Heterotrophic Microplankton Research Articles

<p align="center">Comparison of microplankton heterotrophic-photoautotrophic balance based on the content of ATP and chlorophyll a in the plankton of the northern area of the Black Sea during the autumn and spring seasons

Based on materials on the distribution of microplankton ATP and chlorophyll a concentrations in the euphotic zone of the Black Sea, collected on expeditionary cruises R / V “Professor Vodyanitsky” at October 2016 and at March-April 2017, the heterotrophic photoautotrophic index (HPI) reflecting the ratio of heterotrophic biomass and its photoautotrophic parts of the microplankton community was calculated. The interest in comparing precisely these seasons is due to the fact that they are similar in hydrophysical conditions for the development of the community. Water trophicity was estimated by ATP concentrations as an indicator of the metabolically active microplankton biomass. It has been demonstrated that at the autumn season, the studied waters of the landfill can be estimated as mesotrophic, in the spring they are close to eutrophic. With estimation by HPI, at the autumn season heterotrophic microplankton was dominated in most of the water area, and at spring were parity ratios of heterotrophic and photoautotrophic microplankton biomass.

Future climate scenarios for a coastal productive planktonic food web resulting in microplankton phenology changes and decreased trophic transfer efficiency.

We studied the effects of future climate change scenarios on plankton communities of a Norwegian fjord using a mesocosm approach. After the spring bloom, natural plankton were enclosed and treated in duplicates with inorganic nutrients elevated to pre-bloom conditions (N, P, Si; eutrophication), lowering of 0.4 pH units (acidification), and rising 3°C temperature (warming). All nutrient-amended treatments resulted in phytoplankton blooms dominated by chain-forming diatoms, and reached 13–16 μg chlorophyll (chl) a l−1. In the control mesocosms, chl a remained below 1 μg l−1. Acidification and warming had contrasting effects on the phenology and bloom-dynamics of autotrophic and heterotrophic microplankton. Bacillariophyceae, prymnesiophyceae, cryptophyta, and Protoperidinium spp. peaked earlier at higher temperature and lower pH. Chlorophyta showed lower peak abundances with acidification, but higher peak abundances with increased temperature. The peak magnitude of autotrophic dinophyceae and ciliates was, on the other hand, lowered with combined warming and acidification. Over time, the plankton communities shifted from autotrophic phytoplankton blooms to a more heterotrophic system in all mesocosms, especially in the control unaltered mesocosms. The development of mass balance and proportion of heterotrophic/autotrophic biomass predict a shift towards a more autotrophic community and less-efficient food web transfer when temperature, nutrients and acidification are combined in a future climate-change scenario. We suggest that this result may be related to a lower food quality for microzooplankton under acidification and warming scenarios and to an increase of catabolic processes compared to anabolic ones at higher temperatures.

Small time scale plankton structure variations at the entrance of a tropical eutrophic bay (Guanabara Bay, Brazil)

The dynamics of the plankton compartments at the entrance of Guanabara Bay (SE Brazil) were assessed during a short-term temporal survey to estimate their trophic correlations. Size-fractioned phytoplankton (picoplankton: < 2µm, nanoplankton: 2-20µm and microplankton: > 20µm) biomass and photosynthetic efficiency, composition and abundance of the auto-and heterotrophic nano-and microplankton, and mesozooplankton were evaluated at a fixed station for 3 consecutive days at 3-h intervals, in the surface and bottom (20m) layers. The variability of almost all plankton compartments in the surface layer was directly dependent on temperature, indicating the great influence of the circulation at the entrance of the bay on plankton structure. In the surface layer, the mesozooplankton seems to be sustained by both autotrophic nano-and picoplankton, this last being channeled through the microzooplankton. Near the bottom, both auto-and heterotrophic microplankton are probably supporting the mesozooplankton biomass. Our findings thus suggest that the entrance of Guanabara bay presents a multivorous food web, i.e., a combination of both grazing and microbial trophic pathways.

