Tidal Scales Research Articles

Modeling of phosphate flux induced by flood resuspension on a macrotidal estuarine mudflat (Seine, France)

Coastal marine sediments can be either major scrubbers or eutrophication contributors to surface waters. Standard methods for direct measurement of nutrient fluxes at the sediment-water interface do not consider hydrodynamic forcing although several ex-situ studies suggest that sediment resuspension can dramatically increase dissolved fluxes. We provide a new model to quantify dissolved phosphate (PO43−) resuspension flux (JR) based on physical representation of its identified components: diffusion stimulation by exposure of deeper sediment layer with higher PO43− concentration in the porewater (JD), pore water mixing with overlying water (JM) and net adsorption/desorption from suspended sediments (JK). This approach was applied to field data from a Seine intertidal mudflat periodically submitted to millimetric erosion. On a tidal scale, the model output reveals a JR of 272.3 ± 360.0 μmol m−2 h−1 (± 52% from parameter uncertainty), well above flux calculated by application of Fick's first law (0.15 ± 0.85 μmol m−2 h−1) or by ex situ core incubation (40.8 μmol m−2 h−1). Iron bound phosphorus within suboxic layers buffers PO43− concentrations in superficial sediments leading to negligible contributions of JD and JM to total fluxes. Conversely, JK appears to be the main exchange pathway, even though improvements in turbidity measurement would allow this term to be defined more precisely. Correction required to enhance and control model robustness are described. These results show the importance of considering the dissolved PO43− resuspension flux in dynamic environments.

Tide-salinity patterns reveal seawater-freshwater mixing behavior at a river mouth and tidal creeks in a tropical mangrove estuary

Tide and salinity data collected at minute intervals over multiple semidiurnal tides were used to investigate the source of water (e.g., seawater, river, groundwater and rain) and their relative timing in mixing at the mouth of a river, a tidal creek at mid-estuary and a tidal creek at the shoreline at the head of a tropical mangrove estuary. Our objectives were to document the temporal changes in tide induced water level changes and salinity at each location and to use the relationship between salinity and water level to elucidate the sources of water and the timing of different sources of water in the hydrologic mixing processes. The data trends in tide vs. salinity (T-S) plots for the river mouth revealed mixing with seawater during rising tides and freshwater diluted seawater (brackish) drainage from the mangrove forest during ebb tides. In the mangrove creek at mid-estuary, the data trends in the T-S plots for rising tides initially showed constant salinity, followed by sharp rises in salinity to peak tide caused by seawater intrusion. The salinity decreased precipitously at the start of tidal ebbing due to influx of freshwater (rain) diluted brackish water from the mangrove forest. The data trends in the T-S plots for the tidal creek at the shoreline located at the estuary head showed constant salinity which decreased only near peak rising tide because of river dilution. During tidal ebbing, the salinity further decreased from groundwater influx before increasing to background salinity, which stayed constant to low tide. Establishing T-S patterns for multiple locations in mangrove estuaries over sub-tidal to tidal scales define the expected salinity variations in seawater-freshwater mixing which can be used to (1) establish baseline hydrologic and salinity (hydrochemical) conditions for temporal and spatial assessments and (2) serve to guide short to long-term sampling regimes for scientific studies and estuarine ecosystem management.

Variability of early autumn planktonic assemblages in the strait of Gibraltar: a regionalization analysis

