scispace - formally typeset
Search or ask a question

Showing papers in "Journal of Plankton Research in 1999"


Journal ArticleDOI
TL;DR: The results are consistent with the proposition that components of the DON pool are not only an important potential, direct or indirect N source for phytoplankton, but also that different algal species can exploit these sources with varying capabilities so that different N substrates may selectively stimulate the development of dominantAlgal species.
Abstract: Two experimental series were run to evaluate the potential of algal development on dissolved organic nitrogen (DON) compounds as the sole source of nitrogen (N) nutrition. Mono- cultures of several common Lake Kinneret algae (Pediastrum duplex, Synechococcus sp., Microcystis aeruginosa, Aphanizomenon ovalisporum and Cyclotella sp.) were incubated for 3 weeks in the laboratory with different inorganic (NH4 + , NO3 - ) or organic (hypoxanthine, urea, guanine, ornithine, glucosamine, lysine) nitrogen sources. Even though the cultures were not axenic, marked differences were observed in algal growth response. Pediastrum, Cyclotella and Aphanizomenon grew well on most N sources, and cyanobacterial growth and yield were consistently greatest when urea was the only N source. We also followed algal growth and eventual species dominance in batch samples of GF/F-filtered lake water, supplemented with orthophosphate and different inorganic or organic N compounds and inoculated with concentrated lake phytoplankton. Although no clear impact on phytoplankton growth (as chlorophyll concentration) was observed, in seven out of 11 experiments we could discern changes in the algal species that became dominant in flasks with different organic and inorganic N sources. Our results are consistent with the proposition that components of the DON pool are not only an important potential, direct or indirect N source for phytoplankton, but also that different algal species can exploit these sources with varying capabilities so that different N substrates may selectively stimulate the development of dominant algal species.

195 citations


Journal ArticleDOI
TL;DR: The results suggest that reactive oxygen species generated from C.polykrikoides are responsible for oxidative damage leading to fish kills in red tide dinoflagellate.
Abstract: The underlying toxic mechanisms of the red tide dinoflagellate, Cochlodinium polykrikoides, were studied with respect to the reactive oxygen species-mediated toxic effect. Cochlo- dinium polykrikoides generates superoxide anion (O2 - ) and hydrogen peroxide (H2O2), as measured by the cytochrome c reduction method and scopoletin-peroxidase method, respectively. The capa- bility of C.polykrikoides to generate these oxygen radicals was related to the growth phase: the highest rate in the exponential phase and a gradual decrease in the stationary phase. Other phytoplankton, such as Eutreptiella gymnastica, Heterosigma akashiwo, Prorocentrum micans, Gymnodinium sanguineum and Alexandrium tamarense, also produce H2O2; the rate of H2O2 generation by these species was lower than that of C.polykrikoides. The exposure of liposomal samples to intact or ruptured individuals of C.polykrikoides resulted in severe membrane damage, such as liposomal lipid peroxidation. Cochlodinium polykrikoides-induced lipid peroxidation was significantly reduced by oxygen radical scavengers, superoxide dismutase, benzoquinone, catalase and mannitol. In addition, lipid peroxidation of gill tissue of flatfish exposed to C.polykrikoides increased with increasing algal cell density. These results suggest that reactive oxygen species generated from C.polykrikoides are responsible for oxidative damage leading to fish kills.

189 citations


Journal ArticleDOI
TL;DR: A positive correlation was observed between cellular toxicity and salinity-dependent growth rate, indicating that cell toxin quota may be affected by extrinsic factors, but it is not always a direct functional response to specific environmental stress.
Abstract: Growth and toxin production of a highly toxic clone of the marine dinoflagellate Alexan- drium tamarense, isolated from the lower St Lawrence Estuary (Quebec) in eastern Canada, were studied in unialgal batch cultures under different conditions. Controlled experiments were conducted on the production of paralytic shellfish poisoning (PSP) toxins under conditions of varying light (40, 60, 150, 230 and 470 µmol m-2 s-1), salinity (10, 15, 20, 25 and 30‰) and nitrate concentrations (0, 88, 264, 528 and 880 µmol l-1). The effects of variable environmental factors on both toxin composition (% molar) and cell toxicity (pg STXeq (saxitoxin equivalents) cell-1) were determined through the culture cycle. The toxin profile (% molar; mean ± SD), determined by high-performance liquid chromatography with fluorescence detection (HPLC-FD), remained stable and was consistently dominated by the low-potency N-sulfocarbamoyl toxins C1/C2 (64.0 ± 3%). There were also substan- tial relative amounts of the high-toxicity carbamate derivatives gonyautoxin 1-4 (GTX1-4) (1.7 ± 0.5%), neosaxitoxin (NEO) (16.2 ± 2%) and saxitoxin (STX) (17.8 ± 2%). The cellular toxicity (mean ± SD: 58.8 ± 7 pg STXeq cell -1 ) was essentially independent of light, salinity and nitrate concentration throughout the exponential growth phase, but varied over the growth stages in culture. A positive correlation was observed between cellular toxicity and salinity-dependent growth rate, indicating that cell toxin quota may be affected by extrinsic factors, but it is not always a direct functional response to specific environmental stress.

