Turbid Estuary Research Articles

Hydrodynamics in the Yellow River Estuary via radium isotopes: Ecological perspectives

We used radium isotopes as tracers to characterize coastal hydrodynamics and submarine groundwater discharge (SGD) in the Yellow River Estuary in order to assess the ecological effects in one of the most turbid estuaries in the world. Based on apparent water ages calculated by 224Ra/223Ra activity ratios, we found that the river plume flowed mainly southeast at a flow rate of 5–7kmd−1, while a small portion of the river plume was diverted northeast to the central Bohai Sea at a flow rate of less than 2kmd−1. We estimate that with this flow regime, nutrients would be consumed within about two weeks mostly by microplankton and nanoplankton near shore, and picoplankton further offshore to support an average primary production of 0.14gCm−2d−1. We then used a 226Ra mass balance model to quantify the SGD flux in the study area. The estimated SGD flux was 1.3×109m3d−1 with a range of 2.8×108–3.0×109m3d−1. Even the minimum SGD value was about 3 times higher than the Yellow River discharge at that time. The SGD input of dissolved nutrients was shown to be very important to the estuarine nutrient budget, at least 5 times higher than river input. Sediment regeneration of nutrients proved to be very small relative to the SGD flux in this region.

Suspended particulate matter affects the nutrient budget of turbid estuaries: Modification of the LOICZ model and application to the Yangtze Estuary

This study estimates the importance of the estuarine suspended particulate matter (SPM) on the nutrient budget of the Yangtze Estuary by using extensive field data and a modified version of the LOICZ biogeochemical model that incorporates the partitioning of the nutrients between dissolved and particulate phases. The value of the nutrient partition coefficient in the Yangtze Estuary shows a similar dependence to that in European estuaries. High values of the Net Ecosystem Metabolism are found, with p-r values equal to −10.9 mmol C m-2 day−1 for the non-flood season. If nutrient partitioning was neglected this value would be −2.7 mmol C m-2 day−1, which would have been wrongly interpreted as biological decay and primary production in the turbidity maximum zone and in coastal waters, respectively. The rate of heterotrophy is thus three times higher than when the interaction of SPM with the nutrients is neglected. These results suggest that the estimates of nutrient budgets in turbid estuaries worldwide that ignore the role of SPM in sequestering nutrients, as with most estuarine biogeochemical models, may be incorrect and need to be re-calculated.

Effects of bottom slope, flocculation and hindered settling on the coupled dynamics of currents and suspended sediment in highly turbid estuaries, a simple model

This study aims at gaining basic understanding about two specific phenomena that are observed in the highly turbid estuaries tidal Ouse, Yangtze and Ems, i.e. (1) the accumulation of suspended matter in the deeper parts of the estuaries and (2) the relatively high values of turbidity near the surface in the area of the turbidity maximum. A semi-analytical model is analysed to verify the hypothesis that these phenomena result from bottom slope-induced turbidity currents and from hindered settling, respectively. The model governs the dynamics of residual flow, driven by fresh water discharge, salinity gradients and turbidity gradients. It further uses the condition of morphodynamic equilibrium (no divergence of net sediment transport) to compute the residual sediment concentration. New aspects are that depth variations on flow and mixing processes, as well as flocculation and hindered settling of sediment, are explicitly accounted for. Tides act as a source of mixing and erosion of sediment only, thus processes like tidal pumping are not considered. Model results show that the estuarine turbidity maximum (ETM) shifts in the down-slope direction, compared to the case of a constant depth. Slope-induced turbidity currents, which are directed down-slope near the bottom and up-slope near the surface, are responsible for this shift, thereby confirming the first part of the hypothesis above. The down-slope shift of the ETM is reduced by currents resulting from gradients in depth-dependent mixing, which counteract turbidity currents, but which are always weaker. Including flocculation and hindered settling yields increased surface sediment concentrations in the area of the turbidity maximum, compared to the situation of a constant settling velocity, thereby supporting the second part of the hypothesis. Sensitivity experiments reveal that the conclusions are not sensitive to the values of the model parameters.

