Experimental investigation of the harmonic generation by waves over a submerged plate

Experiments in a wave flume have been performed to analyse the nonlinear interaction between regular gravity waves and a submerged horizontal plate used as breakwater. A new method, based on the Doppler shift generated by a moving probes, has been used to discriminate the incident fundamental mode and the reflected fundamental mode. The relationships of the reflection and transmission coefficients to the wave number at different submergence depth ratios are presented. The accurate discrimination, by this method, of the phase-locked and free modes allows the quantification of the higher harmonics generated by the breakwater and the analysis of the nonlinear interaction between the waves and the submerged plate. The transfer of energy from the fundamental mode to higher harmonics is very large in the cases of small submergence depth ratios. The vortices produced at the edges take part in the production of higher harmonics by interaction with the free surface but involve, at the same time, a dissipation process that increases the efficiency of the breakwater.     

Mapping Submerged Aquatic Vegetation with GIS in the Caloosahatchee Estuary: Evaluation of Different Interpolation Methods

This article evaluates different spatial interpolation methods for mapping submerged aquatic vegetation (SAV) in the Caloosahatchee Estuary, Florida. Data used for interpolation were collected by the Submersed Aquatic Vegetation Early Warning System (SAVEWS). The system consists of hydro-acoustic equipment, which operates from a slow-moving boat and records bottom depth, seagrass height, and seagrass density. This information is coupled with geographic location coordinates from a Global Positioning System (GPS) and stored together in digital files, representing SAV status at points along transect lines. Adequate spatial interpolation is needed to present the SAV information, including density, height, and water depth, as spatially continuous data for mapping and for comparison between seasons and years. Interpolation methods examined in this study include ordinary kriging with five different semivariance models combined with a variable number of neighboring points, the inverse distance weighted (IDW) method with different parameters, and the triangulated irregular network (TIN) method with linear and quintic options. Interpolation results were compared with survey data at selected calibration transects to examine the suitability of different interpolation methods. Suitability was quantified by the determination coefficient (R2) and the root-mean-square error (RMSE) between interpolated and observed values. The most suitable interpolation method was identified as the one yielding the highest R2 value and/or the lowest RMSE value. For different geographic conditions, seasons, and SAV parameters, different interpolation methods were recommended. This study identified that kriging was more suitable than the IDW or TIN method for spatial interpolation of all SAV parameters measured. It also suggested that transect data with irregular spatial distribution patterns such as SAV parameters are sensitive to interpolation methods. An inappropriate interpolation method such as TIN can lead to erroneous spatial representation of the SAV status. With a functional geographic system and adequate computing power, the evaluation and selection of interpolation methods can be automated and quantitative, leading to a more efficient and accurate decision.     

伊乐藻等几种沉水植物的生物量和生产量测定以及竞争态势试验

对1993—1995年在东太湖边池塘中进行的伊乐藻等几种沉水植物的生物量、生产量测定以及竞争态势试验结果进行了分析,阐明了因收割方法不同,水草的单位面积产量及营养物质量不同;并以中位刈割方法为最佳;在竞争试验的环境条件下伊乐藻是优势种。     

内蒙古河套灌区3种沉水植物种子休眠及萌发特性

沉水植物在湖泊生态系统中具有重要作用,它不仅是湖泊生态系统食物链中重要的生产者,同时还对湖泊的营养物沉积和循环有着重要的意义,并由此影响着湖泊的富营养化进程.本文以黄河内蒙古段河套灌区湖泊中常见的3种沉水植物篦齿眼子菜(Potamogeton pectinatus)、穿叶眼子菜(Potamogeton perfoliatus)和狐尾藻(Myriophyllum verticillatum)为研究对象,对其种子休眠及萌发特性进行了初步研究.结果表明:狐尾藻种子休眠率较高但休眠易破除,通过切破种皮、低温层积、硝酸钾和赤霉素处理均可显著提高萌发率;篦齿眼子菜种子休眠性较强,低温层积及赤霉素浸泡处理能够显著提高其萌发率;穿叶眼子菜有较深的休眠特性,破除硬实、硝酸钾、赤霉素及低温层积处理均不能有效解除其休眠.     

超富营养城市湖泊原位生态修复效果分析——以武汉南湖为例

城市湖泊普遍面临富营养化问题,许多湖泊甚至处于超富营养状态。鉴于其在供水、气候调节、景观提升等方面的重要作用,以水质提升为目标的生态修复对于恢复湖泊生态功能、提升城市景观等具有重要意义。文章以超富营养城市湖泊——武汉南湖为例,尝试通过围隔构建、鱼类清除、水生植被重建等生态措施改善示范区水质,探究超富营养湖泊生态修复技术。于2019年8月至12月在示范区进行实验。结果表明：（1）示范区修复措施实施后,挺水植物植株高度显著增加（P<0.01）,芦苇平均株高由0.7 m增加至1.2 m,香蒲的新生叶片在种植后50 d高达1.4 m,二者在种植后90 d,覆盖率均由种植时的10%上升至70%;（2）示范区内透明度由修复前的0.26 m上升至0.77 m,显著高于外湖区（0.26 m）（P<0.01）;（3）示范区内浮游藻类叶绿素a（chl a）由修复前38.90 μg/L降低至3.41 μg/L,显著低于外湖区（101.91μg/L）（P<0.01）;（4）示范区总氮（TN）由4.78 mg/L降低至2.60 mg/L,总磷（TP）由0.76 mg/L降低至0.08 mg/L,二者均略低于外湖区,但均不显著（P>0.05）。总体来看,经过一系列修复措施后,示范区内水生植被得到很好的恢复,水质条件得到一定的改善：透明度提升较多,chl a下降明显。氮、磷等营养水平虽然也有所降低,但与外湖区差异不大,可能是沉积物内源营养释放导致。研究表明,围隔构建、鱼类清除、水生植被重建等生态修复措施虽然可在一定程度上改善富营养湖泊水质状况,但未能有效控制内源负荷,仍需采取底质改良措施以达到降低水体营养含量的目的。相关研究结果可为城市超富营养湖泊生态修复及水质提升提供一定的借鉴。     