164 citations


Journal ArticleDOI
TL;DR: In this paper, the authors present the conceptual bases of universal multifractals and more precisely a stochastic multifractal framework which among different advantages lead in a rather straightforward manner to universal multifractals.
Abstract: A multifractal method of analysis, initially developed in the framework of turbulence and having had developments and applications in various geophysical domains (meteorology, hydrology, climate, remote sensing, environmental monitoring, seismicity, volcanology), has previously been demonstrated to be an efficient tool to analyse the intermittent fluctuations of physical or biological oceanographic data (Seuront et al., Geophys. Res. Lett., 23, 3591-3594, 1996 and Nonlin. Processes Geophys., 3, 236-246, 1996). Thus, the aim of this paper is, first, to present the conceptual bases of multifractals and more precisely a stochastic multifractal framework which among different advantages lead in a rather straightforward manner to universal multifractals. We emphasize that contrary to basic analysis techniques such as power spectral analysis, universal multifractals allow the description of the whole statistics of a given field with only three basic parameters. Second, we provide a comprehensive detailed description of the analysis techniques applied in such a framework to marine ecologists and oceanographers; and third, we illustrate their applicability to an original time series of biological and related physical parameters. Our illustrative analyses were based on a 48 h high-frequency time series of in vivo fluorescence (i.e. estimate of phytoplankton biomass), simultaneously recorded with temper- ature and salinity in the tidally mixed coastal waters of the Eastern English Channel. Phytoplankton biomass, which surprisingly exhibits three distinct scaling regimes (i.e. a physical-biological-physical transition), was demonstrated to exhibit a very specific heterogeneous distribution, in the framework of universal multifractals, over smaller ( 500 m) scales dominated by different turbulent processes as over intermediate scales (10-500 m) obviously dominated by biological processes.

154 citations


Journal ArticleDOI
TL;DR: The study indicated the potential importance of coastal sediments as a source of phytoplankton to their overlying waters and the validity of using marine planktonic diatoms and dinoflagellates for modelling geological events is discussed.
Abstract: Sediment samples from Scottish coastal sites, taken over the last 9 years, were stored in closed containers at 5C. Slurry cultures were used to determine the survival of phytoplankton in these sediments. A range of diatom and dinoflagellate species survived for at least 27 months in these stored samples. A number of species grew for which no resting stage has yet been described: Thalassiosira angulata, T.pacifica, T.punctigera, T.eccentrica, T.minima and T.anguste-lineata. Notable results were survival times of 73 months for Skeletonema costatum, 96 months for Chaetoceros socialis, C.didymus and C.diadema, 109 months for Scrippsiella sp. and 112 months for Lingulodinium polyedrum. A single sample was stored and repeatedly cultured for diatoms over a period of 16 months. The number of species cultured from the sediment declined over this time. Lingulodinium polyedrum cysts isolated from sediments collected at least 18 months previously gave a hatching success of 97%, and cysts isolated from a 9-year-old sample gave a hatching success of 3%. The study indicated the potential importance of coastal sediments as a source of phytoplankton to their overlying waters. The validity of using marine planktonic diatoms and dinoflagellates for modelling geological events is discussed.

147 citations


Journal ArticleDOI
TL;DR: Results show that microcystins are important feeding deterrents against grazing by Daphnia since feeding rates on Planktothrix increased signifi- cantly after an aqueous-methanolic extraction of the major part of microcysts, and copepods persisted in food avoidance, but exhibited high clearance rates onplanktOTHrix after a more lipophilic extraction was applied.
Abstract: Since the cyanobacterium Planktothrix rubescens, which dominates the phytoplankton community in Lake Zurich, is generally considered toxic to zooplankton, we addressed the question whether co-occurring zooplankton species have developed adaptive responses. Artificially shortened filaments (<30 µm in length) of P.rubescens significantly reduced survival of Thamnocephalus platyurus (Crustacea, Branchiopoda, Anostraca) naturally occurring in temporary ponds. In contrast to Thamnocephalus, the survival of co-existing zooplankton was unaffected (Eudiaptomus gracilis) or enhanced (Daphnia hyalina and Cyclops abyssorum). High sensitivity to the microcystins of Plank- tothrix was coupled to strict food avoidance in Eudiaptomus, but not in Thamnocephalus. Daphnia and Cyclops exhibited higher physiological resistance to cyanobacterial toxins, and ingested Plank- tothrix. For the lake zooplankton species, the feeding rates on high-quality algae were not significantly reduced in the presence of Planktothrix. In order to separate the effects of mechanical interference (filament length) versus toxins, clearance rates on Planktothrix filaments were compared to clearance rates on filaments subjected to toxin extraction. The results show that microcystins are important feeding deterrents against grazing by Daphnia since feeding rates on Planktothrix increased signifi- cantly after an aqueous-methanolic extraction of the major part of microcystins. On the other hand, copepods persisted in food avoidance, but exhibited high clearance rates on Planktothrix after a more lipophilic extraction was applied. Both microcystins and a lipophilic, unidentified toxin may contribute to the avoidance behaviour of copepods. For both Daphnia and copepods, the grazing resistance of Planktothrix is mediated by chemical defences rather than by the large size and the rigidity of the filaments.