Microbial population responses in three stratified Antarctic meltwater ponds during the autumn freeze

AbstractThe planktonic microbial communities of three meltwater ponds, located on the McMurdo Ice Shelf, were investigated from the end of January 2008 to early April, during which almost the entire pond volumes froze. The ponds were comprised of an upper mixed layer overlying a salt-stabilized density gradient in which planktonic communities were primarily embedded. Plankton comprised all components of the “microbial loop”, though carnivorous protists were rare. As the ponds froze and light became increasingly limited, it was expected conditions would induce physiological changes altering the functional role of autotrophic and heterotrophic microplankton within the ponds. The results showed that microbial groups responded to the onset of winter by declining in abundance, though an exception was the appearance of filamentous cyanobacteria in the water column in March. As freezing progressed, autotrophs declined more rapidly than heterotrophs and grazing rates and abundances of mixotrophic and heterotrophic organisms increased. Grazing pressure on bacteria and picophytoplankton also increased, in part explaining their decline over time. The results indicate that stressors imposed during freezing select for increasing heterotrophy within the remaining microbial communities, although all components of the food web eventually decline as the final freeze approaches.

Vertical niche partitioning between cryptic sibling species of a cosmopolitan marine planktonic protist

A large portion of the surface-ocean biomass is represented by microscopic unicellular plankton. These organisms are functionally and morphologically diverse, but it remains unclear how their diversity is generated. Species of marine microplankton are widely distributed because of passive transport and lack of barriers in the ocean. How does speciation occur in a system with a seemingly unlimited dispersal potential? Recent studies using planktonic foraminifera as a model showed that even among the cryptic genetic diversity within morphological species, many genetic types are cosmopolitan, lending limited support for speciation by geographical isolation. Here we show that the current two-dimensional view on the biogeography and potential speciation mechanisms in the microplankton may be misleading. By depth-stratified sampling, we present evidence that sibling genetic types in a cosmopolitan species of marine microplankton, the planktonic foraminifer Hastigerina pelagica, are consistently separated by depth throughout their global range. Such strong separation between genetically closely related and morphologically inseparable genetic types indicates that niche partitioning in marine heterotrophic microplankton can be maintained in the vertical dimension on a global scale. These observations indicate that speciation along depth (depth-parapatric speciation) can occur in vertically structured microplankton populations, facilitating diversification without the need for spatial isolation.

Microbial dynamics and flagellate grazing during transition to winter in Lakes Hoare and Bonney, Antarctica

The planktonic microbial communities of Lakes Hoare and Bonney were investigated during transition into winter. We hypothesized that the onset of darkness induces changes in the functional role of autotrophic and heterotrophic microplankton. Bacteria decreased in Lake Hoare during March-April, while in Lake Bonney bacterial abundances varied. Heterotrophic nanoflagellates (HNAN), phototrophic nanoflagellates (PNAN) and ciliates showed no marked decline with the onset of winter. PNAN outnumbered HNAN in both lakes. Grazing rates of HNAN in Lake Hoare ranged up to 30.8 bacteria per cell day(-1). The HNAN community grazed between 3.74 and 36.6 ng of bacterial carbon day(-1). Mixotrophic PNAN had grazing rates up to 15.2 bacteria per cell day(-1), and their daily community grazing exceeded bacterial production. In Lake Bonney East, PNAN grazing rates ranged up to 12.48 bacteria per cell day(-1) and in Lake Bonney West up to 8.16 bacteria per cell day(-1). As in Lake Hoare, the mixotrophic PNAN grazing rates (up to 950 ng C day(-1)) usually exceeded bacterial production. HNAN grazing rates were generally similar to those in Lake Hoare. As winter encroaches, these lakes move progressively towards heterotrophy and probably function during the winter, enabling populations to enter the short austral summer with actively growing populations.

