Eutrophication of an impounded estuarine lagoon

A small lagoon to the east of Hobart is cut off from a larger body of tidal salt water by a causeway, allowing only a limited tidal interchange. Primary treated effluent from a small township is discharged into the lagoon. Causes for noxious odours emanating from the lagoon were investigated. It was found that the effluent, combined with agricultural runoff, has accelerated eutrophication. Wide variations in temperature and salinity occur. Decomposing algal mats account for most of the odours, while rooted aquatics and molluscs appear to play only a small part. Proposed remedies will be costly and still might not achieve their objective.     

Effects of estuarine mudflat formation on tidal prism and large-scale morphology in experiments

Human interference in estuaries has led to increasing problems of mud, such as hyper-turbidity with adverse ecological effects and siltation of navigation channels and harbours. To deal with this mud sustainably, it is important to understand its long-term effects on the morphology and dynamics of estuaries. The aim of this study is to understand how mud affects the morphological evolution of estuaries. We focus on the effects of fluvial mud supply on the spatial distribution of mudflats and on how this influences estuary width, depth, surface area and dynamics over time. Three physical experiments with self-forming channels and shoals were conducted in a new flume type suitable for tidal experiments: the Metronome. In two of the experiments, we added nutshell grains as mud simulant, which is transported in suspension. Time-lapse images of every tidal cycle and digital elevation models for every 500 cycles were analysed for the three experiments. Mud settles in distinct locations, forming mudflats on bars and sides of the estuary, where the bed elevation is higher. Two important effects of mud were observed: the first is the slight cohesiveness of mud that causes stability on bars limiting vertical erosion, although the bank erosion rate by migrating channels is unaffected. Secondly, mud fills inactive areas and deposits at higher elevations up to the high-water level and therefore decreases the tidal prism. These combined effects cause a decrease in dynamics in the estuary and lead to near-equilibrium planforms that are smaller in volume and especially narrower upstream, with increased bar heights and no channel deepening. This trend is in contrast to channel deepening in rivers by muddier floodplain formation. These results imply large consequences for long-term morphodynamics in estuaries that become muddier due to management practices, which deteriorate ecological quality of intertidal habitats but may create potential area for marshes. © 2018 The Authors. Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd.     

Intertidal mudflat properties, currents and sediment erosion in the partially mixed Tamar Estuary, UK

Results are presented from a 1-year campaign to measure the seasonal variability of some key physical and biological properties of intertidal mudflats over a section of the central Tamar Estuary and to relate these to the physical environment. Seasonal variations in physical mudflat properties, such as grain size, density and moisture content were relatively small. With the exception of the particulate organic carbon content in the upper 0.002 m of surface sediment, biological variations were large. Redox potential exhibited considerable seasonal variation and showed that the sediments were less reduced in winter and more reduced in summer. Chlorophyll a and extracellular polymeric substances (EPS) within the surface 0.002 m of sediment (due to the presence of benthic diatoms) were strongly correlated and exhibited a pronounced seasonal pattern, with smallest values during winter and greatest values during late summer and early autumn. EPS had a dominating influence on sediment erosion, as determined from annular flume measurements. Velocity measurements and velocity modelling indicated that during the flood, and for much of the time during benthic diatom bloom conditions of high chlorophyll a and EPS sediment contents, the stresses exerted by tidal currents were too small to cause significant suspension of sediments over much of the middle and upper mudflats. Suspended fine sediment in the turbidity maximum zone was transported down-estuary and deposited in the main channel at low-water (LW) slack. Some of this sediment, in the form of relatively large aggregates, was subsequently transported onto the mudflats during the flooding tide, where slack currents and fast settling velocities may have enhanced sediment deposition there.Responsible Editor: Hans Burchard     

