Autotrophic Pico Research Articles

Eddy-enhanced primary production sustains heterotrophic microbial activities in the Eastern Tropical North Atlantic

Abstract. Mesoscale eddies modulate the ocean's physical, chemical, and biological properties. In cyclonic eddies (CEs), nutrient upwelling can stimulate primary production by phytoplankton. Yet, how this locally enhanced autotrophic production affects heterotrophy and consequently the metabolic balance between the synthesis and the consumption of dissolved organic matter (DOM) remains largely unknown. To fill this gap, we investigated the horizontal and vertical variability in auto- and heterotrophic microbial activity (biomass production and respiration) within a CE that formed off Mauritania and along the ∼ 900 km zonal corridor between Mauritania and the Cape Verde islands in the Eastern Tropical North Atlantic (ETNA). Our results show how the physical disturbances caused by the CE affected the biomass distribution of phyto- and bacterioplankton and their metabolic activities. The injection of nutrients into the sunlit surface resulted in enhanced autotrophic pico- and nanoplankton abundance and generally increased autotrophic activity as indicated by chlorophyll a (Chl a) concentration, primary production (PP), and extracellular release rates. However, the detailed eddy survey also revealed an uneven distribution of these variables with, for example, the highest Chl a concentrations and PP rates occurring near and just beyond the CE's periphery. The heterotrophic bacterial activity was similarly variable. Optode-based community respiration (CR), bacterial respiration (BR) estimates, and bacterial biomass production (BP) largely followed the trends of PP and Chl a. Thus, a submesoscale spatial mosaic of heterotrophic bacterial abundance and activities occurred within the CE that was closely related to variability in autotrophic production. Consistent with this, we found a significant positive correlation between concentrations of semi-labile dissolved organic carbon (SL-DOC; here the sum of dissolved hydrolysable amino acids and dissolved combined carbohydrates) and BR estimates. Extracellular release of carbon as indicated by primary production of dissolved organic carbon (PPDOC) was variable with depth and laterally and not always sufficient to compensate the bacterial carbon demand (BCD: BR + BP), with PPDOC accounting for between 28 % and 110 % of the BCD. Bacterial growth efficiency (BGE: BP / BCD) ranged between 1.7 % and 18.2 %. We estimated the metabolic state to establish whether the CE was a source or a sink of organic carbon. We showed that the CE carried a strong autotrophic signal in the core (PP / CR > 1). Our results suggest that submesoscale (0–10 km) processes lead to highly variable metabolic activities in both photoautotrophic and heterotrophic microorganisms. Overall, we revealed that the CEs not only trap and transport coastal nutrients and organic carbon to the open ocean but also stimulate phytoplankton growth, generating freshly produced organic matter during westward propagation. This drives heterotrophic processes and may contribute to the previously observed net heterotrophy in open Atlantic surface waters.

Influence of warm core eddy on the vertical distribution of autotrophic pico- and nanoplankton in the Bay of Bengal

ABSTRACT The distribution and ecology of autotrophic pico- and nanoplankton were studied in the warm core eddy-affected northern Bay of Bengal (BoB) during the peak south-west monsoon of 2012. Small phototrophs showed diverse response patterns to the contrasting environmental setting in the northern BoB during the south-west monsoon. The convergence of surface waters in the eddy-influenced region was evidenced in terms of a deeper mixed layer and lower concentrations of . Synechococcus and picoeukaryotes were found to be more abundant in coastal waters as compared with the warm core eddy-influenced and offshore waters. Prochlorococcus was more abundant in the eddy-influenced and the offshore regions possibly as a consequence of their ability to survive in low-nutrient/oligotrophic waters. The abundance of Synechococcus increased noticeably in the coastal and subsurface waters due to their affinity towards higher concentrations of nutrients. The present data therefore support the view of a community shift induced by warm core water eddies and show their importance in promoting primary production by Prochlorococcus in the mixed layers of the BoB.