Heterotrophic microplankton in the lower Hudson River Estuary: potential importance of naked, planktonic amebas for bacterivory and carbon flux

The present study is the first to simultaneously document the contributions of bacteria, heterotrophic flagellates, ciliates, and naked, planktonic amebas to the carbon (C) budget of an estu- arine water column, and is also the first study of protistan bacterivory in the lower Hudson River Estu- ary (HRE). Observations were collected at a single near-shore location between June 2006 and May 2009. Bacterial counts and biomass varied approximately 1 order of magnitude on different dates, but were comparable to previous studies of the HRE and other estuaries. Of the 3 heterotrophic protist groups enumerated, heterotrophic nanoflagellates were the least variable and generally had the highest biomass (on average equaling 38% of the bacterial biomass). Counts and biomasses of cili- ates and amebas were highly variable, ranging over at least 3 orders of magnitude between sampling dates. Much of the variability in ameba abundance was consistent with previous observations of sea- sonality. Ciliate biomass averaged 8%, and ameba biomass averaged 15% of the bacterial biomass. Thus, at this location, the importance of amebas as micropredators may be comparable to that of the ciliates, a group generally receiving greater research attention. Ameba ingestion rates could not be measured directly but 3 indirect approaches for calculating ingestion rates produced mean values ranging from 1.2 to 2.5 ng C d -1 ng -1 ameba biomass. Each approach demonstrated that ameba C consumption at the study location was highly variable, but was at times high relative to the bacterial standing stock. Taken together, these data suggest that amebas may be more common and of greater importance in estuarine C-fluxes than generally appreciated.

Similar potential ATP-P production and enzymatic activities in the microplankton community off Concepción (Chile) under oxic and suboxic conditions

The effects of the oxygen minimum zone on the metabolism of the heterotrophic microplankton community (0.22–100 μm) in the Humboldt Current System, as well as the factors controlling its biomass production, remain unknown. Here we compare the effect of four sources of dissolved organic carbon (glucose, oxaloacetate, glycine, leucine) on microbial biomass production (such as ATP-P) and the potential enzymatic activities involved in catabolic pathways under oxic and suboxic conditions. Our results show significant differences ( p < 0.05) in the ATP-P production when induced by the different substrates that are used as dissolved organic carbon herein. The induction of ATP-P production is enhanced from glucose < oxaloacetate < glycine < leucine. Nevertheless, for individual substrates, no significant differences were found between incubation under oxic and suboxic conditions except in the case of leucine. For this amino acid, the induction of ATP-P synthesis was higher under suboxic than oxic conditions. The data sets of all the substrates used showed greater potential ATP-P production under suboxic than oxic conditions. The results of the potential enzymatic activities suggest that malate dehydrogenase has the highest signal of NADH oxidization activity in the microbial assemblage. Furthermore, for all experiments, the malate dehydrogenase activity data set had a significant relationship with ATP-P production. These findings suggest that the microbial community inhabiting the oxygen minimum zone has the same or greater potential growth than the community inhabiting more oxygenated strata of the water column and that malate dehydrogenase is the activity that best represents the metabolic potential of the community.

Pacific bluefin tuna, Thunnus orientalis, larvae utilize energy and nutrients of microbial loop

Prey–predator interactions among planktonic organisms in larval tanks of bluefin tuna and red sea bream were investigated quantitatively, and carbon flow models were constructed from the results of feeding experiments. Two different carbon pathways were uncovered in the rearing water of both fish species. One was from autotrophic nano-plankton (ANP) to rotifers to fish larvae (artificial food chain) and the other was from bacteria to heterotrophic nano-plankton (HNP) to heterotrophic micro-plankton (HMP) (a microbial loop). Surprisingly, bluefin tuna selectively consumed HMP, especially dinoflagellates, throughout the experiment. In the bluefin tuna tank, the microbial loop was linked to the artificial food chain. On the other hand, red sea bream larvae did not consume HMP and the pathway in the red sea bream tank was more straightforward than in the bluefin tuna tank. Energy and nutrients transferred to red sea bream larvae via the artificial food chain only, except for 3 days after hatching. Biomass of micro-protozoa were high (3.2–158.4 ng C ml − 1) in the bluefin tuna tank with higher water temperature (26.0–27.6 °C) and low (3.0–4.9 ng C ml − 1 ) in the red sea bream tank with lower temperature (19.8–20.1 °C). The study suggests that difference in water temperature is a significant factor for the microbial loop in rearing tanks of fish larvae. This is the first report suggesting that a microbial loop, established naturally, contributes energy and nutrient gain to bluefin tuna larvae reared in an artificially controlled environment.

