Patrick Raimbault

Feasibility of using an automated colorimetric procedure for the determination of seawater nitrate in the 0 to 100 nm range examples from field and culture

An automatic colorimetric procedure for measuring nanomolar quantities of nitrate and nitrite in seawater has been improved in sensitivity. Application of this method toin situ investigations in the North Atlantic Ocean in January 1989 and algal culture (Skeletonema costatum) experiments in the laboratory demonstrated that one can record concentrations in the 0 to 100 nM range using classical Technicon AutoAnalyzer equipment. In this working range highest precision obtained was ±2.87 and ±1.24 nM for nitrate and nitrite, respectively.

Biological and geochemical signatures associated with the water circulation through the Strait of Gibraltar and in the western Alboran Sea

Previous hydrological investigations of the Gibraltar Strait have left some questions unanswered as regards the composition of the Mediterranean outflow into the Atlantic Ocean. In the present study, nutrient, oxygen, and chlorophyll distributions are used to characterize the main water masses flowing through the Strait of Gibraltar and the Alboran Sea, with particular reference to Levantine Intermediate Water (LIW) and Western Mediterranean Deep Water (WMDW). High chlorophyll a concentrations, reaching 3 μg L−1 at the subsurface, are observed in the northern part of the anticyclonic Alboran Sea gyre. The salinity subsurface minimum (≃36), located temporarily in the strait, corresponds to high nutrient concentrations (7–8 μg at. L−1). Diagram analysis shows that in the upwelling area off the Spanish coast, nutrients are largely transported into the photic zone, and that photosynthetic nitrate consumption actually takes place. Consequently, the formation of the Alboran Sea O2 extraminimum can be related to the enhanced new production. Most of the lowest O2 values (<4.00 mL L−1) also correspond to a slight nutrient maximum (PO4 and NO3). Comparative diagram analysis of biologically affected parameters (oxygen and nutrients) versus salinity as a conservative property confirms and expands results from the classical θ-S analysis. Oxygen and silicate exhibit the best tracer qualities for the circulation pattern, demonstrating in particular that during the period of observations, water rich in LIW feeds the Mediterranean outflow.

A preliminary methods comparison for measurement of dissolved organic nitrogen in seawater

Abstract Routine determination of dissolved organic nitrogen (DON) is performed in numerous laboratories around the world using one of three families of methods: UV oxidation (UV), persulfate oxidation (PO), or high temperature combustion (HTC). Essentially all routine methods measure total dissolved nitrogen (TDN) and calculate DON by subtracting the dissolved inorganic nitrogen (DIN). While there is currently no strong suggestion that any of these methods is inadequate, there are continuing suspicions of slight inaccuracy by UV methods. This is a report of a broad community methods comparison where 29 sets (7 UV, 13 PO, and 9 HTC) of TDN analyses were performed on five samples with varying TDN and DIN concentrations. Analyses were done in a “blind” procedure with results sent to the first author. With editing out one set of extreme outliers (representing 5 out of 145 ampoules analyzed), the community comparability for analyzing the TDN samples was in the 8–28% range (coefficient of variation representing one standard deviation for the five individual samples by 28 analyses). When DIN concentrations were subtracted uniformly (single DIN value for each sample), the comparability was obviously worse (19–46% cv). This comparison represents a larger and more diverse set of analyses, but the overall comparability is only marginally better than that of the Seattle workshop of a decade ago. Grouping methods, little difference was seen other than inconclusive evidence that the UV methods gave TDN values for several of the samples higher than HTC methods. Since there was much scatter for each of the groups of methods and for all analyses when grouped, it is thought that more uniformity in procedures is probably needed. An important unplanned observation is that variability in DIN analyses (used in determining the final analyte in most UV and PO methods) is essentially as large as the variability in the TDN analyses. This exercise should not be viewed as a qualification exercise for the analysts, but should instead be considered a broad preliminary test of the comparison of the families of methods being used in various laboratories around the world. Based on many independent analyses here, none of the routinely used methods appears to be grossly inaccurate, thus, most routine TDN analyses being reported in the literature are apparently accurate. However, it is not reassuring that the ability of the international community to determine DON in deep oceanic waters continues to be poor. It is suggested that as an outgrowth of this paper, analysts using UV and PO methods experiment and look more carefully at the completeness of DIN conversion to the final analyte and also at the accuracy of their analysis of the final analyte. HTC methods appear to be relatively easy and convenient and have potential for routine adoption. Several of the authors of this paper are currently working together on an interlaboratory comparison on HTC methodology.