Effects of changing nutrient inputs on the ratio of small pelagic fish stock and phytoplankton biomass in the Black Sea

Significant increases in nitrogen and phosphorus inputs to the Black Sea in the second half of the 20th century caused eutrophication and drastically decreasing Si:N and Si:P ratios. Combined with climate change, overfishing of top predators and a huge outbreak of the non-indigenous ctenophore Mnemiopsis, the pelagic food web was strongly modified and its efficiency for channeling primary production to higher trophic levels substantially reduced. We used the ratio between small pelagic fish stock and phytoplankton biomass on the Danube shelf and in the open Black Sea to investigate long-term changes in food web functioning. The ratio had 1) highest values for the pre-eutrophication period when diatoms and copepods dominated the pelagic food web (‘muscle food chain’), 2) decreased during the eutrophication period with stronger prevalence of autotrophic pico- and nanophytoplankton, bacteria, heterotrophic nanoflagellates, microzooplankton, Noctiluca and jellyfish (‘jelly food chain’ with increased importance of the microbial loop), 3) lowest values during the ecological crisis (1989–1992), when small pelagic fish stocks collapsed, and 4) increased after 1993, indicating that the ecosystem went out of the crisis and exhibited a trend of recovery. However, in the last period (1993–2008) the ratio remained close to values observed in the middle eutrophication phase, suggesting that the ecosystem was far from fully recovered. Since early 2000s, fluctuating pelagic fish stocks, with a tendency to decreasing fish landing again, have been observed in the Black Sea. Additionally, the quality of food for the small pelagic fish has deteriorated due to warming trends and the legacy of eutrophication, giving support for the ‘jelly food chain’, exhibiting low energy transfer and prevalence of organisms with high respiration rate and low nutritional value.

Pico and nanoplankton abundance and carbon stocks along the Brazilian Bight.

Pico and nanoplankton communities from the Southwest Atlantic Ocean along the Brazilian Bight are poorly described. The hydrography in this region is dominated by a complex system of layered water masses, which includes the warm and oligotrophic Tropical Water (TW), the cold and nutrient rich South Atlantic Central Water (SACW) and the Coastal Water (CW), which have highly variable properties. In order to assess how pico- and nanoplankton communities are distributed in these different water masses, we determined by flow cytometry the abundance of heterotrophic bacteria, Prochlorococcus, Synechococcus and autotrophic pico and nanoeukaryotes along three transects, extending from 23°S to 31°S and 39°W to 49°W. Heterotrophic bacteria (including archaea, maximum of 1.5 × 106 cells mL−1) were most abundant in Coastal and Tropical Water whereas Prochlorococcus was most abundant in open-ocean oligotrophic waters (maximum of 300 × 103 cells mL−1). Synechococcus(up to 81 × 103 cells mL−1), as well as autotrophic pico and nanoeukaryotes seemed to benefit from the influx of nutrient-rich waters near the continental slope. Autotrophic pico and nanoeukaryotes were also abundant in deep chlorophyll maximum (DCM) layers from offshore waters, and their highest abundances were 20 × 103 cells mL−1 and 5 × 103 cells mL−1, respectively. These data are consistent with previous observations in other marine areas where Synechococcus and autotrophic eukaryotes dominate mesotrophic waters, whereas Prochlorococcus dominate in more oligotrophic areas. Regardless of the microbial community structure near the surface, the carbon stock dominance by autotrophic picoeukaryotes near the DCM is possibly linked to vertical mixing of oligotrophic surface waters with the nutrient-rich SACW and their tolerance to lower light levels.

Summer monsoon onset-induced changes of autotrophic pico- and nanoplankton in the largest monsoonal estuary along the west coast of India.

