Bo Riemann

Qualitative importance of the microbial loop and plankton community structure in a eutrophic lake during a bloom of cyanobacteria.

Plankton community structure and major pools and fluxes of carbon were observed before and after culmination of a bloom of cyanobacteria in eutrophic Frederiksborg Slotssø, Denmark. Biomass changes of heterotrophic nanoflagellates, ciliates, microzooplankton (50 to 140 μm), and macrozooplankton (larger than 140 μm) were compared to phytoplankton and bacterial production as well as micro- and macrozooplankton ingestion rates of phytoplankton and bacteria. The carbon budget was used as a means to examine causal relationships in the plankton community. Phytoplankton biomass decreased and algae smaller than 20 μm replacedAphanizomenon after the culmination of cyanobacteria. Bacterial net production peaked shortly after the culmination of the bloom (510 μg C liter−1 d−1 and decreased thereafter to a level of approximately 124 μg C liter−1 d−1. Phytoplankton extracellular release of organic carbon accounted for only 4–9% of bacterial carbon demand. Cyclopoid copepods and small-sized cladocerans started to grow after the culmination, but food limitation probably controlled the biomass after the collapse of the bloom. Grazing of micro- and macrozooplankton were estimated from in situ experiments using labeled bacteria and algae. Macrozooplankton grazed 22% of bacterial net production during the bloom and 86% after the bloom, while microzooplankton (nauplii, rotifers and ciliates larger than 50 μm) ingested low amounts of bacteria and removed 10–16% of bacterial carbon. Both macro-and microzooplankton grazed algae smaller than 20 μm, although they did not control algal biomass. From calculated clearance rates it was found that heterotrophic nanoflagellates (40–440 ml−1) grazed 3–4% of the bacterial production, while ciliates smaller than 50 μm removed 19–39% of bacterial production, supporting the idea that ciliates are an important link between bacteria and higher trophic levels. During and after the bloom ofAphanizomenon, major fluxes of carbon between bacteria, ciliates and crustaceans were observed, and heterotrophic nanoflagellates played a minor role in the pelagic food web.

Bacterial Secondary Production in Freshwater Measured by 3H-thymidine Incorporation Method

Bacterial secondary production in lake water was measured by3H-thymidine incorporation into DNA. The application of the method to freshwater systems studied required (1) thymidine concentration > 10 nM (10–25 nM) evaluated from isotope dilution by varying the specific activity of labeled thymidine, (2) short incubation periods less than 1 hour, and (3) partial purification of the DNA fraction for measuring3H-thymidine incorporation. During 2 diel studies, bacterial productivity was compared to phytoplankton primary production and extracellular release of organic carbon. Diel changes in bacterial growth suggested substantial activity during the morning and evening. Possible mechanisms of control of bacterial growth, such as extracellular release of organic carbon, are discussed.

Primary production, nutrient assimilation and microzooplankton grazing along a hypersaline gradient

As part of an investigation of the relationship between diversity and productivity, measurements were made in a solar saltern of carbon fixation, nitrate and ammonium uptake and microzooplankton grazing at salt concentrations ranging from 4 to 37%. Elevated photosynthetic pigment concentrations were present in ponds of intermediate (5-11%) and high (>32%) salinity but rates of primary production and nutrient uptake were generally reduced at the highest salinity. Maximum primary production was measured at 8% salinity and chlorophyll-specific carbon fixation also maximised at this salinity. Ammonium was the dominant nitrogen source throughout the salinity gradient; turnover times of ammonium were from 2 to 14 days. Nitrate turnover times were very long ( approximately 100 days) at salinities <22% but at 37% salinity, nitrate was taken up rapidly by the microbial assemblage in the light and turnover times for the ambient nitrate concentrations in the 37%-salinity pond were between 6 and 12 days. There were large changes in C:N uptake ratio. At salinities <11%, the C:N uptake ratio was higher than the Redfield ratio. However, at >22% salinity, the C:N uptake ratio was approximately 1. That is, much more nitrate and ammonium were taken up than would be expected from the observed carbon-fixation rates. Although primary production declined with decreasing phytoplankton diversity along the salinity gradient, there was no clear relationship between heterotrophic activity and microbial biodiversity.

