Ecophysiological studies on citrate-synthase: (I) enzyme regulation of selected crustaceans with regard to temperature adaptation

The characteristics and properties chromatographically purified citrate synthase from the euphausiids Euphausia superba (Antarctica) and Meganyctiphanes norvegica (Scandinavian Kattegat and Mediterranean Sea) and from the isopods Serolis polita (Antarctica) and Idotea baltica (Baltic Sea) were used to elucidate biochemical mechanisms of temperature adaptation. Additionally, maintenance experiments were carried out on the euphausiids to determine mechanisms of short term acclimation. Temperature optima (between 37 and 45°C) were unrelated to genotypic cold adaptation, but the activation energy of the Antarctic krill E. superba (10.9 kJ · mol^-1) was only a quarter of that in other species (41.8–45.1 kJ · mol^-1). The minima of apparent Michaelis constants (total range: 4–20 μmol · 1^-1 oxaloacetate; 7–45 μmol · 1^-1 acetyl-coenzyme A) showed no relation to natural conditions, and no distinct pH optimum occurred at ambient temperatures. In contrast, apparent Michaelis constants and specific enzyme activities were related to maintenance temperatures in M. norvegica, but not in E. superba. The differences between M. norvegica and E. superba can be interpreted as adaptations to the changes in ambient temperature with regard to the respective steno- and eurythermic tolerances of these crustaceans.     

Urea degradation by picophytoplankton in the euphotic zone of Lake Biwa

The ability of photoautotrophic picoplankton Synechococcus to degrade urea was examined in the euphotic zone of Lake Biwa. Samples were divided into pico (0.2–2.0 μm) and larger (>2.0 μm) size fractions by filtration. The rates of urea degradation (the sum of the rates of incorporation of carbon into phytoplankton cells and of liberation of CO₂ into water) measured by radiocarbon urea were 8 and 17 μmol urea m⁻³ day⁻¹ in June and July, respectively, for the picophytoplankton in the surface water, and 196 and 96 μmol urea m⁻³ day⁻¹, respectively for the larger phytoplankton. The rates decreased with depth, somewhat similar to the vertical profiles of the photosynthetic rate. The urea degradation rates were obviously high under light conditions. In daylight, urea was degraded into two phases, carbon incorporation and CO₂ liberation, whereas in the dark it was degraded only into the CO₂ liberation phase. The contribution of picophytoplankton to total phytoplankton in urea degradation was high in the subsurface to lower euphotic layer. Urea degradation activity was higher in the picophytoplankton fraction than in the larger phytoplankton fraction. Shorter residence times of urea were obtained in the upper euphotic zone. The contribution of picophytoplankton to urea cycling was 4% to 35%. The present results suggest that the picophytoplankton Synechococcus is able to degrade urea and effectively makes use of regenerated urea as a nitrogen source in the euphotic layer, and that picophytoplankton play an important role in the biogeochemical nitrogen cycle in Lake Biwa. Received: June 25, 1998 / Accepted: February 10, 1999     

Cascading effects of larval Crucian carp introduction on phytoplankton and microbial communities in a paddy field: top-down and bottom-up controls

Effects of fish predation propagate through aquatic food webs, where the classical grazing food chain and microbial loop are interwoven by trophic interactions. The overall impact on aquatic food webs is further complicated because fish may also exert bottom-up controls through nutrient regeneration. Yet, we still have limited information about cascading effects among fish, zooplankton, phytoplankton, and microbes. In this study, we performed a mesocosm experiment to evaluate effects of fish introduction on plankton communities. Six plots were set in factorial combination with fish introduction and rice straw plowing in a paddy field, and the experiment was continued for 4 weeks. Introduction of fish significantly increased chlorophyll a concentrations in smaller size fractions (<15 μm) and abundances of filamentous bacteria (>5 μm in length) and heterotrophic nanoflagellates in 3–15 μm fraction. Microbes in 0.8–3 μm fraction showed increasing but not significant trends in response to fish introduction. These results indicate cascading effects of fish predation operating via two pathways, one through grazing food chain and the other through microbial food web. Phytoplankton community compositions shifted in similar fashion in all plots until 1 week after fish introduction, and then diverged between plots with and without fish thereafter. Bottom-up effects of fish introduction were suggested by increases of total chlorophyll a and inedible phytoplankton species in response to fish introduction. This study provides an example of how fish predation regulates biomass and structure of phytoplankton and microbial communities.     