The Strait of Gibraltar (SG) is the only connection of the Mediterranean Sea with the global circulation. The SG is an outstanding marine region to explore physical-biological coupling of pelagic communities due to its hydrodynamic complexity, including strong tidal forcing and marked spatial gradients and fronts. The authors have unravelled the role of the fortnightly tidal scale (spring and neap tides) and local processes (upwelling and tidal-topographic mixing) that shape planktonic assemblages in the Strait. To do so, an oceanographic cruise was taken in early autumn 2008 with a high-resolution grid sampling and spring/neap tidal conditions. The planktonic features were captured using different automatic and semi-automatic techniques of plankton analyses (flow cytometry, FlowCAM, LOPC and Ecotaxa) that allowed covering a wide range of sizes of the community from pico- to mesoplankton. The SG was sectorized into two clusters based on the biogeochemical and main water column properties. Cluster 1 (CL1) covered shallow productive areas around Cape Trafalgar (CT). CL1 presented higher concentrations of chlorophyll and nutrients, and phytoplankton was mostly represented by Synechococcus and coastal diatoms while zooplankton had the highest percentage of meroplankton (31%). In contrast, cluster 2 (CL2) covered open ocean waters and presented more oligotrophic features, i.e. nitrogen-depleted waters with lower chlorophyll concentrations and a picoplankton community dominated by Prochlorococcus and holoplankton predominance in mesozooplankton. Under early autumn conditions with overall nutrient-depleted and stratified waters, the CT area emerges as an ecosystem where the constant tidal mixing and nutrients supply is coupled with an active production also being favored by high residence times and finally shaping a plankton community with unique features in the area.

Variation of water mass exchange on tidal scale in Balikpapan Bay

Balikpapan Bay is enclosed water influenced by freshwater from river runoff and saline water from Makassar Strait. The exchange of water mass was examined by 3D numerical model simulation-Hamburg Shelf Ocean Model (HAMSOM) with horizontal resolutions approx. 150 m and 10 vertical layers applied in Balikpapan Bay. The thirteen tidal components, daily river runoff, atmospheric forcing, subsurface temperature, and Salinity in 3D used for model input. The tidal elevation from Geospatial Information Agency (BIG) model fits with this result from 01/03/2020 to 31/03/2020. It has coefficient correlation 0,99 with a significant level of 95% and Root Mean Square Error (RMSE) is 0,1 m. The volume and salt transport in the mouth (Line-A) and middle (Line-B) of bay was examined. The maximum transport in Line-A during spring (neap) high to low tide and low to high tide is −18364.72 m3/s (−1717.57 m3/s) and −17532.27 m3/s (4258.86 m3/s) for volume. Then, 531,947,898.90 kg.psu./s (−45,127,135.38 kg.psu./s) and −536,410,944.50 kg.psu./s (140,700,437.97 kg.psu./s) for salinity. Positive (negative) of water transport is inflow (outflow) to Balikpapan Bay. The net transport in a day during the spring (neap) is −832.45 m3/s (5976.43 m3/s) for volume and −4,463,045.58 kg.psu./s (185,827,573.35 kg.pau./s) for salt. The vertical structure of net volume and salt transport bot in Line-A and Line-B shows the water goes to outer bay in surface and inner bay in subsurface. While in the spring tide the surface deeper than neap tide. It indicated that water mass exchange dominantly influenced by river in surface and tidal in subsurface. It also shows that water mass from inner bay more easy flushing during spring tide than neap tide and vice versa

Temporal and Spatial Variability Scales of Salinity at a Large Microtidal Estuary

The Río de la Plata is a large fluvial–estuarine–sea system discharging into the southwestern Atlantic Ocean, which has relevant features such as high fluvial discharge, microtidal astronomical tidal scales, a relevant meteorological tide, and a strong atmospheric forcing effect, due to its large width. The objective of this study is to advance the understanding of the estuarine hydrodynamics and salt transport, as well as discussing the main characteristics of the spatiotemporal variability of the salinity field. To achieve this, the results of a 3D model of the Río de la Plata and its maritime front were used, simulating an extensive period of 10 years. In this study, the model was validated using vertical salinity profiles collected at different locations in the estuary. The temporal variability of the salinity stratification was characterised at different temporal scales: annual, monthly, and storm. At the same time, the influences of fluvial flow and winds were determined. The correlation analysis between fluvial flow and the salinity field showed that high annual fluvial flows generate an extension of the freshwater area, with larger longitudinal salinity gradients and a shift of the salinity front towards the ocean. The tendency at the monthly scale is not as clear as that observed at the annual scale. The results show that the effect of a storm coming from the northwest is quite similar to that of storms coming from the southwest, especially in the central and southern zones of the Río de la Plata, where mixing increases and stratification decreases, according to the intensity of the storm. The effect of south–southeasterly storms increases the mixing process and reduces stratification; the opposite effect was identified with respect to northeasterly storms, under the influence of which the stratified area increases. Synthesising the obtained results, a global zonification of the vertical salinity stratification for the Río de la Plata is proposed.

