Constraining the propagation of bomb-radiocarbon through the dissolved organic carbon (DOC) pool in the northeast Pacific Ocean

This study extends the 1991-1995 records of marine dissolved organic carbon (DOC) concentrations and Δ¹⁴C values at hydrographic Station M (34°50′N, 123°00′W) with new measurements from a frozen (-20 °C) archive of samples collected between April 1998 and October 2004. The magnitudes and synchronicity of major Δ¹⁴C anomalies throughout the time-series imply transport of DOC from the surface ocean to depths of at least 450 m on the timescale of months. Keeling plots of all measurements at Station M predict a continuum of possible background DOC compositions containing at least 21 μM of -1000‰ (i.e., ≥57,000 ¹⁴C years) DOC, but are more consistent with mean deep DOC (38 μM, -549‰; i.e., 6,400 ¹⁴C years). These results and coral records of surface dissolved inorganic carbon (DIC) Δ¹⁴C were used to estimate pre-bomb DOC Δ¹⁴C depth profiles. The combined results indicate that bomb-¹⁴C has penetrated the DOC pool to depths of ≥450 m, though the signal at that depth is obscured by short-term variability.     

Was a carbon balance measured in the equatorial Pacific during JGOFS?

The likelihood that the carbon fluxes measured as part of the US-JGOFS field program in the equatorial Pacific ocean (EgPac) during 1992 yielded a balanced carbon budget for the surface ocean was determined. The major carbon fluxes incorporated into a surface carbon budget were: new production, particulate organic carbon (POC) and dissolved organic carbon (DOC) export, CaC0₃ export, C0₂ gas evasion, dissolved inorganic carbon (DIC) supply, and the time rate of charge. The ratio of the measured concentration gradients of DOC and DIC provided a constraint on the ratio of POC/DOC export. Uncertainties of ±30–50% for individual carbon flux measurements reduce the likelihood that a carbon balance can be measured during a JGOFS process-type study. As a benchmark, carbon fluxes were prescribed to yield a hypothetical surface carbon budget that was, on average, balanced. Given the typical errors in the individual carbon fluxes, however, there was only about a 30% chance that this hypothetical budget could be measured to be balanced to ±50%. Using this benchmark, it was determined that there was a 95 % chance that the carbon flux measurements yielded a surface DIC budget balanced (to ±50%) during El Nino conditions in boreal spring 1992, when the total organic carbon export rate was - 5 mmol C m-2 day- 1 and the POC export was 3 mmol C m⁻² day⁻¹. In boreal fall 1992, during cold period conditions, there was a 70% chance that the surface carbon DIC budget was balanced when the total organic carbon export rate was 20 mmol C m⁻² day⁻¹ and export was -13 mmol C m-2 day-'. The DOC to DIC concentration gradient ratio of - -0.15, measured in depth profiles down to 100m and in surface waters, was used as an important constraint that most (> 70%) of the organic carbon exported from the euphotic zone was POC rather than DOC. If a balanced surface DIC budget was used to test the compatibility of individual carbon fluxes measured during EgPac, then a three- to four-fold increase in total and particulate organic carbon export between spring and fall is indicated. This increase was not reflected in the POC loss rates measured by drifting sediment trap collections or estimated by²³⁴Th deficiencies coupled with the C/Th measured on suspended particles.     

Major contribution of autotrophy to microbial carbon cycling in the deep North Atlantic’s interior

Current estimates point to a mismatch of particulate organic carbon supply derived from the surface ocean and the microbial organic carbon demand in the meso- and bathypelagic realm. Based on recent findings that chemoautotrophic Crenarchaeota are abundant in the mesopelagic zone, we quantified dissolved inorganic carbon (DIC) fixation in the meso- and bathypelagic North Atlantic and compared it with heterotrophic microbial activity. Measuring ¹⁴C-bicarbonate fixation and ³H-leucine incorporation revealed that microbial DIC fixation is substantial in the mesopelagic water masses, ranging from 0.1 to 56.7 μmol C m⁻³ d⁻¹, and is within the same order of magnitude as heterotrophic microbial activity. Integrated over the dark ocean’s water column, DIC fixation ranged from 1–2.5 mmol C m⁻² d⁻¹, indicating that chemoautotrophy in the dark ocean represents a significant source of autochthonously produced ‘new organic carbon’ in the ocean’s interior amounting to about 15–53% of the phytoplankton export production. Hence, chemoautotrophic DIC fixation in the oxygenated meso- and bathypelagic water column of the North Atlantic might substantially contribute to the organic carbon demand of the deep-water microbial food web.     

