Search Results - (Author, Cooperation:L. Legendre)

2015-12-19

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

*Carbon Cycle ; Carbon Dioxide/*chemistry ; *Oceans and Seas ; Seawater/*chemistry/*microbiology

2018-03-06

Oxford University Press

1054-3139

1095-9289

Biology

Geosciences

Physics

0031-9422

Glycine max ; Gossypium hirsutum ; Leguminosae ; Malvaceae ; Verticillium dahliae ; cell suspension cultures ; elicitation ; oxidative burst. ; phytoalexins

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Biology

Chemistry and Pharmacology

Electronic Resource

1432-1793

Springer Online Journal Archives 1860-2000

Biology

Abstract Time series of chlorophyll a, photosynthetic capacity and many physical parameters were sampled hourly for 167 h in August, 1975, at an anchor station located in the Middle Estuary of the St. Lawrence River, Canada. Sampling was carried out during the transition from neap tides to spring tides. The long-and short-term variations in chlorophyll a are coupled with the advection of water masses which depends on tidal currents. Vertical mixing also influences the chlorophyll a concentration of the cells, since it modifies the physiological state of the phytoplankton. Furthermore, circadian periodicities were observed in the photosynthetic capacity, suggesting that the phytoplankton of this area have a homogeneous light history due to strong vertical mixing. Under these conditions, the photosynthetic capacity is adapted to the mean light intensity in the mixed layer. The semimonthly (M f) variations of the mean light intensity in the mixed layer depend on the M f variations in the vertical mixing, whereas in the short-term, the variations in mean light intensity in the mixed layer are circadian.

Electronic Resource

1432-1793

Springer Online Journal Archives 1860-2000

Biology

Abstract Primary production within the Baie des Chaleurs, Canada, is dominated by a cyclonic gyre system. The pattern of production fits the theoretical expectation of increased productivity associated with colder, nutrient-rich water being brought closer to the surface within the gyre. Upper reaches of the bay, west of the gyre, follow a normal esturine form of circulation with lower rates of production. East of the gyre, in the Gulf of St. Lawrence, primary production is less.

Electronic Resource

1432-1793

Springer Online Journal Archives 1860-2000

Biology

Abstract Sea ice offers a physical support to the growth of microalgae (epontic community). Almost all the studies on ice microflora in the Arctic and the Antarctic have been performed in waters of high salinities, and they generally reported a very high standing crop. From February through May 1978, 15 stations were sampled in the southeastern part of Hudson Bay (Manitounuk Sound), Quebec, Canada. The peculiar physical condition of the water bodies establishes a salinity gradient in the underlying waters from the mouth of the Great Whale River to the upper reaches of Manitounuk Sound, leading to a parallel gradient in the ice dynamics. The main goal of this study was to examine the effect of these gradients on the biomass and the taxonomic composition of the epontic community. Thus, there are two distinct gradients in the ice, one ruled by the surface-water salinity gradient controlling the ice thickness and the settlement efficiency, which triggers the biomass of the epontic community (higher standing crop in the upper reaches of Manitounuk Sound). In addition, the ice salinity gradient influences the taxonomic composition (lower number of microalgae taxa toward the mouth of the Great Whale River). Salinity of the underlying surface waters, following discriminant analyses of species-based clusters of observations, emerges as a major environmental variable controlling the distribution and abundance of species. In addition to adequate light intensity and nutrient supply, salinity of the underlying waters is therefore another limiting factor to the settlement and growth of an ice microflora. This factor is of great potential significance in coastal and esturine glacial waters.

