Carbon isotope and C/N ratios of suspended matter in rivers: an indicator of seasonal change in C4/C3 vegetation

A combination of δ ¹³C values with C/N ratios in suspended matter has been used to examine the seasonal relationship between C₄ and C₃ vegetation along the Loess Plateau, NW China. The C isotopic composition of suspended organic matter in rivers, together with C/N ratios can differentiate between soil and plant material, and can be used to estimate the relative contributions of soil organic C and plant litter to the suspended matter. The relationship between C isotopic composition and C/N ratios indicates that the samples are a mixture of two end members: (1) modern soils with relatively constant δ ¹³C values, low C content and low C/N ratios; (2) plant litter with varying δ ¹³C values, high C content and high C/N ratios. The results reflect the seasonal distribution of C₄/C₃ vegetation within the area studied, as part of the Loess Plateau. The abundance of C₄ grasses is about 20% for the current summer vegetation ecosystem in the eastern part of the Loess Plateau. Hence, the use of δ ¹³C values and C/N ratios of suspended matter in rivers and modern soil may be useful for reflecting seasonal distribution of C₄/C₃ vegetation in catchments. This could be a useful tool for distinguishing between catchments for GIS studies, and long term planning for ecological management of catchment areas.     

Transformation of phosphorus in the Elbe estuary

The distribution of dissolved and particle-bound phosphorus (P) was investigated in the Elbe estuary during March 1995. The forms of particulate P were studied with a sequential extraction technique. Organic P dominated particle-bound P in the outer reaches of the estuary (52%), decreased to a minimum of 21% in the turbidity zone, and increased to 33% further upstream. Fe-bound P was the second most important P species in the outer reaches (27%) and dominated in the turbidity zone (up to 57%) and upstream of the turbidity zone (up to 48%). The P:Fe ratio increased with decreasing salinity, from 0.11 in the outer reaches to about 0.22 at zero salinity. Dissolved inorganic P release from reverine suspended matter was about two to three times larger than release, from marine suspended matter and was dominated by release of Fe-bound P. Dissolved inorganic P release from marine and from riverine organic matter were of equal importance. Because marine suspended matter dominates in the estuary, this suggests riverine organic matter is remineralized much faster than marine organic matter. This is in line with the refractory nature of marine organic matter (no phytoplankton bloom) and the easily degradable character of the riverine suspended matter (phytoplankton bloom) in the Elbe estuary during March 1995.     

The process of glauconitization: chemical and isotopic evidence

Sequential leaching experiments were made on Recent glauconies and clay fractions of the associated mud from off-shore Africa near the estuary of the Congo River. Analyses of major/rare earth elements (REE) and Nd isotopic compositions on the resulting leachate and residue pairs allow identification of at least three important and isotopically distinct components which contributed to the glauconitization process: (1) a detrital component with relatively high ⁸⁷Sr/⁸⁶Sr and relatively low ¹⁴³Nd/¹⁴⁴Nd isotopic ratios; (2) a phosphate phase rich in REE and Sr with sea water Sr and Nd isotopic characteristics; (3) a component rich in organic matter and Ca with a sea water Sr isotopic signature, a relatively low Nd isotopic composition and elevated Sm/Nd ratios. This latter component probably represents the suspended organic and carbonate-rich river load. The detrital and the river components were mixed up in the muddy off-shore sediment, ingested by worms, and integrated into faecal pellets. The resulting material has Sr and Nd isotopic signatures intermediate between those of the detrital and river components, and represents the precursor of the glaucony minerals. During the subsequent dissolution-crystallization process, the glauconitic pellets remain isotopically closed to any external supply, but expulsion of Sr and Nd with increasing degree of maturation is observed without any effect on the Sr and Nd isotopic compositions. At a higher maturation stage (K₂O>4.5%), the Sr and Nd isotopic compositions tend to decrease and increase, respectively, approximating the isotopic composition values of the phosphate-rich phase. Because the Sr and Nd concentrations decrease, the evolution of the glauconies toward lower Sr and higher Nd isotopic compositions can only be explained by expulsion of Sr and Nd of the detrital component with high Sr and low Nd isotopic signatures. Dissolution of the chemically unstable, wormdigested clay material from mud may be responsible for the liberation of these elements. Consequently, the phosphate-rich phase with sea water Sr and Nd isotopic signatures becomes increasingly important for the isotopic characteristics of the maturing glauconite grains, and sea water isotopic signatures can be reached during the stage of mature glauconite (K₂O>6.5%), without chemical exchange with the depositional environment.     