Ingestion of Large Centric Diatoms, Mangrove Detritus, and Zooplankton by Zoeae of Aratus pisonii (Crustacea: Brachyura: Grapsidae)

Ingestion rates of zoeae of Aratus pisonii Milne Edwards (Brachyura: Grapsidae) were determined offering natural plankton-detritus mixtures in laboratory food selection experiments. The food mixtures were sampled in the Itamaracá estuary, north-eastern Brazil, and standardised to a size range of 50–200 μm. Zoeae ingested significant amounts of large centric diatoms (Coscinodiscus spp.), mangrove detritus, tintinnids (Favella ehrenbergi) and adult copepods during feeding experiments. Diatoms were positively selected by A. pisonii zoeae in all three experiments, with ingestion rates of 3.3–21.3 cells zoea⁻¹ day⁻¹. Detritus particles were always more abundant than phytoplankton and zooplankton in the particle size spectrum offered. Detritus was ingested in two of three experiments, with ingestion rates of up to 34.1 particles zoea⁻¹ day⁻¹, being the most important food item during one experiment. Adult copepods (up to 1.8 ind. zoea⁻¹ day⁻¹) and tintinnids (up to 0.4 ind. zoea⁻¹ day⁻¹) were ingested by A. pisonii zoeae during one experiment each. In spite of a wide range of zoeal density, food particle composition, and density, zoeae of A. pisonii displayed a consistent pattern of food selectivity. This hints at a consistent sensory and behavioural mechanism related to capture and handling of food particles, that most likely also affects larval feeding under natural conditions. Although detritus showed to be quantitatively ingested under estuarine conditions, zoeae of A. pisonii preferred large diatoms and ingested zooplankton only occasionally.     

Selective feeding by larvae of the crown-of-thorns starfish, Acanthaster planci (L.)

 The effect of phytoplankton size on feeding rates of planktonic larvae of the crown-of-thorns starfish Acanthaster planci (L.) was evaluated by examining their gut contents under an epifluorescence microscope. Concentrations of coccoid cyanobacteria in natural seawater ranged between 1.73 and 5.33×10⁵ cells ml^-1 and were three to four orders of magnitude greater than that of eukaryotes. Under these conditions, A. planci larvae ingested similar or smaller numbers of cyanobacteria than eukaryotes. Consequently, clearance rates of A. planci larvae on cyanobacteria were approximately three orders of magnitude lower than those on eukaryotes. Cyanobacteria and eukaryotes in the gut of A. planci larvae had mean equivalent spherical diameters (ESD) of 1–2 μm and 3.6–4.6 μm, respectively. Thus, the volume of cyanobacteria ingested was less than 10% of the volume of eukaryotes ingested. Acanthaster planci larvae were fed cultured phytoplankton Dunaliella tertiolecta and suspensions of three different sizes of plastic beads with fluorescence labelling. There was no significant difference in clearance rates on 6 and 20 μm plastic beads. Clearance rates on 1 μm plastic beads were, however, much lower than those on 6 and 20 μm plastic beads. Clearance rates of A. planci larvae on D. tertiolecta (ca. 5 μm ESD) were significantly higher than those on 6 and 20 μm plastic beads. Apart from particle size, this result shows that feeding of A. planci larvae is influenced by other properties of potential food particles. Accepted: 24 May 1996     

The spring mesozooplankton community at South Georgia: a comparison of shelf and oceanic sites

Mesozooplankton (predominantly 200–2000 μm) were sampled at a shelf and an oceanic station close to South Georgia, South Atlantic, during austral spring (October/November) 1997. Onshelf zooplankton biomass was extremely high at 10–16 g dry mass m⁻² (0–150 m), 70% comprising the small neritic clausocalaniid copepod Drepanopus forcipatus. Large calanoid species, principally Calanoides acutus and Rhincalanus gigas, contributed only 8–10%. At the oceanic station, biomass in the sampled water column (0–1000 m) was ∼6.5 g dry mass m⁻² and 4–6 g dry mass m⁻² in the top 200 m. Here, large calanoids composed 40–50% of the standing stock. Antarctic krill (Euphausia superba) occurred in low abundances at both stations. Vertical profiles obtained with a Longhurst Hardy Plankton Recorder indicated that populations of C. acutus and R. gigas, which overwinter at depth, had completed their spring ascent and were resident in surface waters. Dry mass, carbon and lipid values were lower than found in summer but were consistent with overwintered populations. Phytoplankton concentrations were considerably higher at the oceanic station (2–3 mg chlorophyll a m⁻³) and increased over the time on station. In response to this, egg production of both large calanoid species and growth rates of R. gigas approached those measured in summer. Onshelf phytoplankton concentrations were lower (<1 mg m⁻³), and low egg production rates suggested food limitation. Here phytoplankton rations equivalent to 6% zooplankton body C would have been sufficient to clear primary production whereas at the oceanic station daily carbon fixation was broadly equivalent to zooplankton carbon biomass. Accepted: 25 April 1999     