Diel variation in concentration, assimilation and respiration of dissolved free amino acids in relation to planktonic primary and secondary production in two eutrophic lakes

Concentration of dissolved free amino acids (DFAA) and assimilation of the 5 most abundant DFAA (glutamic acid, serine, glycine, alanine and ornithine) were measured at 3-h intervals over 27 h in two Danish, eutrophic lakes. The carbon flux of the amino acid assimilation was compared with the major routes of carbon flux, including primary production, bacterial production and zooplankton grazing. In Frederiksborg Slotssø, the mean DFAA concentration was 275 nM with distinct peaks (up to 783 nM) 3 h after sunrise. Assimilation rates of the 5 amino acids amounted on the average to 2.03 µg Cl−1 h−1, but high values up to 7.41 µg Cl−1 h−1 occurred 3 h after sunrise and at midnight. The mean turnover time of the amino acid pools was 3.2 h. In Lake Mossø, the mean DFAA concentration was 592 nM with peak of 1 161 nM at dusk. The assimilation rate averaged 0.44 µg Cl−1 h−1, and the mean turnover time of the amino acid pools was 39 h. In Lake Mossø, similar turnover times of glutamic acid and serine were determined from the 14C-amino acid tracer technique and Michaelis-Menten uptake kinetics, indicating that the tracer technique gave reliable values of the actual assimilation. The average respiration percentages of the assimilated amino acids were 45% in Frederiksborg Slotssø and 51% in Lake Mossø. Extracellular organic carbon (EOC) released from the phytoplankton contributed DFAA to the water. In Lake Mossø, 81% of the ambient EOC pool was <700 daltons and 9.3% of the EOC was DFAA. This corresponded to about 2.4% of the DFAA pool. Bacterial productivity, determined by means of ‘frequency of dividing cells’ and 35S-SO4 dark uptake techniques gave similar results and constituted 4.5 and 3.7 µg Cl−1 h−1 in Frederiksborg Slotssø and Lake Mossø, respectively. The bacterial productivity suggested that DFAA were essential substrates to the bacteria, especially in Frederiksborg Slotssø. The zooplankton biomass in Frederiksborg Slotssø was six times larger than that in Lake Mossø, but cladocerans were dominant in both lakes. The zooplankton grazing probably was an important regulatory factor for the bacterial productivity.

Zooplankton induced changes in dissolved free amino acids and in production rates of freshwater bacteria

This study examined the importance of zooplankton in the flux of dissolved free amino acids (DFAA) in the water and into bacteria. DFAA release rates were followed in laboratory grazing experiments usingDaphnia galeata andEudiaptomus graciloides as grazers, andScenedesmus acutus andSynechococcus elongatus as food sources. Except for minor initial peaks, DFAAs were released continuously during the first 2 hours and made up 6–12% (in one experiment 50%) of the calculated ingestion rates. During three diel studies in lakes, effects of removal and increase of the density of zooplankton (>200μm) on the pools of DFAA as well as on the bacterial production were followed. During two of the diel studies, higher DFAA pools were measured when 3–4 times the natural zooplankton density was present, and in one study a minor increase also occurred in the bacterial production, compared with results from experiments without zooplankton and with a natural zooplankton density. The increase in bacterial growth coincided with a decline in DFAA. During the third study, neither DFAA nor the bacterial production changed significantly when the zooplankton density was increased 3 times. Removal of zooplankton, however, caused a decline in both DFAA and bacterial production. Our data suggest a close relationship between occurrence of zooplankton and release of DFAA, but the factors regulating the amount of DFAA released and its effect on bacterial growth are not yet understood.

The Role of Mixotrophy in Pelagic Environments

This paper reviews the occurrence and ecological importance of mixotrophic flagellates and ciliates in pelagic environments, particularly in marine ecosystems. Mixotrophy is here defined as the combination of photoautotrophic and heterotrophic nutrition in a single individual, often used in the restricted sense of combining photosynthesis and phagotrophy. Mixotrophic protists represent an alternative strategy that allows a shortcut between the traditional food web and the microbial loop. A large number of reports have been published on the ecological importance of mixotrophic flagellates in freshwater, yet only a few studies have been carried out in seawater. In contrast, most of the knowledge of mixotrophic ciliates comes from marine environments. Results from field studies have demonstrated that both mixotrophic flagellates and ciliates are commonly found in many marine environments, and mixotrophic flagellates can dominate the biomass of photoautotrophs and be responsible for the entire grazing of bacteria or protists. Results from laboratory experiments on factors controlling the degree of photoautotrophy/phagotrophy in flagellates are presented. Finally, we present a hypothesis for a growth strategy of bacterivorous mixotrophic flagellates.