Factors contributing to hypoxia in a highly turbid, macrotidal estuary (the Gironde, France)

Dissolved oxygen (DO) is a fundamental parameter of coastal water quality, as it is necessary to aquatic biota, and it provides an indication of organic matter decomposition in waters and their degree of eutrophication. We present here a 7 year time series of DO concentration and ancillary parameters (river discharge, water level, turbidity, temperature, and salinity) from the MAGEST high-frequency monitoring network, at four automated stations in the central and fluvial regions of the macrotidal Gironde Estuary, one of the largest European estuaries. The spatio-temporal variability of DO at different time scales was first related to the migration and position of the maximum turbidity zone in this extremely turbid estuary. Since 2005, the Gironde Estuary has recorded several borderline hypoxic situations (DO close to 2 mg L(-1)) and a 7 day-long hypoxic event (DO < 2 mg L(-1)) in July 2006. Summer hypoxia occurred exclusively in the fluvial, low salinity, and high turbidity sections of the estuary and was significantly more pronounced in front of the large urban area of Bordeaux (715 000 inhabitants). Detailed analysis of the data at the seasonal, neap-spring and semi-diurnal tidal time scales, reveals that hypoxia in this area occurred: (i) in the maximum turbidity zone; (ii) during the spring to neap tide transition; (iii) at highest water temperature; and (iv) at lowest river discharge; there was also evidence of an additional negative impact of sewage treatment plants of Bordeaux city. Enhancement of respiration by turbidity, temperature and inputs of domestic biodegradable organic matter and ammonia, versus renewal of waters and dispersion of reduced pollutants with the river discharge, appeared as the dominant antagonist processes that controlled the occurrence of summer hypoxia. In the context of long-term environmental changes (increase in temperature and population, decrease in summer river discharge), the occurrence of severe hypoxia could not be excluded in the next decades in the upstream reach of the Gironde Estuary.

Climate-driven chlorophyll-a changes in a turbid estuary: Observations from satellites and implications for management

Significant advances have been made in ocean color remote sensing of turbidity and water clarity for estuarine waters, yet accurate estimations of chlorophyll-a concentrations (Chla in mgm−3) has been problematic, posing a challenge to the research community and an obstacle to managers for long-term water quality assessment. Here, a novel empirical Chla algorithm based on a Red-Green-Chorophyll-Index (RGCI) was developed and validated for MODIS and SeaWiFS observations between 1998 and 2011. The algorithm showed robust performance with two independent datasets, with relative mean uncertainties of ~30% and ~50% and RMS uncertainties of ~40% and ~65%, respectively, for Chla ranging between 1.0 and >30.0mgm−3. These uncertainties are comparable or even lower than those reported for the global open oceans when traditional blue-green band ratio algorithms are used.A long-term Chla time series generated from SeaWiFS and MODIS observations showed excellent agreement between sensors and with in situ measurements. Substantial variability in both space and time was observed in the four bay segments, with higher Chla in the upper bay segments and lower Chla in the lower bay segments, and higher Chla in the wet season and lower Chla in the dry season. On average, river discharge could explain ~60% of the seasonal changes and ~90% of the inter-annual changes, with the latter mainly driven by climate variability (e.g. El Niño and La Niña years) and anomaly events (e.g. tropical cyclones). Significant positive correlation was found between monthly mean Chla anomalies and monthly Multivariate ENSO Index (MEI) (Pearson correlation coefficient=0.43, p<0.01, N=147), with high Chla associated with El Niño and lower Chla associated with La Niña. Further, a Water Quality Decision Matrix (WQDM) was established from satellite observations, providing complementary and more reliable information to the existing WQDM based on less synoptic and less frequent field measurements. The satellite-derived WQDM and long-term time-series data support the decision making efforts of the management agencies that regulate nutrient discharge to the bay. Similar approaches may be established for other estuaries where field data are much more limited than for Tampa Bay.

