Conservative Mixing Line Research Articles

Rapid changes in the surface carbonate system under complex mixing schemes across the Bering Sea: a comparative study of a forward voyage in July and a return voyage in September 2018

Regulated by the rapid changes in temperature, mixing, and biological production during warm seasons, the surface carbonate system in the Bering Sea is subject to significant spatial-temporal variability. However, the seasonal evolution of the carbon cycle and its controls are less clear due to the lack of observations. Here, we present the carbonate data collected during a forward voyage in July and a return voyage in September 2018 across the Bering Sea. For both voyages, we show distinct dissolved inorganic carbon versus total alkalinity (DIC-TA) relationships and partial pressure of CO2 (pCO2) distribution patterns in the Southern Basin (54-57°N), the Northern Basin (57-59°N), the Slope (59-61°N), the Shelf (61-64°N), and the Bering Strait (>64°N). In the Southern Basin, the Northern Basin, and the Slope, surface water was a two end-member mixing of Rainwater and Bering Summer Water (BSW) during the forward voyage and a two end-member mixing of North Pacific Surface Water (NPSW) and BSW during the return voyage. As a result, the observed DIC was almost consistent with the conservative mixing line, with a slight DIC addition/removal of -8.6~5.8 µmol kg-1, suggesting low biological production/respiration during both voyages. Seasonally, the higher factions of NPSW featuring low pCO2 during the return voyage dominated the pCO2 drawdown from July to September in the Southern Basin and the Slope. On the Shelf, the surface water was a two end-member mixing of plume water from the Anadyr River and BSW during both voyages, but the decreased DIC consumption via biological production from 59.9 ± 25.8 µmol kg-1 to 34.8 ± 14.0 µmol kg-1 contributed to the pCO2 increase from July to September. In the Bering Strait, the coastal area was characterized by the influence of plume water from the Anadyr River in July and the coastal upwelling in September. The high biological production in plume water made a strong CO2 sink during the forward voyage, while the upwelling of carbon-enriched subsurface water with minor DIC consumption made the coastal ecosystem a strong CO2 source during the return voyage. In different geographical regions, the observed seawater pCO2 was much lower than the overlying atmospheric CO2, resulting in a net CO2 sink with fluxes of -2.1~-14.0 mmol m-2 d-1 and -2.5~-11.6 mmol m-2 d-1, respectively, during the forward and return voyages.

Geochemistry and evolution of groundwater resources in the context of salinization and freshening in the southernmost Mekong Delta, Vietnam

Study regionCa Mau Province (CMP), Mekong Delta (MD), Vietnam. Study focusGroundwater from deep aquifers is the most reliable source of freshwater in the MD but extensive overexploitation in the last decades led to the drop of hydraulic heads and negative environmental impacts. Therefore, a comprehensive groundwater investigation was conducted to evaluate its composition in the context of Quaternary marine transgression and regression cycles, geochemical processes as well as groundwater extraction. New hydrological insights for the regionThe abundance of groundwater of Na-HCO3 type and distinct ion ratios, such as Na+/Cl-, indicate extensive freshwater intrusion in an initially saline hydrogeological system, with decreasing intensity from upper Pleistocene to deeper Miocene aquifers, most likely during the last marine regression phase 60–12 ka BP. Deviations from the conservative mixing line between the two endmembers seawater and freshwater are attributed to ion-exchange processes on mineral surfaces, making ion ratios in combination with a customized water type analysis a useful tool to distinguish between salinization and freshening processes. Elevated salinity in some areas is attributed to HCO3- generation by organic matter decomposition in marine sediments rather than to seawater intrusion. Nevertheless, a few randomly distributed locations show strong evidence of recent salinization in an early stage, which may be caused by the downwards migration of saline Holocene groundwater through natural and anthropogenic pathways into deep aquifers.