Growth efficiencies of freshwater bacterioplankton

The growth efficiency of freshwater bacteria was examined in continuous cultures. One series of experiments was carried out using generation times from 50 to 200 hours and aged, normal, and enriched media, all of natural origin. Another series of experiments examined the bacterial growth efficiency during the growth season in eutrophic Frederiksborg Slotssø, in relation to changes in the planktonic communities and to factors controlling the bacterial incorporation of 3H-thymidine. Attachment of bacteria to the inner surfaces of the experimental flasks was examined using various types of bottles, adding glass tubes to the bottles, and measuring 3H-thymidine incorporation and direct cell counts of attached and free-living bacteria. Attachment of bacteria varied, and in one example up to 36% of the thymidine incorporation was by attached bacteria after 4 days. It was calculated that 36% of attached bacteria caused an underestimation of the growth efficiency of 11%. The mean growth efficiency tended to decrease with generation time using enriched medium (47 to 19%) and aged medium (35 to 12%), and tended to decrease with medium quality (enriched > normal > aged media) from 37% to 27%. The only significant difference in growth efficiency occurred in relation to generation time, in samples with enriched medium (unpaired t-test, P < 0.05). The overall mean value for all generation times and media was 30% (SEM = 3%, n = 24). From April to October, the growth efficiency was determined 5 times in samples from Frederiksborg Slotssø. The overall mean value was 31% (SEM = 3%, n = 30), and there was no significant change in the growth efficiency during the period measured. In June, three bioassay experiments revealed that carbon limitation controlled bacterial incorporation of 3H-thymidine, whereas additions of phosphate and nitrate did not change the incorporation rates. The narrow range of growth efficiencies obtained in this study (mean 31%, SEM = 2%, n = 54) suggests that changes in substratequality in the media applied and in the eutrophic samples examined causes only subtle changes in the growth efficiency.

Pelagic food web processes in an oligotrophic lake

Major pelagic carbon pathways, including primary production, release of extracellular products (EOC), bacterial production and zooplankton grazing were measured in oligotrophic Lake Almind (Denmark) and in enclosures (7 m3) subjected to artificial eutrophication. Simultaneous measurements at three days interval of carbon exchange rates and pools allowed the construction of carbon flow scenarios over a nineteen day experimental period.The flow of organic carbon was dominated by phytoplankton EOC release, which amounted from 44 to 58% of the net fixation of inorganic carbon. Gross bacterial production accounted for 33 to 75% of the primary production. The lower values of EOC release (44%) and bacterial production (33%) were found in the enclosures with added nutrients. The release of recently fixed photosynthetic products was the most important source of organic carbon to the bacterioplankton. Uptake of dissolved free amino acids was responsible for 52 to 62% of the gross bacterial production. Thus, amino acids constituted a significant proportion of the EOC. Zooplankton (< 50 µm) grazing on algae and bacteria accounted only for a minor proportion of the particulate production in May. Circumstantial evidence is presented that suggests the chrysophycean alga Dinobryon was the most important bacterial remover.The results clearly demonstrated EOC release and bacterial metabolism to be key processes in pelagic carbon cycling in this oligotrophic lake.

Ecological consequences of a manual reduction of roach and bream in a eutrophic, temperate lake

A biomanipulation experiment was carried out in the eutrophic lake, Frederiksborg Slotssø (Denmark). During 1987 and 1988, densities of roach (Rutilus rutilus) and bream (Abramis brama) were reduced, using seine and pounding nets, and large-sized perch (Perca fluviatilis) were added instead. Nutrients, oxygen, phytoplankton, zooplankton, fish and zoobenthos were measured two years after the manipulation and compared with results obtained two years before the manipulation.A total amount of 6524 kg wet weight of roach and bream was removed. Roach and bream constituted 45% of the total fish biomass after the reduction, compared with 78% before the manipulation. Recruitment of roach decreased, and mortality rates of young-of-the-year perch were lower after the fish reduction. After the manipulation, decreases in phytoplankton biomass coincided with increases in zooplankton biomass during spring and autumn periods, although, the mid-summer level of the biomass of cyanobacteria did not change. Inorganic nutrients generally increased, but no significant changes were found, either in the oxygen budget or in the community structure or quantitative distribution of zoobenthos after the fish manipulation. Although the effects of the fish manipulations were not as pronounced as those found in lakes with lower nutrient regimes, the results indicate positive changes in the water quality. Nevertheless, it is probably necessary to continue a fish reduction programme to maintain or further improve the water quality.

Bacterial Growth in Relation to Phytoplankton Primary Production and Extracellular Release of Organic Carbon

Carbon flow in pelagic waters is dominated by phytoplankton photosynthesis and the subsequent transport and decomposition by heterotrophic organisms of the produced organic matter. Present understanding of the interactions between autotrophic and heterotrophic organisms suffers from limited knowledge of the secondary production. One major problem is the part played by bacteria in returning dissolved organic carbon to organic particles which can be utilized by higher trophic levels (Pomeroy 1974). In this context it is especially important to elucidate the dynamic relationship between phytoplankton, bacteria and zooplankton and the extent to which such interactions are controlled by extracellular organic carbon released by the phytoplankton.