137 citations


Journal ArticleDOI
TL;DR: Potentially, nutrient recycling from crustacean zooplankton could enhance N limitation of phytoplankon, but small stoichiometric differ- ences suggest that this effect is probably weak.
Abstract: The carbon (C), nitrogen (N) and phosphorus (P) contents (% of dry weight) of some crust- acean zooplankton were studied in the Baltic Sea. The copepod Acartia sp. had a stable C and N content (48.3 ± 0.8% C, 12.4 ± 0.2% N, C:N ratio 4.5 ± 0.1). The P content was variable (1-2%), probably depending on developmental stage and season. Copepods accumulating fat, like Pseudo- calanus minutus elongatus, had higher and more variable C content (50-60%), and lower N and P content (7-12% N, 0.6-1.5% P). The highest C and lowest N and P contents were found in adult Limnocalanus macrurus. However, the N:P ratio was apparently independent of fat content and between 14 and 27 for all copepods. The cladocerans Bosmina longispina maritima and Evadne nordmanni had lower N content (9.3-10.8%) and higher C:N ratio (5.1-5.7) than Acartia sp. The P content (1.2-1.4%) was similar to Acartia sp. and the N:P ratios (16-19) were in the lower range of that found for the copepods. The N:P ratio was generally somewhat higher in the copepods than in seston, which most of the year had nearly Redfield C:N:P ratios. Potentially, nutrient recycling from crustacean zooplankton could enhance N limitation of phytoplankton, but small stoichiometric differ- ences suggest that this effect is probably weak. The extent is dependent on the structure of the zooplankton community and the gross growth efficiencies. Acartia copepodites, which had nearly Redfield N:P ratios, would have the opposite effect and enhance P limitation in late summer when seston N:P ratios increased.

133 citations


Journal ArticleDOI
TL;DR: Although the ST data coverage was limited, high spring algal specific growth rates, the large change in total community biomass between winter and spring, and small seasonal differences in earlywinter and spring macronutrient concentrations suggest that the structure and functioning of this ecosystem depended on the macronUTrient concentrations entrained during deep winter mixing.
Abstract: Data are synthesized on biomass and fluxes of components of the pelagic food web, downward particulate flux, and associated physical and chemical oceanographic parameters in austral winter and spring in the Subtropical Front (STF) region east of New Zealand. All four food web types (sensu Legendre and Rassoulzadegan, Ophelia, 41, 153-172, 1995) are represented in this region. The STF food web is classified as 'multivorous' in spring and 'herbivorous'/'multivorous' in winter, even though grazing was dominated by microzooplankton. Based on relatively few data over each season, STF community biomass was dominated by large phytoplankton cells (>20 µm) in both seasons. Post- bloom conditions were sampled in spring and highest fucoxanthin vertical fluxes were recorded at this time. A phytoplankton sedimentation event may have occurred just after our mid-October sampling. Subtropical (ST) waters exhibited pronounced seasonality in all plankton compartments. The ST food web is classified as 'multivorous' in both winter and spring. Organic flux in sediment traps was highest in ST water relative to subantarctic (SA) and STF waters. Although the ST data coverage was limited, high spring algal specific growth rates, the large change in total community biomass between winter and spring, and small seasonal differences in early winter and spring macronutrient concentrations suggest that the structure and functioning of this ecosystem depended on the macronutrients entrained during deep winter mixing. Algal populations in SA high-nitrate-low-chlorophyll waters (HNLC) were dominated by small cells. Heterotrophic bacteria dominated community biomass in SA waters in winter and equalled phytoplankton biomass in spring. In contrast to Legendre and Rassoulzadegan's classification of the subarctic Pacific HNLC food web as 'multivorous', the SA food web is classified as 'microbial' tending towards 'microbial loop' in winter (i.e. there was greater carbon uptake by bacteria than phytoplankton) and 'microbial' in spring. Chlorophyll (Chl) a levels in spring were similar to those in winter, but because of a doubling of the C:Chl ratio, algal biomass (as carbon) was 2-fold higher in SA waters in spring, compared with winter. This seasonal increase in algal carbon biomass in spring occurred in spite of a >5-fold increase from winter to spring in microzooplankton biomass, challenging the notion of grazer control of algal biomass. Our findings are compared with Russian data collected in the South Pacific STF region in summer 1985. Aspects of ecosystem func- tioning in the STF region east of New Zealand may hold for other longitudes in the Southern Ocean, although it is likely that some characteristics are regional in nature.

125 citations


Journal ArticleDOI
TL;DR: Besides the potential of the crustacean zooplankton to influence the structure of ciliate communities, ciliates may contribute to the energy demands of copepods and daphnids, especi- ally when phytoplankon resources are limited.
Abstract: A series of single-factor in situ experiments was conducted in a mesotrophic lake in Bran- denburg, North Germany, to study the predatory impact of Eudiaptomus graciloides (adults, cope- podites, nauplii), cyclopoid copepods (adult Diacyclops bicuspidatus, Thermocyclops oithonoides) and daphnids (adult Daphnia hyalina, Daphnia cucullata) on the microbial community (bacteria, autotrophic picoplankton, flagellates, ciliates). All zooplankton species tested reduced the ciliate community significantly and ingestion rates were always higher for ciliates in the 20-55 µm size category as compared to smaller ciliates (10-20 µm). Adult E.graciloides, which exhibited the highest predatory impact on ciliates, differed from cyclopoids and daphnids by their ability to decimate ciliates to very low abundances. Ingestion rates of ciliates by the crustacean zooplankton followed the sequence E.graciloides > daphnids = cyclopoids = copepodites. While top-down control was evident for ciliates, top-down effects down to the autotrophic picoplankton and flagellates were mostly restricted to Daphnia-dominated treatments. Top-down effects were never strong enough to produce negative bacterial growth rates. For all zooplankton tested, clearance rates for ciliates exceeded those for phytoplankton. Besides the potential of the crustacean zooplankton to influence the structure of ciliate communities, ciliates may contribute to the energy demands of copepods and daphnids, especi- ally when phytoplankton resources are limited.

