Philippe Kerhervé

Particulate matter and organic carbon budgets for the Gulf of Lions (NW Mediterranean)

An analysis of sedimentological, sediment trap and hydrological data was performed to investigate the transport and fate of particulate matter in the Gulf of Lions. The sedimentological properties outline the major sedimentary units of the shelf (Rhone prodelta, mid-shelf mud belt, outer shelf) and slope. The geometry of these sedimentary units and the southwestwards increase of particulate fluxes on the slope highlight the influence of the general cyclonic circulation on the dispersion of land-derived particulate matter. Considering the known input and output terms, budgets of particulate matter and organic carbon in the Gulf of Lions are proposed. Inputs were river supply, atmospheric deposition and primary production; outputs were sediment burial and advective export on the slope. Degradation of particulate organic carbon in the water column and at the sediment-water interface was also estimated. Mass and POC budgets were balanced within uncertainties. Nevertheless, these results illustrate the difficulty in establishing budgets based on elements estimated at different time scales, for which the system is not necessarily in a steady state.

Phytoplankton photoadaptation related to some frontal physical processes

Using phytoplankton pigments as biomarkers, we studied light adaptation of phytoplankton communities in a geostrophic front and the surrounding area by (i) onboard kinetic experiments and (ii) detailed analyses of the vertical profiles of a selected photoadaptive index. Kinetic experiments, in which populations were subjected to a sudden light shift (from high to low irradiances and vice versa), clearly demonstrated that frontal phytoplankton populations, especially diatoms (fucoxanthin) are less affected by light shifts than the populations of the adjacent oligotrophic zones. Kinetic experiments also showed that the ratio diadinoxanthin/Chl. a, positively correlated with light intensity, is a useful index to characterize and identify certain time scales of photoadaptation in microalgae. The rate of change of this index was estimated around 0.5 h−1, with significantly higher values for the frontal communities compared to the adjacent area. The rate of variation of this ratio, deduced from in situ measurements, is however one order of magnitude lower than the rate obtained from kinetic experiments. These two approaches allowed us (i) to compare different populations and (ii) to define an upper limit of the adaptation rates. Typical in situ profiles of the diadinoxanthin/Chl. a ratio showed obvious day-night variations. Relative anomalies in these profiles were interpretated in terms of “light histories” of phytoplankton communities and compared with potential causal physical motions (diffusion or advection).

Sub-ambient temperature effects on the separation of monosaccharides by high-performance anion-exchange chromatography with pulse amperometric detection. Application to marine chemistry.

The effects of column temperature in the range 10-45 degrees C using high-performance anion-exchange chromatography (HPAEC) and pulse amperometric detection are described for the determination of monosaccharides. The influence of temperature was tested with an isocratic elution of NaOH at concentrations varying from 2.5 to 20 mM and with a post-column addition of 1 M NaOH. The results showed that small changes of temperature greatly affect retention times and resolution (Rs) of monosaccharides and particularly those of the both pairs xylose-mannose and rhamnose-arabinose which cannot be simultaneously detected at usual room temperature (approximately 25 degrees C). Our results suggest that a subambient temperature of 17 degrees C and an eluent concentration of 19 mM are the more appropriate conditions for an acceptable separation (R(s rha/ara) = 1.02, R(s man/xyl) = 0.70) in a short analytical run time (35 min). The results showed that within the range of temperatures studied, enthalpy and entropy are invariant of temperature indicating that changes in the retention processes are mainly due to temperature than other associated changes in the system. This study demonstrated the importance of controlling temperature during HPAEC of monosaccharides, both to accomplish highly reproducible retention times and to achieve optimal separation of sugars. This method gave acceptable results for detection of marine sugars.