This study presents the response of autotrophic pico- and nanoplankton to southwest monsoon-associated hydrographical transformations in the Cochin backwaters (CBW), the largest monsoonal estuary along the west coast of India. By the onset of the southwest monsoon, the euhaline/mesohaline conditions in the downstream/upstream of CBW usually transform into oligohaline/limnohaline. The flow cytometer analysis revealed the dominance of picoeukaryotes > Synechococcus > nanoautotrophs, with Prochlorococcus either very low or entirely absent. Synechococcus abundance was high during the pre-southwest monsoon (10(6) L(-1)), which dwindled with heavy fresh water influx during the southwest monsoon (10(5) L(-1)). The drastic drop in salinity and faster flushing of the CBW during the southwest monsoon replaced the euhaline/mesohaline strain of Synechococcus with an oligohaline/limnohaline strain. Epifluorescence microscopy analyses showed that, among the two strains of Synechococcus, the phycoerythrin-rich (PE-rich) one was dominant in the mesohaline/euhaline conditions, whereas the phycocyanin-rich (PC-rich) strain dominated in oligohaline/limnohaline conditions. Although Synechococcus abundance diminished during the southwest monsoon, the total abundance of picoplankton community remained virtually unchanged in the upstream due to an increase in the abundance of picoeukaryotes. On the other hand, the autotrophic nanoplankton abundance increased from pre-monsoon levels of av. 3.8 × 10(6)-av. 9.5 × 10(6) L(-1) at the onset of the southwest monsoon. Utilizing suitable multivariate analyses, the study illustrated the differential response and niche preference of various smaller communities of autotrophs to the southwest monsoon-associated hydrographical ramifications in a large monsoonal estuary, which may be applicable to similar such estuaries situated along the Indian coastline.

<i>Blastodinium</i> spp. infect copepods in the ultra-oligotrophic marine waters of the Mediterranean Sea

Abstract. Blastodinium are chloroplast-containing dinoflagellates which infect a wide range of copepods. They develop inside the gut of their host, where they produce successive generations of sporocytes that are eventually expelled through the anus of the copepod. Here, we report on copepod infections in the oligotrophic to ultra-oligotrophic waters of the Mediterranean Sea sampled during the BOUM cruise. Based on a DNA-stain screening of gut contents, 16 % of copepods were possibly infected in samples from the Eastern Mediterranean infected, with up to 51 % of Corycaeidae, 33 % of Calanoida, but less than 2 % of Oithonidae and Oncaeidae. Parasites were classified into distinct morphotypes, with some tentatively assigned to species B. mangini, B. contortum, and B. cf. spinulosum. Based upon the SSU rDNA gene sequence analyses of 15 individuals, the genus Blastodinium was found to be polyphyletic, containing at least three independent clusters. The first cluster grouped all sequences retrieved from parasites of Corycaeidae and Oncaeidae during this study, and included sequences of Blastodinium mangini (the "mangini" cluster). Sequences from cells infecting Calanoida belonged to two different clusters, one including B. contortum (the "contortum" cluster), and the other uniting all B. spinulosum-like morphotypes (the "spinulosum" cluster). Cluster-specific oligonucleotidic probes were designed and tested by fluorescence in situ hybridization (FISH) in order to assess the distribution of dinospores, the Blastodinium dispersal and infecting stage. Probe-positive cells were all small thecate dinoflagellates, with lengths ranging from 7 to 18 μm. Maximal abundances of Blastodinium dinospores were detected at the Deep Chlorophyll Maximum (DCM) or slightly below. This was in contrast to distributions of autotrophic pico- and nanoplankton, microplanktonic dinoflagellates, and nauplii which showed maximal concentrations above the DCM. The distinct distribution of dinospores and nauplii argues against infection during the naupliar stage. Dinospores, described as autotrophic in the literature, may escape the severe nutrient limitation of ultra-oligotrophic ecosystems by living inside copepods.