98 citations


Journal ArticleDOI
TL;DR: Differences in Se requirement between these bloom-forming phytoplankton species suggest that this micronutrient may play a role in structuring phy Topolankton communities in southern Australian waters.
Abstract: This study investigated the selenium (Se) requirements of three phytoplankton species which commonly bloom in southern Australian estuaries. The present study showed that the toxic dinoflagellate Gymnodinium catenatum Graham had an obligate requirement for Se (IV) in culture. After two transfers (~4 weeks 7 generations) in Se-deficient seawater medium, this phytoplankton species exhibited a decline in growth rate (25%) and biomass yield (90%), while complete cessation of cell division occurred under prolonged (8 weeks  12 generations) Se starvation. Addition of 10-9-10-7 M H2SeO3 to nutrient-enriched seawater medium resulted in increased G.catenatum growth and biomass yields in direct proportion to the Se concentrations offered. In contrast to G.catenatum, Se limitation was observed in the dinoflagellate Alexandrium minutum Halim after four transfers (5 weeks 20 generations) in Se-deficient medium. Exponential growth rates of A.minutum decreased slightly (5-10%) when Se was not supplied, but biomass yields decreased as much as 80-90%. The diatom Chaetoceros cf. tenuissimus Meunier showed no evidence of Se limitation even after eight transfers (8 weeks; >60 generations) in Se-deficient medium. Variations in growth rates and biomass yields between transfers provide valuable information about the relative potential for Se limitation in the three species studied. In addition, differences in Se requirement between these bloom-forming phytoplankton species suggest that this micronutrient may play a role in structuring phytoplankton communities in southern Australian waters.

97 citations


Journal ArticleDOI
TL;DR: It is suggested that the high nutrient concentra- tions typically found in ECE waters can alleviate nutrient limitation of phy toplankton growth and couple phytoplankston growth with light and stratification seasonality.
Abstract: We test, through a compilation of published data, whether specific trends in phytoplankton biomass seasonality can be generalized in northern temperate enclosed coastal ecosystems (ECE) and northern temperate open coastal ecosystems (OCE). Bimodal cycles (i.e. displaying two annual peaks) are the most frequent trends in both ecosystems, and whereas they mostly peaked at late winter and fall in OCE, they mostly peaked in spring and late summer, and also over a wider range of months, in ECE. The interaction between restricted depth or tidal stirring and light seasonality in ECE may be responsible for these differences. Moreover, bimodal cycles reached higher biomass peaks in ECE, which can result from higher water column nutrient concentrations. Unimodal cycles (one annual peak) also occurred commonly in both ecosystems, and whereas they peaked throughout winter and spring in OCE, they peaked mostly in summer in ECE. We suggest that the high nutrient concentra- tions typically found in ECE waters can alleviate nutrient limitation of phytoplankton growth and couple phytoplankton growth with light and stratification seasonality. These results reveal broad-scale patterns in phytoplankton seasonality in temperate coastal ecosystems, and help identify physical processes generating these patterns.


Journal ArticleDOI
TL;DR: Primary production, microbial production and the density of planktonic microheterotrophs were estimated at 40 stations in the Okhotsk Sea in July-August 1992 during the seasonal phyto- plankton minimum, and tentative calculations were made of the basin's annual primary production and for the analysis of energy balance in the ecosystem.
Abstract: Primary production, microbial production and the density of planktonic microheterotrophs were estimated at 40 stations in the Okhotsk Sea in July-August 1992 during the seasonal phyto- plankton minimum. The primary production by phytoplankton remained rather high even during this minimum. At most stations it was >0.6-0.8 g m-2 day-1, and in leftover patches of spring diatom 'bloom' it reached >5 g C m-2 day-1. The deep maxima of phytoplankton at the upper boundary of the seasonal thermocline were an ordinary phenomenon. The depth of the euphotic zone was normally 30-50 m in the open sea and 12-25 m at the shelf stations. Any correlations between the phosphate contents in the upper mixed layer and primary production were absent at the stations. There was no adaptation of the phytoplankton to the light deficiency in deep maxima layers. The total numbers of bacterioplankton were 1-1.5 3 106 ml-1 and its biomass was close to 100 mg m-3 in the open sea. All these numbers were 2-3 times greater at the shelf stations. In deep waters, the bacterio- plankton biomass decreased to 10-40 mg m-3. The microbial production in the upper layer was high, at 50-100 mg m-3, decreasing 50-100 times in the deep waters. The numbers of ciliates in the upper water layer varied from 3 to 6 3 103 l-1 and were 1.5-2 times greater than in the shelf areas. Ciliate biomass was 60-100 mg m-3 in the upper mixed layer, and per square metre varied to 1.5-2.5 g. The dominant ciliate taxa belonged to the naked oligotrichid genera Strombidium and Tontonia. Tentative calculations were made of the basin's annual primary production and for the analysis of energy balance in the ecosystem.

Journal ArticleDOI
TL;DR: The synergistic functioning of these processes probably explains the broad patterns of prey ingestion found by in situ studies of Mnemiopsis feeding.
Abstract: Although the lobate ctenophore Mnemiopsis leidyi is an influential planktonic predator, the mechanisms enabling it to capture its characteristically wide range of prey have not been systematic- ally examined. We recorded interactions between free-swimming M.leidyi and two stages (nauplii, adults) of the calanoid copepod Acartia tonsa in order to determine a mechanistic explanation of this feeding process. Prey encounter with Mnemiopsis involved two different processes. The first depended on fluid motions created by the nearly continuous beating of cilia lining the four auricles. These cilia created a low-velocity flow in which A.tonsa nauplii were entrained (94% of naupliar encounters) and transported past the oral lobes onto the tentillae (oral tentacles). The nauplii, although capable of rapid escape responses, generally appeared to be insensitive to the current in which they were carried. The second process relied upon the collision of swimming prey with the inner surfaces of the oral lobes and was not obviously influenced by the auricular feeding currents. Adult A.tonsa were rarely entrained in the auricular flow, but, instead, propelled themselves into contact with the oral lobes (97% of adult encounters). Both prey capture processes functioned simul- taneously. The synergistic functioning of these processes probably explains the broad patterns of prey ingestion found by in situ studies of Mnemiopsis feeding.