Spatial characteristics and removal of dissolved black carbon in the western Arctic Ocean and Bering Sea

In the Arctic, large amounts of black carbon (BC) are released into river water and transported to the oceans as dissolved BC (DBC). However, the cycling and fate of DBC in the ocean is poorly understood. Here, DBC, dissolved organic carbon (DOC), absorbance, and δ18O were analyzed to examine the spatial characteristics and removal of DBC in the western Arctic Ocean and the Bering Sea during the boreal summer. DBC concentrations ranged from 0.67 to 4.18 μmol-C L−1. In the sea-ice free regions, high DBC and DOC concentrations and a254 values corresponded to high δ18O-derived meteoric water contents, indicating that river discharge dominated their spatial patterns. By contrast, sea-ice meltwater (SIM) appeared to dilute DBC and DOC in SIM impacted waters (SIM > 3.5%). The departure of DBC from conservative mixing lines suggested that 47–84% of riverine DBC was removed over the shelves with 7–23% transported to the upper Canada Basin. These results suggest that shelf regions are crucial sites for riverine DBC removal that modulate the delivery of DBC to the open Arctic Ocean.

Isotopic Constraint on the Sources and Biogeochemical Cycling of Nitrate in the Jiulong River Estuary

AbstractNitrate concentrations and isotopic compositions (δ15N‐NO3− and δ18O‐NO3−) in the Jiulong River Estuary (JRE) were determined in 2014 to better understand the transformation and source contributions of nitrate. In contrast to nitrate concentrations, which generally followed a conservative binary mixing pattern, both δ15N‐NO3− and δ18O‐NO3− deviated significantly from the binary isotopic mixing lines during all sampling cruises, indicating a dynamic transformation of nitrate in the JRE. The deviation of nitrate isotopic signatures from conservative mixing lines as well as the slope of the regression between N and O isotopes (δ15N‐NO3− vs. δ18O‐NO3−) had significant seasonal variability, showing the seasonal shifting of the dominant nitrification and sedimentary denitrification in the JRE. Furthermore, the elevated nitrate concentrations and isotopic compositions in the lower JRE during the dry season, along with an isotope regression slope of 1.16, indicated a significant local input of nitrate. Nitrate source apportionment is carried out through the Stable Isotope Analyses in R model with the incorporation of nitrate isotopic fractionation factors for nitrification and denitrification in the JRE. The model results revealed that fertilizer is the largest nitrate source to the JRE, contributing 33%–55% of the nitrate throughout the sampling period. Manure is another major source and contributed 32%–39% of the nitrate to the JRE. Sewage nitrate contributions to the JRE were 7% during April and June but dramatically increased to 18% during the wet season, probably as a result of the rapid flushing of suburban and rural untreated domestic sewage into the river.

Export Flux, Biogeochemical Effects, and the Fate of a Terrestrial Carbonate System: From Changjiang (Yangtze River) Estuary to the East China Sea

AbstractSeasonal variations in the transports of total alkalinity (TAlk) and dissolved inorganic carbon (DIC) from the Lower Changjiang (Yangtze) River/Estuary to the East China Sea were investigated based on a series of field surveys in 2015–2017, including monthly samplings at Datong Station and seasonal mapping cruises in the Changjiang Estuary and the adjacent northwestern East China Sea. In comparison with historical data sets, the Changjiang TAlk flux varied around a nearly stable average over the past 55 years. This is much different from some American rivers, where TAlk export fluxes increased for a century long. To assess effects of riverine carbonate inputs on coastal carbonate chemistry, we compared several cases showing freshwater‐dilution‐induced decline in coastal aragonite saturation state (Ωarag), including rainwater dilution and riverine water dilution. Without riverine carbonate inputs, the effect of a unit of salinity decrease (due to rainwater dilution) on Ωarag was expected to be counteracted by a DIC removal of 10 μmol/kg relative to the baseline value along relevant conservative mixing line, when coastal Ωarag was close to a critical value of 1.5. Considering terrestrial carbonate inputs from Changjiang, however, the freshwater‐dilution‐induced coastal Ωarag suppression decreased by 12%. Our data also showed that more than 10% of wet‐season DIC flux discharged from the Changjiang Estuary was sequestered by biological activities in nearshore areas, while the TAlk flux was rarely affected. This biological alteration effectively transformed the terrestrial carbonate system from a feature of DIC:TAlk >1.0 to the usual seawater feature of DIC:TAlk <0.9.