Unravelling tidal effect on zooplankton community structure in a tropical estuary.

The variability in zooplankton density and species composition in response to tidal oscillations were investigated in the lesser saline upper reaches (E1) and higher saline lower reaches (E2) of the Mahanadi Estuary, located at the mouth of the third largest river in Indian Peninsula. This is the first of its kind high frequency observation over the tidal cycle ranging from highest high tide to lowest low tide in this estuary revealing dynamic variability of zooplankton assemblages. Zooplankton abundance was higher during high tide in comparison to low tide, irrespective of salinity regimes. On the diurnal scale, it was higher at night in comparison to the day at both E1 and E2. The higher abundance of zooplankton groups such as Copepoda, Cladocera, and planktonic larvae during the night at E1 as well as E2 indicating an upward migration. Many of the zooplankton taxa (e.g., Pseudodiaptomus serricaudatus, Pseudodiaptomus sp., Acartia danae, Acrocalanus longicornis, Oithona sp., Corycaeus andrewsi) migrated towards E1 due to tidal effect during high tide and maintained their position even during low tide. In contrast, the prevalence of limnetic taxa (e.g., Brachionus rubens, Polyarthra vulgaris, Bosminopsis deitersi, Moina micrura, Heliodiaptomus sp.) at E2 during low tide indicated a predominant riverine source. The tidal variability of Brachyura (zoea and megalopa) revealed different emergence times that indicated dispersal of zoeas to the adjacent Bay of Bengal and the return of magalopa to the Mahanadi Estuary. Species diversity index was higher during high tide, and prominent at E2. At E1, marine, fresh, marine-brackish, and marine-brackish-fresh zooplankton species dominated during high tide, while brackish-fresh taxa dominated during low tide. Differently, E2 was enriched with marine, marine-brackish, and marine-brackish-fresh taxa during high tide, whereas fresh, brackish-fresh, and marine-fresh dominated during low tide. Salinity and suspended matter influenced the dominant zooplankton taxa at E2 and E1, respectively. Zooplankton assemblages exhibited a pattern of prominent diurnal-spatial variation in comparison to the tidal scale in the Mahanadi Estuary. Overall, this study documented a very high zooplankton diversity (92 taxa belonging to 13 groups) and significant variations in species abundance which highlighted the importance of carrying out sampling over the tidal cycle at contrasting salinity regimes.

Competition Between Atmospheric and Tidally Attenuated Forcing in an Elongated Coastal Lagoon

Water level measurements at four sub-basins of a highly dissipative coastal lagoon were used to study variability at tidal and subtidal scales and to determine whether atmospheric forcing influenced variations in water level at tidal scales. The amplitude of the main astronomic constituents, which were diurnal, attenuated quickly to <5% by the second sub-basin. The attenuation was represented well by an analytical solution that described a damped wave. The semidiurnal constituent with a period of 12 h (S2), however, decreased to 50% of the lagoon’s entrance amplitude and then increased to 60% by the fourth sub-basin. Reproduction of this S2 behavior by the analytical model required unrealistically low signal attenuation, much lower than for the other semidiurnal constituent (12.42 h, M2). This behavior suggested that the S2 amplification was non-gravitational and related to semidiurnal atmospheric forcing, which was dominated by barometric pressure. Such suggestion was confirmed by coherence and wavelet coherence analysis and by the fact that the largest overtide was related to the interaction between S2, instead of M2, and the diurnal constituent. This study documents that a highly dissipative coastal lagoon becomes forced semidiurnally by barometric pressure toward its head. Subtidal water levels were studied with empirical orthogonal functions, revealing 2 modes that explained ~85% of the variance. Mode 1 was related to northerly winds (‘Nortes’) and mode 2 was linked to the Trade winds that dominate the region.