Monsoon-induced changes in the size-fractionated phytoplankton biomass and production rate in the estuarine and coastal waters of southwest coast of India

Changes in the autotrophic pico- (0.2-2 microm), nano- (2-20 microm), and microplankton (>20 microm) biomass (chlorophyll a) and primary production were measured in the estuarine and coastal waters off Cochin, southwest coast of India during the onset and establishment of a monsoon. During this period, the estuary was dominated by nutrient-rich freshwater, whereas the coastal waters were characterized with higher salinity values (>30 psu) and less nutrients. The average surface chlorophyll a concentrations and primary production rates were higher in the estuary (average 13.7 mg m(-3) and 432 mgC m(-3) day(-1)) as compared to the coastal waters (5.3 mg m(-3) and 224 mgC m(-3) day(-1)). The nanoplankton community formed the major fraction of chlorophyll a and primary production, both in the estuary (average 85 +/- SD 8.3% and 81.2 +/- SD 3.2%) and the coastal waters (average 73.2 +/- SD 17.2% and 81.9 +/- 15.7%). Nanoplankton had the maximum photosynthetic efficiency in the coastal waters (average 4.8 +/- SD 3.9 mgC mgChl a m(-3) h(-1)), whereas in the estuary, the microplankton had higher photosynthetic efficiency (average 7.4 +/- 7 mgC mgChl a m(-3) h(-1)). The heavy cloud cover and increased water column turbidity not only limit the growth of large-sized phytoplankton in the Cochin estuary and coastal waters but also support the proliferation of nanoplankton community during the monsoon season, even though large variation in nanoplankton chlorophyll a and production exists between these two areas.

Structure of the summer under fast ice microbial community near Syowa Station, eastern Antarctica

Temporal variations in the microbial community structure of plankton, which is composed of autotrophic and heterotrophic pico-, nano- and microplankton, were investigated during the austral summer of 2005/2006 under fast ice near Syowa Station, eastern Antarctica. Autotrophic algal populations were composed almost entirely of diatoms followed by phytoflagellates such as autotrophic dinoflagellates and cryptophytes. Among the microbial community, heterotrophic biomass was dominated by heterotrophic dinoflagellates and naked ciliates and finally exceeded autotrophic biomass. Qualitative microscopic analysis revealed that heterotrophic dinoflagellates were ingesting large number of diatoms. Synchronizing fluctuation of naked ciliates with phytoflagellates suggested a predator–prey relationship between them. Our results suggest that the pelagic food webs under the extensive ice-covered areas in coastal Antarctic regions are not short but complex.

Seasonal Variations in Virus-Host Populations in Norwegian Coastal Waters: Focusing on the Cyanophage Community Infecting Marine Synechococcus spp

Viruses are ubiquitous components of the marine ecosystem. In the current study we investigated seasonal variations in the viral community in Norwegian coastal waters by pulsed-field gel electrophoresis (PFGE). The results demonstrated that the viral community was diverse, displaying dynamic seasonal variation, and that viral populations of 29 different sizes in the range from 26 to 500 kb were present. Virus populations from 260 to 500 kb and dominating autotrophic pico- and nanoeukaryotes showed similar dynamic variations. Using flow cytometry and real-time PCR, we focused in particular on one host-virus system: Synechococcus spp. and cyanophages. The two groups covaried throughout the year and were found in the highest amounts in fall with concentrations of 7.3 x 10(4) Synechococcus cells ml(-1) and 7.2 x 10(3) cyanophage ml(-1). By using primers targeting the g20 gene in PCRs on DNA extracted from PFGE bands, we demonstrated that cyanophages were found in a genomic size range of 26 to 380 kb. The genetic richness of the cyanophage community, determined by denaturing gradient gel electrophoresis (DGGE) of PCR-amplified g20 gene fragments, revealed seasonal shifts in the populations, with one community dominating in spring and summer and a different one dominating in fall. Phylogenetic analysis of the sequences originating from PFGE and DGGE bands grouped the sequences into three groups, all with homology to cyanomyoviruses present in cultures. Our results show that the cyanophage community in Norwegian coastal waters is dynamic and genetically diverse and has a surprisingly wide genomic size range.