Determination of sub-picomolar levels of platinum in the pristine Krka River estuary (Croatia) using improved voltammetric methodology

Environmental contextPlatinum concentrations in natural waters such as oceans, rivers and lakes are extremely low, hindering studies of Pt distributions and biogeochemical cycles. An improved electrochemical method was used to reliably determine Pt in estuarine conditions at trace concentrations. Platinum displayed a near-conservative behaviour along the salinity gradient of the estuary, with about 90% remaining in the dissolved form. AbstractExtremely low concentrations of platinum in natural waters require very sensitive analytical techniques, with adsorptive cathodic stripping voltammetry (AdCSV) being one of the most frequently used techniques. A ‘fine tuning’ of the voltammetric parameters, along with advanced data treatment based on derivative transformations, allowed us to determine reliably Pt levels down to 50 fM (0.05 pM). By using short modulation and interval times of the differential pulse stripping waveform, and applying a 4th derivative transformation to the voltammograms, the limit of detection (LOD) was lowered down to 10 fM. Although very small concentrations of surface-active substances (e.g. 0.025mgL−1 fulvic acid) strongly influenced the method sensitivity, recoveries of spiked samples were not impacted (~100%). The application of a desorption step (Eds=−1.35V; tds=2s) at the end of the accumulation significantly improved the sensitivity, presumably through the removal of adsorbed surface-active substances. Using this optimised methodology, we determined the Pt distribution in the pristine Krka River estuary in the winter and summer periods by performing both horizontal transects and vertical profiles (salinity ~1 to 39). In surface waters, dissolved Pt concentrations gradually increased towards the seawater end-member (from ~0.15 to ~0.3 pM). A small deviation from the conservative mixing line was observed at salinities below 10, which may reflect changes in Pt redox speciation. In bottom waters, the trend was opposite with dissolved Pt concentrations increasing towards the freshwater end-member, probably owing to progressive accumulation related to seawater residence time. On average, 90% of Pt was present in the dissolved form.

Organic matter along longitudinal and vertical gradients in the Black Sea

We studied organic matter in the central Black Sea and its northeastern and northwestern shelf break within three weeks in November 2013 to test the hypothesis that in situ production could explain lateral and vertical variability in its composition and distribution. The wide spatial coverage over a short period of time achieved during this study revealed longitudinal variability in organic matter characteristics reflecting productivity at the Black Sea surface. Particulate organic matter was dominantly autochthonous in the central Black Sea. Allochthonous influence of river discharge was only traced on the northwestern shelf by high concentrations but low freshness of particulate organic matter. Compared to the NW shelf break and central Black Sea, primary production was high near the NE shelf break, likely fueled by input from the Azov Sea. Vertical patterns were similar throughout the deep Black Sea and appeared to also be governed by in situ processes rather than reflect variability in the surface water. As concentrations of organic matter decreased with depth, its elemental and isotopic composition indicated chemoautotrophic production at the oxic-anoxic interface and organic matter degradation in the benthic boundary layer. Though profiles of dissolved organic carbon indicated a minor source in anoxic deep water, likely linked to chemosynthesis and reflux from sediments, a negative deviation of concentrations from a conservative mixing line between two endmembers suggested net removal of labile dissolved organic carbon.