Evaluation of chlorophyll-a remote sensing algorithms for an optically complex estuary

Estimation of chlorophyll-a concentrations (Chla in mgm−3) in optically complex waters using remote sensing is difficult because traditional empirical (e.g., OCx blue/green band ratios) or semi-analytical algorithms (e.g., GSM or QAA) rely on blue–green wavelengths where water constituents other than phytoplankton often dominate the optical properties and satellite-derived reflectance in these wavelengths often contains substantial uncertainties. Some recent attempts have been made to derive Chla based on red and near-infrared wavelengths to avoid such problems. Using in situ data collected from 9 cruises between 1998 and 2010 in Tampa Bay, a large estuary (~1000km2) located in Florida (U.S.A.), we first tested three Chla algorithms, where each algorithm derived Chla as a function of independent variable x=Rrs(λ2)/Rrs(λ1), x=[Rrs(λ1)−1−Rrs(λ2)−1]∗Rrs(λ3), and x=[Rrs(λ1)−1−Rrs(λ2)−1]/[Rrs(λ4)−1−Rrs(λ3)−1], respectively. Here Rrs(λ) is the remote sensing reflectance, λ is the wavelength from 660 to 760nm, and the positions of the these wavelengths were determined using an objective method while the functional forms of Chla=f(x) were determined through regression between Chla and x. A synthetic chlorophyll index (SCI) using 4 green-red MERIS wavelengths was also tested. Using field measured Rrs(λ) data, all these algorithms showed the potentials to retrieve Chla between ~2 and ~80mgm−3, with 2-band algorithms showing the best performance. However, when satellite-derived Rrs(λ) data were used, none of the four algorithms was applicable to MODIS in this estuary, and only a 2-band algorithm in the form of Rrs(709)/Rrs(665) was applicable to MERIS, with other algorithms significantly degraded. The coefficients of the 2-band algorithm for MERIS were determined through regression between in situ Chla and MERIS-derived Rrs(709)/Rrs(665), with the final form of Chla=10.55x1.51. The algorithm yielded mean relative errors of 35.3% for Chla between 1.0 and 30.0mgm−3 when an in situ dataset from the Environmental Protection Commission of Hillsborough County (EPCHC) was used to tune the algorithm coefficients, which was further validated using another independent dataset. Spatial Chla patterns derived from MERIS for both low and high concentrations also showed reasonable agreement with in situ measurements. In contrast, 3-band and 4-band algorithms yielded significant patchiness in spatial patterns that differed from in situ measurements. Tests in Chesapeake Bay (located on the east coast of the U.S.A.) showed reasonable spatial patterns of Chla using the Rrs(709)/Rrs(665) approach, with a mean relative error of 34.7% for Chla ranging between 1.6 and 21.5mgm−3, suggesting that the approach may be generally applicable to other turbid estuaries once algorithm coefficients are adjusted using local data.

Towards a long-term chlorophyll-a data record in a turbid estuary using MODIS observations

Despite recent advances in using satellite data for continuous monitoring of estuarine water quality parameters such as turbidity and water clarity, estimating chlorophyll-a concentrations (Chla) has remained problematic due to the optical complexity of estuarine waters and imperfect atmospheric correction. This poses a significant challenge to the community as synoptic and frequent Chla “measurements” from satellites are in high demand by various government agencies and environmental groups to help make management decisions. Here, using 10years of in situ and Moderate Resolution Imaging Spectroradiometer (MODIS) measurements from a moderately sized, turbid estuary, Tampa Bay (Florida, USA), we developed and validated a new algorithm specifically designed for retrieving Chla from MODIS data. The algorithm takes the red-to-green remote-sensing reflectance (Rrs(λ)) band ratio of [Rrs(667)+Rrs(678)]/[Rrs(531)+Rrs(547)] as the independent variable, and estimates Chla through the non-linear regression function: Ln(Chla)=1.91Ln(x)+3.40 (R2=0.87, N=97, p<0.01, 1.5<Chla<80mgm−3) where ‘x’ is the band ratio. Validation of the algorithm using two independent datasets collected by different groups and near-concurrent MODIS measurements showed robust algorithm performance for Chla within this range, with mean relative errors of 25.8% and 41.7% for the two datasets. Time-series analyses at representative stations using both in situ and MODIS Chla also showed general agreement, with instances of noticeable discrepancy attributed to different measurement frequencies. The algorithm was implemented to establish a 10-year Chla data record for Tampa Bay in order to serve as a baseline for monitoring future phytoplankton bloom events. The 10-year Chla data record showed substantial variability in both space and time, with generally higher Chla observed during the wet season and in upper bay segments, and Chla minima observed in all bay segments during May and June. These spatial and temporal distributions appear to be regulated primarily by wind and river discharge, which also explain the significant declining trend in Chla since 2005. The established 10-year MODIS-based Chla data record provides complementary information to existing field-based monitoring programs, helping to make nutrient reduction management decisions. Furthermore, preliminary tests of the algorithm for the Chesapeake Bay and for Sea-viewing Wide Field-of-view Sensor (SeaWiFS) measurements suggest possible applicability of the proposed approach to other estuaries and satellite ocean color sensors.