The importance and distinctiveness of small-sized phytoplankton in the Magellan Straits

The distribution of summer phytoplankton across the Straits of Magellan (SOM) was studied with the aims of tracing differences among the distinct subregions of the area and contributing to the knowledge of its biodiversity. Samples collected at 25 stations were observed and counted in light microscopy. Selected samples were observed with transmission electron microscopy. The main unifying feature of the phytoplankton in the SOM was the high abundance and numerical dominance of small-sized (<10 μm) eukaryotic species, among which coccoid cells of <3 μm size were predominant (56.2 ± 30.6 of the total phytoplankton abundance). They mostly belonged to the prasinophyte Pycnococcus provasolii, which was abundant (0.8–6,834 cells × 10³ ml⁻¹) at all stations with the exception of those in proximity to the Atlantic entrances, where it was not recorded. Small-sized (<3 and 3–5 μm) diatoms (Minidiscus trioculatus, Lennoxia faveolata and other undetermined centric species) attained high densities (<3,757 cells 10³ ml⁻¹) especially at stations of the Patagonian sectors, whereas microplanktonic diatoms were only found at the two entrances of the Straits. Dinoflagellates were constituted mainly by >10 μm forms in the Andean subregion and <10 μm naked species in the Patagonian subregion, contributing up to 75.9 and 41.8% of the total carbon in these two areas, respectively. In the Patagonian subregion, flagellates mainly constituted by <5 μm forms and by cryptomonads <10 μm comprised up to 53.9% of the total biomass. Several species identified in this study have never been reported in other investigations in the SOM, while others, including Pycnococcus provasolii and Lennoxia faveolata, have rarely been recorded elsewhere. Overall, the summer phytoplankton of the Straits does not resemble that of any other region of the world’s seas. Although some of the predominant species might have been overlooked elsewhere, their abundance and relative importance apparently constitute a distinctive feature of the SOM.     

Survival of <Emphasis Type="Italic">Enterococcus faecalis</Emphasis> in Seawater Microcosms Is Limited in the Presence of Bacterivorous Zooflagellates

The survival and persistence of growing and starved cells of Enterococcus faecalis in untreated and differentially filtered (20 μm, 5 μm, 3 μm, 1.2 μm, and 0.1 μm) seawater was analyzed in samples taken at different times over a 1-year period by plate counts and scanning electron microscopy. Whereas seawater filtered through a 0.1-μm mesh was not at all or only slightly bactericidal during incubation at 16°C in the dark, culturability of E. faecalis in the other systems decreased as a function of increasing pore size of the filters. Recovery of culturable, glucose pre-starved cells was always higher than that of cells harvested from the exponential growth phase. Electron microscopic analysis showed that the disappearance of enterococci appeared related to the presence and multiplication of various zooflagellates. Received: 25 July 2001 / Accepted: 27 August 2001     

Interannual and between species comparison of the lipids, fatty acids and sterols of Antarctic krill from the US AMLR Elephant Island survey area