Mesopelagic microplankton of the Arabian Sea

The Arabian Sea is notable for its dramatic monsoonal effects on euphotic zone biogeochemical processes and the large spatial extent of its mesopelagic oxygen minimum zone. As part of the US Joint Global Ocean Flux Study Arabian Sea project, we sampled microplankton (organisms 20–200 μm including diatoms, dinoflagellates, ciliates, sarcodines and nauplii) at five depths from 250 to 1000 or 1100 m at six stations during four seasonal cruises in 1995. Abundances of groups of organisms at discrete depths averaged 1–2 l −1 seasonally. Mean seasonal integrated biomass of the assemblage was 29 mg C m −2 during the late Northeast Monsoon, 37 mg C m −2 during the Spring Intermonsoon, 47 mg C m −2 during the late Southwest Monsoon and 49 mg C m −2 during the early Northeast Monsoon. Overall, protozoans dominated the mesopelagic microplankton assemblage. Integrated biomass peaked during the late SW Monsoon at two stations as expected if microplankton responded to surface productivity and mesopelagic organic carbon fluxes. At three stations, microplankton biomass peaked during the early NE Monsoon; this may reflect a continuing response to SW Monsoon productivity signals by these larger, slow-growing organisms. Protozooplankton abundance did not appear to be negatively affected by low (<0.1 ml dissolved O 2 l −1) oxygen, whereas naupliar abundance and biomass were higher where oxygen concentration was higher. Total microplankton biomass was highest where oxygen concentrations and also mesozooplankton biomass were lowest, suggesting that predation also played a role in microplankton distributions. Calculations based on allometric relationships indicated that the mesopelagic heterotrophic microplankton assemblage could, on average, respire 9–38% of the particulate carbon flux that entered the system at 100 m and possibly 18–76% of the flux remaining at 250 m. Microplankton may therefore be significant carbon cyclers in the ocean's vast “twilight zone”.

Diversity and structure of the microplankton community during a coccolithophore bloom in the stratified northern North Sea

Abstract The structure and diversity of the microplankton, between 10 and 200 μm in size, were quantified during a Lagrangian SF 6 tracer experiment in the North Sea (59°N and 03°E) during June 1999. The experiment was conducted within a developing Emiliania huxleyi -rich phytoplankton bloom. Distinct thermoclines characterised the water column. The surface mixed layer extended to between 15 and 30 m, and the subsurface layer reached a depth of between 45 and 55 m. Despite the relatively high abundance of E. huxleyi ( −1 ), a diverse plankton community was observed throughout the study, with 59 autotrophic and 57 heterotrophic microplankton species being identified. Distinct spatial patterns were observed in the plankton, as smaller cells were common in the surface layer and larger species characterised the subsurface waters. The microplankton communities in the two layers, were found to be significantly different from each other. Prorocentrum minimum and P-Nitzschia ‘delicatissima ’ were the dominant microphytoplankton species in surface waters, while the larger diatom species P-Nitzschia ‘seriata’, Chaetoceros radicans , and Rhizosolenia stolterfothii were common in the subsurface layer. The microzooplankton were dominated by oligotrich ciliates and dinoflagellates, with smaller forms such as Strombidium spp., Diplopsalis sp., and Protoperidinium cf. cerasus characterising surface waters, and larger species, Strombidium ovale, Gyrodinium fusiforme , and Gyrodinium sp. dominating the subsurface layer. Significant temporal changes were observed in the composition of the surface-layer microplankton community, as well as in the structure of the subsurface microzooplankton. Measures of diversity indicated that there were significant differences in the microphytoplankton between surface and subsurface waters and that the temporal changes in the surface layer were also significant. These changes coincided with an increase in microzooplankton grazing pressure and a shift in the species composition of the mesozooplankton. Coupled with observations that regenerated nutrients provided an important nitrogen supply for phytoplankton growth during this study, our data suggest that ‘top-down’ control played an important role in enhancing diversity and determining the structure of the plankton in the stratified northern North Sea, during the summer. The significant changes observed in the microplankton during this study, occurred over a relatively short period of ca. 9 days and emphasise the importance of understanding short-term fluctuations in order to make accurate longer-term predictions.