Silver behaviour along the salinity gradient of the Gironde Estuary

Dissolved and particulate Ag concentrations (Ag(D) and Ag(P), respectively) were measured in surface water and suspended particulate matter (SPM) along the salinity gradient of the Gironde Estuary, South West France, during three cruises (2008-2009) covering contrasting hydrological conditions, i.e. two cruises during intermediate and one during high freshwater discharge (~740 and ~2,300 m(3)/s). Silver distribution reflected non-conservative behaviour with 60-70 % of Ag(P) in freshwater particles being desorbed by chlorocomplexation. The amount of Ag(P) desorbed was similar to the so-called reactive, potentially bioavailable Ag(P) fraction (60 ± 4 %) extracted from river SPM by 1 M HCl. Both Ag(P) (0.22 ± 0.05 mg/kg) and Ag(P)/Th(P) (0.025-0.028) in the residual fraction of fluvial and estuarine SPM were similar to those in SPM from the estuary mouth and in coastal sediments from the shelf off the Gironde Estuary, indicating that chlorocomplexation desorbs the reactive Ag(P). The data show that desorption of reactive Ag(P) mainly occurs inside the estuary during low and intermediate discharge, whereas expulsion of partially Ag(P)-depleted SPM (Ag(P)/Th(P) ~0.040) during the flood implies ongoing desorption in the coastal ocean, e.g. in the nearby oyster production areas (Marennes-Oléron Bay). The highest Ag(D) levels (6-8 ng/L) occurred in the mid-salinity range (15-20) of the Gironde Estuary and were decoupled from freshwater discharge. In the maximum turbidity zone, Ag(D) were at minimum, showing that high SPM concentrations (a) induce Ag(D) adsorption in estuarine freshwater and (b) counterbalance Ag(P) desorption in the low salinity range (1-3). Accordingly, Ag behaviour in turbid estuaries appears to be controlled by the balance between salinity and SPM levels. The first estimates of daily Ag(D) net fluxes for the Gironde Estuary (Boyle's method) showed relatively stable theoretical Ag(D) at zero salinity (Ag (D) (0) = 25-30 ng/L) for the contrasting hydrological situations. Accordingly, Ag(D) net fluxes were very similar for the situations with intermediate discharge (1.7 and 1.6 g/day) and clearly higher during the flood (5.0 g/day) despite incomplete desorption. Applying Ag (D) (0) to the annual freshwater inputs provided an annual net Ag(D) flux (0.64-0.89 t/year in 2008 and 0.56-0.77 t/year in 2009) that was 12-50 times greater than the Ag(D) gross flux. This estimate was consistent with net Ag(D) flux estimates obtained from gross Ag(P) flux considering 60 % desorption in the estuarine salinity gradient.

In situ evidence of non-zero reflectance in the OLCI 1020 nm band for a turbid estuary

A spectroradiometer able to measure up to 2500nm has been used to measure the reflectance for the Ocean and Land Color Instrument (OLCI) 1020nm band. In July and October 2010 two measurement campaigns were organized at the Scheldt river to collect reflectance spectra and corresponding Total Suspended Matter (TSM) concentration and turbidity. A wide range of TSM concentration was covered from 15 to 402mgL−1. The measurements show a significant increase in reflectance between 950 and 1150nm, corresponding to a decrease in the pure water absorption coefficient. A high correlation was observed between the reflectance at 1020 and 1071nm and TSM concentration. The results were confirmed by simulations with Hydrolight and by the analysis of airborne Airborne Prism EXperiment (APEX) imagery.

