Dynamics of the buoyant plume off the Pearl River Estuary in summer

Field measurements of salinity, wind and river discharge and numerical simulations of hydrodynamics from 1978 to 1984 are used to investigate the dynamics of the buoyant plume off the Pearl River Estuary (PRE), China during summer. The studies have shown that there are four major horizontal buoyant plume types in summer: Offshore Bulge Spreading (Type I), West Alongshore Spreading (Type II), East Offshore Spreading (Type III), and Symmetrical Alongshore Spreading (Type IV). River mouth conditions, winds and ambient coastal currents have inter-influences to the transport processes of the buoyant plume. It is found that all of the four types are surface-advected plumes by analysing the vertical characteristic of the plumes, and the monthly variations of the river discharge affect the plume size dominantly. The correlation coefficient between the PRE plume size and the river discharge reaches 0.85 during the high river discharge season. A wind strength index has been introduced to examine the wind effect. It is confirmed that winds play a significant role in forming the plume morphology. The alongshore wind stress and the coastal currents determine the alongshore plume spreading. The impact of the ambient currents such as Dongsha Current and South China Sea (SCS) Warm Current on the plume off the shelf has also assessed. The present study has demonstrated that both the river discharge and wind conditions affect the plume evolution.     

Origins and scales of hypoxia on the Louisiana shelf: Importance of seasonal plankton dynamics and river nutrients and discharge

Management plans for the Mississippi River Basin call for reductions in nutrient concentrations up to 40% or more to reduce hypoxia in the Gulf of Mexico (GOM), while at the same time the government is considering new farm subsidies to promote development of biofuels from corn. Thus there are possibilities of both increasing and decreasing river nutrients depending on national priorities. River flow rates which also influence the extent of hypoxia on the shelf may be altered by global climate change. We have therefore developed a series of simulations to forecast ecosystem response to alterations in nutrient loading and river flow. We simulate ecosystem response and hypoxia events using a linked model consisting of multiple phytoplankton groups competing for nitrogen, phosphorus and light, zooplankton grazing that is influenced by prey edibility and stoichiometry, sub-pycnocline water-column metabolism that is influenced by sinking fecal pellets and algal cells, and multi-element sediment diagenesis. This model formulation depicts four areas of increasing salinity moving westward away from the Mississippi River point of discharge, where the surface mixed layer, four bottom layers and underlying sediments are represented in each area. The model supports the contention that a 40% decrease in river nutrient will substantially reduce the duration and areal extent of hypoxia on the shelf. But it also suggests that in low and middle salinity areas the hypoxia response is saturated with respect to nutrients, and that in high salinity regions small increases in nutrient and river flow will have disproportionally large effects on GOM hypoxia. The model simulations also suggest that river discharge is a stronger factor influencing hypoxia than river nutrients in the Mississippi River plume. Finally, the model simulations suggest that primary production in the low salinity regions is light limited while primary production in the higher salinity zones is phosphate limited during the May to October period when hypoxia is prevalent in the Mississippi River plume.     

广州城市近地风场特征研究

利用实测广州市近地风速、风向数据资料,分析广州市近地风场特征。结果表明：（1）城市近地风速微弱,风速范围在0.1~0.71 m.s-1,平均风速约0.25 m.s-1,处于静风状况。城市对盛行风风速的削减达90%左右;（2）在盛行风被极度削弱的同时,珠江江风促成了城区一些区域风速的增强。同时,白云山等北部山林的山风以及大型绿地对周围测点的风场均有影响,这些局地的热力效应促进了大气的流动,增强近地风速,改善近地风速微弱甚至无风的状况。广州大道北联白云山,南通珠江,道路开敞,受到白云山山风和珠江江风的影响,广州大道监测点风速相对较大。有些地方则成为＂风影区＂,监测点基本处于无风状态。随着城市的扩展,城市近地风速微弱,通风不良,风向变化复杂,这对于汽车日益增多,尾气污染激增的城市是极为不利的。因此,城市的河流、山林以及大型绿地对城市近地风场的影响值得关注,值得进一步研究如何利用其布局改善城市的无风状况。     

