Tidal variability of water quality parameters in a mesotidal estuary (Sado Estuary, Portugal)

To establish effective water quality monitoring strategies in estuaries, it is imperative to identify and understand the main drivers for the variation of water quality parameters. The tidal effect is an important factor of the daily and fortnightly variability in several estuaries. However, the extent of that influence on the different physicochemical and biological parameters is still overlooked in some estuarine systems, such as the Sado Estuary, a mesotidal estuary located on the west coast of Portugal. The main objective of this study was to determine how the water quality parameters of the Sado Estuary varied with the fortnightly and the semidiurnal tidal variation. To achieve this goal, sampling campaigns were conducted in May/18, Nov/18 and Jun/19, under neap and spring tidal conditions, with data collection over the tidal cycle. Results were observed to be significantly influenced by the tidal variation, in a large area of the estuary. Flood seemed to mitigate possible effects of nutrient enrichment in the water column. Additionally, significant differences were also observed when considering the different sampling stations. Temperature, Suspended Particulate Matter (SPM) and nutrients showed the highest values at low water. Lastly, the implications of the tidal variability in the evaluation of the water quality according to Water Framework Directive were also discussed, highlighting the importance of studying short-time scale variations and the worst-case scenario to ensure water quality is maintained. These findings are relevant for the implementation of regional management plans and to promote sustainable development.

Would El Niño enhance or suppress the migrating diurnal tide in the MLT region?

Previous observations and simulations are controversial as to whether El Niño will increase or decrease the diurnal tide (DW1) in the upper mesosphere and lower thermosphere (MLT) region. This study revisited the linear response of the MLT DW1 to El Niño during the winter (December-January-February) based on 19-year satellite observations of Sounding of the Atmosphere using Broadband Emission Radiometry (SABER). The MLT DW1 temperature amplitudes decreased by ~10 % during four El Niño winters from 2002 to 2020, consistent with the results from the simulation of the Specified-Dynamics version of the Whole Atmosphere Community Climate Model (SD-WACCM). According to the multiple linear regression analysis, the linear effects of El Niño-Southern Oscillation (ENSO) on tropical MLT DW1 are negative in both SABER observations and SD-WACCM simulations. In the SD-WACCM simulation, Hough mode (1, 1) dominates the DW1 tidal variation in the tropical MLT region. The consistency between the (1, 1) mode in the tropopause region and in the MLT region, as well as the downward phase progression from 15 to 100 km, indicates the direct upward propagation of DW1 from the excitation source in the troposphere. During 7 of 8 El Niño winters from 1979 to 2014, the anomalous amplitudes of the (1, 1) mode are negative in both the tropopause region and MLT region. The suppressed DW1 heating rates in the tropical troposphere (average over ~0–16 km and 35° S–35° N) during the El Niño events contribute to the decreased DW1 tide. The mesospheric latitudinal zonal wind shear anomalies during El Niño winters would lead to a narrower waveguide and prevent the vertical propagation of the DW1 tide. The gravity wave drag excited by convection also plays a role in modulating the MLT DW1 amplitude.

A tidally driven estuary close to an amphidromy

This paper describes the implementation of a high-resolution three-dimensional model of the estuary “Sundalagið Norður”. This estuary is bound by narrow sills in both ends and has a large semidiurnal tidal variation. The proximity to an amphidromic region, results in a periodically varying difference in sea level height between both ends of the estuary, which generates strong semidiurnal tidal currents across the sills. The available observational data support the model results. The model results verify the dominance of tidal forcing with semidiurnally varying volume fluxes into and out of the estuary. The amplitudes of these fluxes vary in strength with the fortnightly variation between spring and neap tides. More surprisingly, the model also indicates a strong fortnightly variation of net fluxes averaged over 25 hours reducing both diurnal and semidiurnal tidal currents. These variations are caused by fortnightly variations in sea level difference between both ends of the estuary, which are verified by comparison with observed sea level variations. This rather surprising result implies that exchanges within the estuary and with its surroundings vary systematically; typically with one week of net northward flow followed by one week of net southward flow. This variation also appears to affect the mixing processes in the estuary and should be taken into account in planning development or activities. More observational data would be beneficial to validate the model more thoroughly and we recommend that a dedicated experiment with combined observations and numerical modeling is implemented.

Deep-Sea DC Resistivity and Self-Potential Monitoring System for Environmental Evaluation With Hydrothermal Deposit Mining

Environmental impact assessment has become an important issue for deep-sea resource mining. The International Seabed Authority has recently developed recommendations for guidelines on environmental assessment of resource mining effects. Several research and development groups have been organized to develop methods for environmental assessment of the seafloor and sub-seafloor under the “Zipangu in the Ocean program,” a part of the Cross-ministerial Strategic Innovation Promotion Program managed by the Cabinet Office of the Japanese government. One attempt planned for long-term environment and sub-seafloor structure monitoring uses a cabled observatory system. To support this observatory plan, we began development of a system to monitor the sub-seafloor resistivity and self-potential reflecting the physicochemical properties of ore deposits and the existence of hydrothermal fluid. The system, which mainly comprises an electro-magnetometer and an electrical transmitter, detects spatio-temporal changes in subseafloor resistivity and in self-potential. Because of the project’s policy changes, cabled observatory system development was canceled. Therefore, we tried to conduct an experimental observation using only a current transmitter and a voltmeter unit. Data obtained during three and a half months show only slight overall apparent resistivity variation: as small as 0.005 Ω-m peak-to-peak. The electrode pair closest to the hydrothermal mound shows exceptionally large electric field variation, with a semidiurnal period related to tidal variation. Results indicate difficulty of explaining electric field variation by seawater mass migration around the hydrothermal mound. One possibility is the streaming potential, i.e., fluid flow below the seafloor, in response to tides. However, we have not been able to perform rigorous quantitative analysis, and further investigation is required to examine whether this mechanism is effective. The system we have developed has proven to be capable of stable data acquisition, which will allow for long-term monitoring including industrial applications.