Measurements of mineral particle optical absorption properties in turbid estuaries: Intercomparison of methods and implications for optical inversions

We examine optical measurements of total particle absorption and mass specific mineral particle absorption collected from two mineral-rich turbid estuaries. Particle absorptions derived from measurements made with profiling radiometers are compared to those obtained using the Quantitative Filter Technique (QFT). A range of different backscattering ratios and pathlength amplification corrections, used to interpret the radiometer and QFT data respectively, are assessed. We also examine what impact the process of removing organic material from QFT samples by combustion has on the measured absorption properties of mineral particles. Our results show that the process of combustion changes the measured absorption properties of mineral particles retained on filters and we show that if the absorption properties of mineral particles are obtained from combusted QFT samples, significant errors can result in optical inversions that require estimates of mass specific mineral particle absorption. In contrast, using mass specific mineral particle absorption coefficients obtained from a regression technique significantly improves the accuracy of our optical inversion results. A correction for mineral particle absorption measurements derived from combusted QFT samples is presented, and assessed by application to an independent dataset.

Origin and composition of particulate organic matter in a macrotidal turbid estuary: The Gironde Estuary, France

At the interface between continent and ocean, estuaries receive particles, and especially particulate organic matter (POM) originating from these two reservoirs, but also produce POM, through autochthonous primary production. The origin and composition of surface POM in the Gironde Estuary (SW France) and the environmental forcing of its variability was investigated using the data set produced by the French Coastal Monitoring Network SOMLIT (Service d'Observation en Milieu LITtoral; monthly like sampling during years 2007–2009). This estuary is considered as a model of macrotidal turbid estuaries. Using elemental and isotopic composition of the POM, we estimated that, at the inner estuary space scale and inter-annual time scale, surface particulate organic carbon (POC) was composed of terrestrial POM originated from the turbidity maximum (96.4%; refractory POC) and flood events (1.6%; labile and refractory POC), and of riverine (0.1%), estuarine (0.8%) and marine (1.1%) phytoplankton, i.e. that POC was 98% and 2% of terrestrial and phytoplankton origin, respectively. However, there was a clear spatial gradient: the phytoplankton contribution increases from ca. 1% in the upper and middle estuary to 8.5% in the lower estuary, where light condition is more favourable to plankton growth. The low contribution of phytoplankton to the POC is a characteristic of the Gironde estuary and contrast with other large temperate estuaries. Statistical analysis indicates that salinity, river flow and SPM concentration, and thus associated hydro-dynamic and sedimentary processes, were the only environmental forcings to the composition of surface POC in this system, at intra- and inter-annual time scale. In contrast, temperature and nutrient concentrations, and thus associated processes, do not force this composition of POC.By combining POC fluxes entering the inner estuary (literature data), POC loss as dissolved organic carbon and CO2 and as sediment trapping within the inner estuary (literature data), and our estimate of the composition of POC flux at the mouth of the estuary (96% and 4% of terrestrial and phytoplankton origin), a first-order net export of POC originating from the Gironde to the continental shelf was estimated: it amounts 48,150tCyr−1, and is composed of 46,200tCyr−1 of terrestrial material and of 1950tCyr−1 of estuarine phytoplankton. POC exported by the Gironde Estuary is thus poorly bioavailable for shelf pelagic and benthic food webs.

Acoustic turbulence measurements of near-bed suspended sediment dynamics in highly turbid waters of a macrotidal estuary