Dredged Material Disposal at the Edge of the Florida Current

A field data collection project was undertaken to investigate the short-term fate of dredged material discharged in the designated Miami Ocean Dredged Material Disposal Site (ODMDS) before dredging of the Miami River and the Miami Harbor Turning Basin begins. the designated ODMDS is located in relatively deep water for discharge sites with a typical bottom depth of 150 metres and is also located in the western boundary region of the Gulf Stream current off Miami. Acoustical backscattering, current, particulate, temperature and salinity data were gathered over a three day period from April 24, 1990 through April 26, 1990. the major generic features of shallow-water discharge plumes were observed to be present: (a) the presence of a rapid convective descending plume portion; (b) impact of that plume portion with the ocean bottom and concomitant generation of a bottom surge; (c) rapid horizontal width growth of the descending plume through entrainment; and (d) retention of a residual plume portion within the water column. A well-mixed upper water column layer extending to a depth of 40 to 60 metres below the surface of the ocean permitted measurements of the plume entrainment coefficient free from bottom boundary, water column density gradient, and vertical current shear effects which are usually present in relatively shallow, e.g. less than 40 metres bottom depth, coastal ocean discharge studies. Entrainment coefficient estimates obtained in this study were between 0.5 to 0.7. the residual water plume material was tracked over one-half hour during each of eight discharge events and was transported in a north-northeast direction.     

Dredged Material Disposal at the Edge of the Florida Current

A field data collection project was undertaken to investigate the short-term fate of dredged material discharged in the designated Miami Ocean Dredged Material Disposal Site (ODMDS) before dredging of the Miami River and the Miami Harbor Turning Basin begins. the designated ODMDS is located in relatively deep water for discharge sites with a typical bottom depth of 150 metres and is also located in the western boundary region of the Gulf Stream current off Miami. Acoustical backscattering, current, particulate, temperature and salinity data were gathered over a three day period from April 24, 1990 through April 26, 1990. the major generic features of shallow-water discharge plumes were observed to be present: (a) the presence of a rapid convective descending plume portion; (b) impact of that plume portion with the ocean bottom and concomitant generation of a bottom surge; (c) rapid horizontal width growth of the descending plume through entrainment; and (d) retention of a residual plume portion within the water column. A well-mixed upper water column layer extending to a depth of 40 to 60 metres below the surface of the ocean permitted measurements of the plume entrainment coefficient free from bottom boundary, water column density gradient, and vertical current shear effects which are usually present in relatively shallow, e.g. less than 40 metres bottom depth, coastal ocean discharge studies. Entrainment coefficient estimates obtained in this study were between 0.5 to 0.7. the residual water plume material was tracked over one-half hour during each of eight discharge events and was transported in a north-northeast direction.     

Temperature,salinity and water-age variations in a tidal creek network,Bushehr Port,Iran

The spatio-temporal variability of temperature and salinity was studied for a creek network and its adjacent coastal waters along the northwestern coast of the Persian Gulf during warm and cold months. Salinity variations and tidal fluctuations were found to be out of phase throughout the creek. Temperature variations at the creek were exhibit a direct correlation with tidal fluctuations during cold months and were inversely related during warm months. The creek water was colder (warmer) than offshore water during the cold (warm) season. The salinity values observed inside this inverse estuary were higher during the warm season than the corresponding values during the cold season due to a change in evaporation rates; while the open water salinity had an opposite pattern. Using salinity as a tracer, the water-age (WA) was calculated, which shows almost linear increase from the mouth to the head. The maximum WA increases from ~10 days in winter to ~30 days in summer due to the corresponding increase in longitudinal salinity gradient. Based on the calculated non-dimensional Peclet number, the diffusion process is more rapid than the advection process in this water body, especially during the cold season.     