Electronic Resource

1432-1793

Springer Online Journal Archives 1860-2000

Biology

Abstract In order to test the ability of phytoplankton to adapt to the high frequency light fluctuations induced by sea surface waves, the green alga Dunaliella tertiolecta was grown under both steady and fluctuating (0.1, 1.0 and 10 Hz) illuminations. The latter conditions reproduced those fluctuations experienced by phytoplankton in the upper photic layer. For each culture, photosynthesis versus irradiance were measured under four incubation frequencies (steady, 0.1, 1.0 and 10 Hz fluctuating illuminations). Results indicated that growth rates were similar for algae grown under steady light and 10 Hz fluctuating light (0.26–0.33 d−1). Cells grown at 0.1 and 1.0 Hz showed lower growth rates (0.17–0.26 d−1). Chlorophyll a and b were significantly higher under 0.1 and 10 Hz frequencies than under steady illumination; at 1.0 Hz, there were no significant differences with steady light. No changes in carotenoids were evidenced at any frequency tested. Photosynthetic measurements showed that algae grown under steady illumination had higher photosynthetic efficiency and capacity when incubated under steady and 0.1 Hz fluctuating light. Photosynthetic characteristics of algae grown under 0.1 Hz illumination did not show any clear responses to fluctuating light. Algae grown under 1.0 or 10 Hz had higher photosynthetic efficiency and capacity than those grown under steady illumination, when incubated under 1.0 and 10 Hz light. This suggests that microalgae grown under high frequency illumination (1.0 and 10 Hz) can adapt their photosynthetic characteristics to the rapidly fluctuating light regime experienced during growth, and that algae grown under steady conditions respond better to steady or slowly fluctuating (0.1 Hz) light. Such an adaptation provides a means of probing the photosynthetic responses of phytoplankton to vertical mixing.

Electronic Resource

1432-1793

Springer Online Journal Archives 1860-2000

Biology

Abstract A biological oceanographic study of phytoplankton in the southeastern section of Hudson Bay was conducted in August and September 1976 in an effort to determine the relationships between phytoplankton and the summer hydrographic conditions in an Arctic coastal environment. Consistently low levels of nitrates were found in the deep waters of the Bay. Some physical oceanographers hypothesize that the deeper waters of Hudson Bay are uncoupled from the Atlantic Ocean, as intensive mixing of incoming oceanic waters occurs in the Hudson Strait. Such a circulation pattern, together with possibly low nutrient regeneration in Hudson Bay, could account for the low levels of nitrate observed in the deep waters. Average chlorophyll: adenosine triphosphate ratios of about 5:1 were observed in the surface coastal waters during this study, suggesting a possible nitrogen-limitation of phytoplankton at the end of the summer. The geographic distribution pattern of the phytoplankton taxa was related to the freshwater inflow from rivers and to the overall production of the phytoplankton. These results are discussed in relation to the previously reported low biological production of Hudson Bay.

Electronic Resource

1432-1793

Springer Online Journal Archives 1860-2000

Biology

Abstract In August 1984, hourly measurements of photosynthetic characteristics were carried out during 96 h, at 5 and 10 m, on a natural population of phytoplankton in the St Lawrence Estuary. Synchronous circadian variations of similar amplitude (max./min.: 2 to 3) were observed at the two depths in both the photosynthetic capacity (P m B ) and the photosynthetic efficiency (αB). Maximum values occurred at around noontime and minima during the night. Estimates of daily specific productivity were computed with and without the observed circadian variability. Large differences (15 to 70%) were evidenced between estimates.

Electronic Resource

1432-1793

Springer Online Journal Archives 1860-2000

Biology

Abstract The simultaneous uptake of nitrogenous nutrients and inorganic carbon was measured in shipboard incubations of natural phytoplankton populations, using tracer additions of 13C-bicarbonate and 15N-labelled nitrogenous substrates. From March 1991 through March 1992, three stations on the Scotian Shelf (eastern Canada) were sampled monthly at ten depths in the euphotic zone. Additions of labelled nitrogen compounds ranged between 0.5 and 98% of ambient concentrations. Most of the C/N (at/at) uptake ratios were lower than the Redfield ratio, suggesting that nitrogen was not limiting. The fixation of carbon with and without addition of nitrate, ammonium or urea was generally similar. Some samples presented significant differences in carbon uptake rate between the four treatments, but these differences were not related to nitrogen enrichment (percent or nitrogen species). Given these results, the double-labelling method appears to be a reliable tool for measuring the simultaneous uptake of carbon and nitrogen by natural phytoplankton.