Daily variability of dissolved inorganic radiocarbon in Sargasso Sea surface water

Surface water samples were collected daily in June 2000 at a site in the Sargasso Sea to observe variability of Δ¹⁴C values in dissolved inorganic carbon (DIC). Temperature, salinity, DIC concentration, alkalinity, and δ¹³C and Δ¹⁴C values of DIC were measured in the samples. Ten Δ¹⁴C measurements averaged 81 ± 8‰ and had a range of 24‰ over the sixteen-day cruise. Δ¹⁴C values were more variable during the latter half of the cruise. Salinity and temperature measurements in the mixed layer throughout the cruise indicate that there were changes in water mass that occurred at our site. We conclude that the daily range of DIC Δ¹⁴C values in the surface ocean at our site is several times greater than the annual change in surface waters in the Sargasso Sea during the last two decades of the 20th century. This points to the importance of obtaining multiple measurements of the surface ocean to adequately define the true variability of DIC Δ¹⁴C measurements.     

Incorporation of aged dissolved organic carbon (DOC) by oceanic particulate organic carbon (POC): An experimental approach using natural carbon isotopes

Incorporation of ¹⁴C-depleted (old) dissolved organic carbon (DOC) on/into particulate organic carbon (POC) has been suggested as a possible mechanism to explain the low Δ¹⁴C-POC values observed in the deep ocean [Druffel, E.R.M., Williams, P.M., 1990. Identification of a deep marine source of particulate organic carbon using bomb ¹⁴C. Nature, 347, 172–174.]. A shipboard incubation experiment was performed in the Sargasso Sea to test this hypothesis. Finely ground dried plankton was incubated in seawater samples from the deep Sargasso Sea, both with and without a biological poison (HgCl₂). Changes in parameters such as biochemical composition and carbon isotopic signatures of bulk POC and its organic compound classes were examined to study the roles of sorptive processes and biotic activity on POC character. Following a 13-day incubation, the relative abundance of the acid-insoluble organic fraction increased. Abundances of extractable lipids and total hydrolyzable amino acids decreased for both treatments, but by a greater extent in the non-poisoned treatment. The Δ¹⁴C values of POC recovered from the non-poisoned treatment were significantly lower than the value of the unaltered plankton material used for the incubation, indicating incorporation of ¹⁴C-depleted carbon, most likely DOC. The old carbon was present only in the lipid and acid-insoluble fractions. These results are consistent with previous findings of old carbon dominating the same organic fractions of sinking POC from the deep Northeast Pacific [Hwang, J., Druffel, E.R.M., 2003. Lipid-like material as the source of the uncharacterized organic carbon in the ocean? Science, 299, 881–884.]. However, the Δ¹⁴C values of POC recovered from the poisoned treatment did not change as much as those from the non-poisoned treatment suggesting that biological processes were involved in the incorporation of DOC on/into POC.     