Asymmetric fluxes of water and sediments in a mesotidal mudflat channel

The hydrodynamics of a small tributary channel and its adjacent mudflat is studied in Willapa Bay, Washington State, USA. Velocity profiles and water levels are simultaneously measured at different locations in the channel and on the mudflat for two weeks. The above tidal flat and channel hydrodynamics differ remarkably during the tidal cycle. When the water surface level is above the tidal flat elevation, the channel is inactive. At this stage, the above tidal flat flow is predominantly aligned along the Bay axis, oscillating with the tide as a standing wave with peak velocities up to 0.3 m/s. When the mudflat becomes emergent, the flow concentrates in the channel. During this stage, current velocities up to 1 m/s are measured during ebb; and up to 0.6 m/s during flood. Standard equations for open-channel flow are utilized to study the channel hydrodynamics. From the continuity equation, a lateral inflow is predicted during ebb, which likely originates from the drainage of the mudflat through the lateral runnels. Both advective acceleration and lateral discharge terms, estimated directly from the velocity profiles, play a significant role in the momentum equation. The computed drag coefficient for bottom friction is small, due to an absence of vegetation and bottom bedforms in the channel. Sediment fluxes are calculated by combining flow and suspended sediment concentration estimated using the acoustic backscatter signal of the instruments. A net export of the sediment from the channel is found during ebb, which is not balanced by the sediment import during flood. When the mudflat is submerged, ebb-flood asymmetries in suspended sediment concentration are present, leading to a net sediment flux toward the inner part of the Willapa Bay. Finally, a residual flow is detected inside the channel at high slack water, probably associated with the thermohaline circulation.     

Emulation of a process-based estuarine hydrodynamic model

Emulation modelling can be an effective alternative to traditional mechanistic approaches for complex environmental systems and, if carefully conceived, can offer significantly reduced run times and user expertise requirements. We present a case study of dynamic emulation for the domain of estuarine water quality modelling, by reporting the development and evaluation of a one-dimensional hydrodynamic model emulator. The proposed “neuroemulator” retains the dynamic nature of the process-based model utilizing a set of artificial neural networks. The underlying hydrodynamic model is routinely used for analysis and management of the northern reach of the San Francisco Bay-Delta estuary, a large complex region of strategic importance for water supply and ecosystem services on the Pacific coast of California, USA. The reduced computational expense of the emulator affords opportunities for direct use, as well as embedded use within other modelling frameworks such as those developed for reservoir operations and socio-hydrology.     

Numerical experiments with a three-dimensional model of an enclosed basin

A three-dimensional numerical model, for simulating velocity and temperature variations in a basin under various forcing mechanisms, is presented. Vertical structure is analyzed using a Galerkin Method with modified Chebyshev polynomials as expansion functions. With the proper choice of these functions, the barotropic and baroclinic parts of the motion are separated naturally in the model. A time-splitting procedure is then implemented; this makes time integration very efficient.Internal seiching due to windstress is simulated in the first experiment. Results are compared with those obtained using two-layered and 3D models. The geostrophic adjustment of a river flowing into a basin is then simulated. When friction and entrainment is neglected, flow adjusts to geostrophic balance over a distance about one third of the width of the basin. Weak turbulence has a large effect on the flow. For example, with a vertical eddy coefficient of 0.001 m² s⁻¹, isothermals are distorted so that their angle of intersection with the basin axis is large. Geostrophic adjustment still takes place over one third of the width but, due to frictional effects the flow is now quite large all over the lake.     

Coagulation and diffusion of enclosed nuclei

Summary Decay experiments were carried out with ultra small nuclei, the sedimentation loss of which is negligible. It was found that the coefficient of the linear diffusion term in the decay equation varies as the square root of the diffusion coefficient. An interpretation of this relation, which takes convection into account, is suggested. Previous decay experiments are examined in the light of the new relation.

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Zusammenfassung Es wurden Schwundexperimente mit ultra-kleinen Kernen ausgeführt, bei denen der Verlust durch Sedimentation vernachlässigbar ist. Es wurde gefunden, dass der Koeffizient des linearen Diffusions-Gliedes in der Gleichung für das Verschwinden von Kernen sich wie die Quadratwurzel aus dem Diffusionskoeffizienten ändert. Eine Interpretation dieser Beziehung, welche Konvektion berücksichtigt, wird vorgeschlagen. Frühere Schwundexperimente werden im Lichte dieser neuen Beziehung geprüft.