Seasonal distributions of organic nutrients on the Louisiana continental shelf and their implications for nutrient limitation and hypoxia formation

Nutrients delivered by the Mississippi and Atchafalaya Rivers to the Louisiana continental shelf cause local eutrophication and feed phytoplankton growth, resulting in a massive region of bottom water hypoxia each summer. Much of what is known about the drivers of this hypoxia is based on extensive datasets of inorganic nutrients. We measured dissolved organic and particulate organic carbon, nitrogen and phosphorus in surface water along the Louisiana continental shelf between the Mississippi and Atchafalaya Rivers during three cruises in March, May and July 2004 to develop a more comprehensive understanding of interactions between nutrients and eutrophication. We specifically sought to determine if sufficient organic P is present to relieve the inorganic P limitation that occurs during the spring and early summer. DOP is added to shelf waters in situ, but drawdown of DOP concentrations below conservative mixing lines indicate DOP is also being used as a source of P for growth. Calculations of the amount of DOP potentially utilized towards primary production reveal there is not enough to relieve P-limitation. Total nitrogen to total phosphorus ratios were high, averaging 86, 72 and 51 in March, May and July, respectively. Additionally, correlations between DIN and total nitrogen (TN) were identical to those between DIP and total phosphorus (TP), indicating that the use of either TN:TP or DIN:DIP ratios is equally relevant in assessing nutrient limitation in this region, but DIN:TP ratios are clearly underestimates. Organic matter on the shelf is best correlated to TN and TP loads measured upstream in the Mississippi River one month prior. The combination of low organic P concentrations and high total N:P ratios indicated P-limitation of phytoplankton biomass on the Louisiana shelf during the spring and early summer of 2004. This P-limitation occurs during the period of hypoxia formation as a result of excess N loads delivered by the rivers, supporting dual controls of both N and P to reduce the hypoxia.

Trace metal geochemistry of organic carbon-rich watercourses draining the NW German coast

Numerous small watercourses are draining the hinterland of the NW German coast. The waters mainly originate from marsh and fen areas and have yellow to deep brownish color. During their flow path, the waters exhibit gradients in salinity (0.2–3), pH (6.2–8.8), particulate organic carbon (5–25%), and iron oxides (7–12%), which alter the concentrations of most dissolved and particulate trace metals. For example, dissolved Fe is rapidly removed from solution at increasing salinities by flocculation, whereas dissolved U is removed in the very low-salinity zone by Fe- and organic-rich colloids. The waters at the flood-gate of Neuharlingersiel, where a composite sample of the entire study area is collected before the freshwater is discharged into the marine-dominated tidal flat area in front of the mainland dike, have the following average trace metal concentrations: dissolved Fe 11 μM, Mn 4 μM, Mo 10 μM, U 1.8 μM, V 75 μM and particulate Fe 7 mg kg−1, Mn 1200 mg kg−1, Mo 2 mg kg−1, U 1.7 mg kg−1, V 140 mg kg−1. After passing the flood-gate most Mo- and U-salinity distributions fit well to the conservative mixing line that connects the seawater of the Wadden Sea to the low-salinity river water entering the tidal flat area. In contrast, dissolved Fe, Mn, and V are removed from solution, especially at intermediate salinities, which may be due to mixing of freshwater with seawater depleted in these elements, flocculation and/or scavenging by particulate matter. In general, processes similar to those in estuarine systems alter the geochemical signatures on transects from land to sea. Compared to average river water, the watercourses are enriched in dissolved and particulate Fe, Mn, Mo, U, and V. Due to the fast removal of dissolved Fe in the salinity gradient and the high concentrations of Mo and U in seawater, the watercourses only form a source for Mn, V, terrestrial organic carbon, and particulate Fe for the Wadden Sea.

Calculation of river-seawater endmembers and differential trace metal scavenging in the Columbia River plume

A simple model is presented for calculating river and seawater mixing zone endmembers. This model allows construction of a conservative mixing line with respect to trace metals scavenged from surface waters, in the absence of direct endmember sampling. Results of the model suggest a transition from non-conservative to conservative behavior of the trace metals gallium (Ga) and zirconium (Zr) along a transect extending offshore of the Columbia River, indicating environmental controls on differential scavenging. Mechanistic explanation of observed differential scavenging is supported by wind data indicating northward transport of Columbia River plume waters, followed by southwest, offshore, transport. Columbia River plume waters transported to the southwest at the time of sample collection were therefore flanked offshore by aged plume waters returning from the north. While upwelling conditions provide nutrients during southerly transport of plume waters, northward plume transport is associated with downwelling, low nutrient conditions. Low phytoplankton abundance during periods of northward plume transport reduces trace metal scavenging, therefore dilution of river waters with ocean surface waters of comparatively low Ga and Zr concentrations results in near-conservative mixing of Ga and Zr in offshore, aged, plume waters. Results of this work suggest dynamic modification of trace metal residence time within close geographic proximity established by physical and biologic controls, dictated by local winds. Lower trace metal scavenging rates, resulting in a longer residence time, are suggested during northward transport of Columbia River plume waters, compared to higher scavenging rates, and thus shorter residence times, during southerly transport of plume waters. Gallium, demonstrating high analytical precision, reduced contamination risk, and decreased complications of upwelled sources relative to Zr, is proposed to provide identification of aged Columbia River plume waters.