Spatial and temporal variations in terrestrially-derived organic matter from sediments of the Delaware estuary

Terrestrially-derived organic matter in sediments of the Delaware Estuary originates from riverine transport of soils and fresh litter, sewage and industrial wastes, and marsh export of organic matter. The quantity, composition, and spatial distribution of terrigenous organic matter in sediments was determined by elemental (C and N), lignin, and stable carbon isotope analyses. Sediments in the upper Delaware Estuary had low organic carbon content and high lignin content. In contrast, sediments in the lower Delaware Estuary had high organic carbon content and low lignin content. There was a slight decrease in the proportion of syringyl and cinnamyl phenols relative to vanillyl phenols between the upper estuary and lower estuary. Differences in lignin and stable carbon isotope compositions between sediments of the Delaware Estuary and sediments of the Broadkill River estuary (an adjoining salt-marsh estuary) supported previous observations that marshes do not export substantial quantities of organic matter to estuaries. Additional results suggested that lignin-rich sediments were concentrated in the upper estuary, most likely in the zone of high turbidity. Furthermore, algal material diluted lignin-rich sediments, particularly in the lower estuary. The weaker algal signal in bottom sediments compared to that in suspended particulate matter suggested algal material was decomposed either in the water column or at the sediment-water interface. Physical sorting of sediments prior to deposition was also indicated by observations of compositional differences between the upper and lower estuary bottom sediments. Finally, seasonal variations in primary productivity strongly influenced the relative abundance of terrestrial organic matter. In fall, however, the proportion of lignin was greatest because of a combination of greater inputs of terrestrially-derived organic matter, lower river discharge, and a decrease in algal biomass.     

Distribution of biochemical constituents in the surface sediments of western coastal Bay of Bengal: influence of river discharge and water column properties

Biochemical composition of surface sediment samples from off major and minor rivers along the east coast of India revealed that spatial distribution of sediment organic carbon (SOC) composition was mainly governed by differential characteristics of discharged water and associated biogeochemical processes in the water column. The northwest (NW) region of coastal Bay of Bengal was influenced by discharges from Ganges river while peninsular (monsoonal) rivers influenced the southwest (SW) region. The NW region characterized by low nutrients suspended particulate matter (SPM), high phytoplankton biomass in the water column and high SOC while contrasting to that observed in the SW region. The isotopic ratios of SOC (?22 ‰) in the NW region were close to that of organic matter derived from phytoplankton (?23 ‰) suggesting in situ production is the major source whereas terrigeneous source contributed significantly in the SW region (?19.6 ‰). Though low in situ biological production in the SW region, relatively higher total carbohydrates (TCHO) were found than in the NW and insignificant difference of total and free amino acid concentrations between NW and SW were resulted from faster removal of organic matter to the sediment in association with SPM in the SW region. Higher proteins concentrations than total amino acids indicate that nitrogenous organic matter is preserved in the former form. The protein to TCHO ratio was lower in the SW suggesting significant contribution of aged and non-living organic matter in this region.     

Silica Mass-Balance and Retention in the Riverine and Estuarine Scheldt Tidal System (Belgium/The Netherlands)

An annual budget for dissolved silica (DSi) and biogenic silica (BSi) was constructed for the Scheldt estuary and for the entire riverine and estuarine Scheldt tidal system (Belgium/The Netherlands) using previously published silica concentrations and fluxes for the period 2003–2005. The annual estuarine DSi mass-balance was established, based on seasonal fluxes estimated using measured DSi concentrations and (fully transient) model simulations of conservative transport. The annual BSi mass-balance was deduced from measured BSi contents in the suspended particulate matter and annual mud fluxes taken from the literature. The Scheldt estuary acted as a net sink not only for the BSi carried by the tidal river as well as that produced by diatoms in the estuary, but also for large amounts of BSi imported from the coastal zone. This results in the retention of dissolved and biogenic silica higher than that of DSi alone, which is in contrast with the classical consideration that rivers act as a source of BSi for the coastal zone. DSi and silica (DSi + BSi) retentions amounted to, respectively, 28 and 64 % in the estuary, and 33 and 66 % in the entire tidal system. This study highlights thus the predominant role of the estuary in the entire Scheldt tidal system when dealing with silica dynamics, as well as the importance of including BSi when investigating estuarine silica retention.     