Electronic Resource

1432-1793

Springer Online Journal Archives 1860-2000

Biology

Abstract A laboratory system has been developed to simulate wave-induced irradiance fluctuations that occur in the top few metres of the sea under sunny surface conditions. A principle of operation is that the fluctuations are produced after refraction of light by water waves generated in the tank. Simulated irradiance consists of repetitive high-amplitude “flashes” resulting from a lens-effect of the waves. Statistical properties of flashes are similar to those recorded at sea. In our laboratory simulation, the flashes that exceed an amplitude level of 1.5 Ē (where Ē is the time-averaged irradiance) have a mean frequency of about 230 min-1, and a typical duration of 5 to 20 ms. The frequency of the flashes decreases exponentially with increasing amplitude level. The system simulates timeaveraged irradiance comparable to full sunlight at shallow depths (∼800 μmol quanta m-2 s-1 over the visible spectral region), and provides light with a reasonable spectral composition. The simulation method was designed for studying responses of phytoplankton, but applications involving optical aspects can also be foreseen.

Electronic Resource

1432-1793

Springer Online Journal Archives 1860-2000

Biology

Abstract Photosynthetic and optical properties of the marine chlorophyte Dunaliella tertiolecta Butcher were studied in response to irradiance fluctuations caused by surface-wave focusing. The experimental conditions simulated the prominent features of the light field (high average irradiance, spectral composition and statistical properties) in the uppermost few meters of the water column under sunny surface conditions. The properties of algae grown under high-frequency fluctuations were compared with control cells grown under constant light at the same average irradiance (∼800 μmol quantam-2s-1). No significant differences were found for a number of parameters, including growth rate, cellular chlorophyll a and pigment ratios, photosynthetic unit size and density of Photosystem I reaction centers, the rate of photosynthesis at the growth irradiance, dark respiration, and in vivo fluorescence of chlorophyll a per cell. Photosynthetic parameters were not affected by whether the incident light for oxygen exchange measurements was fluctuating or constant. This was the case whether the cells had been previously acclimated to either fluctuating or constant irradiance. Such a photosynthetic response indicates that cells are accomplishing a time integration of the fluctuating light. In addition, although D. tertiolecta is capable of dramatically changing its optical properties in response to low or high growth irradiance levels, the refractive index of the cells, the efficiency factors for light absorption and scattering by individual cells, and chlorophyll-specific absorption and scattering coefficients of cell suspensions, were all very similar under high irradiance, whether or not wave focusing was present.

Electronic Resource

1432-1793

Springer Online Journal Archives 1860-2000

Biology

Abstract Epifluorescence microscopy was used to analyze the stomach contents of bivalve larvae collected in the Baie des Chaleurs (western Gulf of St. Lawrence, Canada) in order to document food-particle sizes, compare feeding among taxa, and compare the diet with the in situ phytoplankton community. Stomach contents were mainly composed of small autotrophic flagellates (〈5 μm) and cyanobacteria (〈2 μm), reflecting the microbial food web which characterizes these waters. More than half (55%) of all veligers examined contained algal cells of 5 to 15 μm, whereas only 3% had cells of 15 to 25 μm. Differences in the size ranges of ingested algal cells among similar-sized larvae of different species suggests that veligers actively selected food particles. Among the smallest veligers (185 to 260 μm), scallops (Placopecten magellicanus) and mussels (Mytilus edulis) ingested more 〈5 μm and 5 to 15 μm algae than clams (Mya arenaria). Among larger veligers (261 to 405 μm), clams contained significantly more 〈5 μm cells than mussels, whereas mussels contained significantly more 5 to 15 μm algae than clams. Algal cells of 15 to 25 μm were preferentially ingested by mussel veligers. Feeding also differed between different-sized veligers within taxa, i.e. the smallest clam veligers ingested fewer of 5 to 15 μm algae than the larger size classes. Mussel veligers ingested significantly more 15 to 25 μm and fewer 〈5 μm cells as their size increased. The dominance of ultraplankton in the nearshore waters of Baie des Chaleurs and in the stomach contents suggests that veliger larvae may be an important export path for carbon produced by small phytoplankton.