Analysis of the Monthly and Spring-Neap Tidal Variability of Satellite Chlorophyll-a and Total Suspended Matter in a Turbid Coastal Ocean Using the DINEOF Method

Missing spatial data is one of the major concerns associated with the application of satellite data. The Data INterpolating Empirical Orthogonal Functions (DINEOF) method has been proven to be an effective tool for filling spatial gaps in various satellite data products. The Ariake Sea, which is a turbid coastal sea, shows the large spatial and temporal variability of chlorophyll-a (Chl-a) and total suspended matter (TSM). However, ocean color satellite data for this region usually have large gaps, which affects the accurate analysis of Chl-a and TSM variability. In this study, we applied the DINEOF method to fill the missing pixels from the regionally tuned Moderate Resolution Imaging Spectroradiometer (MODIS)-Aqua (hereafter, MODIS) Chl-a and MODIS-derived TSM datasets for the period 2002–2017. The validation results showed that the DINEOF reconstructed data were accurate and reliable. Furthermore, the Empirical Orthogonal Functions (EOF) analysis based on the reconstructed data was used to quantitatively analyze the spatial and temporal variability of Chl-a and TSM at both monthly and individual events of spring-neap tidal scales. The first three EOF modes of Chl-a showed seasonal variability mainly caused by precipitation, the sea surface temperature (SST), and river discharge for the first EOF mode and the sea level amplitude for the second. The first three EOF modes of TSM exhibited both seasonal and spring-neap tidal variability. The first and second EOF modes of TSM displayed spring-neap tidal variability caused by the sea level amplitude. The second EOF mode of TSM also showed seasonal variability caused by the sea level amplitude. In this study, we first applied the DINEOF method to reconstruct the satellite data and to capture the major spatial and temporal variability of Chl-a and TSM for the Ariake Sea. Our results demonstrate that the DINEOF method can reconstruct patchy oceanic color datasets and improve spatio-temporal variability analysis.

Spatial and Temporal Variation of the Hydrological Parameters in the Wouri-Nkam Section of the Cameroon Estuary, Central African Atlantic Coast

This study aimed to provide relevant knowledge about the dynamics of the hydrological parameters in the river-estuary continuum of the Wouri-Nkam river estuary for a sustainable management program. The hydrological parameters were recorded in eleven stations spanned out on the basis of population density and human activities. Water quality parameters (Temperature, salinity, dissolved oxygen, pH, Total dissolved solutes, Redox potential and conductivity) were collected in subsurface water using a multiple parameter. Surface currents and morphometric (depth and width) parameters were recorded using a drifter, sonar depth and GPS. The field measurements took placed between 18/05/2019 to 08/09/2020 and were divided into six (06) cruises. The data were later subjected to an analysis of variance (ANOVA) and Principle Component Analysis using XLSTAT 2017 (2.7 version) software. Results obtained revealed that, the water quality parameters were spatially more stable notsignficant at (df = 9, p 0.05) with a relatively low temperature (25.5°C - 27°C) during the wet period. The limit of the frontal zone extended to S5 (Bonalokan, 8.25 km from S1) during the snapshot of the dry period, spring phase and flood tide conditions. Inversely, during wet period, this extension reduced to S1 (Bridge) and relatively increases slightly to S3 (Bonangang) during the neap phase and ebb tides of this season. This result revealed a change in the axial gradient of about eight (08) and four (04) kilometers during the seasonal and tidal scales (lunar and diurnal periods) respectively. These changes were also accompanied by changes in the water quality signatures, that may affect the fishery distribution and compositions. However, futures studies to buttress the results of this investigation should focus on longer time series sampling methods and model developments.

Tidal variation of total suspended solids over the Yangtze Bank based on the geostationary ocean color imager

Hourly mapping by a Geostationary Ocean Color Imager was used to reveal the spatial pattern and tidal variation of total suspended solids (TSS) over the Yangtze Bank in the Yellow and East China Seas during the winter. The TSS form a tongue-shaped structure, which decreases further offshore in a stepwise manner. The stepwise change is separated by two fronts of TSS, which are located near the 20-m and 50-m isobaths. The tidal variation of TSS concentration during the study period is evident and can be divided into three stages: decay, maintenance, and growth. Compared with the relatively stationary TSS during the maintenance stage, drastic changes exist during the decay and growth stages. In terms of tide-induced mixing, the dynamic analysis shows that both the topography and the tidal currents play an important role in the spatio-temporal variation of TSS during the tidal period. In particular, spatial distribution is primarily determined by the topography, whereas the temporal variations in tidal scale are determined by the tidal currents.