Particulate matter grain-size characteristics and flocculation in a partially mixed estuary

The concentration and grain size of the natural and deflocculated inorganic suspended particulate matter were measured along the length of the Miramichi Estuary and interpreted with respect to flocculation and transport properties. Changes in particulate matter concentration are associated with regular changes in grain-size characteristics. In the turbidity maximum region of the estuary the suspended matter occurs mostly as large flocculated particles whereas, in the waters with lower particle concentrations, a larger proportion of the material occurs as fine material. At higher concentrations natural floc modes and inorganic grain modes vary simultaneously but at low concentrations the two modes vary inversely. This modal relationship and the variation in organic matter within the estuary is proposed to result from variation in inorganic—organic composition of flocs. Increase in settling rates due to flocculation is believed to increase the trapping effect of the estuarine circulation that produces the turbidity maximum.     

Seasonal Dynamics of Dissolved Trace Metals in the Scheldt Estuary: Relationship with Redox Conditions and Phytoplankton Activity

Seasonal dynamics of dissolved trace metals (Cd, Co, Cu, Ni and Zn) and its relationship with redox conditions and phytoplankton activity has been studied in the Scheldt estuary, during nine surveys carried out between May 1995 and June 1996. Seasonal profiles of dissolved trace metals and general estuarine water quality variables are compared, to identify the geochemical and biological processes responsible for the observed trace metal distributions. In keeping with previous studies, the behavior of dissolved Cd, Cu, and Zn can be explained by the presence of anoxic headwaters and the restoration of dissolved oxygen within the estuary. In the river water, the concentration of dissolved Cu and Zn is generally low, except during winter when dissolved oxygen is present in the water column, although highly undersaturated. Mobilization of particle-bound Cd, Cu, and Zn occurs as dissolved oxygen increases with increasing salinity, possibly because of oxidation of metal sulfides in the suspended matter. The geochemistry of dissolved Co is also related to the redox conditions but in an opposite way. Dissolved Co is mobilized in the anoxic upper estuary, along with the reduction in Mn (hydro) oxides, and subsequently coprecipitated with Mn (hydro) oxides when dissolved oxygen is restored. Conservative behavior is observed for dissolved Ni within the estuary. In the middle estuary, Cd and Zn are readsorbed during phytoplankton blooms, as suggested by the low concentrations of these metals during the most productive periods in spring and early summer. The removal may be caused by direct biological uptake and/or increased adsorption to suspended matter because of the pH increase associated with algae blooms. In the lower estuary, chemical gradients are much weaker and dilution with seawater is the dominant process.     

The influence of phytoplankton community composition on primary productivity along the riverine to freshwater tidal continuum in the San Joaquin River,California

Differences in phytoplankton community composition along a riverine to, freshwater tidal continuum was an important factor affecting the primary productivity and quantity of phytoplankton biomass available to the San Francisco Estuary food web downstream. The relative contribution of riverine and freshwater tidal phytoplankton was determined using measurements of primary productivity, respiration, and phytoplankton species composition along a riverine to freshwater tidal gradient in the San Joaquin River, one of two major rivers that flow into, the San Francisco Estuary. Chla-specific net primary productivity was greater in the freshwater tidal habitat and was correlated with both a higher growth efficiency and maximum growth potential compared with the river upstream. Cluster analysis indicated these differences in growth parameters were associated with differences in species composition, with greater percent diatom and green algal species biomass upstream and flagellate biomass downstream. Correlation between the chla specific net productivity and phytoplankton species composition suggested the downstream shift from riverine diatom and green algal species to flagellate species contributed to the seaward increase in net primary productivity. Environmental conditions, such as specific conductance and water transparency, may have influenced primary productivity along the riverine to freshwater tidal continuum through their effect on both species composition and growth rate. Data suggest light was not the sole controlling factor for primary productivity in this highly turbid estuary; phytoplankton growth rate did not increase when riverine plankton communities from low light conditions upstream were exposed to higher light conditions downstream. This study suggests that the availability of phytoplankton biomass to the estuarine food web may be influenced by management of both phytoplankton growth and community composition along the riverine to freshwater tidal continuum.     