Seasonality and coexistence of autotrophic pico- and nanoplankton and zooplankton in three boreal lakes

Seasonality and coexistence of autotrophic pico- and nanoplankton and zooplankton in three boreal lakes

Structure of planktonic microbial food web in a brackish stratified Siberian lake

The distribution of primary components of the microbial community (autotrophic pico- and nanoplankton, phototrophic bacteria, heterotrophic bacteria, microscopic fungi, heterotrophic flagellates, ciliates and heliozoa) in the water column of Lake Shira, a steppe brackish-water, stratified lake in Khakasia, Siberia (Russia), were assessed in midsummer. Bacterioplankton was the main component of the planktonic microbial community, accounting for 65.3 to 75.7% of the total microbial biomass. The maximum concentration of heterotrophic bacteria were recorded in the monimolimnion of the lake. Autotrophic microorganisms contributed more significantly to the total microbial biomass in the pelagic zone (20.2–26.5%) than in the littoral zone of the lake (8.7–14.9%). First of all, it is caused by development of phototrophic sulphur bacteria at the oxic-anoxic boundary. The concentrations of most aerobic phototrophic and heterotrophic microorganisms were maximal in the upper mixolimnion. Heterotrophic flagellates dominated the protozoan populations. Ciliates were minor component of the planktonic microbial community of the lake. Heterotrophic flagellates were the most diverse group of planktonic eucaryotes in the lake, which represented by 36 species. Facultative and obligate anaerobic flagellates were revealed in the monimolimnion. There were four species of Heliozoa and only three of ciliates in the lake.

Functional response and particle size selection of Halteria cf. grandinella, a common freshwater oligotrichous ciliate

In laboratory experiments, we studied the growth and feeding characteristics of Halte- ria cf. grandinella, a common and widespread oligotrichous ciliate in freshwater plankton. Particle- size-dependent feeding rates were measured with fluorescent latex micropheres (0.22 to 4.23 µm diameter) and natural food organisms (bacteria, Synechococcus sp., Chlorella minutissima) in short- term feeding experiments. H. cf. grandinella ingested all but the smallest (0.22 µm) particles offered and demonstrated a concentration dependent type-2 functional response. Maximum clearance rates were obtained with 2.76 µm latex beads (0.6 µl h -1 ). Clearance rates declined for smaller and larger particle sizes but even with the suboptimal size classes, 0.47 and 1 µm, high maximal ingestion rates were measured (19 and 13 beads ciliate -1 min -1 , respectively). The maximum clearance rates for bac- teria and algae were close to those of similar sized latex beads, and no discrimination of particles based on properties other than size could be detected. The physiological state of the ciliates did not seem to have an impact on the relative pattern of the particle-size-dependent functional response curves, but the absolute feeding rates decreased nearly 10-fold after prolonged starvation. The experiments demonstrated that this small filter-feeding ciliate is an omnivorous species which is able to efficiently exploit the planktonic prey size spectrum from 0.5 to 5 µm, covering heterotrophic and autotrophic pico- and nanoplankton in the diet.

Size-fractionated phytoplankton biomass and species composition in the Indian sector of the Southern Ocean during austral summer

During the late austral summer of 1994, Antarctic waters were characterized by low phytoplankton biomass. Along the 62°E meridian transect, between 49°S and 67°S, chlorophyll (Chl.) a concentration in the upper 150 m was on average 0.2 mg m −3. However, in the Seasonal Ice Zone (SIZ) chlorophyll a concentrations were higher, with a characteristic deep chlorophyll maximum. The highest value (0.6 mg Chl. a m −3) was measured at the Antarctic Divergence, 64°S, corresponding to the depth of the temperature minimum (∼100 m). This deep biomass maximum decreased from South to North, disappeared in the Permanently Open Ocean Zone (POOZ) and reappeared with less vigour in the vicinity of the Polar Front Zone (PFZ). In the SIZ, the upper mixed layer was shallow, biomass was higher and the >10 μm fraction was predominant. In this zone the >10 μm, 2–10 μm and <2 μm size fractions represented on the average 46%, 25.1% and 28.9% of the total integrated Chl. a stock in the upper 100 m, respectively. The phytoplankton assemblage was diverse, mainly composed of large diatoms and dinoflagellate cells which contributed 42.7% and 33.1% of the autotrophic carbon biomass, respectively. Moving northwards, in parallel with the decrease in biomass, the biomass of autotrophic pico- and nanoflagellates (mainly Cryptophytes) increased steadily. In the POOZ, the picoplanktonic size fraction contributed 47.4% of the total integrated Chl. a stock. A phytoplankton community structure with low biomass and picoplankton-dominated assemblage in the POOZ contrasted with the relatively rich, diverse and diatom-dominated assemblage in the SIZ. These differences reflect the spatial and temporal variations prevailing in the Southern Ocean pelagic ecosystem.