Remplacement de Sphaeroma serratum par S. hookeri dans l'Etang de Berre par suite de sa dessalure

The inflow of the river Durance into the Etang de Berre has resulted in a great change in hydrological conditions. Air photographs of this fresh-water influx and the rapid decrease in salinity explain a most important change in ecological factors. Salinity values are about 9 to 13‰. The observed variations are caused by water flow and meteorological conditions, especially wind. Populations of the genus Sphaeroma in the Etang de Berre were notably affected by the decrease in salinity. Sphaeroma serratum has completely disappeared and S. hookeri has taken its place. This substitution of species is not due to interspecific competition, since S. hookeri has moved into the habitats which S. serratum has left. This change of milieu observed in the Etang de Berre iepresents a remarkable natural experiment, in which salinity separates 2 distinct ecological niches.     

Sources,distribution, and decomposition stages of sedimentary organic matter in estuaries and its adjacent areas

Lignin-derived phenols, C/N ratios and ratios of refractory to total organic matter were used to study the sources, distribution, and diagenesis of sedimentary organic matter along estuary, river bank, and salt marsh systems. The Changjiang Estuary showed a higher signal of terrestrial organic matter at the river mouth which decreased farther offshore. The locations along the Qiantang River were affected by their adjacent surroundings, and the Andong salt marsh showed higher terrestrial organic matter signal at the upper marsh compared to the lower marsh. All studied areas showed slight increases in organic matter decomposition farther downstream and towards the sea. Study of these three different systems will improve our understanding on their ecotoxicological impact. The Changjiang Estuary had higher pollutant levels near the river mouth, which decreased farther offshore due to dilution. The midstream Qiantang River was most likely affected by pollution from their adjacent surroundings. Pollutants were likely to be accumulated in the Andong salt marsh due to the presence of marsh plants and small-sized particles. Organic pollutants were likely to be decomposed during transport along the systems. The ability of the sediments to release inorganic pollutants was determined by whether these systems were oxic or anoxic.     

Effects of wind-driven circulation on river intrusion in Lake Tegel: modeling study with projection on transport of pollutants

Prediction of mixing intensity of water masses in riverine Lake Tegel (Berlin, Germany) can be used to trace the fate of pollutants that enter the lake through several inflows. Because the contributions of each inflow have not yet been quantified and because the lake features complex bathymetry and numerous islands, a hydrodynamic circulation model with high spatial resolution and dynamic wind forcing is useful. We applied the two-dimensional version of the Princeton Ocean Model to separate the influences of wind and river discharge on the currents and mass transport in Lake Tegel. For model validation, we compared the simulation results with 1?year of electrical conductivity data, which was used as a conservative tracer to distinguish between water from the River Havel and water supplied by a smaller second inflow. Calculation of currents alone is insufficient to investigate water exchanges between rivers and lakes, especially when several islands create multiple pathways for river intrusion. Therefore, mass transport simulations are applied. Our calculations based on archetypical scenarios indicate that the proportion of (polluted) water from the River Havel in the main basin of Lake Tegel fluctuates with river discharge and wind, which either amplify or neutralize each other.     

Alongshore transport of a toxic phytoplankton bloom in a buoyancy current: Alexandrium tamarense in the Gulf of Maine

We examined the mechanisms controlling blooms of the toxic dinoflagellate Alexandrium tamarense Lebour and the concomintant patterns of shellfish toxicity in the southwestern Gulf of Maine, USA. During a series of cruises from 1987 to 1989, hydrographic parameters were measured to elucidate the physical factors affecting the distribution and abundance of dinoflagellates along this coast. In 1988 and 1989 when toxicity was detected in the southern part of this region, A. tamarense cells were apparently transported into the area between Portsmouth, New Hampshire, and Cape Ann, Massachusetts, in a coastally trapped buoyant plume. This plume appears to have been formed by the outflow from the Androscoggin and Kennebec Rivers. Flow rates of these rivers, hydrographic sections, and satellite images led us to conclude that the plume persisted for about a month, and extended alongshore for several hundred kilometers. The distribution of cells followed the position of the plume as it was influenced by wind and topography. When winds were downwelling-favorable (to the southwest), cells were moved alongshore to the south, and were held to the coast; when winds were upwelling-favorable (to the northeast),the plume sometimes separated from the coast, advecting the cells offshore. In 1987 when no plume was present, A. tamarense cells were scarce, and no toxicity was recorded at the southern stations. The alongshore advection of toxic cells within a coastally trapped buoyant plume can explain the details of the temporal and spatial patterns of shellfish toxicity along the coast. We hypothesize that (1) the source of the A. tamarense populations is in the north, possibly associated with the Androscoggin and Kennebec estuaries, that (2) toxicity patterns follow a predictable relationship with river flow volume and timing of flow peaks and that (3) wind stresses directly influence the distribution of low salinity water and the dinoflagellate cells. Local, in situ growth of dinoflagellates can be an important factor initiating toxic dinoflagellate blooms. However, these data demonstrate the significant role of alongshore transport of established populations of A. tamarense in controlling the location and timing of paralytic shellfish poisoning (PSP) outbreaks in May and June along the southwestern coast of the Gulf of Maine.     