Stable isotopes (13C/12C and 15N/14N) insettling organic matter of the northwestern Mediterranean Sea: biogeochemical implications

Within the framework of the High Frequency Flux (HFF) experiment (MATER programme), time-series sediment traps have been deployed for two months on the continental slope off Marseilles to measure downward particle fluxes at a high frequency sampling rate (two and six days). Combined isotopic analyses of carbon and nitrogen have been performed on selected samples. Both isotopic tracers have been used for the first time on organic material of the Mediterranean Sea (Gulf of Lions) to determine the main biological sources and to address the biogeochemical processes that affected this material. Settling particles were characterised by very low values of delta N-15 (near 0 parts per thousand) and delta C-13 (near -24.5 parts per thousand) that indicate the existence of a mixed material with two sources that differ according to the considered element (C or N). The terrestrial source slightly dominates the carbon pool because of its higher C:N ratio, whereas the nitrogen pool may mostly originate from N-2-fixing cyanobacteria (delta N-15 = 0 parts per thousand, low C:N ratio). These preliminary data suggest that dissolved atmospheric N, may act as a significant new nitrogen source in the Mediterranean Sea.

Determination of marine monosaccharides by high-pH anion-exchange chromatography with pulsed amperometric detection

Abstract In this study, a method was developed for the determination of ten monosaccharides in marine particulate matter utilizing high-pH anion-exchange chromatography (HPAEC) with pulsed amperometric detection (PAD) Samples were analyzed using a Dionex CarboPac PA1 column with a flow-rate of 1 ml/min and addition of 380 m M NaOH post-column. The effect of NaOH concentration (between 0.5 and 50 m M ) on the monosaccharide separations expressed by the capacity factor ( k ′) was tested. The results showed that one isocratic elution was unfit to discriminate properly arabinose, fructose, fucose, galactose, glucosamine, glucose, mannose, ribose, rhamnose and xylose. Thus, two isocratic elutions, at 2.5 and 15 m M NaOH, were necessary to separate and quantify with significant selectivity (α) and resolution terms ( R s ), respectively, glucose, mannose, xylose and fucose, rhamnose, arabinose, glucosamine, galactose, fructose and ribose. The method is linear for all sugars over the concentration range tested (25–50 ng per injection or 1–10 mg/g) expected in marine concentrated samples, and reproducibility was found to be satisfactory (1.7–4.8%), except for ribose (27%). Monosaccharide determinations from two kinds of marine matrix (hydrolyzates of surface sediment and of suspended particulate matter) are presented.

Biogeochemistry and dynamics of settling particle fluxes at the Antikythira Strait (Eastern Mediterranean)

Abstract For the first time, a 12-month trap experiment was conducted on both sides of the strait between Crete and Antikythira Island (Eastern Mediterranean Sea) from June 1994 to June 1995 as part of the PELAGOS experiment. Analyses of major chemical constituents, including carbohydrates and stable lead isotopes and Scanning Electron Microscope studies were performed on the trap samples. Total mass fluxes varied between 1 and 1273 mg m −2 d −1 . The lowest fluxes observed were in summer and autumn 1994, when stratification of the water column was at its deepest. In general, mass fluxes exhibited very low values throughout this experiment confirming the strong oligotrophy of this area. The mean contents of the major constituents (carbonates, opal, lithogenic fraction) were quite similar during the survey and between traps, with the exception of organic carbon contents, which were highest (7–10%) in summer 1994, i.e. during the period of lowest mass fluxes. During the first 6-month deployment (summer–autumn 1994) there was an important mass flux peak, which was depleted in organic carbon, at the Ionian near-bottom trap. This event coincided with a violent wind episode, which may have caused the resuspension of particles, which were then transported down the steep continental slope on the Ionian side of the strait. A smaller peak in mass flux occurred at the Aegean near-bottom trap, coincident with rainfall. Both these events indicate that environmental factors can control flux variations in an oligotrophic environment. During the second 6-month deployment (winter–spring 1995) there was another important increase in mass fluxes, which occurred at all three traps, although in the Ionian traps mass flux peaks were delayed by one to two sampling intervals. The distance between the two mooring sites gives a rough estimate of a minimum horizontal advection speed of 2 cm s −1 for this particulate transfer from the Aegean to the Ionian area. This estimate is in good agreement with the measured current velocities.