Electronic Resource

1432-1793

Springer Online Journal Archives 1860-2000

Biology

Abstract Within the framework of a general study on time variability in the St. Lawrence Estuary, phytoplankton was sampled at 10-min intervals for 13 h. A symmetric, cosine, tapered filter was applied to the time series of cell counts, the cyclical trend being interpreted in relation to physical variables. Autocorrelation coefficients were computed for residual phytoplankton data, in order to investigate fine-scale effects in the series. The main pattern (trend) of variation in the phytoplankton data had a frequency of about 1 cycle/190 min and was related to physical variations, following therefore the tidal movement of water masses in the estuary. On the other hand, the correlogram suggests that phytoplankton is aggregated in patches of an average diameter of about 0.75 km.

Electronic Resource

1432-1793

Springer Online Journal Archives 1860-2000

Biology

Abstract Tests have been carried out, both on phytoplankton cultures and in the field, on the relation between the ratio of 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU)-enhanced to normal fluorescence (F D /F N ) and the specific photosynthetic rate (P/B). In all cases observed, significant linear correlations were found between these two values. Differences in this relation were observed according to species and physiological conditions. Nutrient stress that occurs in batch cultures decreases both F D /F N and P/B, while lowered light intensity has a different effect on both, increasing F D /F N and decreasing P/B. This is interpreted as indicating that F D /F N is an index of photosynthetic efficiency at different light levels and an indicator of the specific photosynthetic rate (P/B) when physiological conditions vary at a given light intensity. The practical use of DCMU-enhanced fluorometry to estimate instantaneous P/B values in the field is discussed, stressing the frequent calibrations needed, as in all in situ fluorometry studies.

Electronic Resource

1432-1793

Springer Online Journal Archives 1860-2000

Biology

Abstract In 1981 two large (1 200 1) seawater samples from the St. Lawrence Estuary were kept under constant temperature and light conditions for periods of 50 and 68 h, respectively. In both tank experiments, semidiurnal variations in NH4 were observed that could be related to cyclical NH4 uptake by the phytoplankton. Semidiurnal cycles in photosynthetic efficiency (αB) and intracellular chlorophyll a in the tank, phased on tides at sea, were also evidenced in both experiments. These results support the hypothesis that variations in phytoplankton photosynthetic activity, which are possibly endogenous, can be phased on semidiurnal variations in vertical tidal mixing (variations of the mean light conditions in the mixed layer). In addition, observed variations in intracellular chlorophyll a suggest the possibility of endogenous cycles of phytoplankton light and shade adaptation.

Electronic Resource

1432-1793

Springer Online Journal Archives 1860-2000

Biology

Abstract To study the nocturnal feeding of euphausiids during vertical migrations and its impact on the phytoplankton, a phytoplankton-rich water mass (drogue marked) drifting over a dense krill scattering layer (acoustic 104kHz) in the lower St. Lawrence estuary was monitored over 46 h in July 1982. Phytoplankton 〉20 μm was abundant and mostly concentrated at the bottom of the photic layer above the pycnocline. Less than 42% of the particulate carbon was due to phytoplankton. The krill scattering layer was about 2 to 3 km in width, elongated along the 100-m bathymetric contour, and absent when the bottom was shallower than 50 m. Its upper day depth was 50 m. At deeper depths, its vertical distribution frequently changed from unimodal to polymodal shapes and viceversa, often with large concentrations of zooplankton just above the bottom. Typical vertical migrations were observed on both days. At night the scattering layer had a lower scattering strength. Most of it was below the thermocline but net catches showed that large concentrations of euphausiids (up to 57 individuals m-3) crossed it. Stomach pigment content of Thysanoessa raschi was generally low, but mean stomach fullness was always high. They were more opportunistic than herbivorous. From stomach fullness and the presence of a food bolus in mouth parts, feeding in surface waters appeared to be intensive, but gut content indicated that food was not processed there. It is therefore suggested that individuals underwent vertical interchanges across the thermocline while feeding during the night. Meganyctiphanes norvegica had significant herbivorous activity during the night. The grazing pressure impact of the scattering layer on phytoplankton was negligible.

Electronic Resource