On the advection of turdidity in the saint lawrence middle estuary

The suspension transport away from the extensive turbidity zone of the St. Lawrence estuary is largely determined by the channel topography. The suspended sediments are advected downstream by a 40 km long turbid plume which takes its source in a turbidity maximum at the head of the estuary and flows downstream partly confined by the South Channel. During the ebbing phase of tidal cycles, the turbid waters of the plume are forced downstream through narrow converging sections of the channel, and slowed down through more opened diverging regions, particularly down-stream of the St. Roch Traverse. These, large fluctuations in stratification modulate the vertical transport of suspended material from the bottom to the surface layer. Midway down the estuary, dispersion of the plume occurs along a frontal zone which seasonally migrates 30 km or more in response to changes in fresh water discharge. The plume is reinforced and the turbidity gradient is intensified by local injections of inshore waters from Ste. Anne Bay, a subtidal platform highly enriched in suspended material by intertidal exchanges with large mudllats. Lateral erosion of the plume and cross-channel transport of suspended matter from the South into the North Channel is made possible by large horizontal shears developing in the central part of the middle estuary during the early flood. These are created by a one-hour tidal phase difference between the North and the South Channel.     

Boron isotopes geochemistry of the Changjiang basin rivers

We report analyses of B isotopic compositions in water and suspended particulate matter collected in the Changjiang and its main tributaries. We showed that four sources control the dissolved boron budget; namely atmospheric deposition, evaporite dissolution, anthropogenic inputs and silicate weathering. The contribution of silicate weathering to the dissolved B load ranges from 40% to 50% for the Changjiang main channel and from 45% to 88% for the main tributaries. The isotopic composition of dissolved boron derived from silicate weathering range from −3‰ up to +9‰ suggesting that isotopic fractionation occurs during silicate weathering. The boron isotopic composition of suspended particulate matter range from −11.4‰ to −6‰. Boron derived from silicate weathering is preferentially carried out by the dissolved load which accounts for 30-96% of the total boron. We show that the isotopic compositions of both the dissolved load and suspended particulate matter are controlled by the competition between boron leaching and boron uptake into secondary phases. The first process is characterized by a loss of boron relative to the bedrock without apparent isotopic fractionation whereas the last one is associated to a large isotopic fractionation which enriches the dissolved boron in heavy isotope.     

Organic matter sources and transport in an agriculturally dominated temperate watershed

The quality, quantity, and origin of suspended organic matter were studied in the highly agricultural Upper Scioto River in Central Ohio. Late summer baseflow conditions were compared to late autumn high flow conditions. Variables examined in the suspended matter were the total suspended solids concentration, semi-quantitative concentrations of lignin, carbohydrate concentrations, total organic C, total and organic P, and δ-¹³C. Also examined were ratios of C to N, organic C to organic P ratios and fluxes of total organic C. The primary hypothesis of this research was that the quality (or biodegradability) and quantity of organic matter in the Upper Scioto River would increase during autumn stormflow conditions due to inputs of fresh terrestrial organic matter. The autumn suspended matter was also expected to reflect C4 plant contributions from corn organic matter. Results show that the quality and quantity of organic matter were greater during summer, as reflected in low molar ratios (178:1) of organic C to organic P, and higher organic C content of the suspended matter in summer. Summer suspended matter was 3.6% organic C and autumn suspended matter was 2.3% organic C. Carbon to N molar ratios in both seasons were very close to the Redfield ratio (6.6:1 in summer and 6.7:1 in autumn). Total suspended matter and total organic C concentrations were lower in autumn (8.7 mg/l⁻¹ TOC and 17.7 mg/l⁻¹ TSS) than in summer (17.5 mg/l⁻¹ TOC and 39.0 mg/l⁻¹ TSS), but the fluxes were greater in autumn due to greater stream flow. Stable isotope analyses suggested a phytoplankton or C3 plant source (most likely corn) for summer organic C (mean δ¹³C of −24.8‰) and a phytoplankton or C4 plant source for autumn organic matter (δ¹³C=−21.5‰).     