Dissolved carbon and 33-133-133-1anomalies in the water column caused by hydrocarbon seeps on the northwestern Gulf of Mexico slope

Alkalinity, dissolved inorganic carbon (DIC), and ¹³C profiles from seep sites on the northwestern Gulf of Mexico upper slope show anomalously negative ¹³C values of up to –4.5 PDB, increased levels of DIC of up to 2.45 mmole/liter, and slight alkalinity rises of up to 2.54 meq/liter, relative to water column profiles from a seep-free site (0.63, 2.04 mmole/liter, and 2.39 meq/liter). The observed DIC enrichments coupled with the¹³C-depletions are attributed to the release of CO₂ by microbial oxidation of crude oil in the seep environment, and its migration into the water column. The ¹³C composition of the migrating CO₂ is estimated to be –26.0 on the basis of dissolved carbon inventory. Manifestation of DIC and ¹³C anomalies in the water column caused by hydrocarbon seepage holds promise to be useful for hydrocarbon reconnaissance surveys over large offshore tracts on account of the simplicity of sampling acquisition, and rapidity of analytical techniques in the laboratory.     

Dissolved inorganic carbon (DIC) and its carbon isotopic composition in sediment pore waters from the Shenhu area,northern South China Sea

The Shenhu area is one of the most favorable places for the occurrence of gas hydrates in the northern continental slope of the South China Sea. Pore water samples were collected in two piston cores (SH-A and SH-B) from this area, and the concentrations of sulfate and dissolved inorganic carbon (DIC) and its carbon isotopic composition were measured. The data revealed large DIC variations and very negative δ ¹³C-DIC values. Two reaction zones, 0–3 mbsf and below 3 mbsf, are identified in the sediment system. At site SH-A, the upper zone (0–3 mbsf) shows relatively constant sulfate and DIC concentrations and δ ¹³C-DIC values, possibly due to bioturbation and fluid advection. The lower zone (below 3 mbsf) displays good linear gradients for sulfate and DIC concentrations, and δ ¹³C-DIC values. At site SH-B, both zones show linear gradients, but the decreasing gradients for δ ¹³C-DIC and SO₄ ²⁻ in the lower zone below 3 mbsf are greater than those from the upper zone, 0–3 mbsf. The calculated sulfate-methane interface (SMI) depths of the two cores are 10.0 m and 11.1 m, respectively. The depth profiles of both DIC and δ ¹³C-DIC showed similar characteristics as those in other gas hydrate locations in the world oceans, such as the Blake Ridge. Overall, our results indicate an anaerobic methane oxidation (AMO) process in the sediments with large methane flux from depth in the studied area, which might be linked to the formation of gas hydrates in this area.     

Dissolved organic carbon in the southern Baltic Sea: Quantification of factors affecting its distribution

During a cruise of r/v ‘Oceania’ in May 2006, seven vertical dissolved organic carbon (DOC) concentration profiles were produced against a background of CTD, chlorophyll a (chl a) and phaeopigment concentration profiles. The results indicate distinct vertical and spatial DOC fluctuations, ranging from 248 ± 7 μmol C dm⁻³ at 70 m depth at the westernmost station G/06 to 398 ± 5 μmol C dm⁻³ at 5 m depth at station A/06 in the western Gulf of Gdańsk. DOC concentrations were the highest at 10 m depth, where phytoplankton activity was relatively intensive, as reflected by the active chl a concentration distribution. DOC concentrations decreased towards the sea bottom.     

Annual cycle of fCO2 under sea-ice and in open water in Prydz Bay, East Antarctica