Antarctic euphausiids, Euphausia superba, E. tricantha, E. frigida and Thysanoessa macrura were collected near Elephant Island ¦ during 1997 and 1998. Total lipid was highest in E. superba small juveniles (16 mg g⁻¹ wet mass), ranging from 12 to 15 mg in other euphausiids. Polar lipid (56–81% of total lipid) and triacylglycerol (12–38%) were the major lipids with wax esters (6%) only present in E. tricantha. Cholesterol was the major sterol (80–100% of total sterols) with desmosterol second in abundance (1–18%). 1997 T. macrura and E. superba contained a more diverse sterol profile, including 24-nordehydrocholesterol (0.1–1.7%), trans-dehydrocholesterol (1.1–1.5%), brassicasterol (0.5–1.7%), 24-methylenecholesterol (0.1–0.4%) and two stanols (0.1–0.2%). Monounsaturated fatty acids included primarily 18:1(n−9)c (7–21%), 18:1(n−7)c (3–13%) and 16:1(n−7)c (2–7%). The main saturated fatty acids in krill were 16:0 (18–29%), 14:0 (2–15%) and 18:0 (1–13%). Highest eicosapentaenoic acid [EPA, 20:5(n−3)] and docosahexaenoic acid [DHA, 22:6(n−3)] occurred in E. superba (EPA, 15–21%; DHA, 9–14%), and were less abundant in other krill. E. superba is a good source of EPA and DHA for consideration of direct or indirect use as a food item for human consumption. Lower levels of 18:4(n−3) in E. tricantha, E. frigida and T. macrura (0.4–0.7% of total fatty acids) are more consistent with a carnivorous or omnivorous diet as compared with herbivorous E. superba (3.7–9.4%). The polyunsaturated fatty acid (PUFA) 18:5(n−3) and the very-long chain (VLC-PUFA), C₂₆ and C₂₈ PUFA, were not present in 1997 samples, but were detected at low levels in most 1998 euphausiids. Interannual differences in these biomarkers suggest greater importance of dinoflagellates or some other phytoplankton group in the Elephant Island area during 1998. The data have enabled between year comparisons of trophodynamic interactions of krill collected in the Elephant Island region, and will be of use to groups using signature lipid methodology.     

Trophic behavior of Euphausia superba Dana in laboratory conditions

Summary Experiments conducted at US Palmer Station, 1980 on antarctic krill, Euphausia superba, attempted 1) to quantify and describe behavioral features of ingestion, food clearance and egestion, and 2) to test the hypotheses that feeding and swarming are mutually exclusive events, and that feeding is a cyclical, diel phenomenon. Ingestion was quantified in a large flow-through aquarium. Food clearance and egestion were estimated visually and fluorometrically in E. superba individually kept in one liter jars. Ingestion was directly proportional to chlorophyll concentration (0.65–11.5 gChl/l) and did not change significantly as a function of krill density (200–9000 krill/m³). Ingestion and egestion did not show significant diel trends or mean day-night differences in a 16 h light-8 h dark cycle. Our results suggested that feeding may occur in nature: at swarm densities, within a wide range of phytoplankton concentrations, and may be sustained throughout the diel cycle. We propose that feeding and swarming are co-occurring events. The theoretical basis at the individual and group levels, and its implications, are briefly discussed.In memory of Mary Alice McWhinnie (1922–1980)This research was supported by the University of Chile, National Science Foundation and Chilean Antarctic Institute     

The seasonal abundance and vertical distribution of the <3-μm phytoplakton in the north basin of Lake Biwa

The seasonal changes in the size-fractionated chlorophylla concentrations (<3 μm, 3 to 25 μm, and >25 μm) were investigated at a pelagic site of the north basin of Lake Biwa during June to December 1985. Autofluorescing plankton cells in the <3-μm fractions were also examined using the fluorescein isothiocyanate staining epifluorescence microscopic technique. The <3-μm phytoplankton (usually dominated by chroococcoid cyanobacteria except for a few cases dominated by small eukaryotes) showed a clearly different pattern of seasonal change compared with the larger fractions. That is, from August to early September, chlorophylla of the larger fractions declined considerably, while the <3-μm chlorophylla did not decrease significantly. Moreover, cyanobacterial cell density in the <3-μm fraction showed a maximum value (2–3.5×10⁵ cells·ml⁻¹) during this period. The relative contribution of the <3-μm chlorophylla to the total chlorophylla increased from <5% to 45% during the course of this change. No clear vertical trend in the distribution and composition of the <3-μm phytoplankton was found, except that relatively large cyanobacteria (>4 μm³) appeared at a depth of 15m but not at 0,5 and 10 m from late July to August. These large cells were also found in November and December. The drastic seasonal change of phytoplankton size structure occurring in this basin was discussed in relation to grazing, nutrient depletion and sinking. Contribution from Otsu Hydrobiological Station, Kyoto Univeristy (No. 308, foreign language series).     