Analysis of Wind-Induced Thermocline Oscillations of Lake Tanganyika

An analysis is presented of the wind-induced thermocline oscillations of Lake Tanganyika, East Africa. The region undergoes a four month dry season and the wet season for the rest of the year. The dry season is characterised by nearly constant high southeasterly winds, while for the rest of the year mild wind blows generally from the northeast. Observations show that the dry season high winds cause tilting of the thermocline, being higher/lower than normal at the southern/northern ends of the lake. The thermocline tries to restabilise itself after the cessation of dry season winds and oscillates for the rest of the year. A non-linear reduced-gravity model is used to study the thermocline oscillations of the lake. The numerical simulations satisfactorily represent the oscillations, their period and amplitude. Different forcing conditions (thermocline depth, wind stress and stability) are used in the model and their effect on the period and amplitude of the oscillations are studied. The amplitude of oscillations ranges from 15 to 45 m, while their period varies from 3 to 4 weeks according to the variation in the model parameters. Wavelet transform is used to study the evolution of periods of oscillations with depth in the time series of observations and along the length of the lake using model simulations. Wavelet spectra presents several dominant modes including the semidiurnal, diurnal, synoptic, intraseasonal variability, besides the modes representing the wind-induced thermocline oscillations.     

Estimating river flow effects on water ages by hydrodynamic modeling in Little Manatee River estuary, Florida, USA

The water age in a tidal river in Florida, Little Manatee River, has been investigated in this study by the application of a three-dimensional hydrodynamic model. In response to a pulse dye release in the upper end of the river boundary, the hydrodynamic model determines the water age for a given location by recording the time for the dye to reach the given river location. The hydrodynamic model uses horizontal curvilinear orthogonal grids to represent the complex river system that includes several bayous and tributaries. The model has was calibrated and verified in previous study by using two continuous data sets for a 6 month period. Satisfactory model verifications indicate that the hydrodynamic model is capable of quantifying the mixing and transport process for calculating the water age in the tidal river. For 17 freshwater inflow scenarios in the Little Manatee River, the hydrodynamic model was applied to simulate water ages along the main channel of the river at 2-km interval. Flow rates in the 17 scenarios varying from 0.26 to 76.56 m³/s cover the range of the observed flows in the Little Manatee River. Water ages from model predictions range from the minimum 1.2 days under the maximum 76.56 m³/s inflow condition to the 50 days under the minimum 0.26 m³/s inflow condition. Empirical regression equations at three selected stations, with the correlation coefficient R² above 0.96, were derived from numerical model simulations to correlate water ages to freshwater inflows. The empirical water-age equation derived from hydrodynamic model simulations can be used to provide quick and low-cost estimations of water ages in response to various inflow scenarios for studying physical–chemical and biological processes in the river.     

Modeling the interaction between tides and storm surges for the Taiwan coast

An unstructured grid, two-dimensional hydrodynamic model was established and applied to the coast of Taiwan to investigate the tide-surge interaction. Tidal elevations at the open boundaries coupled with a global ocean tidal model and the meteorological conditions using a cyclone model are used to drive the model. The model was calibrated and verified with the observed tidal levels at six tidal stations for seven typhoon events to ascertain the capability and feasibility of the model. The results show reasonable agreement between the simulated and observed tidal levels. The validated model was then applied to probe the influence of tide-surge interaction on phase, water levels, and storm surge height. We found that the tide-surge interaction influenced both the magnitude and timing of the surge, which depended on the typhoon path. The storm surge heights at different tidal stations were significantly influenced by wind stresses and directions. The water level rise due to the storm surge during high tide was greater at neap tide than at spring tide. Changing tidal ranges altered the prediction of the surge enough to induce the changes in peak water levels.     