The annual cycle of dissolved nutrients and the fugacity of CO₂ (fCO₂), calculated from the concentration of dissolved inorganic carbon (DIC) and pH, was studied over a 14-month long period (December 1993 to February 1995) at a site in Prydz Bay near Davis Station, Vestfold Hills, East Antarctica. Significant spring decreases in fCO₂ began under the sea-ice in mid-October, when both water column and sea-ice algal activity resulted in the removal of nutrients and DIC and increased pH. Minimum fCO₂ (<100 μatm) and lowest nutrient and DIC concentrations occurred in December and January. The low summer fCO₂ values were clearly the result of biological activity. The seasonal depletion of dissolved nitrate reached 85% in mid-summer when chlorophyll-a concentrations exceeded 15 mg m⁻³. Oceanic uptake of carbon dioxide from the atmosphere, calculated from the fugacity difference and daily wind speeds, averaged more than 30 mmol m⁻² day⁻¹ during the summer ice-free period. This exchange replaced approximately half of the DIC consumed by biological activity. Apparent nutrient utilisation ratios (C/N/P) were close to Redfield values. In autumn fCO₂ began to rise, continuing slowly well into winter, and reaching a maximum close to modern atmospheric values between July and September. This increase can be attributed to a combination of local remineralisation of organic carbon in the water column and the steady increase in the mixing depth of the water column. At first glance, this suggests that air–sea equilibration occurred in winter despite the sea-ice cover, perhaps by horizontal circulation from regions outside the pack ice, or through openings in the ice. However, the persistent 15 to 20% undersaturation of dissolved oxygen throughout the winter suggests an alternate explanation. The late winter fCO₂ level may represent a characteristic established by global circulation, so that as a result of increasing atmospheric CO₂ concentrations, these Antarctic waters are in transition from being a winter-time source of CO₂ to the atmosphere to becoming a sink. Our fCO₂ observations emphasize the need to address seasonal variations in assessing Antarctic contributions to the oceanic control of atmospheric CO₂.     

夏季黄海不同水团溶解有机物分布特征

依据2017年8—9月对黄海海域溶解有机物(DOM)的调查,探讨了夏季黄海海水中溶解有机碳(DOC)和有色溶解有机物(CDOM)的空间分布特征。在表层海水中,受陆源影响较大的近岸海域CDOM含量相对较高,北黄海冷水团区域由于水产养殖的饵料引起DOC浓度升高,且该部分DOC以无色为主。DOC浓度随深度逐渐降低,而CDOM逐渐升高,该特征在冷水团区域更为显著,因此DOC和CDOM在冷水团区域的表底差异远大于浅水区的非冷水团区域。陆源输入和初级生产是引起表层DOC升高的主要原因,而光漂白则引起CDOM降低,同时光漂白还导致表层水体中CDOM分子量和芳香性低于底层。底层溶解氧饱和度在冷水团为80%～93%,均表现为弱不饱和状态。层化不仅阻碍了O₂向底层水体输送,还抑制了DOC和CDOM的垂向混合,这是引起冷水团区域表底层DOC和CDOM差异较大的主要原因。     

海底地下水排放对典型红树林蓝碳收支的影响

海底地下水排放(Submarine Groundwater Discharge,SGD)是陆海相互作用的重要表现形式之一,其携带的物质对近岸海域生源要素的收支有重要影响。本文利用222Rn示踪技术估算了我国典型红树林海湾—广西珍珠湾在2019年枯季(1月)SGD携带的碳通量。调查发现,地下水中222Rn活度、溶解无机碳(DIC)和溶解有机碳(DOC)的平均浓度均高于河水和湾内表层海水。利用222Rn质量平衡模型估算得到珍珠湾SGD速率为(0.36±0.36) m/d,SGD输入到珍珠湾的DIC和DOC通量分别为(2.41±2.63)×107 mol/d和(1.96±2.20)×106 mol/d。珍珠湾溶解碳的源汇收支表明,SGD携带的DIC和DOC分别占珍珠湾总DIC和总DOC来源的91%和89%。因此,SGD携带的DIC和DOC是珍珠湾DIC和DOC的主要来源,是海岸带蓝碳收支和生物地球化学循环过程中的重要组成。     

Cycling of dissolved and particulate nitrogen and carbon in the Framvaren Fjord, Norway: stable isotopic variations