Bioattenuation in Groundwater Impacted by Landfill Leachate Traced with δ13C

The impact on groundwater imparted by the infiltration of high dissolved organic carbon (DOC) leachate from capped, unlined landfills can be attenuated by biogeochemical reactions beyond the waste source, although such reactive loss in the aquifer is difficult to distinguish from conservative advective dispersion. Compound-specific measurement of δ(13)C in carbon species, including CH(4), dissolved inorganic carbon (DIC), and the major DOC compounds (acetate, humic acid, and fulvic acid) provides a constraint in this assessment that can assist in exercises of modeling and prediction of leachate transport. The Trail Road municipal landfill near Ottawa, Ontario, Canada, hosts an unlined sector which produces a highly enriched leachate (DOC >4500 mg/L) that provides a good site to examine reactive attenuation within the receptor aquifer. Acetate, a sentinel component of leachate DOC (~1000 mg C/L), is absent in impacted groundwater. Mass balance calculations together with reaction modeling suggest continued acetate fermentation with calcite control on DIC and δ(13)C(DIC) evolution. In groundwater within 50 m of the landfill, methane concentrations are elevated (~10 mg/L), consistent with acetate fermentation, whereas δ(13)C(CH4) measurements in deeper groundwater range down to -51‰ compared with -60‰ in the landfill demonstrating oxidative loss. DOC in the deep aquifer is remarkably depleted to values less than -40‰ suggesting methanotrophic bacteria selectively consume isotopically light CH(4) to fix carbon. Continued reaction of leachate DOC in groundwater is demonstrated by evolution away from conservative mixing lines on diagrams of δ(13)C vs. concentrations of DIC and DOC.

Seasonal variability and controls on chromophoric dissolved organic matter in a large river‐dominated coastal margin

The spring‐to‐summer seasonal transition in the spatial and vertical distribution of chromophoric dissolved organic matter (CDOM) optical properties was examined during surveys conducted in March, May, July, and August of 2005 in the waters influenced by the Mississippi‐Atchafalaya River system of the northwestern Gulf of Mexico. Strong seasonal river influences and a well‐correlated inverse relationship between CDOM absorption at 412 nm and salinity during spring of 2005 suggests conservative mixing in the water column between the riverine freshwater sources and the oceanic end members. Trends in the CDOM absorption and the spectral absorption slope (S) and deviations from the end‐member conservative mixing line in surface and bottom waters during the summer suggest apparent sources and sinks of CDOM. Elevated CDOM in lower‐salinity surface waters at many stations appears to be linked to high concentrations of dissolved oxygen (DO; >10 mg L−1) and elevated chlorophyll concentrations. Effects of photo‐oxidation in surface waters were observed as increasing S and enhanced CDOM loss in the ~27‐32 salinity range and were attributed to seasonal stratification and the increased solar radiation during the summer. In the bottom waters, elevated CDOM levels in the ~32‐36 salinity range were associated with low DO and relatively higher bacterial abundance; this suggests a potential CDOM source due to microbial remineralization of organic matter in the hypoxic zone in bottom waters. Seasonal dynamics associated with CDOM loss in surface waters and CDOM gains in bottom waters may be another pathway influencing hypoxia in coastal waters.