Lignin geochemistry of sediments from the Narragansett Bay Estuary

Cupric oxide oxidation has been employed to characterize the lignin geochemistry of Narragansett Bay sediments. Lignin concentrations throughout the estuary are low when expressed on a carbon-normalized basis, but can be characterized as enriched when expressed on a mass-normalized basis. This implies substantial dilution of the sedimentary lignin by inputs of lignin-poor carbon. Lignin concentrations do not correlate with the ¹³C isotopic composition of the sedimentary organic matter. These results are consistent with a sediment lignin component consisting of varying amounts of vascular plant debris and lignin-depleted organic matter, the latter originating from both marine (planktonic) and terrestrial (uncharacterized) sources. Compositional plots of lignin-derived phenols show that sediments in the upper estuary are influenced to a greater extent by gymnosperm lignin sources than those in the mid-and lower estuary. Given the extent to which the upper estuary is affected by pollution sources, inputs from anthropogenic discharges are the most likely cause of these compositional differences. However, an evaluation of processed paper products as an “anthropogenic” lignin source indicates that the lignin content of these materials is insufficient to account for the levels found in the sediments. Subsurface lignin compositions at an upper estuary site reveal that lignin originating from the inferred anthropogenic sources disappears at a depth shallower than that which would be expected based on the distribution of other trace organic pollutants (hydrocarbons and several synthetic organic compounds). We speculate that differences in either the depositional history or the degree of preservation of these two compound classes are responsible for the observed trends.     

Determination of the origin of suspended matter and sediments in the Elbe estuary using natural tracers

The clay mineral composition, the concentrations of carbonates, the proportions of carbon and oxygen isotopes in carbonates and organic matter, as well as the concentrations of different nonanthropogenic metals were used to determine the origin of different grain size fractions of sediments and suspended matter in the Elbe estuary. Analysis of the smectite/kaolinite proportion revealed that solid material ≤2 μm from the North Sea is transported up the river, about 40 km beyond the most upstream position of the salt wedge. In the 2–20 μm fraction, the¹⁶O/¹⁸O ratio in carbonates and the kaolinite/chlorite proportion demonstrate a transport of North Sea material between 40 and 20 km upstream of the marine water limit. The transport behaviour of the 20–63 μm grain size fraction could be determined by the hafnium concentration, representative for the heavy mineral zircon. In this case, the transport distance beyond the salt wedge was up to 20 km. No information was available on the origin of the fine organic matter, whereas the coarser fractions were derived primarily from debris of salt marsh vegetation. The results demonstrate that in the Elbe estuary mixing between marine and fluvial solid material occurs upstream of the salt wedge and is significantly responsible for the observed decrease in the concentration of various pollutants in sediments and suspended matter along the estuary. The cause of the upstream particle transport is probably a scour lag mechanism based on asymmetries of the flood- and ebb-tide current distribution, especially their differing maximum velocities.     

Lower Seine River and estuary (France) carbon and oxygen budgets during low flow

Ecological processes driving the oxygen budget were investigated in the downstream part of the Seine River and its estuary. Phytoplankton and bacterioplankton production were measured along longitudinal profiles (11 to 17 stations) in a range of low discharges from 300 m³ s⁻¹ in 1993 and 1995 to 140 m³ s⁻¹ in 1996. Values representative of the water column were based on investigations carried out during two tidal cycles. Net primary production was invariably greatest in the freshwater estuary, from Poses to Rouen (from 500 to 1,000 μg C l⁻¹ d⁻¹ between PK 202 and 240) and decreased sharply downstream (from 10 to 25 μg c l⁻¹ d⁻¹ between PK 250 and 310). This decrease was mainly due to the deterioration of the light conditions with the increase in depth and suspended matter concentrations. Heterotrophic activity was maximum in the reach where primary production declined. Judging by the production:respiration ratio (P:R), the system appeared clearly heterotrophic in the Seine River immediately downstream of the Paris region due to high allochthonous organic pollution by the incompletely treated Parisian effluents and in the part of the estuary characterized by intense degradation of autochthonous material. Because the effluents are not treated by a nitrification step, the oxygen consumption due to nitrification was much higher than expected from the P:R ratio. Oxidation of ammonium represented an oxygen consumption of between 1 and 14 g O₂ m⁻² d⁻¹, almost equalling the sum of heterotrophic respirations that were barely balanced by photosynthesis. The reaeration flux at the water-atmosphere interface was deduced from the calculations and a reaeration coefficient was estimated.     