Assimilating in situ and remote sensing observations in a highly variable estuary-shelf model

To improve the forecasting performance in dynamically active coastal waters forced by winds, tides, and river discharges in a coupled estuary-shelf model off Hong Kong, a multivariable data assimilation (DA) system using the ensemble optimal interpolation (EnOI) method has been developed and implemented. The system assimilates the Conductivity-Temperature-Depth (CTD) profilers, time-series buoy measurement, and remote sensing sea surface temperature (SST) data into a high-resolution estuary-shelf ocean model around Hong Kong. We found that the time window selection associated with the local dynamics and the number of observation samples are two key factors in improving assimilation in the unique estuary-shelf system. DA with a varied assimilation time window based on the intra-tidal variation in the local dynamics can reduce the errors in the estimation of the innovation vector caused by the model-observation mismatch at the analysis time, and improve greatly simulation in both the estuary and coastal regions. Statistically, the overall root-mean-square error (RMSE) between the DA forecasts and not-yet-assimilated observations for temperature and salinity have been reduced by 33.0% and 31.9% in the experiment period, respectively. By assimilating higher resolution remote sensing SST data instead of lower resolution satellite SST, the RMSE of SST is improved by ~18%. Besides, by assimilating real-time buoy mooring data, the model bias can be continuously corrected both around the buoy location and beyond. The assimilation of the combined buoy, CTD, and SST data can provide an overall improvement of the simulated three-dimensional solution. A dynamics-oriented assimilation scheme is essential for the improvement of model forecasting in the estuary-shelf system under multiple forcings.

Estimating groundwater inputs from Sankarabarani River Basin, South India to the Bay of Bengal evaluated by Radium (226Ra) and nutrient fluxes

Sankarabarani river basin gains significance due to presence of major industrial, agricultural, urban development and tourist related activities has influenced the water quality in the estuarine environment. Investigations about river water quality has been attempted but not more studies focus about the evaluation of groundwater discharge a significant process that connects groundwater and the coastal seawater have been attempted. For the present study, radium (226Ra) a naturally occurring isotope was measured at three locations and used as effective tracers for estimating the groundwater discharge along with nutrient inputs to the Bay. Groundwater samples representing north east monsoon (December, 2017) has been collected during tidal variation in three locations (Location A- away from the coast towards inland, Location B-intermediate between Location A and the coast and Location C-at the estuary). 226Ra mass balance calculated groundwater fluxes irrespective of tidal variations were 2.27×108 m3/d, 2.19×108 m3/d and 5.22×107m3/d for A, B and C locations respectively. The nutrients like Dissolved inorganic nitrogen (DIN), Dissolved inorganic Phosphate (DIP) and Dissolved Silica (DSi) were found to be influencing the coastal groundwater by contributing fluxes to the sea of about 679.33 T mol/day. The study suggests increasing radium and nutrient fluxes to the Bay altering the coastal ecosystems would result in surplus algal blooms creating hypoxia.

Effect of tidal variation on the growth of Anadara senilis L. (1758) in the marine protected area of Bamboug (Senegal)

An in-situ study was conducted in the Sine Saloum Delta with the objective of studying the growth rate of Anadara senilis according to size and tide variation. The study revealed a minor allometry with values of 2.60 and 2.53 in the consistently submerged area and intermittently submerged area, respectively. The Von Bertalanffy linear growth parameters estimated from the ELEFAN in R Software are L∞ = 58.1 mm; K = 0.20, t0 = -0.70 and Φ’ = 2.83 in the intermittently submerged area and L∞ = 53.2 mm; K = 0.31; t0 = - 0.46 and Φ’= 2.94 in the consistently submerged area. The growth rate is inversely proportional to the size of the individual. The results of this study should help to develop strategies for the sustainable management of A. senilis in Sine Saloum Delta.

Geographic and environmental contributions to genomic divergence in mangrove forests

ABSTRACTAssessing the relative importance of geographic and environmental factors to the spatial distribution of genetic variation may provide relevant information about the processes that maintain genetic variation in natural populations. With a globally wide but very restricted habitat distribution, mangrove trees are an interesting model for studies aiming to understand the contributions of these factors. Mangroves occur along the continent-ocean interface of tropical and subtropical latitudes, regions considered inhospitable to many other plant types. Here, we used landscape genomics approaches to investigate the relative contributions of geographic and environmental variables to the genetic variation of two black mangrove species, Avicennia schaueriana and Avicennia germinans, along the South American coast. Using single nucleotide polymorphisms, our results revealed an important role of ocean currents in the gene flow of A. schaueriana and isolation by environment as the pattern that best explains the genetic differentiation of A. germinans. Additionally, for both species, we observed significant correlations between genetic variation with evidence of selection and precipitation regimes, tidal variation, solar radiation and temperature patterns. These discoveries expand our knowledge about the evolution of mangrove trees and provide important information to predict future responses of coastal species to the expected global changes for this century.