Journal ArticleDOI
TL;DR: This work examines two classes of ecosystem models capable of representing a fundamental duality in the way planktonic communities are viewed, and examines an alternative food web structure in which total herbivore abundance is represented by a single state variable, G, and where grazing pressure is distributed among taxa in proportion to their abundances.
Abstract: There is a fundamental duality in the way we view planktonic communities. On one hand, we see these communities as composed of a diverse mixture of taxonomically and biogeochemically distinct species; pelagic ecosystem models capable of predicting the ocean's role in the global carbon cycle will need to resolve these multiple plankton taxa. However, we have also come to appreciate that superimposed on this diversity are regular patterns of size structure, where plots of abundance within size classes typically show a power-law dependence on size. A truly complete model descrip- tion of planktonic community structure must reflect both aspects of this duality. Here I examine two classes of ecosystem models that are capable of representing this duality. The first structure is based on multiple (n) couplets of phytoplankton (P) and zooplankton (Z) (an 'n(PZ)' model). Within certain regions of parameter space, this model structure can produce anomalous oscillatory behaviors, the mathematical origin of which is explored in detail. I then examine an alternative food web structure in which total herbivore abundance is represented by a single state variable, G, and where grazing pressure is distributed among taxa in proportion to their abundances. This '(nP)G' system is stabilized by the redistribution of grazing intensity among taxa in response to changes in their densities. This 'distributed grazing' model also naturally produces size spectra of plankton abundance; this last observation argues that the model with distributed grazing can help in uncovering and repre- senting the mechanistic basis for the genesis and maintenance of planktonic size spectra of phyto- plankton and bacteria.

Journal ArticleDOI
TL;DR: The A.taylori bloom can be considered a manifestation of large-scale proliferation in a restricted area, where coupling between resting cysts in the sediment and bloom outbreak is not a major factor compared to the interaction of local environmental conditions with the planktonic organism's life history.
Abstract: A recurrent, prolonged and singular bloom of Alexandrium taylori Balech in an open beach (La Fosca, Spain, NW Mediterranean) is described. Alexandrium taylori appears at several places along a wide area of the NW Mediterranean (Costa Brava) during the summer, reaching concentra- tions up to 10 5 cells l -1 , but it only proliferates persistently, massively (densities >10 6 cells l -1 ) and recurrently during August in La Fosca beach. The A.taylori bloom can be considered a manifestation of large-scale proliferation in a restricted area, where coupling between resting cysts in the sediment and bloom outbreak is not a major factor compared to the interaction of local environmental conditions with the planktonic organism's life history. From observations of environmental conditions (the environmental window) and the multiscale spatio-temporal distributions and life history of A.taylori, we describe the bloom dynamics and answer some critical questions about the different phases of the bloom. Some of these answers are: (i) the source of the A.taylori population is wide- spread offshore and is not located directly at the beach; (ii) high cell densities are reached and main- tained with a moderate in situ growth and low loss rates; (iii) temporary cysts act as a reserve of the population.

Journal ArticleDOI
TL;DR: The vertical migration of a zooplankton community dominated by the euphausiid Meganyctiphanes norvegica was monitored between 16 and 23 September 1997 with a 153 kHz Acoustic Doppler Current Profiler (ADCP) and a MOCNESS net.
Abstract: The vertical migration of a zooplankton community dominated by the euphausiid Meganyctiphanes norvegica was monitored between 16 and 23 September 1997 with a 153 kHz Acoustic Doppler Current Profiler (ADCP) and a MOCNESS net. The sampling period covered a phase in the lunar cycle when the rise of the moon (full moon) coincided initially with sunset and then became progressively later. On 16 September 1997, a lunar eclipse occurred 45 min after sunset, lasting for ~2 h. At dusk, the ADCP observed the upward vertical migration of two principal backscat- tering bands ~10 min apart with vertical velocities of up to 7 cm s-1. After a period at the surface, a more diffuse band subsequently sank at a slower rate (1-2 cm s-1) to a depth of 75-100 m. Net samples showed that the earlier band consisted mainly of the pteropod Cavolinia inflexa, whilst the later band was mostly euphausiids, predominantly M.norvegica. This species was also the major constituent of the band that sank. The timing of upward migration was relatively constant over the sampling period, but there was an increasing delay of the secondary sinking until 21 September. This showed as a strong correlation between the onset of sinking and the time of moonrise. The lunar eclipse on 16 September perturbed this pattern, such that animals did not sink soon after their arrival at the surface, as occurred on 17 September, but remained at the surface until the end of the umbra. This suggests that M.norvegica can perceive moonlight and that this influences vertical migration. Evidence that the behaviour is not solely mediated by this exogenous factor, however, is seen in the pattern that emerged after 21 September, when midnight sinking occurred at a relatively constant time after sunset and before moonrise. These observations support the hypothesis that moonlight is a Zeitgeber for an endogenous rhythm that synchronizes secondary sinking behaviour with the lunar cycle.