The input of nutrients by the Rhône river into the Mediterranean Sea: Recent observations and comparison with earlier data

The nutrient input by the Rhone river into the Mediterranean Sea was measured from June 1994 to May 1995. The various fractions of N and P (dissolved and particulate, organic and inorganic) as well as chlorophyll and dissolved silicate concentrations were measured twice a month at Arles, 40 km upstream of the river mouth, in this period. In addition, some samples were taken when the river was in high flood.The study shows that nitrogen is mainly present as nitrate (76%). Dissolved and particulate organic nitrogen represent 8 and 9%, ammonium 5% and nitrite 1% of total nitrogen respectively. Almost half of the phosphate is particulate phosphate, the largest part of which is calcium-bound phosphate. Orthophosphate represents 31% of total phosphate.We estimated the total input of nitrogen, phosphate and silicate as 115–127 kt y-1 (N), 6.5–12.2 kt y-1 (P) and 135–139 kt y-1 (Si) by different methods. For nitrogen an annual input of 92.3 to 96.1 kt of nitrate is found, 1.3 to 1.5 kt of nitrite, 6.3 to 6.7 kt of ammonium, 9.7 to 9.8 kt of dissolved organic nitrogen and 5.3 to 12.7 kt of particulate nitrogen; for phosphate the annual imput was 2.7 to 3.0 kt of orthophosphate, 1.3 to 1.7 kt of dissolved organic phosphate & polyphosphate and 2.5 to 7.5 kt of particulate phosphate. While the N-input was mainly in the form of nitrate, a large part of the phosphate input was particulate-P. Comparisons to previous estimations show that the mean annual nitrate concentration in the Rhone has increased by about 50% during the last two decades.

Wet-oxidation and automated colorimetry for simultaneous determination of organic carbon, nitrogen and phosphorus dissolved in seawater

Abstract We developed a simple and reliable method which allows simultaneous determination of organic forms of carbon (DOC), nitrogen (DON) and phosphorus (DOP) dissolved in seawater. Conversion of dissolved organic matter (DOM) to inorganic products (carbon dioxide, nitrate+nitrite and soluble reactive phosphate) is performed by a persulfate wet-oxidation in low alkaline condition. After oxidation, the concentration of the inorganic products dissolved in the sample was measured automatically by colorimetry using a 3-channel Technicon AutoAnalyzer system. A number of pure organic compounds were tested in the concentration range encountered in coastal and open ocean, indicating a high efficiency of the digestion procedure. The recovery range is similar to that obtained by other wet-oxidation procedures and by high-temperature catalytic oxidation techniques. Direct comparisons with usual methods used for separate determination of DOC, DON and DOP indicated a high efficiency of the procedure. Reproducibility tests demonstrated a very good precision (around 5%) for lagoonal and coastal waters, while precision was sometimes around 10–25% in oligotrophic oceanic waters, especially for DOP where values approached limits of detection for measuring phosphate. This method is highly suitable for routine analysis and especially appropriate for shipboard work.

The coupled physical‐new production system in the equatorial Pacific during the 1992–1995 El Niño

We investigate the coupling between the physics and new production variability during the period April 1992 to June 1995 in the equatorial Pacific via two cruises and simulations. The simulations are provided by a high-resolution Ocean General Circulation Model forced with satellite-derived weekly winds and coupled to a nitrate transport model in which biology acts as a nitrate sink. The cruises took place in September-October 1994 and sampled the western Pacific warm pool and the upwelling region further east. The coupled model reproduces these contrasted regimes. In the oligotrophic warm pool the upper layer is fresh, and nitrate-depleted, and the new production is low. In contrast, the upwelling waters are colder, and saltier with higher nitrate concentrations, and the new production is higher. Along the equator the eastern edge of the warm pool marked by a sharp salinity front, also coincides with a “new production front”. Consistent with the persistent eastward surface currents during the second half of 1994, these fronts undergo huge eastward displacement at the time of the cruises. The warm/fresh pool and oligotrophic region has an average new production of 0.9 mmol NO3 m−2 d−1, which is almost balanced by horizontal advection from the central Pacific and by vertical advection of richer water from the nitrate reservoir below. In contrast, the upwelling mesotrophic region shows average new production of 2.1 mmol NO3 m−2 d−1 and the strong vertical nitrate input by the equatorial upwelling is balanced by the losses, through westward advection and meridional divergence of nitrate rich waters, and by the biological sink.

Observations of open‐ocean deep convection in the northwestern Mediterranean Sea: Seasonal and interannual variability of mixing and deep water masses for the 2007‐2013 Period

We present here a unique oceanographic and meteorological data set focus on the deep convection processes. Our results are essentially based on in situ data (mooring, research vessel, glider, and profiling float) collected from a multiplatform and integrated monitoring system (MOOSE: Mediterranean Ocean Observing System on Environment), which monitored continuously the northwestern Mediterranean Sea since 2007, and in particular high-frequency potential temperature, salinity, and current measurements from the mooring LION located within the convection region. From 2009 to 2013, the mixed layer depth reaches the seabed, at a depth of 2330m, in February. Then, the violent vertical mixing of the whole water column lasts between 9 and 12 days setting up the characteristics of the newly formed deep water. Each deep convection winter formed a new warmer and saltier “vintage” of deep water. These sudden inputs of salt and heat in the deep ocean are responsible for trends in salinity (3.3 ± 0.2 × 10−3/yr) and potential temperature (3.2 ± 0.5 × 10−3 C/yr) observed from 2009 to 2013 for the 600–2300 m layer. For the first time, the overlapping of the three “phases” of deep convection can be observed, with secondary vertical mixing events (2–4 days) after the beginning of the restratification phase, and the restratification/spreading phase still active at the beginning of the following deep convection event.