     

Impact of a power plant on a subtropical estuarine environment

The development of electricity generating stations around Biscayne Bay, Florida, has resulted in a considerable discharge of cooling water into this subtropical sea. The impact of this on the biology of the area has been studied for the last four years by a team of scientists; this report summarizes their general conclusions.     

Tidal and meteorological forcing of sediment transport in tributary mudflat channels

Field observations of flow and sediment transport in a tributary channel through intertidal mudflats indicate that suspended sediment was closely linked to advection and dispersion of a tidal salinity front. During calm weather when tidal forcing was dominant, high concentrations of suspended sediment advected up the mudflat channel in the narrow region between salty water from San Francisco Bay and much fresher runoff from the small local watershed. Salinity and suspended sediment dispersed at similar rates through each tidal inundation, such that during receding ebbs the sediment pulse had spread spatially and maximum concentrations had decreased. Net sediment transport was moderately onshore during the calm weather, as asymmetries in stratification due to tidal straining of the salinity front enhanced deposition, particularly during weaker neap tidal forcing. Sediment transport by tidal forcing was periodically altered by winter storms. During storms, strong winds from the south generated wind waves and temporarily increased suspended sediment concentrations. Increased discharge down the tributary channels due to precipitation had more lasting impact on sediment transport, supplying both buoyancy and fine sediment to the system. Net sediment transport depended on the balance between calm weather tidal forcing and perturbations by episodic storms. Net transport in the tributary channel was generally off-shore during storms and during calm weather spring tides, and on-shore during calm weather neap tides.     

The effect of long-term aerial exposure on intertidal mudflat erodibility

Intertidal zones by definition are exposed to air at low tide, and the exposure duration can be weeks (e.g. during neap tides) depending on water level and bed elevation. Here we investigated the effect of varying exposure duration (6 h to 10 days) on intertidal mudflat erosion (measured using the EROMES device), where the effects of water content and biofilm biomass (using chlorophyll-a content as a proxy, Chl-a μg g⁻¹) were taken into account. Sediments were collected between spring and summer (in October 2018, January 2019 and February 2019) from an intertidal site in the Firth of Thames, New Zealand. Longer exposure duration resulted in more stable sediments [higher erosion threshold (Ƭ_cr, N m⁻²) and lower erosion rate (ER, g m⁻² s⁻¹)]. After 10 days, exposure increased Ƭ_cr by 1.7 to 4.4 times and decreased ER by 11.6 to 21.5 times compared with 6 h of exposure. Chl-a and water content changed with exposure duration and were significantly correlated with changes in Ƭ_cr and ER. The stability of sediments after two re-submersion periods following exposure was also examined and showed that the stabilizing effect of exposure persisted even though water content had increased to non-exposure levels. Re-submersion was associated with an increase in Chl-a content, which likely counteracted the destabilizing influence of increased water content. A site-specific model, which included the interplay between evaporation and biofilm biomass, was developed to predict water content as a function of exposure duration. The modelled water content (W_Mod.) explained 98% of the observed variation in water content (W_Obs.). These results highlight how the exposure period can cause subtle changes to erosion regimes of sediments. An understanding of these effects (e.g. in sediment transport modelling) is critical to predicting the resilience of intertidal zones into the future, when sea-level rise is believed to exacerbate erosion in low-lying areas. © 2020 John Wiley & Sons, Ltd.     