The reaction pathways of nitrogen and carbon in the Framvaren Fjord (Norway) were studied through stable isotope analysis (δ¹⁵N and δ¹³C) of dissolved inorganic and particulate organic matter (POM). The variations in the isotopic compositions of the various C and N pools within the water column were use to evaluate the historical deposition of material to the sediments. The high δ¹⁵N-NH₄⁺ at the O₂/H₂S interface, as a consequence of microbial uptake between 19 and 25 m, results in extremely depleted δ¹⁵N-particulate nitrogen (PN) of approximately 1‰ within the particulate maximum at approximately 19 m. The carbon isotopic distribution of dissolved inorganic carbon (DIC) and particulate organic carbon (POC) within the interface suggests that the distinct microbial flora (Chromatium sp. and Chlorobium sp.) fractionate inorganic carbon to different degrees. The extremely light δ¹³C-POC within the interface (−31‰) appears to be a result of carbon uptake by Chromatium sp. while δ¹³C-POC of −12‰ is more indicative of Chlorobium sp. Nitrogen isotopic mass balance calculations suggested that approximately 75% of the material sinking to the sediments was derived from the dense particulate maximum between 19 and 25 m. The sediment distribution of nitrogen isotopes varied from 2‰ at the surface to approximately 6‰ at 30 cm. The nitrogen isotopic variations with depth may be an indicator of the depth or position of the O₂/H₂S interface in the fjord. Low sediment δ¹⁵N indicated that the interface was within the photic zone of the water column, while more enriched values suggested that the interface was lower in the water column potentially allowing for less fractionation during biological incorporation of dissolved inorganic nitrogen. Results indicate that the dense layers of photo-autotrophic bacteria in the upper water column impart unique carbon and nitrogen isotopic signals that help follow processes within the water column and deposition to the sediments.     

Dissolved organic carbon in sediments from the eastern North Atlantic

Profiles of dissolved organic carbon (DOC) were measured in the pore water of sediments from 1000, 2000 and 3500 m water depth in the eastern North Atlantic. A net DOC accumulation in the pore waters was observed, which followed closely the zonation of microbial respiration in these sediments. The concentration of pore water DOC in the zone of oxic respiration was elevated relative to that in the bottom ocean water. The resulting upward gradient across the sediment–water interface indicated a steady state diffusive benthic flux, F_DOC, of 0.25–0.44 mmol m⁻² day⁻¹ from these sediments. Subsequent increase in the concentration of DOC in the pore water occurred only in the sediments from 1000 and 2000 m water depth that supported anoxic respiration, leading to a deep concentration maximum. By contrast, in the sediments from 3500 m water depth, a deep concentration minimum was measured, coincident with minimal postoxic respiration in this near-abyssal setting. The gradient-based F_DOC represented approximately 14% of the total remineralized organic carbon (TCR=sum of F_DOC and depth-integrated organic carbon oxidation rate) in the sediments from 1000 and 2000 m water depth, while it was 36% of the TCR in the sediments from 3500 m water depth. A covariance of particulate organic carbon (POC) and pore water DOC with depth in the sediments was evident, more consistently at the deepest site. While the covariance can be related to biotic processes in these sediments, an alternative interpretation suggests a possible contribution of sorption to the biotic control on sedimentary organic carbon cycling. The steady state diagenetic conditions in which this may occur can be conceivable for some organic-poor deep-sea locations, but direct evidence is clearly required to validate them.     

Seasonal and depth-related dynamics of prokaryotes and viruses in surface and deep waters of the northwestern Mediterranean Sea

The study site located in the northwestern Mediterranean Sea was visited nine times in 2005–2006 to collect water samples from the epi- (5 m), meso- (200, 600 m), and bathypelagic (1000, 2000 m) zone. Total abundance of prokaryotes and viruses was determined by flow cytometry (FCM). Prokaryotic abundance in the epi-, meso-, and bathypelagic varied between 0.9 and 15.9×10⁵, 0.6 and 2.1×10⁵, and 0.3 and 1.3×10⁵ ml⁻¹, respectively. Variation of viral abundance in the epi-, meso-, and bathypelagic was between 1.2 and 57.5×10⁶, 0.5 and 3.5×10⁶, and 0.4 and 1.3×10⁶ ml⁻¹, respectively. The fraction of low (LNA) and high (HNA) nucleic acid prokaryotes averaged 42.9% and 57.1% throughout the water column and did not differ between depth layers. Throughout the water column the fraction of low, medium, and high fluorescent viruses (Vir-LF, Vir-MF, Vir-HF) averaged 66.3%, 30.2%, and 3.5%. Vir-LF and Vir-MF did not differ between depth layers; however, Vir-HF showed a preference for surface waters. The fraction of LNA cells decreased in the epi- and increased in the bathypelagic with decreasing stratification. The fraction of Vir-LF viruses increased in the epipelagic and decreased in the bathypelagic with increasing prokaryotic abundance. Also, the relationship between viral abundance and the bacterial community was different in surface and deep waters. The data suggest that different mechanisms of interaction between viruses and their prokaryotic hosts prevail at the surface and in deep waters.     

