The benthic macrofauna of the St. Lucia Estuary during the 2005 drought year

The St. Lucia Estuary is the largest estuarine system in Africa. The estuary is part of the Greater St. Lucia Wetland Park, which has been declared a World Heritage Site. This ecosystem has been subjected to severe drought conditions over the last four to five years, resulting in its mouth being closed off from the ocean in June 2002 for a period of over four years. The main aim of this study was to document the effects of the prevailing drought on the macrofauna of the system, since the last work on this benthic component had been undertaken over a decade ago, during a normal-to-wet phase. Macrofauna samples together with physico-chemical data were collected at representative sites in the Narrows, and the South and North lakes in February, April, August and October 2005. The drought exerted a strong influence on the system, leading to hypersaline conditions developing in its northern regions (maximum of 126 at Hell's Gate), and to the loss of aquatic habitat. Ordinations and clustering indicated that the macrofauna of the system could generally be separated into three clusters viz. (1) the Narrows and the southern portion of South Lake, (2) the northern half of South Lake, and (3) the North Lake–False Bay complex. Multivariate correlations indicated weak relationships between macrofaunal community structure and physico-chemical parameters. The distinction in macrofaunal assemblages between these clusters was probably caused by these habitats being physically separated at the peak of the drought, with no water flow between them, thereby preventing exchange of planktonic larvae and retarding colonisation of habitats. There was a northward decline in taxonomic richness and diversity of macrofauna in the system, which correlated positively with water depth and negatively with the biomass of microphytobenthos. It is evident that the drought structured macrofauna communities primarily through its effects on water depth and habitat fragmentation. The results of this investigation provide valuable information regarding the effects of droughts on estuarine–lake systems and the possible mechanisms by which they occur.     

珠江磨刀门河口潮波振幅梯度与上下游动力边界的关系异变研究

强人类活动目前已经成为河口演变的主要动力。阐明流量驱动下河控型河口潮波传播演变过程及机制,对河口治理及人类活动的影响评价具有重要指导意义。以珠江磨刀门河口为例,研究了径潮动力阶段性演变特征。采用流量驱动的R_TIDE数据驱动模型探究了该区潮波振幅梯度和上下游动力边界(即上游流量和口门振幅)关系的变化规律。结果表明,在强人类活动影响下,各潮位站M₂分潮振幅明显上升(三灶站除外),且具有季节性差异和阶段性变化,灯笼山-马口河段的M₂分潮振幅沿程平均增大约0.07 m,河段潮波振幅梯度平均增大约4.61×10^–6 m^–1。研究潮波振幅梯度与上下游动力边界的阈值的关系表明,阈值效应主要出现在甘竹-马口河段。在强人类活动影响下,潮波振幅梯度阈值增大,相应的流量阈值增大,而振幅阈值基本不变。在达到振幅阈值之前,由于底床摩擦效应,大潮振幅衰减效应大于小潮,而在超过振幅阈值后,地形辐聚效应成为影响潮波振幅梯度变化的主要因素,大潮振幅衰减效应小于小潮。阈值的变化主要与流量、地形、摩擦等多因子耦合作用有关,当地形辐聚效应和底床摩擦效应达到平衡时,潮波振幅梯度与上下游动力边界之间则出现阈值效应。该现象的发现可为河口海岸防灾减灾和水资源管理等实际问题提供重要理论支撑。     

The dynamics of a bathymetrically arrested estuarine front

The mixing and circulation associated with a bathymetrically arrested estuarine front was studied using hydrographic and current data. A quasisteady front, exhibiting strongly convergent surface flows, is formed along the steeply sloping inner margins of the flood tide delta during each semidiurnal tide cycle. This front separates the brackish ambient water within a deep estuarine basin from the incoming oceanic tidal water. The position of the front is dependent on the local water depth and the difference in density between the two water masses. Beneath the surface there is an inclined frontal interface where static stability is very low and vertical mixing intense. A vertically integrated horizontal momentum equation was derived for flow in the upper layer and an estimate made as to the value of the associated entrainment coefficient.     