Spatial and Temporal Variability of Sediment Organic Matter Recycling in Two Temperate Eutrophicated Estuaries

This paper deals with the spatial and seasonal recycling of organic matter in sediments of two temperate small estuaries (Elorn and Aulne, France). The spatio-temporal distribution of oxygen, nutrient and metal concentrations as well as the organic carbon and nitrogen contents in surficial sediments were determined and diffusive oxygen fluxes were calculated. In order to assess the source of organic carbon (OC) in the two estuaries, the isotopic composition of carbon (δ ¹³C) was also measured. The temporal variation of organic matter recycling was studied during four seasons in order to understand the driving forces of sediment mineralization and storage in these temperate estuaries. Low spatial variability of vertical profiles of oxygen, nutrient, and metal concentrations and diffusive oxygen fluxes were monitored at the station scale (within meters of the exact location) and cross-section scale. We observed diffusive oxygen fluxes around 15 mmol m^?2 day^?1 in the Elorn estuary and 10 mmol m^?2 day^?1 in the Aulne estuary. The outer (marine) stations of the two estuaries displayed similar diffusive O₂ fluxes. Suboxic and anoxic mineralization was large in the sediments from the two estuaries as shown by the rapid removal of very high bottom water concentrations of NO _x ^? (>200 μM) and the large NH₄ ⁺ increase at depth at all stations. OC contents and C/N ratios were high in upstream sediments (11–15 % d.w. and 4–6, respectively) and decreased downstream to values around 2 % d.w. and C/N ≤ 10. δ ¹³C values show that the organic matter has different origins in the two watersheds as exemplified by lower δ ¹³C values in the Aulne watershed. A high increase of δ ¹³C and C/N values was visible in the two estuaries from upstream to downstream indicating a progressive mixing of terrestrial with marine organic matter. The Elorn estuary is influenced by human activities in its watershed (urban area, animal farming) which suggest the input of labile organic matter, whereas the Aulne estuary displays larger river primary production which can be either mineralized in the water column or transferred to the lower estuary, thus leaving a lower mineralization in Aulne than Elorn estuary. This study highlights that (1) meter scale heterogeneity of benthic biogeochemical properties can be low in small and linear macrotidal estuaries, (2) two estuaries that are geographically close can show different pattern of organic matter origin and recycling related to human activities on watersheds, (3) small estuaries can have an important role in recycling and retention of organic matter.     

Spectral Irradiance and Phytoplankton Community Composition in a Blackwater-Dominated Estuary,Winyah Bay,South Carolina,USA

We investigated spatial and temporal relationships between spectral irradiance and phytoplankton community composition in the blackwater-influenced estuary Winyah Bay, South Carolina. Upstream, high concentrations of chromophoric dissolved organic matter (CDOM) absorbed blue wavelengths, resulting in a predominantly red light field. Green light prevailed downstream near the lower-CDOM coastal ocean, and phytoplankton community composition was distinct from upstream and mid-estuarine communities. Diatoms were abundant throughout the estuary in January, August, and October, cryptophytes dominated in July, and chlorophytes were abundant in December 2006. Only diatoms and chlorophytes showed significant covariation with the spectral attenuation coefficient (k(λ)): Chlorophytes showed positive relationships with k(442) (blue light) while diatoms were negatively correlated with k(442) and k(490) (violet to blue). Phytoplankton community composition in Winyah Bay appears to be driven by strong horizontal flow rather than gradients in spectral irradiance, but results indicate that water color is likely to play a greater role in blackwater-influenced estuaries with longer residence times.     

