Pierre Le Corre

The seasonal variations of nutrients in the upper waters of the Bay of Biscay region and their relation to phytoplankton growth

Abstract The upper 500 m of the Bay of Biscay were investigated during four cruises from 1972 to 1976. According to Bary s classification, transition and southern waters were detected. The Bay of Biscay is characterized by a homogeneous water mass, relatively rich in nutrients, with numerous gyres and slow horizontal movements. Nitrogen, phosphorus, and silicon uptake during a phytoplankton bloom have been calculated and related to the chlorophyll distribution. During the period N/P uptake ratios were close to Redfield s values but a lack of silicon appeared after the first phase of phytoplankton growth. The winds are important in the renewal of nutrients in the upper waters. An interpretation of the seasonal cycle of nutrients in superficial and subsuperficial water masses is proposed.

Rapid and simultaneous analysis of trace metals (Cu, Pb and Cd) in seawater by potentiometric stripping analysis

Abstract A potentiometric stripping method for a simultaneous measurement of copper, lead and cadmium in oceanic waters using a TraceLab PSU22 unit has been developed. Compared to the earlier TraceLab PSU20 unit, the PSU22 has a sampling rate of the potential during the stripping step that is three times lower. This results in the decrease of the detection limit by about four times. The detection limits obtained with a pre-electrolysis time of 15 min were 44 ng l−1 (0.7 nM), 3 ng l−1 (14 pM) and 1 ng l−1 (9 pM) for Cu, Pb and Cd, respectively. The precision, accuracy and reproducibility of the method agree well with those obtained by other techniques for trace metals analysis in seawater. Vertical profiles of Cd in the Mediterranean waters obtained by AA and PSA were concordant.

NITROGEN UPTAKE AND ASSIMILATION KINETICS IN ALEXANDRIUM MINUTUM (DYNOPHYCEAE): EFFECT OF N‐LIMITED GROWTH RATE ON NITRATE AND AMMONIUM INTERACTIONS1

Uptake and assimilation kinetics of nitrate and ammonium were investigated along with inhibition of nitrate uptake by ammonium in the harmful dinoflagellate Alexandrium minutum Halim at different nitrogen (N)–limited growth rates. Alexandrium minutum had a strong affinity for nitrate and ammonium (Ks=0.26±0.03 and 0.31±0.04 μmol·L−1, respectively) whatever the degree of N deficiency of the cells. Ammonium was always the preferred form of nitrogen taken up (=0.42–0.50). In the presence of both forms, nitrate uptake was inhibited by ammonium, and inhibition was particularly marked in N‐sufficient cells (Imax∼0.9 and Ki=0.31–0.56 μmol·L−1). In the case of N assimilation, ammonium was also the preferred form in N‐deficient cells (=0.54–0.72), whereas in N‐sufficient cells, both N sources were equally preferred (=0.90–1.00). The comparison of uptake and assimilation rates highlighted the ability of A. minutum to significantly store in 1 h nitrate and ammonium in amounts sufficient to supply twice the daily N requirements of the slowest‐growing N‐deficient cells. Nitrogen uptake kinetic parameters of A. minutum and their ecological implications are discussed.

Remote sensing observations of biological material by LANDSAT along a tidal thermal front and their relevancy to the available field data

Abstract Two images recorded on two successive summer days by LANDSAT satellite over the western approaches to the English Channel show bright pattern of complex shape the origin of which is puzzling. Among the wavelength bands available on LANDSATs multispectral scanner, these patterns are apparent only in the green region of the spectrum, and they are located towards the stratified side of a well marked tidal thermal front. Spectral signature analysis and available knowledge on hydrography and plankton in the area are used to derive a proposed interpretation. Phytoplankton would accordingly be the best candidate for being responsible for the observed patterns.

The ratios of nitrate, phosphate, and silicate during uptake and regeneration phases of the Moroccan upwelling regime

Abstract Upwelling off Morocco was investigated in the summers of 1972 and 1973. In the northern zone, superficial waters result from the mixture of three water masses. The southern area is characterized by a ‘bell’ of cold water brought to the continental shelf, where nutrient regeneration is intense. Using models of water mass formation, nutrient anomalies (nitrogen, phosphorus, and silicon) are calculated in both uptake and regeneration phases and corresponding ratios are evaluated. Deviations from Redfields ratios were observed in superficial North Atlantic central waters bathing the continental shelf, where inorganic phosphorus and silicon appear to regenerate faster than nitrogen. In the photic layer no limitation on phytoplankton growth by silicon was detected.