The diversity of bacteria,eukaryotic cells and viruses in an oligotrophic lake

An in situ transmission electron microscopic study of biomass samples concentrated from oligotrophic lake water revealed a variety of virus-infected microbial cells and many free viruses and virus-like particles. The most abundant group of microorganisms in screened and filtered water-column samples were 2 μm or less in diameter, and included representatives of several oligotrophic genera, Prosthecomicrobium, Ancyclobacter, Caulobacter and Hyphomicrobium. Among the prokaryotic host cells, which included both heterotrophs and autotrophs, on the basis of electron microscope observations, approximately 17% were infected with bacteriophage or bore adherent phage particles on their surfaces. Several bacterial morphotypes were observed among the prokaryotic hosts. Water samples passed through a 20-μm Nitex screen allowed us to concentrate and examine the larger host cells as well, including several species of single-celled algae and two amoeba species. The infected algal cells included those Chlorella-like in appearance, photosynthetic flagellates and others that could not be positively identified. About one-third of the eukaryotic cells were infected by viruses that were larger (150–200 nm) and structurally more complex than bacteriophages (50–60 nm). None of the viruses have been isolated, but when 0.2 μm filtrate from a biomass sample was spotted onto lawns of four representative heterotrophs and a Chlorella, the clearing observed was taken as evidence of lysis. Cyanobacterial lawns showed no plaques. Thin sections of two amoeba showed food vacuoles containing what appeared to be virus particles of a type seen in certain prokaryotic and eukaryotic cells in the biomass. Received: 26 January 1996 / Received revision: 10 July 1996 / Accepted: 5 August 1996     

Comparisons of morphology and neritic distributions of <Emphasis Type="Italic">Euphausia crystallorophias</Emphasis> and <Emphasis Type="Italic">Euphausia superba</Emphasis> furcilia during autumn and winter west of the Antarctic Peninsula

Euphausia crystallorophias and E. superba larvae often overlap in distribution in Antarctic coastal regions. Here, we describe the morphology and ecology of E. crystallorophias furcilia stages F3–F6, with emphasis on characteristics that distinguish them from E. superba, based on samples collected west of the Antarctic Peninsula during autumn and winter 2001 and 2002. During autumn most E. crystallorophias occurred as F4s (53%) and F5s (35%), while E. superba occurred in all furcilia stages (F1–F6). During winter, F6 was the dominant stage (>67%) for both species. On average, body lengths of E. crystallorophias larval stages were significantly greater than those of E. superba. During autumn, densities of the two species were similar (range: 0.003–11.8 m^–3) at many on-shelf stations, with lower densities during winter. Where both species occurred, >58% of E. crystallorophias furcilia were collected between 50 and 100 m depth, while 82% of E. superba were shallower (25–50 m). Younger stages of E. crystallorophias occurred more frequently (54% of F3s) in water >100 m than older stages (11% of F6s). Thus, many larval E. crystallorophias were vertically segregated from E. superba, thereby reducing grazing competition between the young of these morphologically similar species.     

Abundance of picophytoplankton in the halocline of a meromictic lake, Lake Suigetsu, Japan

Numerous (0.5 to 4.8 × 10⁵ cells/ml), small phytoplankton (smaller than 0.5–1 × 1–2 μm in cell size, picophytoplankton) were distributed in the halocline (depth 2–12 m, 4–14 practical salinity units) of the saline meromictic lake, Lake Suigetsu (35°35′ N, 135°52′ E), located in the central part of the coast of Wakasa Bay along the Japan Sea in Fukui Prefecture, Japan. Vertical distribution of phytoplankton revealed that the maximum number of picophytoplankton was always observed near or a little deeper than the oxic-anoxic boundary layer (depth 5–6 m); they were dominant phytoplankton in the water layer deeper than the oxic-anoxic boundary from July to late September 2005. Spectral analysis of autofluorescence emitted from the particle fractions smaller than 5 μm measured with a spectrofluorometer and from individual cells measured with a microscope photodiode array detector revealed that the major component of picophytoplankton was phycoerythrin-rich, unicellular cyanobacteria (picocyanobacteria). Eukaryotic phytoplankton about 2.5 μm in diameter were also found, but the numbers were low. Fluorescence intensity of chlorophyll a at 685 nm (room temperature) emitted from the particle fractions smaller than 5 μm was increased by the addition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea. These observations indicated that at least some picophytoplankton had a functional photosystem II in the halocline where sulfide, the potential inhibitor of oxygenic photosynthesis, was always present. The large abundance together with their physiological potency suggest that picophytoplankton are one of the important primary producers in the halocline of Lake Suigetsu. Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