Sediment–turbulence interactions near the bed are still poorly understood in highly turbid estuaries, especially in the presence of fluid mud layers. This results primarily from the difficulty in measuring co-located velocity and suspended sediment concentration (SSC) at sufficiently high rate to resolve small turbulent flow scales. In this paper, we show how a set of commercial acoustic and optical backscattering systems known as ADCPs, ADVs and OBSs, can be deployed and used in a complementary way to perform large-scale profilings of tidal current and SSC combined with high-resolution velocity and SSC measurements in the highly turbid near-bed zone. The experiment was done in the Gironde estuary (France) which is well known for its turbidity maximum zone characterized by high SSC values, above 1 g l −1 near the surface. A first simple inversion method is proposed to convert the backscattered acoustic intensity measured with ADV into SSC data in the highly turbid near-bed zone. Near-bed SSC data from the OBS are used to compensate for the important acoustic sediment attenuation effect at an acoustic frequency of 6 MHz. No a priori knowledge of acoustic backscattering properties of mud suspensions is required with this calibration procedure. We obtain an attenuation coefficient for mud suspensions of 0.28 m 2/kg at 6 MHz leading to a good agreement between the SSC timeseries from the three ADV receivers and the OBS over the entire tidal cycle. The obtained SSC data are then analyzed with respect to the near-bed velocity, Reynolds shear stress and turbulent kinetic energy (TKE) timeseries in order to identify the relevant sediment transport processes during the tidal cycle. Significant differences in bed shear stress and TKE levels are found between ebb and flood stages with effects on near-bed sediment dynamics. During the ebb, maximum levels of tidal current, bed shear stress and TKE are associated with a reduction of near-bed sediment concentration (from 400 kg m −3 down to 100 kg m −3). Bed liquefaction process is assumed to occur at this moment with the presence of highly concentrated mud layer and a possible lutocline at a distance of less than 20 cm above the bed. During the first 1.5 h of flood, turbulent activity remains moderate. The near-bed flood current is then inhibited very abruptly while a sudden increase in SSC occurred above the bed. Assuming that the ADV is able to estimate relevant turbulent erosion fluxes, the co-located velocity and SSC are multiplied and compared with settling flux measurements made onboard under quiescent water conditions. The mean sediment settling fluxes (averaged over 3 min) increase with SSC and are in relative good coherence with fluxes in quiescent water below the hindered regime, for SSC below 15 g l −1. Reducing averaging time from 3 min to 30 s allows to increase the range of turbulent fluxes and SSC values, up to 99 g l −1. At this scale, fluxes keep increasing quasi-linearly at higher SSC, suggesting the inhibition (delay or reduction) of the hindered settling regime as previously shown by Gratiot et al. (2005) from laboratory experiments. However, the 3-min averaged concentration field remained too low to conclude definitively on the effectiveness of such a process. Further analysis conducted at higher SSC regimes and under fully verified equilibrium are necessary.

Tide-induced fine sediment resuspension in the well-mixed Hangzhou Bay, East China Sea, China

ABSTRACTHangzhou Bay is a typical funnel-shaped high turbid estuary along a mesotidal coast of the East China Sea. In this paper, field studies are undertaken in an attempt to better understand tide-induced fine sediment transport processes in the Bay. Field observations were made mainly of current velocities (speeds and directions) and suspended sediment concentrations at six different relative depths and two stations in September 1992. Results show that (1) high near-bed suspended sediment concentrations varied from 1·81 to 4·00 kg m−3 during the spring tide and from 1·04 to 8·00 kg m−3 during the neap tide; (2) tide-induced near-bed fine sediment resuspension processes are indicated by several periodic suspended sediment concentration peaks; and (3) a hysteresis effect, or suspension-lag, occurs in suspended sediment concentration and resuspension events. Tide-induced fine sediment processes are mainly responsible for very high concentration suspensions of fine sediments in Hangzhou Bay. The occurrence of the suspension-lag phenomenon is likely to be common in other similar muddy tidal environments.

Copper addition by organic matter degradation in the freshwater reaches of a turbid estuary

This study reports on the relationship between copper (Cu) behavior and organic matter (OM) transformation along the turbidity gradient in the freshwater reaches of the Gironde Estuary. During a one-year survey, surface water and suspended particulate matter (SPM) were sampled at least monthly at three sites along the Garonne Branch, representing the main fluvial branch of the Gironde Estuary. Additionally, a longitudinal high resolution profile was sampled along the Garonne Branch, covering the turbidity gradient from the river water endmember to the maximum turbidity zone (MTZ). Seasonal variability and spatial distribution of Cu in both the dissolved phases (<0.2μm, Cu0.2 and <0.02μm, Cu0.02) and particulate Cu (CuP) clearly suggested Cu0.2 addition during summer, that increased the Cu0.2 concentrations by a factor ~2, mainly manifested by an increase in the Cu0.02 fraction. At the annual timescale (2004), this internal Cu reactivity increased Cu0.02 fluxes in the Garonne Branch by ~20% (3.6tyear−1), with the equivalent of ~2.9tyear−1 derived from the CuP fraction and ~0.7tyear−1 from the colloidal (0.02–0.2μm) fraction, without involving and/or affecting the CuC18 (hydrophobic metal-organic complexes) fraction.Combining data on Cu speciation with the results obtained by several independent techniques (DOC and POC measurements, 3D-fluorescence, and TEM) suggested close relationships between Cu behavior and OM transformation/restructuration along the turbidity gradient in the Garonne Branch. The observed Cu0.02 addition was related to increasing humification (humification index HIX increased from 9 to 12, network formation) and labile OM degradation (Iγ/Iα ratio decreased from 0.70 to 0.44), going along with decreasing DOC and POC concentrations. Mass-balances suggest that in the studied system, degradation of OM may account for the release of ~25μmol potentially bioaccessible Cu0.02 per mole of particulate organic carbon mineralized.

