific ingestion and clearance rates. The ingestion rates varied between 0.68 and 14.26 NTZ ind⁻¹ h⁻¹, and with the predator–prey specific handling time of 2.83 min the U_max was 21.18 NTZ ind⁻¹ h⁻¹. The increase in the prey concentration between approximately 700 and 3000 NTZ ml⁻¹ did not increase the uptake of prey, and at the lowest Pfiesteria NTZ concentrations the feeding efficiency of Strombidium sp. was lowered, possibly indicating a situation of threshold feeding. When data from direct observations of ingested Pfiesteria NTZ were compared with values of total NTZ loss from the experimental water during the experiment, ingestion was found to represent only a fraction of the total NTZ loss in the presence of ciliates. This discrepancy was concluded to be due to other grazer related factors than actual ciliate grazing. The control of the initial growth of Pfiesteria community, in a pre-bloom situation, would require only a small ciliate abundance (less than 5 ml⁻¹), but when the Pfiesteria NTZ are scarce, relatively more ciliates are needed to limit the population growth of the dinoflagellate community because of the apparent feeding threshold. It is concluded that the formation of non-toxic P. piscicida blooms require periods of low grazing pressure or a means to escape grazing.     

Distribution, abundance, behavior and metabolism of Periphylla periphylla, a mesopelagic coronate medusa in a Norwegian fjord

The distribution, behavior and metabolism of the mesopelagic jellyfish, Periphylla periphylla (Péron & Lesueur), were investigated in Lurefjorden, Norway. Field studies, conducted in 1998–1999 with plankton nets and a remotely operated vehicle, indicated that 80-90% of the dense (up to 2.5 m^–3) population migrated 200–400 m vertically each day throughout the year. In situ observations with red light revealed that swimming rates and feeding activity varied with age and time of day. Detection of turbulence and contact with surfaces caused this medusa to conceal one or all of its tentacles in the stomach or to shed nematocyst-laden tissue from the tentacles. Stomachs of medusae collected with nets were often full of prey entangled with the sloughed tissue. Stomachs of medusae captured individually with ROV samplers were empty or contained only a few prey in their stomachs (typically, 1–4 copepods Calanus spp. or chaetognaths Eukrohnia hamata Möbius per medusa). Low rates (0.4–5.6 l O₂ mg C^–1 h^–1) of oxygen consumption of P. periphylla suggested that this species was sustained by relatively few (1–34) prey d^–1.     

Optimization of glycerol feeding for clavulanic acid production by Streptomyces clavuligerus with glycerol feeding

Glycerol at 10–20 g l^–1 increased clavulanic acid production by Streptomyces clavuligerus in shake-flask culture. The biosynthesis of clavulanic acid continued for longer by feeding glycerol and production increased to 250 mg l^–1 compared with 115 mg l^–1 without feeding. In fermenter batch culture, degradation of clavulanic acid began after 72 h. With glycerol feeding in fed-batch culture, clavulanic acid production was not only increased further to about 280 mg l^–1 but also remained stable up to 130 h.     

Bioconversion of compactin into pravastatin by Streptomyces sp.

Streptomyces sp. Y-110, isolated from soil, modified compactin to pravastatin, a therapeutic agent for hypercholesterolemia. In a batch culture, the highest production of pravastatin was 340 mg l^–1 from 750 mg compactin l^–1 in 24 h. By intermittent feeding of compactin into the culture medium, both the compactin concentration and its conversion increased to 2000 mg l^–1 and 1000 mg pravastatin l^–1, respectively, with the conversion rate of 10 mg l^–1 h^–1. Continuous feeding of compactin increased production of pravastatin to 15 mg l^–1 h^–1.     

Factors influencing the phytoplankton steady state assemblages in a drinking-water reservoir (Ömerli reservoir, Istanbul)

In situ growth of heterotrophic nanoflagellates (HNF) in Lake Donghu, a eutrophic shallow lake in mainland China, was studied from January 1999 to March 2000 using a modified Weisse protocol. The study results indicated that the growth rates of HNF showed pronounced seasonal variation (–0.37–1.25 d^–1), reaching the maximum during spring to early summer. When the water temperature was higher than 25.5°C, HNF growth was inversely proportional to water temperature. There was an effect by bacterial abundance and autotrophic picoplankton on HNF growth that depended on location. HNF biomass was the highest in late spring, and the HNF production ranged from –2.25 to 35.45 mg l^–1 d^–1 with mean of 3.17 mg l^–1d^–1. When considered in the context of biomass and production data for zooplankton in Lake Donghu, it was evident that HNF contributed significantly to the total zooplankton production in Lake Donghu. These in situ studies indicate that temperature and food supply are the major determinants of HNF abundance and productivity.