Topographic monitoring of a middle estuary mudflat, Humber estuary, UK--anthropogenic impacts and natural variation

Annual topographic surveys were carried out at the Saltend mudflat (Humber estuary, UK) between 1998 and 2006. These surveys formed part of an ongoing monitoring programme to examine the potential effects on the mudflat topography of the construction and operation of a waste water treatment works (WwTW) development by Yorkshire Water. Of particular concern was the potential disruption to the sedimentological regime within the special protection area (SPA) and candidate special area of conservation (cSAC) which could affect the invertebrate communities and ornithological functioning of the site. In addition to the development of the WwTW located to the extreme north-west of the site, a port extension removing 10ha of the Saltend intertidal mudflat (outside the SPA but immediately south east of the WwTW) also occurred between 1999 and 2006. Minimal change was noted across the site following the construction and operation of the WwTW between 1998 and 2000. However, the construction of the bund in closer proximity to the SPA and cSAC masked any potential impact the WwTW could have had across the site after 2000. Profiles and contour mapping indicate that significant mudflat accretion occurred in the immediate area of the bund, with a general increase recorded across the western section of the site since 2000. In contrast the alternations to channel planform and subsequent rapid accretion of the mudflat to the east of the jetty, being a significant distance from the developments, are attributed to natural cyclical changes.     

巢湖富营养化研究进展

本文系统论述了20年来巢湖富营养化的研究过程,根据不同阶段对巢湖研究的特点,将巢湖富营养化研究过程划分为三个阶段:初步调查阶段、深入研究阶段和应用研究阶段. 综述了巢湖富营养化研究的8个方面的内容,包括流域自然和社会环境的调查、水域生态调查与评价、流域水污染源调查与评价、巢湖富营养化机理研究、水质区划与容量研究、流域的非点源污染研究、生态环境现状调查研究和内源污染控制研究等. 文中指出了近10多年来缺少对巢湖的基础研究,形成背景资料的断裂,10多年前的研究成果不能完全满足当今巢湖富营养化防治的需要. 目前,巢湖仍然处于全湖富营养化状态,急需对巢湖进一步开展基础研究,为从根本上治理巢湖污染寻求对策. 最后指出了一些主要的研究方向.     

Estuarine macrofauna responses to continuous in situ nutrient addition on a tropical mudflat

A field experiment to assess the effects of continuous nutrient addition on the macrobenthic community was carried out on an estuarine mudflat on the northeast coast of Brazil. The experiment began on 5 October 2005 and ended on 8 February 2006. Macrofauna was compared at approximately four-week intervals in triplicate plots with three levels (Control – C, Low Dose – LD and High Dose – HD) of weekly fertilizer additions for 17 weeks. Inorganic fertilizer (N–P–K) was applied on nine randomly defined quadrangular plots (4 m² each). All measurements were calculated from species abundances. Multivariate analyses as well as the univariate indices (richness, abundance and Shannon–Wiener index) showed statistically significant differences between the enriched and control areas during the period of the experiment. The expected gradual response based on the succession model of Pearson and Rosenberg was not observed. The nutrient doses used were high enough to cause severe decreases in abundance, richness and evenness, and an increase in dominance.     

Atrazine effects on meiobenthic assemblages of a modular estuarine mesocosm

Atrazine is a widely used herbicide in the US found at levels ranging from <10 ng/L to 62.5 microg/L in estuaries throughout the southeast. Effects of atrazine on estuarine meiobenthic assemblages chronically exposed to environmentally relevant concentrations are unknown. The purpose of our research was to assess effects of atrazine on meiobenthos at concentrations near the proposed USEPA SWQC (26 microg/L) using modular estuarine salt marsh mesocosms as a field surrogate. Indigenous copepod and nematode densities were assessed after 28 days of exposure in transplanted colonization chambers. Cluster analysis showed a group characterized by low copepod densities, mostly atrazine exposed chambers, and a group containing all but one control chamber. The later group included chambers with high densities of the copepods Paronychocamptus wilsoni and Enhydrosoma baruchi. Compared to controls, copepod densities was approximately 70% lower in atrazine chambers, with three of the most common copepod species (E. baruchi, Onychocamptus sp. and P. wilsoni) showing an average of 50-70% reduction in population densities (p<0.05). Although nematode density did not differ between atrazine and control chambers, the nematode-to-copepod ratio was significantly higher in atrazine (9.95+/-7.61; p=0.011) than in control chambers (0.61+/-0.35). Our findings suggest that chronic exposures over multiple generations to atrazine at concentrations near the proposed USEPA SWQC could have significant effects on the abundance and composition of estuarine meiobenthic copepod assemblages.     