Nutrient and organic matter patterns across the Mackenzie River, estuary and shelf during the seasonal recession of sea-ice

Suspended material, nutrients and organic matter in Mackenzie River water were tracked along a 300 km transect from Inuvik (Northwest Territories, Canada), across the estuarine salinity gradient in Kugmallit Bay, to offshore marine stations on the adjacent Mackenzie Shelf. All particulates measured (SPM, POC, PN, PP) declined by 87–95% across the salinity gradient and levels were generally below conservative mixing. Organic carbon content of suspended material decreased from 3.1% in the river to 1.7% in shelf surface waters while particulate C:N concurrently decreased from 17.1 to 8.6. Nitrate and silicate concentrations declined by more than 90% across the salinity gradient, with nitrate concentrations often below the conservative mixing line. Phosphate concentrations increased from 0.03 μmol/L in the river to 0.27 μmol/L over shelf waters, thereby changing the inorganic nutrient regime downstream from P to N limitation. Dissolved organic carbon decreased conservatively offshore while dissolved organic N and P persisted at high levels in the Mackenzie plume relative to river water, increasing 2.7 and 25.3 times respectively. A deep chlorophyll- a maximum was observed at two offshore stations and showed increases in most nutrients, particulates and organic matter relative to the rest of the water column. During river passage through the Mackenzie estuary, particulate matter, dissolved organic carbon and inorganic nutrients showed sedimentation, dilution and biological uptake patterns common to other arctic and non-arctic estuaries. Alternatively, inorganic content of particles increased offshore and dissolved organic N and P increased substantially over the shelf, reaching concentrations among the highest reported for the Arctic Ocean. These observations are consistent with the presence of a remnant ice-constrained (‘stamukhi’) lake from the freshet period and a slow flushing river plume constrained by sea-ice in close proximity to shore. Nutrient limitation in surface shelf waters during the ARDEX cruise contributed to the striking deep chlorophyll- a maximum at 21 m where phytoplankton communities congregated at the margin of nutrient-rich deep ocean waters.

The estuarine mixing behaviour of peatland derived dissolved organic carbon and its relationship to chromophoric dissolved organic matter in two North Sea estuaries (U.K.)

The mixing behaviour of dissolved organic carbon (DOC) was investigated in two U.K. estuaries with peatland derived river inputs (Tyne and Tweed), under varying discharge regimes and in different seasons. DOC removal (i.e. non-conservative) was common to both estuaries at low salinities, but DOC addition also occurred in the Tyne at higher salinities due to inputs from a large sewage treatment works and polluted urban rivers. DOC in freshwater endmembers was in the range 608.1–2326.5 μM (mean 1229.6 μM, n = 7) for the Tyne and was 226.1 and 668.2 μM on two occasions in the Tweed; the high DOC concentrations reflecting the peatland nature of the catchments and high river discharge. DOC removal in the Tyne estuary was in the range 0–58% ( n = 7) (one transect exhibits 0% removal as pollution sources of DOC dominate medium to high salinity samples so it was not possible to extrapolate a conservative mixing line back to low salinities). Highest percent removals were at highest river flows and correspondingly high turbidity and DOC concentrations in the Tyne estuary. In contrast, DOC removal in the Tweed estuary was consistently 6% ( n = 2), and appeared independent of river flow, turbidity and DOC concentration on the two transects conducted. Both estuaries exhibited a strong correlation between the chromophoric dissolved organic matter (CDOM) absorption coefficient at 350 nm ( a 350) and DOC which was seasonally robust; Tweed ( r 2 = 0.98, p < 0.001, n = 20), Tyne ( r 2 = 0.85, p < 0.001, n = 87) and Tyne without potentially anthropogenic or sedimentary derived DOM source samples ( r 2 = 0.93, p < 0.001, n = 74). The robust nature of these relationships implies that remotely sensed CDOM characteristics have the potential to improve the spatio-temporal resolution of estuarine DOC. This study highlights the variation in estuarine removal of peatland derived DOC between the two estuaries and in the Tyne estuary it shows how differences in river regime can result in significant changes in DOC removal at low salinities. Our findings demonstrate the need to conduct DOM sampling under varying seasons and hydrological regimes to accurately quantify the removal of terrigenous DOM in estuaries and its subsequent flux to the oceans.