Journal ArticleDOI
TL;DR: The presence of TEP appears to enhance, rather than depress, macrozooplankton grazing, and TEP-clusters, aggregates which formed from TEP and nano-sized particles normally too small for the filtering apparatus of E.pacifica served as an alternative food source for the euphausiid, reducing their ingestion of cells.
Abstract: The hypothesis that ubiquitous, sticky transparent exopolymer particles (TEP) formed from phytoplankton exudates will adhere to and coat the feeding structures of marine zooplankton grazers, and thus depress feeding on phytoplankton, was tested using the euphausiid, Euphausia pacifica, as a model organism. During two feeding experiments, E.pacifica were offered cells of the diatom Thalassiosira weissflogii, TEP, or both TEP and T.weissflogii cells. Ingestion rates on cells were lower in the presence of TEP. However, contrary to the hypothesis, grazing on cells was not inhibited by TEP. Rather, TEP-clusters, aggregates which formed from TEP and nano-sized particles normally too small for the filtering apparatus of E.pacifica to retain, served as an alternative food source for E.pacifica, reducing their ingestion of cells. These clusters were very similar in form to the TEP actually available to marine grazers in nature. TEP-clusters were similar to cells in size and food quality, and were grazed at similar rates. When feeding on TEP-clusters, euphausiids short circuit the food web by feeding on nano- and picoplankton directly, bypassing the microbial loop. Thus, the presence of TEP appears to enhance, rather than depress, macrozooplankton grazing.

Journal ArticleDOI
TL;DR: The possible effects of five Microcystis aeruginosa strains on the filtering and feeding behavior of Daphnia galeata were investigated to clarify the mechanisms of the inhibitory effect on the ingestion rate of the animals.
Abstract: The possible effects of five Microcystis aeruginosa strains (two unicellular and three colony forming) on the filtering and feeding behavior of Daphnia galeata were investigated to clarify the mechanisms of the inhibitory effect on the ingestion rate of the animals. Of the tested cyanobacterial strains, three inhibited Daphnia's food ingestion. This was primarily caused by affecting the frequency and food transport efficiency of the maxillary contractions. As a result, Daphnia's swallowing rate, and in one case the amount of food contained in the swallowed boluses, were lower than in suspen- sions of well-ingestible food sources. In contrast, there is no indication that Microcystis can influence thoracic appendage beat or rejection rate. It is likely that colony-forming and unicellular strains can affect the ingestion process due to different factors. The mucilage of the colony-forming Microcystis strains HUB-W75 and HUB-W334 possibly produced a mechanical hindrance of the maxillules, which resulted in an abnormal movement and consequently in the inhibitory effects described above. This was related to the sugar composition of the mucilage polysaccharides, but not to the cellular micro- cystin content or occurrence of accompanying bacteria. The mucilage-lacking strain HUB 5-2-4, however, must contain another factor which interfered with the maxillules' movement in an unknown way.

Journal ArticleDOI
TL;DR: Individual carbon contents of eight commonCopepod species from the Andaman Sea were determined and equations derived are expected to be general charac- teristics, which are applicable for the calculation of copepod biomass and production in tropical areas.
Abstract: Individual carbon contents of eight common copepod species from the Andaman Sea were determined. Length-carbon regressions are presented for four calanoids (Acrocalanus gibber, Centropages furcatus, Temora discaudata and Euchaeta spp.), two cyclopoids (Oncaea spp. and Corycaeus spp.) and two harpacticoids (Macrosetella gracilis and Microsetella spp.). The copepod specimens were obtained from different localities and times of the year during 1996-1997. The regres- sion coefficients are good in calanoid and cyclopoid copepods, but poor in the harpacticoids. The slope values range from 2.3 to 3.8 in the calanoids to 1.2 and 1.6 in the harpacticoids, while the cyclopoids have slope values of 2.0 and 2.9. The equations derived from this study are expected to be general charac- teristics, which are applicable for the calculation of copepod biomass and production in tropical areas.

Journal ArticleDOI
TL;DR: A strong influence of silicate metabolism on the photosynthetic efficiency of the reaction centres of PSII assessed by PAM fluorescence implies that silicate, as well as other nutrients, has to be considered as a possible cause for variable photosynthesis efficiency of diatoms.
Abstract: Pulse-amplitude-modulation (PAM) fluorometry was used to investigate the effects of varying the silicate concentration on different fluorescence characteristics of batch and chemostat cultures of the diatom Thalassiosira weissflogii. The fluorescence signals, measured both on- and off- line, were used to calculate the actual (fP) and potential (fP0) photochemical efficiencies of the reaction centres of photosystem II (PSII). Also the relative electron transport rate of the reaction centres of PSII (Je) was calculated. Fluorescence decreased in silicate-limited cells and increased rapidly after silicate addition. The decrease was caused by strong non-photochemical quenching (qN) in silicate-limited cells. Continuous recording (on-line) of the minimum and maximum fluorescence provided data with high temporal resolution, revealing that first recovery reactions of silicate-limited cells occurred 20 min after the addition of silicate. Based on these observations, we assume a strong influence of silicate metabolism on the photosynthetic efficiency of the reaction centres of PSII assessed by PAM fluorescence. This implies that silicate, as well as other nutrients, has to be considered as a possible cause for variable photosynthetic efficiency of diatoms.