Composition and significance of picophytoplankton in Antarctic waters

Filter fractionated picophytoplankton from Antarctic coastal waters (summer 2001) represented only 7–33% of total phytoplankton, even though total stocks were low (average Chl a = 0.32 μg l⁻¹, range = 0.13–1.03 μg l⁻¹). Though all cells passed a 2 μm filter, electron microscopy revealed most cells were over 2 μm, principally Parmales, Phaeocystis sp., and small diatoms. CHEMTAX analysis of HPLC pigment data suggested type 8 haptophytes (e.g. Phaeocystis sp. plus Parmales and pelagophytes) contributed 7–58% of picoplanktonic chlorophyll a, type 6 haptophytes (e.g. coccolithophorids) 18–59%, diatoms 0–18% (mostly type 2 diatoms, e.g. Pseudonitzschia sp., 0–15%), prasinophytes 0–17%, with cell fragments of cryptophytes 0–40%, and dinoflagellates 0–11%. Only stocks of type 8 haptophytes and prasinophytes differed significantly due to successional changes. Zeaxanthin concentrations exceeded estimates from previous cyanobacterial counts and may derive from non-photosynthetic bacteria.     

Reproduction,grazing, and development of the large subarctic calanoid <Emphasis Type="Italic">Eucalanus bungii</Emphasis>: is the spring diatom bloom the key to controlling their recruitment?

The response of large calanoid, Eucalanus bungii, to environmental fluctuation, particularly in relation to the spring diatom bloom in the Oyashio region, western subarctic Pacific Ocean, was examined by investigating egg production, grazing, development and starvation tolerance. Mean in situ egg production rate increased with ambient chlorophyll-a concentration, ranging from 0 to 47 eggs female⁻¹ d⁻¹, while no diurnal synchronous spawning behavior was observed. Under the spring bloom condition, E. bungii showed prey preference for less mobile and larger-sized prey (≥30 μm ESD) and bloom-forming diatom Thalassiosira spp. accounted for >80% of ingested carbon. In the laboratory, E. bungii was successfully reared from newly hatched nauplii to adult with the diatom, Thalassiosira nordenskioldi, as a food resource. Nauplii newly hatched from eggs reached the adult stage in ca. 150 days (5°C) with a sigmoidal developmental pattern and no sexual difference in development pattern. Starvation experiments indicated that the starved copepodids (C1–C4) became more vulnerable to high temperature with the progression of developmental stage, suggesting that the post-bloom condition with low food availability and increased temperature is harsh for their copepodids. The results of this study in conjunction with previous findings suggest that E. bungii is well adapted to utilize large-sized phytoplankton, such as a bloom-forming diatoms and, therefore, their recruitment processes, including egg production, development and mortality would be strongly affected by the duration and intensity of the spring bloom.     

Some observations on the diet and food selection ofDaphnia hyalina (Cladocera) in an eutrophic lake

Summary The food composition in the guts ofDaphnia hyalina was compared with the phytoplankton composition in the lake water on several dates during the year. The guts contained almost exclusively algae; detritus was not an important food item. ExceptOscillatoria agardhii, all large algal forms were generally avoided. On a whole,Scenedesmus was the most preferred taxon, but as numbers or biomass ingestedO. agardhii was more important. The filamentous blue-green algae>60 m (range: 60–1500 m) were strongly avoided byDaphnia. The observed size selective feeding is probably the result of limitations in their filtering apparatus rather than active selection.     

Spring phytoplankton assemblages in the Southern Ocean between Australia and Antarctica