Dissolved organic carbon export and subsequent remineralization in the mesopelagic and bathypelagic realms of the North Atlantic basin

Dissolved organic carbon (DOC) data are presented from three meridional transects conducted in the North Atlantic as part of the US Climate Variability (CLIVAR) Repeat Hydrography program in 2003. The hydrographic sections covered a latitudinal range of 6°S to 63°N along longitudes 20°W (CLIVAR line A16), 52°W (A20) and 66°W (A22). Over 3700 individual measurements reveal unprecedented detail in the DOC distribution and systematic variations in the mesopelagic and bathypelagic zones of the North Atlantic basin. Latitudinal gradients in DOC concentrations combined with published estimates of ventilation rates for the main thermocline and North Atlantic Deep Water (NADW) indicate a net DOC export rate of 0.081 Pg C yr⁻¹ from the epipelagic zone into the mesopelagic and bathypelagic zones. Model II regression and multiple linear regression models applied to pairwise measures of DOC and chlorofluorocarbon (CFC-12) ventilation age, retrieved from major water masses within the main thermocline and NADW, indicate decay rates for exported DOC ranging from 0.13 to 0.94 μmol kg⁻¹ yr⁻¹, with higher DOC concentrations driving higher rates. The contribution of DOC oxidation to oxygen consumption ranged from 5 to 29% while mineralization of sinking biogenic particles drove the balance of the apparent oxygen utilization.     

Daily variability of dissolved inorganic radiocarbon at three sites in the surface ocean

We report radiocarbon measurements of dissolved inorganic carbon (DIC) in surface water samples collected daily during cruises to the central North Pacific, the Sargasso Sea and the Southern Ocean. The ranges of Δ¹⁴C measurements for each cruise (11–30‰) were larger than the total uncertainty (7.8‰, 2-sigma) of the measurements. The variability is attributed to changes in the upper water mass that took place at each site over a two to four week period. These results indicate that variability of surface Δ¹⁴C values is larger than the analytical precision, because of patchiness that exists in the DIC Δ¹⁴C signature of the surface ocean. This additional variability can affect estimates of geochemical parameters such as the air–sea CO₂ exchange rate using radiocarbon.     

Isotopic composition of dissolved inorganic carbon in subsurface sediments of gas hydrate-bearing mud volcanoes, Lake Baikal: implications for methane and carbonate origin

We report on the isotopic composition of dissolved inorganic carbon (DIC) in pore-water samples recovered by gravity coring from near-bottom sediments at gas hydrate-bearing mud volcanoes/gas flares (Malenky, Peschanka, Peschanka 2, Goloustnoe, and Irkutsk) in the Southern Basin of Lake Baikal. The δ¹³C values of DIC become heavier with increasing subbottom depth, and vary between ?9.5 and +21.4‰ PDB. Enrichment of DIC in ¹³C indicates active methane generation in anaerobic environments near the lake bottom. These data confirm our previous assumption that crystallization of carbonates (siderites) in subsurface sediments is a result of methane generation. Types of methanogenesis (microbial methyl-type fermentation versus CO₂-reduction) were revealed by determining the offset of δ¹³C between dissolved CH₄ and CO₂, and also by using δ¹³C and δD values of dissolved methane present in the pore waters. Results show that both mechanisms are most likely responsible for methane generation at the investigated locations.     