Physical and biological contributions to the western tropical North Atlantic Ocean carbon sink formed by the Amazon River plume

Dissolved inorganic carbon (DIC) and total alkalinity (TA) were measured in the upper 1000 m of the western tropical North Atlantic Ocean (WTNA; study area 3–15°N, 40–59°W) in January–February and July–August 2001. Concentrations of DIC and TA in surface samples (0–10 m) influenced by the Amazon River plume were up to 400 μmol C kg−1 (∼20%) lower than oceanic surface samples. In this region, physical dilution by river water dominates DIC and TA inventories, driving CO2 partial pressure (pCO2) well below atmospheric levels. Nevertheless, DIC concentrations at most plume‐influenced stations were 10–90 μmol C kg−1 below levels expected from conservative mixing of seawater with low‐salinity, low‐CO2 Amazon River water. In this otherwise oligotrophic region, the diazotrophs Trichodesmium spp. and Richelia intracellularis were often abundant, supporting a link between increased carbon drawdown and nitrogen fixation in the outer plume. Net community production in the plume must surpass the fluxes of inorganic carbon from below and air‐sea CO2 replacement to leave biologically mediated DIC deficits, which is possible under observed conditions. Biological activity lowers plume pCO2 30–120 μatm below the conservative mixing line, and contributes to a CO2 deficit in the northern WTNA that outlasts the plume's physical structure.

Impact of phytoplankton and bacterial production on nutrient and DOM uptake in the Rhône River plume (NW Mediterranean)

From April 1998 to April 1999, 4 cruises were conducted to investigate the evolution of physical and biological characteristics of the Rhone River plume. Particular emphasis was placed on the relationships between phytoplanktonic or bacterial production and environmental conditions. For most of the year, distributions of nutrients, phytoplankton and bacterial activities were conservative along the salinity gradient, indicating the dominance of physical dilution processes. In April 1999, nutrient-salinity relationships showed strong deviations from the conservative mixing line which were associated with a phytoplankton bloom. Deviations from the conservative mixing line were used to estimate the relative contributions of physical and biological processes in the removal of riverine derived material. During the phytoplankton bloom, there was a specific organisation of the microbial populations and their activities at intermediate salinity. Maximal productions were associ- ated with maxima in ammonium and phosphate uptake rates for bacteria, but with maxima in nitrate uptake rates for phytoplankton. The relative accumulation of dissolved organic carbon (DOC) and nitrogen (DON) at high salinity, and the seasonal variability in their distribution, resulted in a poten- tial competition between bacteria and phytoplankton for key mineral nutrients highlighted by an increase in the DOC:DON ratio up to 10. This study contributes to our understanding of the relation- ship between phytoplankton, bacteria and dissolved organic matter and helps to elucidate the factors controlling the productivity of the system. In particular, the impact of wind and riverine output need to be considered alongside seasonal variability in the chemical and hydrodynamic environment.

The biogeochemistry of inorganic carbon and nutrients in the Pearl River estuary and the adjacent Northern South China Sea