Release of dissolved amino acids by flagellates and ciliates grazing on bacteria

Release of amino acids was examined in the laboratory in the form of dissolved primary amine (DPA) by two marine planktonic protozoa (the oligotrichous ciliate, Strombidium sulcatum and the aplastidic flagellate Pseudobodo sp.) grazing on bacteria. DPA release rates were high (19–25 × 10 −6 and 1.8–2.3 × 10 −6 μmol DPA cell −1 h −1 for flagellates and ciliates, respectively) during the exponential phase, when the ingestion rates were maximum. Release rates were lower during the other growth phases. The release of DPA accounted for 10 % (flagellates) and 16 % (ciliates) of the total nitrogen ingested. Our data suggest that the release of DPA by protozoa could play an important role in supporting bacterial and consequently autotrophic pico- and nanoplankton growth, especially in oligotrophic waters, where the release of phytoplanktonic dissolved organic matter is low.

Toxic effects of tri-butyl-tin (TBT) on autotrophic pico-, nano-, and microplankton assessed by a size fractionated pollution-induced community tolerance (SF-PICT) concept

The effects of tri-butyl-tin (TBT) on autotrophic pico-, nano and microplankton were assessed by a marine enclosure experiment. In accordance with the pollution-induced community tolerance (PICT) concept, an increased tolerance was found for microplankton in enclosures amended with 0.01, 0.6 and 10 nM TBT. In contrast, nanoplankton showed increased tolerance only in an enclosure amended with 10 nM TBT. Neither the entire phytoplankton community nor picoplankton showed increased tolerance. These observations are discussed in terms of the ecology of pico-, nano- and microplankton. In all enclosures amended with TBT, primary production by nanoplankton declined, and primary production by picoplankton increased: changes that are interpreted as indirect effects of TBT. A comparison of TBT levels found in polluted coastal areas and in off-shore areas indicates that phytoplankton communities might be affected by TBT. Size fractionated measurements of TBT tolerance in phytoplankton communities were proven to increase the sensitivity of the PICT concept.

Distribution patterns of autotrophic pico- and nanoplankton and their relative contribution to algal biomass during spring in the Atlantic sector of the Southern Ocean

Flow cytometry and epifluorescence microscopy were employed to determine the abundance, distribution and size structure of autotrophic pico- and nanoplankton on a series of transects along 6°W between the Polar Front and the receding ice-edge during 12 October–21 November 1992. Abundances ranged from < 1.0 to > 20.0 × 10 6 cells 1 −1 in the upper 100-m layer. Lowest surface concentrations were found at the receding ice-edge between 56° and 59°30′S. Abundances increased with advancing season within the Polar Front region (47°−50°30′S) and in the Southern Antarctic Circumpolar Current zone (ACC, 51°−54°30′S) but not south of the ACC-Weddell Gyre Boundary (54°30′−58°30′S). Autotrophic pico- and nanoplankton contributed up to 90% to total chlorophyll a in regions with low chlorophyll a concentrations (0.2−0.4 μg 1 −1) but less than 50% in regions where phytoplankton biomass accumulated (> 1.8 μg chl l −1, Polar Front region), indicating a shift in dominance towards larger size classes in areas of high chlorophyll a concentrations. Referring to cell numbers, this shift towards larger size fractions was observed within the autotrophic pico- and nanoplankton. The build up of phytoplankton biomass to bloom levels was primarily due to the accumulation of diatoms. Hence, regional variability in magnitude of the spring bloom is due to diatoms while the standing stock of autotrophic pico- and nanoplankton is less variable. We hypothesize that neither micronutrients (e.g. Fe) nor grazing suppresses abundance of pico- and nanophytopiankton below a “background” concentration of about 2.0−4.0 × 10 6 cells 1 −1 during spring and early summer in wide areas of the Southern Ocean. Our observations confirm the hypothesis that both groups represent abundant and stable constituents of the phytoplankton assemblage in the Southern Ocean.

