Gustave Cauwet

Carbon inputs of the Rhône River to the Mediterranean Sea: Biogeochemical implications

Since the damming of the Nile, the Rhone River is the main freshwater and sediment supplier to the Mediterranean Sea. We estimated for the period 1987–1996, the dissolved and particulate organic carbon (DOC and POC), dissolved inorganic carbon (DIC), and total suspended matter (TSM) fluxes of the Rhone River to the Mediterranean Sea to be 1.1 ± 0.2, 1.6 ± 0.5, 16.2 ± 0.3 × 1010 moles C yr−1, and 9.9 ± 6.4 × 106 t yr−1, respectively. PIC flux was estimated to be 8.2 ± 5.4 × 109 moles C yr−1. On the basis of literature data, we estimated that nearshore bacterial respiration of Rhone derived labile‐POC and ‐DOC (LPOC and LDOC) might produce in a few days ∼0.21 and 0.12 × 1010 moles CO2 yr−1, respectively. Extended to the whole Mediterranean, this study suggests that bacterial respiration of labile organic carbon derived from Mediterranean rivers might rapidly (days) produce 2.6‐11 × 1010 moles CO2 yr−1. On the continental shelf, up to 4.7 × 1010 moles of organic carbon introduced by primary production and Rhone export would escape each year to sedimentation and bacterial mineralization and would be exported off the shelf. Moreover, as total carbon fixed by phytoplankton exceeds (+ 5.2 × 1010 moles C yr−1) the CO2 produced by bacterial respiration (on average), the biological system on the shelf, could be considered as an autotrophic system and then a sink for atmospheric CO2. However, these numbers need further examination because of the the large uncertainties associated currently to the bacterial growth efficiency values (± 100%)

Carbon inputs and distribution in estuaries of turbid rivers: the Yang Tze and Yellow rivers (China)

Particulate and dissolved organic and inorganic carbon concentrations were measured in the Yang Tze (Changjiang) and Huanghe (Yellow) river estuaries, during two different seasons. During the winter, the organic matter in the suspended load of both estuarine systems was dominantly of terrestrial origin, while in the summer, there were significant contributions from aquatic production. In these turbid rivers, there is a strong positive correlation between the concentrations of total suspended matter and particulate organic carbon, probably because biological production is inhibited when turbidity exceeds a certain limit. Dissolved organic carbon (DOC) does not exhibit a conservative behavior in these estuarine systems. High dissolved organic carbon values in bottom water samples suggest remobilization of DOC from near-surface, bottom sediments. Dissolved inorganic carbon concentrations were very high in the Huanghe River, and the trend in the calculated saturation indices for calcite suggests precipitation of carbonate in this estuarine system. There is no such evidence for carbonate precipitation in the Yang Tze River.

Influence of sedimentological features on the distribution of trace metals in marine sediments

Abstract The most important parameters influencing trace metal distribution in sediments are reviewed. The accent is put on sedimentation rate, grain size distribution and the knowledge of past events through sedimentological and mineralogical parameters. It is concluded that chemical data must be compared to sedimentological data before being interpreted.

Organic matter characterization in the Northern adriatic sea with special reference to the sea surface microlayer

Abstract A study of the organic matter in the sea surface microlayer of the northern Adriatic Sea was performed along the transect from the Istrian peninsula to the Po River Estuary. The sources and nature of the organic matter were determined using a multiparameter approach: (1) screening by hydrological and biological parameters, (2) characterization by global parameters such as surfactant activity (SA) and particulate organic carbon (POC) and (3) a molecular approach using organic substances as markers (fatty acids and hydrocarbons). The hydrobiological parameters indicate a homogeneous surface layer down to a depth of 10 m and autochthonous production with an increase in the area under the direct influence of the Po River. Surfactant activity and POC show a similar trend along the transect. The correlation between SA and POC is better than the correlation between SA and chlorophyll-α indicating that the dissolved organic matter pool is produced by previous populations rather than by active phytoplankton organisms. SA measurements indicate the predominance of polar polymeric material. The ‘molecular marker’ approach confirms the predominant contribution of autochthonous production to the total pool of organic matter, and a substantial increase at the station under the direct influence of the Po River where bacterial activity is also increased and organic matter of terrestrial origin is abundant in the particulate matter. Hydrocarbon concentrations increase along the transect towards the Po River Estuary but they are, in general, low, of the same order as open Mediterranean waters, indicating a comparable level of contamination. Enrichment factors in the sea surface microlayer are different with respect to the nature and state of compounds (dissolved or particulate). The accumulation is discussed in terms of sources and processes of formation of the microlayer.

Automatic determination of dissolved organic carbon in seawater in the sub-ppm range

Abstract The improvement of a dissolved organic carbon (DOC) analysis technique is described. This continuous flow system, based on both persulfate and UV oxidations, allows the automatic determination of DOC in sea and estuarine waters. The use of FID detection gives a very high sensitivity, permitting the detection of small variations in carbon content. The efficiency of the method and its sensitivity are tested, together with the influence of initial pH and duration of nitrogen bubbling. The system can also be used to determine total or inorganic carbon.