Sources of oxygen demand in the lower San Joaquin River,California

Dissolved oxygen concentration below 5 mg 1⁻¹ has characterized the lower tidal portion of the San Joaquin River downstream of Stockton, California, during the summer and fall for the past four decades. Intensive field research in 2000 and 2001 indicated low dissolved oxygen concentration was restricted to the first 14 km of the river, which was deepened to 12 m for shipping, downstream of Stockton. The persistent low dissolved oxygen concentration in the shipping channel was not caused by physical stratification that prevented aeration from vertical mixing or respiration associated wigh high phytoplankton biomass. The low dissolved oxygen concentration was primarily caused bynitrification that produced up to 81% of the total oxygen demand. Stepwise multiple regression analysis isolated dissolved ammonia concentration and carbonaceous oxygen demand as the water quality variables most closely associated with the variation in oxygen demand. Between these two sources, dissolved ammonia concentration accounted for 60% of the total variation in oxygen demand compared with a maximum of 30% for carbonceous oxygen demand. The Stockton wastewater treatment plant and nonpoint sources upstream were direct sources of dissolved ammonia in the channel. A large portion of the dissolved ammonia in the channel was also produced by oxidation of the organic nitrogen load from upstream. The phytoplankton biomass load from upstream primarily produced the carbonaceous oxygen demand. Mass balance models suggested the relative contribution of the wastewater and nonpoint upstream load to the ammonia concentration in the shipping channel at various residence times was dependent on the cumulative effect of ammonification, composition of the upstream load, and net downstream transport of the daily load.     

Properties of fluorescent dissolved organic matter in the Gironde Estuary

The isolation, characterization and study of the properties of aquatic dissolved organic matter (DOM) still represent a challenge because of the heterogeneity, complexity and low concentration of organic material in natural waters. Based on its ability to interact with contaminants and thus to modify their transport and bioavailability, DOM is of interest for environmental purposes. The objective of this work was to better characterize DOM in the Gironde Estuary (southwestern France). The estuary represents an exchange zone between the continent and the Atlantic Ocean and conditions the transfer of organic and inorganic substances from the continental to the oceanic environment. Several samples were collected along the estuary during three cruises in 2002 and 2006. They were analysed using excitation–emission matrix (EEM) spectroscopy, a sensitive technique that allows direct analysis of water samples. Fluorescent DOM and dissolved organic carbon (DOC) did not behave conservatively in this estuarine system, i.e. the organic material did not undergo simple dilution from the upstream to the downstream part of the estuary. A seasonal variability in DOC content was pointed out, whereas few seasonal variations in DOM fluorescence were observed. DOM sources and processing in the estuary were further evaluated by determining two fluorescence indices – the humification index (HIX) and the index of recent autochthonous contribution (BIX). By applying these indices, the relative degree of humification (HIX) and autotrophic productivity (BIX) could be assessed. Based on the fluorescence and DOC results, the estuary was divided into three zones depending on salinity (S) and characterized by specific DOM: (i) A turbid zone of low salinity (S < 5) and high suspended particulate matter concentration with increase in the intensities of the α′ and α fluorophores, characteristic of humic-like compounds. (ii) A mid-estuarine zone (5 < S < 25) characterized by low autotrophic productivity and containing strongly degraded organic material, as shown by the low values of BIX and high values of HIX. (iii) A higher salinity area (S > 25) characterized by increased autotrophic productivity and a marked marine influence, and associated with high and low values of BIX and HIX, respectively. The HIX and BIX indices were shown as useful tools for readily defining and classifying DOM characteristics in estuarine waters.     

Transformation of particulate organic matter at the water-bottom boundary in the Russian Arctic seas: Evidence from isotope and radioisotope data

Complex biogeochemical studies including the determination of isotopic composition of C_org in both suspended particulate matter and surface horizon (0–1 cm) of sediments (more than 260 determinations of δ¹³C-C_org) were carried out for five Arctic shelf seas: White, Barents, Kara, East Siberian, and Chukchi. The aim of this study is to elucidate causes that change the isotopic composition of particulate organic carbon at the water-sediment boundary. It is shown that the isotopic composition of C_org in sediments from seas with a high river runoff (White, Kara, and East Siberian) does not inherit the isotopic composition of C_org in particles precipitating from the water column, but is enriched in heavy ¹³C. Seas with a low river runoff (Barents and Chukchi) show insignificant difference between the value of δ¹³C-C_org in both suspended load and sediment because of a low content of the isotopically light allochthonous organic matter (OM) in particulates. Complex biogeochemical studies with radioisotope tracers (¹⁴CO₂, ³⁵S, and ¹⁴CH₄) revealed the existence of specific microbial filter formed from heterotrophic and autotrophic organisms at the water-sediment boundary. This filter prevents the mass influx of products of OM decomposition into water column, as well as reduces the influx of a part of OM contained in the suspended particulate matter from water into sediment.