Measurement of Copper in Sea-water by Constant Current Stripping Analysis (CCSA) With a Rotating Gold Disk Electrode

A chronopotentiometric stripping method [constant current stripping analysis (CCSA)] using a rotating gold disk electrode for measurements of copper in sea-water is described. Compared with a CCSA method using a stationary gold film electrode, the new method has a sensitivity 2.5 times higher and a detection limit of 11 ng l–1 after a 5 min deposition time, and eliminates from the analysis the time needed for gold plating. The precision and accuracy of this method agree well with those obtained by other techniques. Vertical profiles of copper obtained by two different chronopotentiometric stripping methods, CCSA and potentiometric stripping analysis (PSA) in west Brittany coastal waters show that the measurement of copper by CCSA coupled with a gold disk electrode appears to be less affected by organic matter than those made by PSA with a mercury electrode.

Laboratory study on degradation of fatty acids and sterols from urban wastes in seawater

Changes of particulate and dissolved fatty acids and sterols were studied during an in vitro experiment on the degradation, in seawater, of organic matter of urban effluent origin. Incubation of seawater-effluent mixture was done in dark at 15°C and the fatty acids and sterols were analysed by HPLC and GC. Decomposition of total fatty acids and sterols was rapid, with a loss of 90% in the former and 70% in the latter during the first week. Over two months, 95% of both groups were decomposed. Fractionation showed that the dissolved and particulate neutral lipids which are representative of anthropogenic inputs were rapidly degraded, whereas polar lipids, containing bacterial fatty acids initially and polyunsaturated fatty acids later, were synthesized. A synthesis of sterols (cholesterol and βsitosterol) was also observed, especially in the dissolved fraction. These variations are related to successive development of bacterial and bacteriophagal populations. The sterols, and particularly the 5β-stanols, appear to be good tracers for organic matter of urban effluents decomposing in the coastal environment.

A sampling device for collection and analysis of volatile halocarbons in coastal and oceanic waters

A simple device for collection of sea water samples for measurement of volatile halocarbons has been developed. This device enables collection of samples without contact with the atmosphere and by introducing the sample directly into the analytical circuit eliminates secondary handling of the samples and the risks of contamination associated with it. The device is chemically inert and samples can be stored without contamination for periods of up to 16 h in the case of chlorofluorocarbons (CFCs) and CCl4 and several days in the case of other volatile organic halocarbon compounds before analyses. Standard deviations, as a percentage of the average concentrations measured, varied between 1.4 and 5.7% for a range of halocarbons in coastal waters and were about 2% for CFC-11 and CFC-12 in the oceanic surface waters. Detection limits were less than 0.17 ng l–1 for volatile halocarbons and 0.015 pmol l–1 for CFC-11 and CFC-12. The sampling device has been routinely used for collection of estuarine, coastal and oceanic waters. Results obtained in the coastal waters of Douarnenez Bay and the oceanic waters of the north-east Atlantic are presented as examples.

On the presence of a cadmium-rich subsurface water mass in the western Mediterranean and its influence on the distribution of cadmium in the surface waters

Abstract Vertical distribution of cadmium (27–133 pM) at 13 stations in the Alboran Sea (western Mediterranean) showed profiles that were different from those typically known from oceanic waters. A subsurface cadmium-rich layer, related to the presence of Winter Intermediate Water (WIW), was a prominent feature. The WIW is formed in the northwestern coastal sector of the western Mediterranean Sea, and the high cadmium concentrations in this water mass are therefore likely to be of coastal origin. The surface concentrations of cadmium at all stations in the Alboran Sea were higher and increased, but not progressively, from west to east, with a substantially higher concentrations around the Almeria–Oran front than on either side. As shown by the cadmium–salinity diagram and the rates of diffusive transfer of cadmium from the subsurface into the surface waters, flux from WIW is an important source of cadmium for surface waters in the Alboran Sea, with a marked increase in the rates near the front.