Zooplankton community modulated by spatial and tidal changes in the Bahía Blanca Estuary, Argentina

The variability of the zooplankton community over a spatial and a tidal scale, the relationship between physico-chemical variables, and the abundance of zooplankton were studied in the temperate and turbid Bahía Blanca Estuary (Southwestern Atlantic, Argentina). Samples were taken by pumps during 12-h tidal cycles, at 3-h intervals, from two depths and three sites across the main channel in the inner and middle estuary zones. The zooplankton was dominated by the copepods Acartia tonsa and Euterpina acutifrons, and larvae of the invasive Pacific oyster Crassostrea gigas. The physico-chemical variables that most influenced the zooplankton community were salinity, temperature, suspended particulate matter, and chlorophyll a Most taxa showed higher abundances in the inner estuary, which corresponds to a turbid and shallow area. The abundance of A. tonsa was higher in the inner estuary while that of E. acutifrons was higher in the middle estuary. This spatial distribution is consistent with the tidal distribution of these species as in the inner estuary the abundance of A. tonsa peaked at ebb tide and that of E. acutifrons peaked at flood tide. Significant differences in the zooplankton community structure both across the channel and between depths were detected by multivariate analyses, but no clear patterns were found in the abundance of each taxon by a species-level analysis. The results provide insight into the patterns of zooplankton distribution and abundance in a temperate, turbid, and human-impacted coastal ecosystem, considering different spatial and short-term scales. These results will be useful to design efficient sampling programmes in highly dynamic environments.

Interactions between barotropic tides and mesoscale processes in deep ocean and shelf regions

The interactions between barotropic tides and mesoscale processes were studied using the results of a numerical model in which tidal forcing was turned on and off. The research area covered part of the East Atlantic Ocean, a steep continental slope, and the European Northwest Shelf. Tides affected the baroclinic fields at much smaller spatial scales than the barotropic tidal scales. Changes in the horizontal patterns of the M2 and M4 tidal constituents provided information about the two-way interactions between barotropic tides and mesoscale processes. The interaction between the atmosphere and ocean measured by the work done by wind was also affected by the barotropic tidal forcing. Tidal forcing intensified the transient processes and resulted in a substantial transformation of the wave number spectra in the transition areas from the deep ocean to the shelf. Tides flattened the sea-surface height spectra down to ~ k−2.5 power law, thus reflecting the large contribution of the processes in the high-frequency range compared to quasi-geostrophic motion. The spectra along sections parallel or normal to the continental slope differ from each other, which indicates that mesoscale turbulence was not isotropic. An analysis of the vorticity spectra showed that the flattening was mostly due to internal tides. Compared with the deep ocean, no substantial scale selectivity was observed on the shelf area. Particle tracking showed that the lengths of the Lagrangian trajectories increased by approximately 40% if the barotropic tidal forcing was activated, which contributed to changed mixing properties. The ratio between the horizontal and vertical scales of motion varied regionally depending on whether barotropic tidal forcing was included. The overall conclusion is that the barotropic tides affect substantially the diapycnal mixing.

Relationships Between Chlorophyll‐a and Suspended Sediment Concentration in a High‐Nutrient Load Estuary: An Observational and Idealized Modeling Approach