Eutrophication in the Krka estuary

Abstract Changes in longitudinal distribution of eutrophication parameters in brackish and marine layers of a stratified karstic estuary have been investigated from December 1988 until October 1989 on a gradient between a pristine and a moderately polluted area. In the estuary, only marine phytoplankton blooms were found to develop. Near the head of the estuary, blooms develop below the halocline. Marine phytoplankton is fed by nutrients released from the decay of sinking freshwater phytoplankton. Phytoplankton growth is P-limited. Hypoxia near the bottom follows the decay of occasionally large quantities of allochtonous freshwater and of autochthonous marine phytoplankton. In the lower estuary, blooms have been observed to develop in the brackish layer above the halocline due to high salinity in the layer and the existence of a local anthropogenic nutrient input.

Carbon inputs and biogeochemical processes at the halocline in a stratified estuary: Krka River, Yugoslavia

Abstract Suspended matter, particulate and dissolved organic carbon and dissolved inorganic carbon were measured seasonally from 1987 to 1989 in the Krka Estuary. Carbon inputs by the river are low, because of the small river flow and the low content of suspended matter of its water. The permanent stratification of the estuary creates an unusual environment, with a very sharp salinity gradient at the freshwater-seawater interface. The accumulation of particulate organic carbon at the interface and the production of dissolved organic carbon suggest that an enhanced biological activity influences the organic matter budget.

Occurrence of organic colloids in the stratified estuary of the Krka River (Croatia)

Along a vertical profile of a stratified estuary, we determined organic carbon in the following fractions: low molecular weight (LMW-OC 0·7 μm). The results showed abundant concentrations of organic colloids accounting for: 40% of total organic carbon (TOC) in the overlying brackish water (salinity 2), 22% in the underlying marine water (salinity 38) and 19% (±5%) for intermediate salinity samples (salinity 3–31). Upon contact with seawater (in the salinity range 2–5) both DOC and COC concentrations decreased drastically (44 and 47 μM respectively), whereas POC slightly increased (11 μM), being consistent with previous laboratory and field studies, and indicating that at low salinities, the colloidal fraction is actively involved in the DOC physicochemical aggregation. In the halocline, we observed that organic colloids and relatively degraded particles accumulated together in the lowest part of the interface (salinity 31). Because previous studies in the same area have shown accumulation of degraded cells and fragments in the bottom of the halocline, we suggest that large colloids (COC in this study) at the interface probably originated from fragmentation of non-living organisms or aggregates. Furthermore, it is likely that these processes which are enhanced in a highly stratified estuary, are partly responsible for the DOC non-conservative dilution observed along this profile.

C18 Sep-Pak extractable trace metals in waters from the Gulf of Lions

Abstract Dissolved trace-metal (Cd, Cu, Fe, Ni and Pb) interactions with C18 Sep-Pak extractable organic matter have been investigated in the Rhone mouth and in the Gulf of Lions in July 1989. Pb and Cd do not show any complexation while Ni and Fe are slightly complexed in surface waters near the Rhone mouth. Up to 40% of total dissolved Cu is complexed by the extractable organic matter in coastal waters and, in open sea waters a subsurface maximum of 64% of complexed Cu was observed, apparently linked to the maximum of primary production. Our results combined with other data on the same area suggest that in the euphotic zone of the western Mediterranean Sea, organic matter directly connected to primary production and associated degradation processes is contributing to this complexation as well as to the enrichment of colloidal fraction and the increase of the discrepancy between concentrations of dissolved organic carbon according to the method of Sugimura and Suzuki (1988) and by traditional techniques.

Ultrafiltration of seawater with a zirconium and aluminum oxide tubular membrane: application to the study of colloidal organic carbon distribution in an estuarine bottom nepheloid layer

For separation of organic colloids in fresh and saline waters, a cross-flow ultrafiltration system was built with a new type of membrane made of zirconium and aluminum oxides. This membrane had a tubular design (16 cm long, 4 cm wide) and a porosity of 10 nm. The suitability of the system for organic carbon studies was first tested by laboratory experiments. In the conditions determined, we obtained acceptable values of dissolved organic carbon (DOC) blank (less than 4 μmol Cl−1) and minimal material adsorption on the membrane. Our special unit was applied to seawater samples collected from the bottom nepheloid layers (BNL) off the Rhone Delta (France). Material defined as colloidal matter in the size range of 0.01–0.7 μm was isolated by filtration on GF/F filters, followed by ultrafiltration. Organic carbon was measured in the so-called dissolved (DOC 0.7 μm) fractions. COC displayed a wide range of concentrations, from 7 μmol Cl−1 (COC/DOC = 7%) offshore, up to 88 μmol Cl−1 (COC/DOC = 49%) near the mouth, whereas truly dissolved carbon remains constant (tDOC = 89 ± 10 μmol Cl−1). In fact, a strong negative correlation was found between COC concentrations near the sediment and the distance from the mouth, indicating that in the area investigated, the major part of the colloidal material does not reach the open ocean. Furthermore, COC concentrations were positively correlated with POC concentrations, suggesting a particulate origin of colloids in the Rhone BNL.