塔里木河下游植物群落结构特征分析

对塔里木河下游经30a断流后的应急输水河畔植被进行外业调查,分析了调查区植物群落的种类组成、垂直结构、密度结构以及优势种群的格局分布,旨在了解输水后研究区植物群落在结构特征方面恢复程度。结果表明:调查样地内有植物13种,其中胡杨(Populus euphratica)、柽柳(Tamarix)、黑果枸杞(Lycium ruthenicum)、花花柴(Karelinia caspica)和疏叶骆驼刺(Alhagi sparsifolia)为优势种。不同地下水条件物种的重要值不同,表现在不同离河距离具有不同的物种组成结构,随地下水埋深不同植物群落由以胡杨和胀果甘草为主到以柽柳、黑果枸杞、花花柴和疏叶骆驼刺等抗旱耐盐碱植物为主。乔木层密度随着离河距离加大而减小,灌木层和草本层密度随离河距离加大呈先增加后减小的趋势。群落垂直结构随离河距离加大由乔-灌-草3层逐渐转变为灌-草2层,随离河距离加大趋于简单。优势种群胡杨在离河300m以内呈聚集分布,聚集强度随离河距加大而增强;柽柳灌丛在离河50m呈均匀分布,离河150m以外呈聚集分布,但离河750m段有向随机分布发展趋势。植物群落结构特征在不同的离河距离具有差异,说明经过输水下游植物群落的恢复在结构特征方面有响应,为区域生态治理和保护提供科学依据。     

Numerical analysis on the Brisbane River plume in Moreton Bay due to Queensland floods 2010–2011

During the Queensland floods in the summer of 2010–2011, a flood-driven Brisbane River plume extended into Moreton Bay, Queensland, Australia, and then seaward, travelling in a northward direction. It covered approximately 500 km $^{2}$ . This paper presents a three- dimensional hydrodynamic numerical model investigation into the behaviour of the Brisbane River plume. The model was verified by using satellite observations and field measurement data. The present study concludes that the high river discharge was the primary factor determining the plume size and its seaward extensions. A notable finding was that the plume was a bottom-trapped type rather than a buoyant type. Further, the southerly winds were found to have moderately confined the alongshore extension of the plume, and had caused the plume to mix thoroughly with the ocean water.     

A Numerical Study of the Dynamics of the Riverine Thermal Bar in a Deep Lake

A numerical model based on a Finite Volume formulation of the Navier–Stokes equations is used to simulate a range of scenarios leading to a thermal bar formed by a river inflow to an idealised deep lake. The results presented here show that small riverine salinity increases have a profound effect on the dynamics of the thermal bar, suppressing horizontal propagation of the plume and raising the possibility of a thermal bar which is capable of sinking to great depths. This finding is particularly relevant to Lake Baikal in Siberia, where the vigorous deep-water renewal is still not fully understood. An analysis of the buoyancy forces governing the depth of penetration of the thermal bar plume shows that realistic salinity gradients are an important factor in determining the circulation of Baikal waters. Observations of the saline curtailment of the thermal bar's horizontal propagation also reveal a potential for reduced productivity in the ecosystem of any temperate river delta during the Spring renewal period.     