Spectrophotometric determination of dissolved organic carbon in some South Island streams and rivers (note)

Absorbance at 360 nm and dissolved organic carbon (DOC) concentration were measured on 47 filtered water samples collected from streams and rivers of the west coast of South Island, New Zealand. The regression equation (DOC (g m^‐3) = 59.6 Abs_{1 cm} + 1.9) calculated from the data, reliably predicted DOC concentration within the range 1.6–43.2 gm^‐3. This relationship is similar to that found fsr some Venezuelan and south‐eastern United Stales waters indicating that it may have widespread utility for estimating DOC concentrations in soft waters where DOC is dominated by humic substances     

Dissolved nutrient balance and net ecosystem metabolism in a Mediterranean-climate coastal lagoon: San Diego Bay

The temporal and spatial variability of dissolved inorganic phosphate (DIP), nitrogen (DIN), carbon (DIC) and dissolved organic carbon (DOC) were studied in order to determine the net ecosystem metabolism (NEM) of San Diego Bay (SDB), a Mediterranean-climate lagoon. A series of four sampling campaigns were carried out during the rainy (January 2000) and the dry (August 2000 and May and September 2001) seasons. During the dry season, temperature, salinity and DIP, DIC and DOC concentrations increased from oceanic values in the outer bay to higher values at the innermost end of the bay. DIP, DIC and DOC concentrations showed a clear offset from conservative mixing implying production of these dissolved materials inside the bay. During the rainy season, DIP and DOC increased to the head, whereas salinity decreased toward the mouth due to land runoff and river discharges. The distributions of DIP and DOC also showed a deviation from conservative mixing in this season, implying a net addition of these dissolved materials during estuarine mixing within the bay. Mass balance calculations showed that SDB consistently exported DIP (2.8–9.8 × 10³ mol P d⁻¹), DIC (263–352 × 10³ mol C d⁻¹) and DOC (198–1233 × 10³ mol C d⁻¹), whereas DIN (5.5–18.2 × 10³ mol N d⁻¹) was exported in all samplings except in May 2001 when it was imported (8.6 × 10³ mol N d⁻¹). The DIP, DIC and DOC export rates along with the strong relationship between DIP, DIC or DOC and salinity suggest that intense tidal mixing plays an important role in controlling their distributions and that SDB is a source of nutrients and DOC to the Southern California Bight. Furthermore, NEM ranged from −8.1 ± 1.8 mmol C m⁻² d⁻¹ in September to −13.5 ± 5.8 mmol C m⁻² d⁻¹ in January, highlighting the heterotrophic character of SDB. In order to explain the net heterotrophy of this system, we postulate that phytoplankton-derived particulate organic matter, stimulated by upwelling processes in the adjacent coastal waters, is transported into the bay, retained and then remineralized within the system. Our results were compared with those reported for the heterotrophic hypersaline coastal lagoons located in the semi-arid coast of California–Baja California, and with those autotrophic hypersaline systems found in the semi-arid areas of Australia. We point out that the balance between autotrophy and heterotrophy in inverse estuaries is dependent on net external inputs of either inorganic nutrients or organic matter as it has been indicated for positive estuaries.     

Resuspension-induced partitioning of organic carbon between solid and solution phases from a river–ocean transition

Laboratory experiments were conducted to evaluate the net exchange of organic carbon (OC) between sediments and overlying water during episodes of resuspension. Surface sediment samples collected from six locations within the Hudson River Estuary and the Inner New York Bight were resuspended in their respective bottom waters for periods ranging from 30 s to 2 h. After resuspension, dissolved organic carbon (DOC) concentration generally reached levels greater than that predicted by conservative mixing of pore water and bottom water, indicating net release of OC from the sediment particles. The amount of OC released during the 1-h extractions comprised ≤0.1% of the total sediment pool, but correlated positively (R²=0.65, P<0.052) with the amount of particulate organic carbon (POC) found in the high-density fraction of the sediment matrix. This suggests that the mineral-bound fraction of sedimentary OC was the major source for the excess DOC released into solution, and that across various sedimentary environments, only a small (but fairly constant) fraction of the total sedimentary POC may be poised for rapid transfer to the water column.