Variations of phytoplankton assemblages were studied in November–December 2001, in surface waters of the Southern Ocean along a transect between the Sub-Antarctic Zone (SAZ) and the Seasonal Ice Zone (SIZ; 46.9°–64.9°S; 142°–143°E; CLIVAR-SR3 cruise). Two regions had characteristic but different phytoplankton assemblages. Nanoflagellates(<20 μm) and pico-plankton (∼2 μm) occurred in similar concentrations along the transect, but were dominant in the SAZ, Sub-Antarctic Front (SAF), Polar Front Zone (PFZ) and the Inter-Polar Front Zone (IPFZ), (46.9°–56.9°S). Along the entire transect their average cell numbers in the upper 70 m of water column, varied from 3 × 10⁵ to 1.1 × 10⁶ cells l⁻¹. Larger cells (>20 μm), diatoms and dinoflagellates, were more abundant in the Antarctic Zone-South (AZ-S) and the SIZ, (60.9°–64.9°S). In AZ-S and SIZ diatoms ranged between 2.7 × 10⁵ and 1.2 × 10⁶ cells l⁻¹, dinoflagellates from 3.1 × 10⁴ to 1.02 × 10⁵ cells l⁻¹. A diatom bloom was in progress in the AZ-S showing a peak of 1.8 × 10⁶ cells l⁻¹. Diatoms were dominated by Pseudo-nitzschia spp., Fragilariopsis spp., and Chaetoceros spp. Pseudo-nitzschia spp. outnumbered other diatoms in the AZ-S. Fragilaropsis spp. were most numerous in the SIZ. Dinoflagellates contained autotrophs (e.g. Prorocentrum) and heterotrophs (Gyrodinium/Gymnodinium, Protoperidinium). Diatoms and dinoflagellates contributed most to the cellular carbon: 11–25 and 17–124 μg C l⁻¹, respectively. Small cells dominated in the northern region characterized by the lowest N-uptake and new production of the transect. Larger diatom cells were prevalent in the southern area with higher values of N-uptake and new production. Diatom and nanoflagellate cellular carbon contents were highly correlated with one another, with primary production, and productivity related parameters. They contributed up to 75% to the total autotrophic C biomass. Diatom carbon content was significantly correlated to nitrate uptake and particle export, but not to ammonium uptake, while flagellate carbon was well correlated to ammonium uptake, but not to export. Diatoms have contributed highly to particle export along the latitudinal transect, while flagellates played a minor role in the export.     

Chla-specific productivity of picophytoplankton not higher than that of larger phytoplankton off the South Shetland Islands in summer

Size-fractionated chlorophyll a (Chla)-specific productivity (μgC μgChla ⁻¹ h⁻¹) was measured at 11 stations off the northern coast of the South Shetland Islands during summer. The Chla-specific productivity of the 2- to 10 or 10- to 330-μm fraction was highest at 100% and 23% light depths. The Chla-specific productivity of the 2- to 10-μm fraction was generally highest, and that of the <2 or 10- to 330-μm fraction was sometimes highest at 12% and 1% light depths. Temperature was less than 3°C within the euphotic zone at all stations. The hypothesis of Shiomoto et al., according to which Chla-specific productivity of picophytoplankton (<2 μm) is not significantly higher than that of larger phytoplankton (>2 μm) in water colder than 10°C, was supported on condition that light is not limited for larger phytoplankton. Received: 16 September 1997 / Accepted: 8 December 1997     

An Under-appreciated Component of Biodiversity in Plankton Communities: The Role of Protozoa in Lake Michigan (A Case Study)

Recent technological advances have led to the discovery that free-living, planktonic protozoa are ubiquitous in nature and appear to be important components of pelagic food webs (e.g., fluorescent straining, flow cytometry). Despite this, limited information exists tying their seasonality to rate processes that drive succession patterns. The abundance, and seasonal growth and grazing loss of an entire protozoan assemblage were evaluated in Lake Michigan. The protozoan assemblage was species-rich (100 taxa) and abundant throughout the year in Lake Michigan. Nano-sized protozoa (Hnano and Pnano, <20 μm in size) ranged in abundance from 10² to 10³ cells ml⁻¹, while micro-protozoa (Hmicro and Pmico, >20 and <200 μm in size) ranged in abundance from 4 to 17 cells ml⁻¹. The biomass of Hnano and Hmicro by itself represented more than 70–80% of crustacean zooplankton biomass, while Pnano and Pmicro constituted nearly 50% of phytoplankton biomass. Protozoa exhibited growth rates comparable to other components of the plankton in Lake Michigan, and some populations grew at rates similar to maximum rates determined in the laboratory (rates of 1–2 day⁻¹). Overall, it appears that macro-zooplankton predation is a major loss factor counter-balancing growth with only small differences between the two rate processes (<0.1 day⁻¹). Discrepancies between growth and grazing loss in the spring were likely attributed to sedimentation losses for larger species of tintinnids and dinoflagellates (Codonella, Tintinnidium, and Gymnodinium) that can account for their occurrence in the deep chlorophyll layer. In the summer, carnivory among similar sized species (Chromulina and small ciliates) may be additional loss factors impinging on the protozoan assemblage.