AbstractThe Guadalquivir estuary is a high‐nutrient load environment, yet it has a reduced primary production because the high suspended sediment concentration (SSC) causes light attenuation. High‐resolution observations revealed relationships between the SSC and Chlorophyll‐a (Chla) concentration from seasonal to intratidal time scales. Local maxima of time‐averaged Chla levels occurred at locations with relatively low SSC. In the upper (lower) part of the estuary, larger Chla concentrations were observed during the wet (dry) season. In contrast to longer time scales, SSC and Chla exhibited in‐phase oscillations during the spring‐neap cycle. Both SSC and Chla were found to peak at maximum ebb and flood. There was no positive correlation at certain apogean neap tides, and instead, Chla exhibited daily variations. An idealized model was developed, which mimicked and helped to explain SSC‐Chla relationships at different time scales. The model accounts for the vertical advection of Chla and SSC, tidal resuspension, radiation‐mediated growth, and effects of the change in stratification stemming from tides and SSC. The specific growth rate of the Chla biomass is inhibited due to high SSC throughout the year. The in‐phase relationship at spring‐neap and tidal scales seems to be due to resuspension of algal microorganisms attached to sediments. Daily variability of Chla during apogean neap tides emerges because tidal shear stress is low and the strong decline in the suspended particulate matter reduces the light attenuation. The light‐mediated growth of part of the biomass, still remaining in the upper layers of the water column, is then driven by the day‐night cycle.

Interaction between seismicity and deformation on different time scales in volcanic areas: Campi Flegrei and Stromboli

Abstract. We study oscillations recorded at Stromboli and Campi Flegrei by different sensors: seismometers, strainmeters and tiltmeters. We examine both the high-frequency (&gt;0.5 Hz) portion of the spectrum and very long period signals up to tidal scales. In this context, seismicity and deformation are investigated on different time scales (from minutes to days/years) in order to identify the basic elements of their interaction, whose understanding should provide new insights on the predictive models. In this work, the strict relation of tides and volcanic processes is shown. At Stromboli, indeed the transition from the stationary phase to the non-stationary phase seems to have a tidal precursor that is related to the duration of the crisis. The subsequent volcanic activity is interpreted as the response of the volcano to restore the equilibrium condition. The moveout from equilibrium produces, first, variations in the standard statistics of explosions, then leads to effusive stage and to a pressure drop in the shallow feeding system. That process induces the nucleation of a gas bubble and the excitation of low frequencies. Campi Flegrei seismicity shows a correlation between the diurnal solar solid tide and the energy released by the long period signals, indicating that the whole mechanism is modulated on a tidal scale. In other words, in the case of Stromboli, a departure from the equilibrium state is marked by solid tide variations in a certain frequency band. On the other hand, at Campi Flegrei diurnal to annual solid tides modulate an increase of volcanic activity.

Local Variability of CO2 Partial Pressure in a Mid-Latitude Mesotidal Estuarine System (Tagus Estuary, Portugal)

Estuaries play a crucial role in regional carbon cycling. Until now, accurate estimations of the impact of environmental variables on estuarine air–water CO2 fluxes have been mostly characterized by a low spatial-temporal sampling resolution. This study reports on the variations of CO2 partial pressure (pCO2) and related environmental parameters, at both tidal and seasonal temporal scales, in the surface seawater of a station located in the lower section of the Tagus estuary, Portugal. The study was carried out from February to December 2007. Air–water CO2 fluxes suggest that the lower estuary acted as a relatively weak source of CO2 to the atmosphere, with an average rate of 7.2 mol∙m−2∙year−1, with highest fluxes occurring in winter. Over a tidal cycle, pCO2 was mainly influenced by tidal-induced mixing. Results suggest an influence of upper and central estuary inputs with higher pCO2 values. pCO2 varied seasonally, with values decreasing from ~890 µatm in winter to ~400 µatm in summer and increasing again to ~990 µatm in autumn. The generalized linear model (GLM) applied to the data set explained 69.3% of the pCO2 variability, pointing to the thermodynamic effect of temperature and biological activity as the most relevant processes in CO2 dynamics. Tidal variation of pCO2 corresponded to ~35% of its seasonal variability, denoting the importance of tide conditions on the dynamics of inorganic carbon. Results showed distinct patterns in the dynamics of CO2 at the tidal scale. This outcome suggests that disregarding tidal variability in the use of seasonal data sets may lead to significant errors in annual carbon budget estimations.