Characteristics and sources of the settling particulate organic matter in the South Adriatic basin

In the South Adriatic Pit a mooring equipped with two time-series sediment traps and two Aanderaa RCM7 current meters was deployed in the frame of EEC-MAST II Mediterranean Targeted Project 1, Subproject Euromarge-AS. The first trap was located 35 m above the bottom (mab) and the second 500 mab. Sediment deposition fluxes were recorded biweekly over 18 months (1 April 1994 to 31 October 1995) in the deeper sediment trap, and over 12 months (15 November 1994 to 31 October 1995) in the 500 mab trap. The settling material and surficial sediment have been analysed for inorganic and organic carbon, d 13 C composition, total nitrogen, and biogenic silica, in order to infer the sources of material. The total mass fluxes at both depths were characterized by a high variability and did not present a clear seasonal trend. Values obtained in the 500 mab trap were generally 2‐3 times lower but occasionally similar. The fluctuations were similar in both traps. Based on analyses of inorganic and organic constituents it appears that the fresh organic matter produced in the surface layer rapidly sank through the water column. Strong seasonal variations of biogenic constituents occurred. During spring‐early summer an input of fresh organic matter prevailed due to the blooms of mainly siliceous phytoplankton. Late summer and winter samples were characterized by more degraded matter originating from calcareous phytoplankton. A significant imprint of terrigenous organic matter was observed in summer and winter 1994, most likely because an unusually high Po River inflow was the most important fresh water source for the Adriatic. A vertical budget of organic carbon has been calculated; only 2.7% of mean annual primary production reaches the 500 mab trap, suggesting that remineralization processes take place mainly in the upper part of the water column. Comparison between 35 mab trap particle fluxes and bottom rain (calculated as sum of remineralization fluxes and burial) indicates that a strong near-bottom input of material between the deepest trap and the surficial sediment exists due to the lateral advection. # 1999 Elsevier Science Ltd. All rights reserved.

River organic matter shapes microbial communities in the sediment of the Rhône prodelta

Microbial-driven organic matter (OM) degradation is a cornerstone of benthic community functioning, but little is known about the relation between OM and community composition. Here we use Rhône prodelta sediments to test the hypothesis that OM quality and source are fundamental structuring factors for bacterial communities in benthic environments. Sampling was performed on four occasions corresponding to contrasting river-flow regimes, and bacterial communities from seven different depths were analyzed by pyrosequencing of 16S rRNA gene amplicons. The sediment matrix was characterized using over 20 environmental variables including bulk parameters (for example, total nitrogen, carbon, OM, porosity and particle size), as well as parameters describing the OM quality and source (for example, pigments, total lipids and amino acids and δ13C), and molecular-level biomarkers like fatty acids. Our results show that the variance of the microbial community was best explained by δ13C values, indicative of the OM source, and the proportion of saturated or polyunsaturated fatty acids, describing OM lability. These parameters were traced back to seasonal differences in the river flow, delivering OM of different quality and origin, and were directly associated with several frequent bacterial operational taxonomic units. However, the contextual parameters, which explained at most 17% of the variance, were not always the key for understanding the community assembly. Co-occurrence and phylogenetic diversity analysis indicated that bacteria–bacteria interactions were also significant. In conclusion, the drivers structuring the microbial community changed with time but remain closely linked with the river OM input.

Floating Microplastics in the Northwestern Mediterranean Sea: Temporal and Spatial Heterogeneities

Mediterranean sea is one of the most sampled areas for floating microplastics (MPs). However, only few investigations have been conducted at small spatial and temporal scales in coastal areas. To fill this gap, MPs ( 300 μm), at different seasons in the Rhone area and every month during one year in the Tet area. After removal of organic matter, MPs were examined under a dissecting stereo microscope. Preliminary results show highly variable MP concentrations even at small scale. Indeed, concentrations ranged from 0.05 to 0.6 items.m−3 for the Tet area with an average of 0.2 items.m−3 and from 0.1 to 0.5 items.m−3 with an average of 0.3 items.m−3 for the Rhone area. Concentrations can change by a factor of 4 between two consecutive days at the same location and by a factor of 3 between two consecutive trawls on the same day. Fibers are the most abundant shape (40–90%), followed by fragments (0–50%). Foams and films are less represented (0–20%). FTIR analysis indicates that fragments and films were mostly polyethylene (PE) and polypropylene (PP), while foams were essentially made of polystyrene (PS). Fiber analysis is ongoing. Occasional presence of lint composed of hundreds of fibers can partly explain the high differences observed at small scales, as well as fast changing river inputs. These extended observations of floating MPS in the NW Mediterranean coastal environment underlines the necessity of performing replicate sampling to get a better insight into the spatial and temporal distribution patterns of these worrying pollutants.