High penetration of ultraviolet radiation in the south east Pacific waters

We investigated the penetration of solar ultraviolet radiation (UVR) in the surface waters of the south east Pacific (08 - 35 degrees S, 142 - 73 degrees W) from October to December 2004 during the BIOSOPE cruise. In the hyper-oligotrophic waters of the South Pacific Gyre ( near Easter Island), diffuse attenuation coefficients for downward irradiance, K-d(lambda), at 305 nm (UV-B), 325, 340 and 380 nm (UV-A) were 0.083, 0.055, 0.039 and 0.021 m(-1), respectively. The corresponding 10% irradiance depths, Z(10%)(lambda), were 28, 42, 59 and 110 m, respectively. These UVR penetrations are the highest ever reported for oceanic waters and are equal to those measured in the clearest fresh waters. UV-extended inherent optical property (IOP) and radiative transfer (RT) models allowed reliable estimations of K-d(lambda) with the Case 1 water assumption when two values of chromophoric dissolved organic matter (CDOM) absorption spectral slope coefficient (S) were used, i.e. 0.017 nm(-1) at 325, 340 and 380 nm, and 0.023 nm(-1) at 305 nm.

Biomass, metabolism and nutrition of zooplankton in the Gulf of Lions (NW Mediterranean)

Abstract Zooplankton biomass was measured at 20 stations located along five coastal–offshore transects in the Gulf of Lions (NW Mediterranean) during spring 1998 and winter 1999, using two nets equipped with 200 and 80 μm mesh. Biomass was twice as abundant in spring. It increased toward the coast and the west of the gulf and was positively correlated to chlorophyll, primary production and organic particulate matter and negatively to temperature and salinity. The enrichment effect of Rhone river input and the oligotrophic influence of the north Mediterranean current (NMC) were the main distribution factors of zooplankton biomass. Higher biomass was obtained in coastal stations with the 80 μm net, which sampled more efficiently young larval stages and nauplii. Zooplankton metabolic activity (respiration, ammonium and phosphorus excretion) was studied at the inner and outer stations of each transect. Oxygen/nitrogen (O/N) ratio was lower in spring than in winter, indicating that more protein material was used for food, to complete the energy needed for the spring enhancement of secondary production. Nitrogen and phosphorus excreted by zooplankton accounted for 31% and 10% of the primary production requirements in spring and for 32% and >100%, in winter. Grazing impact and secondary production were calculated from respiratory requirements using a K 2 value of 0.4 and an assimilation rate of 0.8. Grazing impact ranged between 9% and 69% of the primary production in spring and between 6% and 83% in winter, depending on the stations. The average secondary production was 54 mg C m –2 d –1 in spring and 19 mg C m –2 d –1 in winter, which represents 11% and 12% of the primary production, respectively.

The Peruvian upwelling near 15°S in August 1986. Results of continuous measurements of physical and chemical properties between 0 and 200 m depth

Continuous measurements of temperature, salinity, nutrients, oxygen, partial pressure of CO2, pH, flourescence of chlorophyll, and light scattering were undertaken during the Paciprod cruise in August 1986. Underway measurements were performed on two tracks perpendicular and parallel to the Peruvian coast near 15°S, and continuous vertical profiles were obtained on two transects perpendicular to the coast. The upwelling was intense, as indicated by the thickness of the Eckman layer and the large extent of cold water and nutrients. Offshore upwellings were marked by decreases of temperature and increases in the nutrient content of the mixed layer. The surface water was largely undersaturated with oxygen and supersaturated with respect to CO2. Partial pressures of CO2 as high as 1000 μatm was observed at the source of the upwelling. Oxygen-poor waters were found immediately below the thermocline. Water layers with near zero nitrate concentration show that the denitrification was intense in the equatorial subsurface water beyond the shelf. The occurrence of these low nitrate cores situated westward of upward tilts of the isotherms was linked to the circulation. An increase of fluorescence and ligth scattering accompanied the decrease in nitrate and nitrite concentrations in oxygen-depleted water. Denitrification was lower in the shelf water and in the less saline subantarctiv water. The mixed layer indicated a high nutrient-low chlorophyll situation on the whole area studied. Four main surface zones with different features were distinguished along a transect perpendicular to the coast. The fluorescence is relatively high and increases seaward in the first zone situated between the upwelling center and 22 km offshore. This corresponds to the biological conditioning described by MacIsaacet al. (Deep-Sea Research, 32, 503–529, 1985). In the second and third zones, the biomass is low, silicate or nitrate concentrations are probably limiting, and grazing may control production, especially in the third zone where ammonium appears. In the subtropical water, the fourth zone, the biomass is very low and the N/Si/P ratios in sea water are approximately 4/3/1.