Effects of short-term hydrological processes on benthic macroinvertebrates in salt marshes: A case study in Yangtze Estuary, China

Understanding the effects of hydrological processes on the benthic macroinvertebrates in salt marshes can provide theoretical basis for species diversity restoration, coastal environment protection, and comprehensive exploitation and utilization of salt marsh ecosystems. In this study, 4 fixed-point sampling sites were set up in the salt marsh of the East Nanhui tidal flat in the Yangtze Estuary for benthic macroinvertebrate survey, hydrological monitoring and sedimentary parameter collection over two short-time scales (semidiurnal and fortnightly cycles). Based on the results of these surveys, we analyzed the effects of hydrological processes on the benthic macroinvertebrates over different timescales. The results showed that benthic macroinvertebrates assemblages varied significantly over the semidiurnal and fortnightly tide periods but not between sites. The number of species and the abundance of the benthic macroinvertebrates during spring tide period were significantly lower than that during neap tide period, although the biomass during both tidal periods were not significantly different. There was no significant variation in the number of species, abundance, and biomass of benthic macroinvertebrates over the semidiurnal tidal scale in general with few exceptions. However, there were significant differences in most of the hydrological and sedimentary parameters between the spring and neap tide periods, as well as between semidiurnal tides in these two periods. Two principal components, the intensity of hydrological processes (PC1) and the physicochemical properties of water and sediment (PC2), were derived from principal component analysis on hydrological factors and sedimentary physicochemical parameters. The results show that PC1 had a significant effect on abundance of benthic macroinvertebrate community; while PC2 had a significant effect on biomass. The best combined environmental factors, which exhibited significant correlations with the characteristics of the benthic macroinvertebrates and also their taxonomic classes, varied across the sampling periods. This study indicates that short-term hydrological processes can not only directly affect the benthic macroinvertebrates, but also indirectly affect the communities by altering sedimentary physicochemical factors. There were significant differences in the effects of the hydrological processes on the benthic macroinvertebrate community over the semidiurnal and fortnightly tidal scales, and it could be inferred that the scale effect still exists in the short-time scale.

On extracting high-frequency tidal variability from HF radar data in the northwestern Bay of Bengal

ABSTRACTFirst results from a systematic harmonic analysis of HF radar (HFR) derived ocean surface current observations in the northwestern Bay of Bengal (BoB) during 2010 is presented. The daily-averaged HFR currents compared reasonably well to composite daily surface currents from multiple satellites with correlation coefficient of 0.90 (0.69) for zonal (meridional) component. A set of sequential daily currents demonstrated sustained northward (southward) alongshore flow during February–April (October–December) with peak magnitude of about 1.8 (1.2) m/s. On tidal scales, harmonic analyses of zonal and meridional components at nearshore and offshore locations indicated that among semi-diurnal tidal components, M2 dominates over S2 and N2; time-scales of which were verified from available coastal tide gauges nearby. Amplitudes of semi-major axis for M2 and K1 tidal ellipses are 7.16 (6.13) and 4.02 (3.30) cm/s in nearshore (offshore) location indicating relatively stronger tidal currents in nearshore location. Finally, significant shallow water constituents S4, MS4 and M3 (M4, 2SM6 and M6) at nearshore (offshore) location are identified, which are due to non-linear interaction of tidal currents with bathymetry. Both semi-diurnal and shallow water tidal currents show dominance along isobaths in offshore region, which turn progressively across-isobath as they move nearshore.

In situ high frequency long term measurements of suspended sediment concentration in turbid estuarine system (Seine Estuary, France): Optical turbidity sensors response to suspended sediment characteristics