Abundance, biomass and growth rates of pelagic microorganisms in a tropical coastal ecosystem

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 18:175-185 (1999) - doi:10.3354/ame018175 Abundance, biomass and growth rates of pelagic microorganisms in a tropical coastal ecosystem Petra Wallberg1,*, Per R. Jonsson2, Ron Johnstone1 1Department of Zoology, Stockholm University, S-106 91 Stockholm, Sweden 2Tjärnö Marine Biological Laboratory, S-452 96 Strömstad, Sweden *E-mail: petra.wallberg@itm.su.se ABSTRACT: In situ growth rates of ciliates and heterotrophic nanoflagellates were determined after size fractionation during rainy and dry seasons at 2 coastal stations off Zanzibar Island, Tanzania. In addition, bacterial and primary production and the biomass components in the pelagic microbial community were measured. The bacterial and primary production, and growth rates of heterotrophic nanoflagellates were significantly higher during the rainy season. For the ciliate community the growth rate between the seasons did not differ, but the number of positive growth estimates increased by a factor of 5 in the rainy seasons compared with the dry seasons. Further, the diversity of ciliates increased with the amount of rain, and the size distribution indicated a shift towards a community with larger species. Data obtained from these experiments are summarised in a carbon budget for each season. Carbon budgets indicate that the carbon flow through the heterotrophic microbial food web was approximately 3 times higher during the rainy season compared with the dry season. However, due to the relatively lower production of larger diatoms, heterotrophic microplankton may be relatively more important as a carbon source for higher trophic levels, such as copepods, during the dry season. KEY WORDS: Ciliates · Microbial food web · Growth rates · Carbon budget · East Africa Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 18, No. 2. Publication date: August 09, 1999 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 1999 Inter-Research.

On the Quantitative Importance of Heterotrophic Microplankton in the Northern German Wadden Sea

To assess the importance of heterotrophic microplankton in the Wadden Sea, seasonal distribution and biomass of the main subgroups, that is, heterotrophic dinoflagellates, (separated into thecate and athecate forms), tintinnids, and aloricate ciliates, were studied in 1989 and 1990 in a total of six surveys covering the whole area of the northern German Wadden Sea. Heterotrophic microplankton biomass exhibited high spatial and temporal variation, ranging from 0 μg Cl−1 to 66 μg Cl−1, with maximum concentrations in spring., Mean stocks were lowest in winter (1.6 μg Cl−1) and highest in spring (11.7 μg Cl−1); intermediate concentrations were found in summer (8.5 μg Cl−1). In winter, the heterotrophic microplankton was dominated by tintinnids. In spring and summer, aloricate ciliates and dinoflagellates made up the largest part of the biomass. A pronounced feature was a shift within the dinoprotist group from athecate to thecate forms in summer. In spring, maxima of athecate dinoflagellate carbon were associated with blooms ofPhaeocystis globosa, indicating a close trophic relationship. From rough estimates of the daily grazing potential, based on microheterotrophic biomass and conversion factors from the literature, it may be concluded that heterotrophic microplankton temporarily share a main role in the transfer of food and energy to higher trophic levels within the pelagic system of the Wadden Sea.

Microzooplankton biomass distribution in Terra Nova Bay, Ross Sea (Antarctica)

This work describes the spatial and vertical distribution of microzooplankton (20–200 μm) abundance and biomass of the upper layers (0–100 m), collected during the first oceanographic Italian expedition in Antarctica (1987/1988) in Terra Nova Bay (Ross Sea). Biomass was estimated by using biovolume calculations and literature conversion factors. Sampling was carried out at three depths, surface, 50 and 100 m. The dominant taxa were made up of tintinnid ciliates, ciliates other than tintinnids, larvae of micrometazoa and heterotrophic dinoflagellates. The abundance of the total microplankton fraction had its absolute maximum in the center of Terra Nova Bay at the surface with 31 042 ind. dm −3. The areal and vertical distribution of heterotrophic microplankton biomass differs from that of abundance. On the basis of hydrological conditions, phytoplankton composition and biomass and microzooplankton biomass and structure it is possible to identify three groups of stations: 1—northern coastal stations (intermediate chlorophyll maxima, microphytoplankton prevalence, low microzooplankton biomass); 2—central stations (high surface chlorophyll, nanoplankton prevalence, high abundance of microzooplankton); 3—northern stations (deeper pycnocline, nanoplankton prevalence, high microzooplankton biomass at intermediate depths).