Wind selectivity of migratory flight departures in birds

Optimal migration theory predicts that birds minimizing the overall time of migration should adjust stopover duration with respect to the rate of fuel accumulation. Recent theoretical developments also take into account the wind situation and predict that there is a time window (a set of days) during which birds should depart when assisted by winds but will not do so if there are head winds. There is also a final day when birds will depart irrespective of wind conditions. Hence, the wind model of optimal migration theory predicts that birds should be sensitive to winds and that there should be a correlation between departures and winds blowing towards the intended migration direction. We tested this assumption by tracking the departures of radio-tagged passerines during autumn migration in southern Sweden. Our birds were moderately to very fat when released and therefore energetically ready for departure. There was a significant correlation between direction of departure and wind direction. We also found that during days when birds departed there was a significantly larger tail wind component than during days when birds were present but did not depart. Our results show that passerines do take the current wind situation into account when departing on migratory flights. We also briefly discuss possible clues that birds use when estimating wind direction and strength. The inclusion of wind is an important amendment to optimal migration theory of birds and should be explored further. Received: 1 March 1999 / Received in revised form: 4 October 1999 / Accepted: 16 October 1999     

珠江(广州河段)表层沉积物中重金属的分布特征及相关性研究

随着广州市截污工程的实施,珠江(广州河段)外源污染得到有效控制后,沉积物内源污染将成为影响河流水质的重要因素。在珠江(广州河段)表层沉积物中8种重金属质量分数监测的基础上,对该河段沉积物中重金属的沿程分布特征和横向分布特征进行了全面的分析,并对所测8种重金属之间的相关性进行了研究。结果表明:从沿程分布来看,这8种污染元素的分布特征基本相似,元素的高值区和低值区大致吻合。从横向分布来看,绝大多数污染物在一个断面的最高值和最低值出现的点位基本上是重合的,各种污染物质量分数的最高值绝大部分出现在靠近岸边的点位,并且大部分集中于左岸。而最低值出现在河道中间的比例较高。在所测定的8种元素中,除了Hg和As之外,其它各种元素之间均呈现较明显的正相关关系,特别是Cr和Cd、Cu和Cd、Cr和Cu、Cr和Ni之间的相关性显著。     

Modeling the impact of natural and anthropogenic nutrient sources on phytoplankton dynamics in a shallow coastal domain, Western Australia

The influence of different nutrient sources on the seasonal variation of nutrients and phytoplankton was assessed in the northern area of the Perth coastal margin, south–western Australia. This nearshore area is shallow, semi-enclosed by submerged reefs, oligotrophic, nitrogen-limited and receives sewage effluent via submerged outfalls. Analysis of 14 year of field observations showed seasonal variability in the concentration of dissolved inorganic nitrogen and phytoplankton biomass, measured as chlorophyll-a. For 2007–2008, we quantified dissolved inorganic nitrogen inputs from the main nutrient sources: superficial runoff, groundwater, wastewater treatment plant effluent, atmospheric deposition and exchange with surrounding coastal waters. We validated a three-dimensional hydrodynamic-ecological model and then used it to assess nutrient-phytoplankton dynamics. The model reproduced the temporal and spatial variations of nitrate and chlorophyll-a satisfactorily. Such variations were highly influenced by exchange through the open boundaries driven by the wind field. An alongshore (south–north) flow dominated the flux through the domain, with dissolved inorganic nitrogen annual mean net-exportation. Further, when compared with the input of runoff, the contributions from atmospheric-deposition, groundwater and wastewater effluent to the domain’s inorganic nitrogen annual balance were one, two and three orders of magnitude higher, respectively. Inputs through exchange with offshore waters were considerably larger than previous estimates. When the offshore boundary was forced with remote-sensed derived data, the simulated chlorophyll-a results were closer to the field measurements. Our comprehensive analysis demonstrates the strong influence that the atmosphere–water surface interactions and the offshore dynamics have on the nearshore ecosystem. The results suggest that any additional nutrient removal at the local wastewater treatment plant is not likely to extensively affect the seasonal variations of nutrients and chlorophyll-a. The approach used proved useful for improving the understanding of the coastal ecosystem.     

Transoceanic bird migration: Evidence for detection of wind direction

Summary Further analysis of radar tracks of migrating birds over the Atlantic Ocean (Larkin et al., 1979) indicates that birds sometimes fly at lower air speeds when winds are in the direction of flight than in crosswinds or head winds. The birds are able to estimate wind direction over open ocean by a mechanism which is presently open to speculation.