Journal ArticleDOI
TL;DR: A great intraspecific variabil- ity in host susceptibility to viruses present in natural samples is found, with viral titres ranging over one or two orders of magnitude for the same samples incubated on different M.pusilla strains.
Abstract: To assess the role of viruses in the bloom dynamics of Micromonas pusilla in the Gulf of Naples (Mediterranean Sea), variations of host and virus abundance were followed over one annual cycle and in late winter-spring of three consecutive years. Micromonas pusilla was recorded from autumn to spring, with peak values up to 6.6 3 103 cells ml-1, but was undetectable in summer. Free M.pusilla viruses were detectable in all seasons, with concentrations from 0.02 viruses ml-1 to 1.9 3 103 viruses ml-1, exceeding host abundances only in one case. We found a great intraspecific variabil- ity in host susceptibility to viruses present in natural samples, with viral titres ranging over one or two orders of magnitude for the same samples incubated on different M.pusilla strains. Over the winter-spring periods, a highly dynamic situation was evident, with wide fluctuations for both host and virus abundances from one week to another. In some cases, peak host concentrations were accom- panied by an increase in viral numbers, whereas in other cases the respective fluctuations were uncoupled. Although fluctuations of M.pusilla abundance could be influenced by viral infection, there was no evidence that viruses were able to terminate host blooms. The summer decline of M.pusilla populations did not appear to be related to the impact of viral infection.

Journal ArticleDOI
TL;DR: The raphidophyte flagellate Chattonella marina was successfully cultured from Boston Bay (South Australia), coincident with mass mortality of farmed bluefin tuna in April 1996, and the different light adaptation phenotypes were still apparent after long-term culturing under similar physiological conditions.
Abstract: The raphidophyte flagellate Chattonella marina was successfully cultured from Boston Bay (South Australia), coincident with mass mortality of farmed bluefin tuna (Thunnus maccoyii) in April 1996. Grown under laboratory conditions at 150 µmol m-2 s-1 irradiance, optimal growth (>0.5 day-1) occurred at a temperature of 25°C and a salinity of 30 p.s.u., but good growth (>0.3 day-1) also occurred between a temperature of 10 and 30°C and at a salinity of 15-45 p.s.u. However, cultures grew much faster at an irradiance of 450 µmol m-2 s-1 (1.08 day-1). While Australian C.marina had similar temperature and salinity requirements as well-studied Japanese cultures from the Seto Inland Sea, the Australian strains exhibited a light saturation level for growth four times higher than that reported from Japan (150 µmol m-2 s-1). An adaptation to higher light intensities was reflected in higher concentrations of microsporine-like amino acids in the Australian strains. The different light adaptation phenotypes were still apparent after long-term culturing under similar physiological conditions. Potential growth habitats for this ichthyotoxic flagellate in the Australian region and impli- cations for finfish aquaculture industries are discussed.

Journal ArticleDOI
TL;DR: Overall, the dynamics of the near-bottom planktonic communities was characterized by a low biomass of heterotrophic and autotrophic bacteria, phytoplankton and ciliates in contrast to previous water column studies.
Abstract: Microbial planktonic communities (i.e. bacteria and protozoa), phytoplankton, dissolved organic carbon (DOC) and particulate organic carbon (POC) were seasonally examined at Medes Islands (Northwestern Mediterranean) to assess their variation in abundance and composition throughout the year in a near-bottom littoral ecosystem. From October 1995 to November 1996, samples were collected between two and six times per month at 0.5 m above the bottom. Mean DOC and POC values throughout the year were 2560 ± 180 (SE) and 387 ± 35 µg C l -1 , respectively. All year, detrital organic carbon (detrital = total POC - live carbon) represented the main POC fraction, and mean live carbon was 24 ± 9 µg C l -1 . Winter and spring had maximum values of POC, and spring and summer had maximum values of DOC. Heterotrophic bacteria, with a mean abundance of 5.16 ± 0.08 3 105 cells ml-1, were the main contributor to live carbon (26 ± 7%). During winter, heterotrophic bacterial biomass decreased 40% due to a decrease in mean biovolume per cell. Synechococcus sp. and Prochlorococcus sp. abundance were 2.24 ± 0.09 3 104 and 1.05 ± 0.07 3 104 cells ml-1, respec- tively. However, while Synechococcus sp. were present all year, Prochlorococcus sp. were not observed from April to July. Mean phytoplankton (i.e. diatoms and dinoflagellates) abundance was 2.06 ± 0.40 3 104 cells l-1 with biomass at a maximum during the winter months, the period with the lowest temperature and the highest nutrient concentration. The size composition of live carbon showed two clearly distinct periods: from December to March, live carbon was dominated in biomass by microplankton, while from April to November, pico- and nanoplankton cells were dominant. Overall, the dynamics of the near-bottom planktonic communities was characterized by a low biomass of heterotrophic and autotrophic bacteria, phytoplankton and ciliates in contrast to previous water column studies. This pattern and the high temporal heterogeneity of the different planktonic communities are discussed in relation to the physical and chemical characteristics of the environment, as well as to the potential role that benthic communities may be exerting in the control of the near- bottom planktonic communities.

Journal ArticleDOI
TL;DR: Bacterial grazing was measured from June to August 1995 in Lake Ortrasket, a deep brown-water lake in northern Sweden, where Mixotrophic chrysophytes were the dominating bacterivores at all times.
Abstract: Bacterial grazing was measured from June to August 1995 in Lake Ortrasket, a deep brown-water lake in northern Sweden. Mixotrophic chrysophytes were the dominating bacterivores at all times, grazin ...