Vertical distribution of first stage larvae of the blue crab,Callinectes sapidus,at the mouth of Chesapeake Bay

The vertical distribution of stage I blue crab larvae, near the mouth of Chesapeake Bay, was examined over four diurnal cycles. Each of two stations was occupied for 30 hours twice during the summer of 1979.On each of the four cruises, peak larval abundance occurred after a night time high slack tide, suggesting a synchronized hatch of blue crab larvae. 90–99% of all larvae collected were taken in the neuston layer. The apparent timing of the hatches to coincide with the beginning of an ebb tide and the concentration of larvae in the neuston layer strongly suggests seaward transport of these early stage larvae and the probability of offshore development.     

Simulation of the Fate of Faecal Bacteria in Estuarine and Coastal Waters Based on A Fractionated Sediment Transport Model

A two-dimensional depth-integrated numerical model is refined in this paper to simulate the hydrodynamics, graded sediment transport process and the fate of faecal bacteria in estuarine and coastal waters. The sediment mixture is divided into several fractions according to the grain size. A bed evolution model is adopted to simulate the processes of the bed elevation change and sediment grain size sorting. The faecal bacteria transport equation includes enhanced source and sink terms to represent bacterial kinetic transformation and disappearance or reappearance due to sediment deposition or re-suspension. A novel partition ratio and dynamic decay rates of faecal bacteria are adopted in the numerical model. The model has been applied to the turbid water environment in the Bristol Channel and Severn estuary, UK. The predictions by the present model are compared with field data and those by non-fractionated model.     

A Model Simulation of Future Oceanic Conditions along the British Columbia Continental Shelf. Part II: Results and Analyses

An ocean circulation model for the British Columbia continental shelf is run with future initial conditions and forcing fields downscaled from the North American Regional Climate Change Assessment Program archive. Average seasonal sea surface temperatures for the period 2065 to 2078 are projected to increase by between 0.5° and 2.0°C with respect to analogous averages from 1995 to 2008. Seasonal sea surface salinities are projected to decrease by as much as 2.0 over the same period, though there are some regions where and periods when small increases are projected. Though stronger winter winds result in larger Haida Eddies, slightly stronger summer winds along the western Vancouver Island shelf do not result in appreciable changes to either the cross-shelf upwelling or to the magnitude of Juan de Fuca Eddies or the timing of their formation. However, increased flows are projected in some seasons for the Rose Spit, Middle Bank, and Goose Island Bank eddies. More precipitation over the watersheds emptying into coastal waters produces larger freshwater discharges and, in particular, a stronger estuarine flow in Juan de Fuca Strait and a stronger Vancouver Island Coastal Current. Generally increasing winds and decreasing density mean that the winter minus summer range of sea surface heights is projected to increase all along the coast.     

利用粗糙集理论进行遥感分类信息提取

以辽宁省双台子河口湿地为研究对象,以Landsat 8和HJ-1-A/HJ-1-B的多时相遥感影像为数据源,根据研究区现状,将研究区分为旱地、芦苇、水田、碱蓬、混合植被、水面、滩涂、居民点、养殖塘九个类型.利用时间序列的归一化植被指数提取植被与非植被的分类阈值,采用粗糙集理论和多时相遥感影像,对植被和非植被分别进行分类规则的获取,建立了研究区决策树分类模型.为了进行精度评价,利用相同的训练点又进行了同样基于像元的最大似然法分类.最后利用混淆矩阵对上述两种方法进行了精度评估,基于粗糙集的决策树分类法与最大似然法总体分类精度分别为93.70%和91.62%,Kappa系数分别为0.92和0.90,两项指标值基于粗糙集理论法均比最大似然法有所提高.这为构建决策树分类模型进行湿地地表分类信息提取提供了一条新的研究思路.