Light as a driver of phytoplankton growth and production in the freshwater tidal zone of a turbid estuary

Light is usually the main driver of phytoplankton growth in turbid estuaries, but it has received far less attention than nutrients as a bottom-up factor. This study presents the first experimental analysis of light limitation of phytoplankton growth and production and its seasonal variability in the freshwater tidal reaches of the turbid Guadiana estuary, SE Portugal/SW Spain. Natural phytoplankton communities were exposed to different photosynthetically active radiation (PAR) intensities. Short-term incubations with addition of 14HCO 3 − were used to estimate photosynthetic parameters and long-term incubations allowed the evaluation of the effects of light on phytoplankton composition and growth. Light limitation of phytoplankton growth occurred throughout the year in the freshwater tidal reaches of the estuary and no photoinhibition was observed at least up to 615 μmol photons m −2 s −1. In the summer, co-limitation by nutrients prevented a positive response of phytoplankton to light enrichment. Diatoms were the most light-limited group, whilst cyanobacteria were the only group acclimated to low-light conditions. Green algae and dinoflagellates responded positively to higher PAR exposures. High saturating irradiances, high light-saturated rates of primary production and low photosynthetic efficiencies suggest that phytoplankton community was not acclimated to the low-light conditions that prevail in the Guadiana estuary.

Sinking of particulates 234Thxs, 7Be and 210Pbxs in the Changjiang estuary, China

The time scale of transport and reaction processes in the ocean can be evaluated using natural radionuclides of different half-lives. In the present study, we evaluated the residence times of particulate reactive radionuclides 7Be, 210Pbxs and 234Th in the Changjiang (Yangtze River) estuary, which is a high turbidity estuary, in June, 2006. The residence times of particulate 7Be, 234Thxs and 210Pbxs in the Changjiang estuary were calculated to be 0.7–20.4, 0.18–4.1 and 0.6–89.8 d, respectively. The obtained residence times of those nuclides are comparable with those of other estuaries and bays. For 234Thxs and 7Be, the residence times decreased with increasing concentrations of suspended particulate matter (SPM); however, those of 210Pbxs increased. Moreover, scavenging residence times of 234Th and 7Be increased from coast to offshore, while those of 210Pbxs decreased.

Particle–water interactions of organic nitrogen in turbid estuaries

The hypothesis that particle–water interactions of organic nitrogen (ON) in turbid estuaries are controlled by specific fractions of suspended particulate matter (SPM) has been tested. Suspended particles from the turbidity maximum zone of a temperate, macro-tidal estuary were separated by gravitational settling to yield permanently suspended (PSPM; 46–51% of the total SPM load) and resuspendable (RSPM) particle fractions. Abiotic and biotic laboratory incubations were carried out in triplicate over 48 h, on the separate fractions to quantify the partitioning of ON under representative estuarine conditions, using acidic, basic and neutral 14C-labelled amino acids as proxies for ON. Under abiotic conditions, about 10% of arginine (basic) sorbed to both SPM fractions within 48 h, while sorption did not occur for aspartic acid (acidic) and glycine (neutral). Partitioning of the amino acids with both SPM fractions under biotic conditions was much greater and most uptake had occurred by 24 h, when about 44% of added amino acid was in the particulate phase. By 48 h, 33–60% of activity on the particles had been lost as 14CO 2, and the mean amount of dissolved amino acid remaining, in all biotic experiments, was 14 ± 3%. For abiotic experiments with arginine, K ds at 48 h were of the order 10 2 mL g − 1 , although were much higher (10 3 mL g − 1 ) when RSPM concentrations were markedly increased. The biotic experiments consistently gave K ds that were of the order 10 3 mL g − 1 . Thus, uptake of DON onto the SPM was bacterially-controlled and the general paradigm, that the removal of DON to particles is an abiotic process that restricts its bioavailability, does not hold for estuarine SPM.