Among-lake and within-lake variations of autotrophic pico- and nanoplankton biomass in six Quebec lakes

Les variations de la biomasse du picoplancton autotrophe (APP) et du nanoplancton autotrophe (ANP) ont ete examinees dans six lacs des Laurentides et des Cantons de l'Est (Quebec) dont le statut trophique, la geologie du bassin versant et la physico-chimie de l'eau etaient differentes. Les variations de la biomasse des deux composantes du phytoplancton ont ete evaluees en fonction des changements dans les caracteristiques limnologiques. Dans chaque lac, l'echantillonnage a ete realise a trois profondeurs correspondant aux strates epi-, meta- et hypolimnetique et a cinq periodes au cours de l'ete. La biomasse de APP et sa contribution a la biomasse totale des algues de taille 1) etait d'environ 20-30 μg.L -1 . Les valeurs moyennes des biomasses de APP (75-365 μg L -1 ou 16-80 μg C.L -1 ) et ANP (46-249 μg.L -1 ou 7-37 μg C. L -1 ) notees dans les lacs du Quebec le long de notre plage de niveau trophique (TP: 7-73 μg.L -1 ) etaient du meme ordre de grandeur que celles rapportees pour d'autres ecosystemes d'eau douce. La majeure partie de la variation dans la biomasse de APP et ANP etait reliee aux variations inter-lacs dans les elements nutritifs (NO 3 , TP, TDP), la conductivite ou le pH.

Among-lake and within-lake variations of autotrophic pico- and nanoplankton biomass in six Quebec lakes

Among-lake and within-lake variations of autotrophic pico- and nanoplankton biomass in six Quebec lakes

Diurnal and seasonal variation in the contributions of autotrophic pico-, nano- and microplankton to the primary production of an upland lake

Diurnal and seasonal variation in the contributions of autotrophic pico-, nano- and microplankton to the primary production of an upland lake

Density‐dependent effects of protozoans on specific growth rates in pico‐ and nanoplanktonic assemblages

We experimentally assess nutrient feedback of oligotrichous ciliates and phagotrophic flagellates on growth in naturally occurring Mediterranean autotrophic microbial populations as well as on the microphytoplankton. Dialysis bags were used to separate cultured protozoa from the &lt;7‐µm plankton filtrates yet allow nutrient exchange. Growth rates in microbial autotrophic populations increased with protozoan densities added to bags. Microbial growth half‐saturation constants, expressed in terms of protozoan densities added, varied from 11 to 17 ciliates ml−1 and 2–5 × 103 flagellates ml−1. The “transdialysis contact” of protozoa with microbial planktonic populations led to growth rates 2 to &gt; 10 times higher than those observed without added protozoa. These findings suggest that in situ, autotrophic pico‐ and nanoplanktonic growth is limited by protozoan standing stocks. Lower generation times and half‐saturation constants were obtained for autotrophic nanoplankton (∼3 h, Kµ = 10 ciliates ml−1 and 1.7 × 103 phagotrophic flagellates ml−1, respectively). Microphytoplankton (e.g. diatoms and dinoflagellates) showed no significant functional responses to the addition of microprotozoa, suggesting a possible dependence of the latter on new rather than on recycled nutrients.