Sustained fecundity when phytoplankton resources are in short supply: Omnivory by Calanus finmarchicus in the Gulf of St. Lawrence

Despite low ambient concentrations of phytoplankton, Calanus finmarchicus sustained relatively high rates of egg production (11–45 eggs female−1 d−1) in the open waters of the Gulf of St. Lawrence in late June–early July. These rates were comparable to egg production rates observed in high‐chlorophyll waters of the lower St. Lawrence estuary. Contrary to classical, linear food‐chain models, variations in egg production of C. finmarchicus were therefore not predictable from variations in standing stocks of autotrophic cells. Egg production rates were independent of the concentration of chlorophyll in situ, the abundance of autotrophic microplankton cells, in situ gut fluorescence, and the rate of ingestion of autotrophic particles. Heterotrophic microplankton—including ciliates and dinoflagellates presumed to be heterotrophic—constituted a disproportionate fraction of the ration of C. finmarchicus in the open Gulf of St. Lawrence, despite their relatively low cell densities. Evidence suggests that heterotrophic microplankton frequently provided a prey resource sufficient for net lipid synthesis as well as for egg production.

The western North Atlantic bloom experiment

An investigation of the spring bloom was carried out in the western North Atlantic (40–50°W) as one component of the multi-nation Joint Global Ocean Flux Study (JGOFS) North Atlantic Bloom Experiment (NABE). The cruise track included an extended hydrographic section from 32 to 47°N and process studies at two week-long time-series stations at 40 and 45°N. Biological and chemical data collected along the transect indicated that the time-series stations were located in regions where the spring bloom was well developed; algal biomass was high and surface nutrient concentrations were reduced from maximum wintertime levels. Despite similarities in the vertical structure and magnitude of phytoplankton biomass and productivity, the two stations clearly differed in physical, chemical and other biological characteristics. Detailed depth profiles of the major autotrophic and heterotrophic microplankton groups (bacteria, phytoplankton, microzooplankton) revealed a strong vertical coherence in distribution at both sites, with maximum concentrations in the upper 50 m being typical of the spring bloom. Ultraplankton (< 10 μm) were an important component of the primary producers at 40°N, whereas larger netplankton (diatoms, dinoflagellates) were more important at 45°N. Silicate depletion was clearly evident in surface waters at 45°N, where diatoms were most abundant. Despite the relative importance of diatoms at 45°N, dinoflagellates dominated the biomass of the netplankton at both sites; however, much of this community may have been heterotrophic. Bacterial biomass and production were high at both stations relative to phytoplankton levels, particularly at 45°N, and may have contributed to the unexpectedly high residual ammonium concentrations observed below the chlorophyll maximum layer at both stations. Microzooplankton grazing dominated phytoplankton losses at both stations, with consumption as high as 88% of the daily primary production. Grazing losses to the mesozooplankton, on the other hand, were small (<10%), but mesozooplankton contribution to the vertical flux of organic matter (fecal pellets) was important at 45°N. F-ratios estimated by 15N tracer methods and sediment trap fluxes were similar and suggeste that ∼30% of the daily primary production was lost by direct sedimentation during the observation period. Numerous similarities in bloom characteristics were noted between the western and eastern Atlantic study sites.

Community structure and bottom-up regulation of heterotrophic microplankton in arctic LTER lakes

Microplankton community structures and abundance was assessed in lakes at the Toolik Lake LTER site in northern Alaska during the summers of 1989 and 1990. The microplankton community included oligotrich ciliates, but rotifers and zooplankton nauplii comprised greater than 90% of total estimated heterotrophic microplankton biomass. Dominant rotifer taxa included Keratella cochlearis, Kellicottia longispina, Polyarthra vulgaris, Conochilus unicornis and a Synchaeta sp. Microplankton biomass was lowest in highly oligotrophic Toolik Lake (< 5 µgCl−1 at the surface) and highest (up to 55 µCl−1) in the most eutrophic lakes, experimentally fertilized lakes, and fertilized limnocorrals, consistent with bottom-up regulation of microplankton abundance.