Three-dimensional modelling of estuarine turbidity maxima in a tidal estuary

A new numerical model for simulating estuarine dynamics is introduced here. This model, called General Estuarine Transport Model (GETM), has been specifically designed for reproducing baroclinic, bathymetry-guided flows where the tidal range may exceed the mean water depth in large parts of the domain such that drying and flooding processes are relevant. Several physical and numerical features of the model support exact and stable results for such domains. For the physics, high-order turbulence closure schemes guarantee proper reproduction of vertical exchange processes. Among the specific numerical features, generalised vertical coordinates, orthogonal curvilinear horizontal coordinates, high-order TVD advection schemes and stable drying and flooding algorithms have been implemented into GETM. The model is applied here to simulate the dynamics of estuarine turbidity maxima (ETMs), a complex feature present in most tidal estuaries. First, idealised simulations for a two-dimensional domain in the xz space will be shown to reproduce the basic generation mechanisms for ETMs. Then, a realistic three-dimensional simulation of the Elbe estuary in Northern Germany will be carried out. It is demonstrated that for a given forcing situation the model reproduces a stable ETM at the correct location.Responsible Editor: Phil Dyke     

Short-term dispersion and turbulence in a complex-shaped estuarine embayment

Initial dispersion of material in complex-shaped embayments is examined using observations and scaling based in Crail Bay, Pelorus Sound, New Zealand in autumn of 2005. These observations show the highly variable nature of dispersive transport in an embayment with multiple headlands. Acoustic current profiler-derived typical flow speeds were around 0.05 m s⁻¹ which resulted in drifter-derived short-term (<6 h) horizontal eddy diffusivities of the order of 1 m² s⁻¹ which is somewhat larger than the empirical paradigm. Microstructure estimates of the turbulent kinetic energy dissipation rate were in the range 10⁻⁹–10⁻⁷ m² s⁻³, with some evidence that sidewalls influence the variability and that headlands increase the dissipation rate by at least and order of magnitude. A new parameter relating horizontal diffusivity to circulation and the tide is proposed. This and other scaling comparisons indicate that the headlands in Crail Bay create similar effects to those studied in other systems. However, the long decay times estimated for eddies here implies that they likely interact with other headlands, unlike some previously studied examples.     

Stochastic analysis of estuarine hydraulics

A new methodology is presented for the solution of the stochastic hydraulic equations characterizing steady, one-dimensional estuarine flow. The methodology is predicated on quasi-linearization, perturbation methods, and the finite difference approximation of the stochastic differential operators. Assuming Manning's roughness coefficient is the principal source of uncertainty in the model, stochastic equations are presented for the water depths and flow rates in the estuarine system. Moment equations are developed for the mean and variance of the water depths. The moment equations are compared with the results of Monte Carlo simulation experiments. The results confirm that for any spatial location in the estuary that (1) as the uncertainty in the channel roughness increases, the uncertainty in mean depth increases, and (2) the predicted mean depth will decrease with increasing uncertainty in Manning'sn. The quasi-analytical approach requires significantly less computer time than Monte Carlo simulations and provides explicit     

Estuarine pollution as a probable cause of increase of estuarine birds

A long-term increase of estuarine birds does not always appear to be a good indication of the improvement of the environmental quality of an estuary. This paper shows that an incontestable ecological deterioration in a part of the Scheldt estuary has led probably to a greater food supply for intertidal birds. They respond to this phenomenon by visiting the area in increasing numbers, and the period of migration of birds on other feeding grounds nearby differs from theirs.