The Zhu-jiang (Pearl River) estuary and its adjacent continental shelf in the Northern South China Sea (SCS) is unique in that its drainage basin is located entirely in a subtropical zone with heavy population development, and therefore represents an important regime for biogeochemical studies on how large rivers influence continental shelves. The near-zero salinity end member has high nutrient concentrations (silicate 130–140 μM, nitrate 75–100 μM and phosphate 0.2–1.2 μM) and relatively high total dissolved inorganic carbon (DIC) (1500 μM) and alkalinity (∼1650 μM) values. Water column DIC, alkalinity, and nutrient in the estuary are largely controlled by mixing of waters from different tributaries with different drainage basin chemistry, anthropogenic influence, and degree of estuarine recycling. Biological uptake of nutrients and inorganic carbon occur in the outer estuary and inner shelf areas supported by riverine nutrients. The N/P and Si/P ratios are generally very high within the estuary. The summertime area-integrated biological production rate of 0.8 gC m −2 d −1 is estimated based on the depletion of DIC and alkalinity relative to the conservative mixing line and a plume travel time. This estimate agrees reasonably well with 14C based primary production rates (PP) and with that from effective river phosphate flux. Biological production decreases about 10-fold in the open continental shelf and slope and is largely supported by mixing with subsurface water. A comparison of DIC, phosphate, and nitrate concentrations in the surface mixing layer and at the bottom of the euphotic zone with the 14C-based PP (0.13 gC m −2 d −1) suggests that the surface water residence time in the Northern SCS is ∼1.3 years. The N/P, Si/P, and Si/C ratios are 15, 25, and 0.15, respectively. The subtropical Pearl River study is also compared to other large rivers with regard to differences in both natural processes (i.e., weathering rates) and anthropogenic influences (i.e., nutrient input) between these different river–estuary systems.

Behaviour of the optical properties of coloured dissolved organic matter under conservative mixing

The optical properties of coloured dissolved organic matter (CDOM) can be used, in some environments, to trace water masses and provide information about the dynamics of the dissolved organic fraction in natural waters. This work presents the results from a modelling exercise, laboratory experiment and field data, which describe the variations in the optical properties of CDOM during mixing. The exponential slope coefficient ( S) is frequently used to characterise different CDOM pools; however, its behaviour during conservative mixing of two different CDOM types is often misunderstood. Identification of a theoretical conservative mixing line allows the rapid identification of non-conservative processes (e.g. in situ production, flocculation and degradation) acting on the pool during mixing. The results suggest that some of the patterns reported in the literature could purely be a result of conservative mixing rather than a product of non-conservative processes.

DOC cycling in a temperate estuary: A mass balance approach using natural 14C and 13C isotopes

We measured dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), and their corresponding △14C and γ13C values in order to study the sources and fates of DOC in the York River Estuary (Virginia, U.S.A.). The d△14C and γ13C values of DOC and DIC at the freshwater end‐member indicate that during periods of moderate to high flow, riverine DOC entering the York was composed of decadal‐aged terrestrially organic matter. In nearly all cases, DOC concentrations exceeded conservative mixing lines and were therefore indicative of a net DOC input flux from within the estuary that averaged 1.2 µM L−1 d−1.

Spatial and temporal variation of chlorophyll a, primary productivity and chemical hydrography in the southern East China Sea

Chemical hydrography, chlorophyll a distribution and primary productivity in the southern East China Sea from the inner shelf to the offshore region were examined for their temporal and spatial variability based on observations along a cross-shelf transect during 1991–1995. In summer, all surface waters were depleted in nutrients and low in chlorophyll, resulting in widespread subsurface chlorophyll a maxima. In other seasons, shelf waters were usually nutrient-laden, but phytoplankton biomass was limited by light availability in the turbid inner shelf water and the short exposure time of the upwelled water in the outer shelf. The main nitrogen source in the inner shelf was from river runoff. Nitrate deficit, defined as the negative deviation from the conservative mixing line, was well correlated with phytoplankton biomass. In spite of the seasonal and spatial variability of chlorophyll a distribution, the euphotic zone integrated value of chlorophyll a was correlated well with that of primary production in shelf waters. This relationship was used to convert chlorophyll a data from 28 cruises to primary production. The results showed that the lowest productivity on the shelf was in summer with little spatial variation. The elevated primary productivity in the inner shelves in spring was due to a mild spring bloom. On the other hand, the elevated primary productivity in the middle and outer shelves in autumn was due to the intensification of upwelling during the transition of the monsoon. The annual mean values of the euphotic zone integrated primary production in the shelf waters were quite uniform (513–576 mgC m −2 d −1) with an overall mean of 549±84 mgC m −2 d −1. The offshore Kuroshio Water showed a weak seasonal variation (276–309 mgC m −2 d −1) with an annual mean of 292±15 mgC m −2 d −1.