Journal ArticleDOI
TL;DR: Three different species of marine diatoms were isolated from the Adriatic Sea and studied in vitro for the production of extracellular carbohydrates, finding that at low N/P ratios, S.costatum and Chaetoceros sp.
Abstract: Three different species of marine diatoms, Nitzschia closterium (Ehr) Smith, Chaetoceros sp. (Ehr) and Skeletonema costatum (Grev) Cleve, were isolated from the Adriatic Sea and studied in vitro for the production of extracellular carbohydrates. Variations of the nitrogen (N)/phosphorus (P)/silicon (Si) ratios in the growth medium affect the accumulation and release of carbohydrates. In the N.closterium cultures at high N/P ratio, the rate of extracellular polysaccharide release was higher both in rapidly growing cells (2.14 µg per 106 cells day-1) and in stationary phase (1.0 µg per 106 cells day-1) compared to S.costatum and Chaetoceros sp. Instead, at low N/P ratios, S.costatum and Chaeto- ceros sp. produced large amounts of extracellular polysaccharides during the logarithmic phase, compared to N.closterium, with values of 10 µg per 106 cells day-1 for S.costatum and 2.8 µg per 106 cells day-1 for Chaetoceros sp., respectively; in particular, only S.costatum was able to produce extra- cellular carbohydrates (1.2 µg per 106 cells day-1) during the stationary phase of growth. Under Si limitation, any of the three diatom species produce extracellular polysaccharides both in the loga- rithmic and stationary phase of growth. The potential ecological significance of these findings is discussed.

Journal ArticleDOI
TL;DR: The results show that, at least during an important part of the winter-spring bloom period of the Catalano-Balearic Sea, high phytoplankton biomass offshore of the Catalan Front is not associated with the diatom dominance typically found in coastal waters and other offshore areas of the NW Mediterranean, which has biogeochemical implications in relationship with vertical export fluxes of particulate matter.
Abstract: The qualitative and quantitative composition of the phytoplankton assemblages of the Catalano-Balearic Sea (NW Mediterranean) was examined in relationship with frontal boundaries and short-term meteorological forcing originated by an intense wind storm. Data were obtained during February 1990, before and after strong NW winds, from several transects across the Catalan (continental side) and North Balearic (Balearic Islands side) shelf/slope fronts. High chlorophyll concentrations and diatom dominance inshore of the Catalan Front were typical of the winter-spring phytoplankton maximum. However, offshore of the Catalan Front, diatoms were scarce and the phytoplankton assemblage was dominated by coccolithophorids. In spite of weak density stratifi- cation, a deep chlorophyll maximum (DCM) was well developed, before the storm, offshore of the Catalan Front. The wind event shifted the North Balearic Front 15 km further offshore and appeared to cause a pulse of new production, as suggested by increased oxygen concentrations in the vicinity of the DCM. The results presented here indicate the existence of a persistent association between the distribution of distinct phytoplankton assemblages and hydrographic characteristics of the studied area. Our results also show that, at least during an important part of the winter-spring bloom period of the Catalano-Balearic Sea, high phytoplankton biomass offshore of the Catalan Front is not associ- ated with the diatom dominance typically found in coastal waters and other offshore areas of the NW Mediterranean. This finding has biogeochemical implications in relationship with vertical export fluxes of particulate matter.

Journal ArticleDOI
TL;DR: The varied responses of dominant zooplankton species appeared to be the result of a combination of factors, and laboratory feeding studies indicate that M.relicta preferred Cladocera over Copepoda, with the following order of feeding preference: Bosmina longirostris > D.longiremis > Daphnia thorata > L.ashlandi > Leptodiaptomus ashlandi.
Abstract: We documented major changes in the zooplankton community of Flathead Lake following the appearance of Mysis relicta. The three common cladocerans found in the lake decreased in abundance, most notably Daphnia longiremis which virtually disappeared from the lake. Copepods were also affected by M.relicta, especially Diacyclops thomasi which decreased in abundance by an order of magnitude. The only macrozooplankton species which seemed to benefit from the presence of M.relicta was Leptodiaptomus ashlandi. The varied responses of dominant zooplankton species appeared to be the result of a combination of factors. Laboratory feeding studies indicate that M.relicta preferred Cladocera over Copepoda, with the following order of feeding preference: Bosmina longirostris > D.longiremis > Daphnia thorata > L.ashlandi > D.thomasi. The two zooplank- ton species which declined most following the appearance of M.relicta showed the greatest degree of habitat overlap with M.relicta. Daphnia longiremis and D.thomasi, together with M.relicta, are cold stenotherms and were concentrated in the hypolimnion during the summer. Finally, another trait shared by D.longiremis and D.thomasi was the absence of a diapause stage, which made them vulner- able to predation by M.relicta throughout the year.

Journal ArticleDOI
TL;DR: It is concluded that food availability (Chl a concen- tration), rather than temperature, limits the egg production of A.bifilosa in the present study area in the northern Baltic Sea.
Abstract: Egg production of a brackish water calanoid copepod Acartia bifilosa was measured in the laboratory in different chlorophyll (Chl) a concentrations (0-24 µg l-1) and temperatures (4-24°C), and the cephalothorax length and carbon content of females were determined. Egg production was positively correlated both with Chl a concentration and with temperature; highest egg production was obtained with 14-20 µg Chl a l-1 and at 13-18°C. There was also a significant positive correlation between egg production and female length-specific carbon content (µg C µm-1). However, no correl- ation was observed between egg production and cephalothorax length of females. Female carbon content changed during the 3 day experiments; carbon content was positively related to Chl a concen- tration and negatively related to temperature. We conclude that food availability (Chl a concen- tration), rather than temperature, limits the egg production of A.bifilosa in the present study area in the northern Baltic Sea. Further, both food concentration and temperature affect egg production not only through the direct effect on the numbers of eggs produced per female, but also through their effect on female carbon content.