Managed realignment as compensation for the loss of intertidal mudflat: A short term solution to a long term problem?

Intertidal mudflats are critical to the functional ecology of estuaries yet large areas are being lost as a result of land claim, erosion and coastal squeeze. This study examines whether managed realignment (at Paull Holme Strays, Humber estuary) can realistically achieve compensation for the loss of intertidal mudflat in the long term. Typical estuarine species quickly colonised the site with the total number of species recorded from the site as a whole being almost equal to that in the reference area within one year. Comparable biomass between the two areas was achieved after 2 years. However, organism abundance remains an order of magnitude lower within the realignment site compared to outside. Community structure within the realignment has changed from one characterised by terrestrial/freshwater organisms and early colonising species to one composed of typically estuarine species. However, the developing benthic communities only represent those typical of the estuary in areas of low elevation and high inundation frequency. Rapid accretion has favoured saltmarsh colonisation in much of the realignment site and this is expected to increase as accretion proceeds with invertebrate colonisation being inhibited by increasing elevation. Hence, realignment to restore intertidal mudflats can only be a short term solution in sites of high tidal elevation and in a dynamic and turbid estuary with high natural accretion rates, such as the Humber.

Tidal straining effect on the suspended sediment transport in the Huanghe (Yellow River) Estuary, China

Tidal straining effect on sediment transport dynamics in the Huanghe (Yellow River) estuary was studied by field observations and numerical simulations. The measurement of salinity, suspended sediment concentration, and current velocity was conducted during a flood season in 1995 at the Huanghe river mouth with six fishing boats moored at six stations for 25-h hourly time series observations. Based on the measurements, the intra-tidal variations of sediment transport in the highly turbid river mouth was observed and the tidal straining effect occurred. Our study showed that tidal straining of longitudinal sediment concentration gradients can contribute to intra-tidal variability in sediment stratification and to asymmetries in sediment distribution within a tidal cycle. In particular, the tidal straining effect in the Huanghe River estuary strengthened the sediment-induced stratification at the flood tide, thus producing a higher bottom sediment concentration than that during the ebb. A sediment transport model that is capable of simulating sediment-induced stratification effect on the hydrodynamics in the bottom boundary layers and associated density currents was applied to an idealized estuary to demonstrate the processes and to discuss the mechanism. The model-predicted sediment processes resembled the observed characteristics in the Huanghe River estuary. We concluded that tidal straining effect is an important but poorly understood mechanism in the transport dynamics of cohesive sediments in turbid estuaries and coastal seas.

Impact of solid waste disposal on nutrient dynamics in a sandy catchment

Groundwaters impacted by mature landfill leachate are generally enriched in ammonium. In order to assess the dynamics of ammonium exchanges between leachates and the water system inside a sandy permeable catchment we measured ammonium, nitrate and chloride concentrations in the stream and in sediment pore waters of the streambed of a landfill impacted aquifer. Geophysical investigation methods complemented the biogeochemical survey. The studied zone is a 23 km² catchment located in a coastal lagoon area sensitive to eutrophication risk. Ammonium concentrations in the river were up to 800 µmol l − 1 during low water period in summer. Three surveys of the river chemistry showed a regular increase in ammonium, nitrate and chloride concentrations along a 1 km section of the watercourse, downstream the landfill, implying that the leachate plume exfiltrates along this section. Sediment cores collected within this section showed all an increase in ammonium concentrations with depth in pore waters as a consequence of the landfill leachate dispersion, as attested by a simultaneous increase in chloride concentrations. Nitrate enrichment in the river water was due to nitrification of ammonium at the interface between groundwater and streamwater. The apparent nitrification rate obtained was within values reported for turbid estuaries, although the river contained very little suspended particulate matter. Actually, pore water chemistry suggests that nitrification occurred for the most part in subsurface permeable sediments, rather than in stream water. The overall topographic, hydrological, geochemical, and geoelectrical data set permit to estimate the extension of the chloride and ammonium plume. The estimation of the apparent ammonium plume velocity is 23 m year − 1 whereas the chloride plume velocity should be 50 m year − 1. The river is the outlet of the impacted groundwaters. Considering that the input of ammonium from the landfill is balanced by the present day output via the river, the residence time of ammonium in the aquifer is between 7 and 18 years.