Phyto- and protozooplankton biomass during austral summer in surface waters of the Weddell Sea and vicinity

Phyto- and protozooplankton were sampled in the upper 10 m of the water column in austral summer during a cruise of RV Polarstern from January 6 to February 20 1985 in the eastern Bransfield Strait vicinity and in the northern, southeastern (off Vestkapp, twice: I and II) and southern Weddell Sea (Vahsel Bay across the Filchner Depression to Gould Bay). The plankton assemblages are discussed in relation to physical, chemical and biological factors in the different geographical areas in summer. Phytoplankton biomass (Phytoplankton carbon, PPC) ranged from 4–194 μg carbon/l and consisted on average of 65% diatoms and 35% autotrophic flagellates. Whereas in the northwest phytoplankton assemblages were dominated by small nanoflagellates (78% of PPC), higher biomass of diatoms (54–94% of PPC) occurred at the other sampling sites. In general autotrophic flagellates and small pennate diatoms dominated at oceanic stations; in neritic areas large centric diatoms prevailed. Chlorophyll a concentrations ranged from 0.25–3.14/μg chl a/l with a mean of 1.13/gmg chlorophyll a/l and an average phytoplankton carbon/chlorophyll a ratio of 39. Protozooplankton biomass (Protozooplankton carbon, PZC) ranged from 0–67 μg carbon/l and consisted of 49% ciliates, 49% heterotrophic dinoflagellates and 2% tintinnids. Heterotrophic dinoflagellates were more important in the northern investigation areas (58%–84% of PZC). Ciliates dominated the protozooplankton in the southeast and south (56%–65% of PZC); higher abundances of tintinnids were observed only in the south (11% of PZC). The most remarkable feature of the surface waters was the high protozooplankton biomass: protozooplankton amounted to 25% on an average of the combined biomass of PPC plus PZC for the entire investigation period. Protozoan biomass in the southeastern and southern Weddell Sea occasionally exceeded phytoplankton biomass. Temperature, salinity, and inorganic nutrients were generally lower in the southern regions; at most of these stations a meltwater layer occurred in the upper meters of the water column. We suggest that this physical regime allows a well developed summer system with a high proportion of heterotrophic microplankton. In the eastern Bransfield Strait, in the northern Weddell Sea and close to the coast off Vestkapp (I), however, early summer conditions occurred with less protozooplankton contribution.

Trophic interactions among heterotrophic microplankton, nanoplankton, and bacteria in Lake Constance

A considerable portion of the pelagic energy flow in Lake Constance (FRG) is channelled through a highly dynamic microbial food web. In-situ experiments using the lake water dilution technique according to Landry & Hasset (1982) revealed that grazing by heterotrophic nanoflagellates (HNF) smaller than 10 µm is the major loss factor of bacterial production. An average flagellate ingests 10 to 100 bacteria per hour. Nano- and micro-ciliates have been identified as the main predators of HNF. If no other food is used between 3 and 40 HNF are consumed per ciliate and hour. Other protozoans and small metazoans such as rotifers are of minor importance in controlling HNF population dynamics.

Microbial autotrophic and heterotrophic eucaryotes in Antarctic waters: relationships between biomass and chlorophyll, adenosine triphosphate and particulate organic carbon

M~croscopical examination of near-surface eucaryotic microbial populations in circumcontinental waters of Antarctica indicated that nanoplankton ((20 btm diameter) dominated in regions with low chlorophyll concentrations ( 20 pm diameter) was generally less significant. The variation in phytoplankton biomass was the result primarily of changes in cell density of pennate diatoms in the East Wind Drift, and of centric diatoms in the Weddell Sea and the Scotia Ridge region. Autotrophic and heterotrophic carbon as determined by microscopical analysis were compared with data for total particulate carbon, chlorophyll a, and adenosine triphosphate. Estimates for the C:chl ratio of autotrophs increased with decreasing concentrations of chlorophyll a, with mean values of 46 in bloom waters and 144 in 'blue water'. A C:ATP ratio for heterotrophic nanoplankton was estimated to be about 100, while that for heterotrophic microplankton may be lower. Algorithms, incorporating concentrations of chlorophyll a and ATP, are described which allow estimates of autotrophc and heterotrophic microbial biomass.