改性粘土辅助沉水植物修复技术维持清水稳态的原位研究

在富营养湖泊治理实践中,修复沉水植被被认为是改善水质的长效措施,而壳聚糖改性粘土是短期快速改善水质的有效手段.本研究利用改性粘土辅助沉水植被修复,旨在探索改善水质的长效方案.2011年5-11月在太湖梅梁湾开展了四组不同处理(对照、水草、水草+粘土、粘土)围隔实验,在水草(盖度13.0%)和水草+粘土(盖度52.3%)围隔中不同程度重建了苦草群落.实验期内每3 d一次的水质监测表明,粘土处理可显著改善水质,水体总氮(TN)、总磷(TP)、正磷酸盐(PO34--P)和叶绿素a(Chl.a)含量分别比对照下降了20.7%、74.6%、31.0%和80.4%,透明度(SD)升高了90.4%;粘土辅助植被修复改善水质效果最长稳,水体TN、TP、PO34--P和Chl.a含量分别比对照下降了36.2%、64.0%、28.6%和71.1%,SD升高了76.4%;低盖度苦草群落单独处理对水质改善效果不显著.在三种处理中,粘土辅助植被修复改善底质效果最好,使间隙水的TN、TP、PO34--P、NH4+-N分别比实验前下降了15.6%、61.7%、55.8%和82.8%.本研究表明改性粘土辅助沉水植被修复可作为重富营养水体中水质改善的整合技术,但其长期生态效应仍需谨慎评估.     

Nitrifying and Denitrifying Bacteria in Epiphytic Communities of Submerged Macrophytes in a Treated Sewage Channel

A quantification of nitrifying and denitrifying bacteria present in different compartments (water, sediments, submerged macrophytes) of a treated sewage channel was made to estimate their influence on the nitrogen balance and to assess the significance of macrophytes for nitrification and nitrogen conversions in general. Considerable numbers of autotrophic and heterotrophic nitrifying and denitrifiying bacteria were found to be present in the epiphytic communities of different species of submerged macrophytes of a treated sewage channel. Comparing the influence of the different compartments on total stream nitrification and denitrification it could be concluded that dense beds of submerged macrophytes particularly positively influence nitrification. Epiphytic nitrifiers were estimated to be as important for the total nitrification as nitrifiers in the sediment. Denitrification was mainly taking place in the sediment. The influence of the suspended nitrifiers and denitrifiers on the nitrogen balance was assumed to be negligible.     

湖泊生态恢复的关键因子分析

中国是一个多湖泊的国家。由于经济快速发展及湖泊资源不合理的开发利用,中国湖泊的污染问题和生态系统退化相当普遍。特别是由于氮、磷等营养元素的富集造成的水体富营养化,导致蓝藻水华频繁发生,甚至出现了饮用水危机事件。由于缺乏基础理论的指导,中国湖泊富营养化治理曾经走过弯路。在没有实现控源截污的条件下,片面强调生态恢复来净化湖泊水环境,一度成为富营养化湖泊治理的主流思想。实际上,湖泊生态恢复是有条件的,而对这些条件的诊断和分析是开展湖泊生态恢复的前提和基础。通过对太湖水生植物分布及其影响因子分析,确定沉水植物恢复的核心条件是水下光照条件。水下光照条件受富营养水平、悬浮物浓度与水深等因子的影响。只有当一个水域的真光层深度接近水深的情况(比值>0.8),恢复水生植物才有可能。改善水下光照条件,包括降低水深,提高透明度,消除风浪等措施,实际上,都是增加真光层深度与水深的比值。在上述生态恢复条件不具备的情况下,湖泊治理与恢复的工作更多地应该聚焦在控源截污方面。这对中国湖泊污染治理与生态恢复具有普遍的意义。     

Macrophyte management: An integrated perspective

A concept of aquatic macrophyte management that integrates the positive and negative aspects of vegetation in lakes and rivers is discussed. This integrated approach involves three factors: macrophyte control, macrophyte enhancement, and identification and resolution of the conflicts created by multiple use of a waterbody. The primary decision in macrophyte management programmes must be whether to optimise for single‐purpose or for multipurpose use of the waterbody. Both technical (macrophyte control and enhancement) and social (conflict resolution) procedures are required to solve problems associated with the macrophyte status of multipurpose waterbodies.