The aim of this study is to investigate the complex response of optical turbidity sensors (side- and back-scattering sensors) to Suspended Particle Matter (SPM) characteristics and the consequences when investigating SPM dynamics from long-term high frequency monitoring networks. Our investigation is based on the analysis of a unique dataset of monthly 12 h cycle measurements of SPM characteristics such as turbidity, concentration, floc size distribution, floc density and organic matter content in the macrotidal Seine Estuary (France) between February 2015 and June 2016. Results reveal that despite calibration to a Formazin standard, turbidity sensor response to SPM concentrations (in the range of 7–7000 mg L−1) are strongly variable, from the tidal scale to the annual scale and in different compartments of the Seine Estuary. The variability in the calibration relationships is related to changes in the sensor sensitivity according to (i) the sensor intern technology (mainly due to optical geometry) and (ii) the variability in inherent optical properties (IOP) of SPM.Side-scattering optical instruments (measuring scattering at 90°) provide at the annual scale a more stable optical response than backscattering sensor (measuring scattering at angles larger than 100°) for a wide variety of floc size and density in the estuarine environment, while at the tidal scale the backscatter sensors are the most accurate. Sensor sensitivity is strongly affected by floc characteristics, i.e. their median size D50, dry density ρ and the scattering efficiency Qb. Results highlight that the median particle diameter contribute to modify the scattering efficiency Qb as well as the dry density: Qb increases with increasing floc size, and for a given floc size, Qb increases with floc density.In this study, the results are next applied to turbidity data from the long-term automated monitoring network in order to estimate SPM concentrations and estimate the related uncertainties.

Nghiên cứu sự biến động sinh khối phiêu sinh thực vật và hàm lượng ôxy hòa tan để quản lý đàn cá trên kênh Nhiêu Lộc - Thị Nghè, Thành phố Hồ Chí Minh

The study was conducted during one-year cycle, with six sampling periods in January, March, July, September and November 2015 to identify the phytoplankton and dissolved oxyen (DO) fluctuations at the Nhieu Loc - Thi Nghe (NL-TN) canal in spatial, temporal and tidal scales, thereby proposing solutions to manage the fish stock in the canal. The NL-TN canal is devided into three sections including downstream, midstream and upstream, with nine sampling stations. The collected samples represented the highest and lowest tide periods of the month. The results showed that the phytoplankton biomass reached highest value in upstream (5,2-6,1 gDW/m3) and lowest in downstream (4,4-4,9 gDW/m3); however, there is no correlation between high biomass phytoplankton and low DO in early morning in spatial scale. In terms of temporal aspect, phytoplankton biomass reached the highest in July (6.9 gDW/m3) and the lowest in September (3.5 gDW/m3); however, there is no correlation between phytoplankton and DO biomass over time. Therefore, the monitoring of phytoplankton biomass with the frequency of 2 months/time is not effective in predicting correlative fluctuations of phytoplankton indicator with DO deficiency for fish stock management. Considering tidal fluctuations, high and low tide levels had significant influence on the annual average phytoplankton biomass difference. Considering the DO aspect, DO shows the lowest level in the early morning of May at the low tide period, which coincides with the high mortality of fish stock in the canal at the beginning of the rainy season, posing a requirement for DO monitoring and management at the risky times with DO deficiency for fish stock management.

Tidal and subtidal hydrodynamics and energetics in a constricted estuary

The dynamics of coastal plain estuaries are mainly associated with variable tidal forcing and local winds in combination with bathymetric complexity and coastline irregularity. Specific features, such as constricted areas, can potentially affect and energize the hydrodynamics of these types of systems. Particularly, tidal range and tidal currents can be significantly amplified where the incoming tidal wave becomes constricted. In this work, the impact of a narrow constriction on a mesotidal estuary was analysed at tidal and subtidal time scales. Tidal hydrodynamics, energy fluxes and energy dissipation were determined for the entire Cádiz Bay (southwestern Spain) using the Delft3D numerical model. Field observations were used to analyse tidal propagation and energy dissipation along the bay constriction and to calibrate and test the numerical model. The results indicate that the presence of the constriction transformed and distorted the tide and increased the tidal range and flow velocities along the channel, with implications on energy dissipation. The tidal currents were oriented along-channel at the central part of the constriction, although abrupt bathymetric changes at the channel inner boundary provoked a sudden rotation of the flow. Although the energy fluxes were higher for spring tides and were strongly influenced by winds, the energy dissipation was controlled by bed shear stresses and vertical dispersion. The significance of this energy dissipation was that it destabilized the water column, which resulted in a weakly stratified system with implications on water quality. At a subtidal scale, the residual water volume exchange was the result of the combined effects of the neap/spring tides, wind and waves, whereas tides were dominant at the tidal scale.