Vertical Profile of PAHs and Dredging Depth of River Sediments

Sediment dredging is an effective engineering measure to reduce the negative effects of PAHs pollution on water environment. The dredging depth is a key parameter in environmental dredging engineering. A guidance of environmental dredging depth needed to reduce the toxicity risk was developed which was specifically designed for the removal of river sediments contaminated by PAHs based on the mean effect range median quotients (mean-ERM-q). The dredging depths for removing river sediments were calculated based on the vertical profile of PAHs content and the quality guidelines of sediment. Pinghu water system was carried out to determine the dredging depths of sediments from nine rivers by the proposed method. Each sediment core was collected from different river. The results showed the profiles of PAHs in sediment cores were irregular and diverse due to river dredging events and human activities in different periods. The risk assessment of PAHs toxicity showed one of nine cores with moderate-high probability, 4 of nine cores with low-moderate probability and the others with low toxicity risk. The achievement can offer a reference to the dredging engineering in other similar river systems.     

Short-term impact of blue mussel dredging (Mytilus edulis L.) on a benthic community

The short-term effect of mussel dredging in a brackish Danish sound was studied. A commercial dredging track was identified and an analysis of the species composition inside the track and at an adjacent control area showed that dredging changed the community structure by reducing the density of polychaetes. In order to investigate the extent and the duration of the dredging impact experimental dredging was conducted. The experimental dredging removed 50% of the mussels in two dredged areas. Immediately after dredging, a significantly lower number of species was measured inside the mussel beds in dredged areas compared to control and boundary areas. This effect lasted for at least 40 days. The analysis of the species composition showed that the dredged area had a significantly lower density, particularly of polychaetes compared to the boundary area. An increased number of species was recorded outside the mussel beds just after dredging, but this effect lasted for less than 7 days. After dredging, brown shrimps, C. crangon invaded the dredged areas. This species is an important predator of smaller invertebrates, and it is suspected that it was feeding on small vulnerable polychaetes exposed at the sediment surface after dredging. The dredging process was observed to form 2–5-cm deep furrows in the seabed, but the sediment texture and the organic content of the sediment was not affected. The biomass accumulation of individual blue mussels was significantly lower in the dredged area compared to the boundary area. This indicates that the disturbance of the mussel bed structure reduced growth and that the lowering of intraspecific food competition caused by a reduced density of mussels did not increase the accumulation of biomass in the mussels which remained in the dredged area.     

Detecting the effects of physical disturbance on benthic assemblages in a subtropical estuary: A Beyond BACI approach

The effects of dredging on the benthic communities in the Noosa River, a subtropical estuary in SE Queensland, Australia, were examined using a ‘Beyond BACI’ experimental design. Changes in the numbers and types of animals and characteristics of the sediments in response to dredging in the coarse sandy sediments near the mouth of the estuary were compared with those occurring naturally in two control regions. Samples were collected twice before and twice after the dredging operations, at multiple spatial scales, ranging from metres to kilometres. Significant effects from the dredging were detected on the abundance of some polychaetes and bivalves and two measures of diversity (numbers of polychaete families and total taxonomic richness). In addition, the dredging caused a significant increase in the diversity of sediment particle sizes found in the dredged region compared with elsewhere. Community composition in the dredged region was more similar to that in the control regions after dredging than before. Changes in the characteristics of the sedimentary environment as a result of the dredging appeared to lead to the benthic communities of the dredged region becoming more similar to those elsewhere in the estuary, so dredging in this system may have led to the loss or reduction in area of a specific type of habitat in the estuary with implications for overall patterns of biodiversity and ecosystem function.     

巢湖双桥河底泥疏浚过程中浮游植物功能群分类研究

浮游植物功能群分类分析可以精确地反映水体环境变化对水生生物群落的影响。为探究巢湖双桥河底泥疏浚工程对浮游植物群落及水生生态系统的影响,对2017年6月—2018年3月双桥河底泥疏浚期间的水体理化因子、浮游植物群落结构和功能群划分开展了研究。研究发现,底泥疏浚工程能够大量削减双桥河水体中的氮、磷营养盐,但疏浚后的双桥河仍处于中-富营养状态;双桥河浮游植物群落依据其生态功能可划分为M、H1、H2、MP、S1、L_O、X2、X1、J、F、N、G、X3、C、D、Y、W1、L_M、E、T、S_N、K、P等23个功能群,绝对优势功能群为以微囊藻为主的M功能群和以硅藻为主的MP、P功能群。底泥疏浚工程对双桥河浮游植物种类的影响不大,但藻类密度和生物量在疏浚后有显著地下降,浮游植物多样性指数也有提升。将浮游植物功能群生物量与环境因子进行冗余分析(RDA)和Pearson分析发现,水环境因子变化对双桥河浮游植物功能群产生了较大影响,双桥河浮游藻类优势功能群由底泥疏浚前的M、MP、P、W1、Y类转变为底泥疏浚后的C、F、J、M、MP、P、Y类。其...     

Dredging impact on the benthic community of an unaltered inlet in southern Spain

The impact of dredging on macrobenthic communities was studied in an unaltered zone, the Getares inlet of Algeciras Bay (SW Spain). The data obtained before, during and after dredging in a time series spanning 5 years revealed the re-establishment of directly affected communities and of physicochemical substrate characteristics within 1 month of the end of dredging, although 2 years later there was a confusing biological impoverishment of the whole inlet. After 4 years, there was a high degree of population re-establishment, both on the bottoms directly affected by the works and on neighbouring areas, that was partly due to the hydrodynamic conditions. Before this type of activity is undertaken, each case should be studied regarding viability, the environmental medium where it will take place, the best time of year, and the type of dredging to be used.Communicated by H.-D. Franke     

基于遥感技术的鄱阳湖采砂对水体透明度的影响

采砂是一项具有巨大生态影响的经济活动。其引起沉积泥沙的再悬浮,降低水体透明度,减弱水下光环境以及产生一系列的生态影响。旨在利用遥感技术监测鄱阳湖自2001年开始的采砂活动对水体透明度的影响。时间序列的IANDSAT影像用于船只的识别和同期的水体透明度反演,MODIS影像用于估计湖区2000—2005年6—10月期间水体透明度的动态变化。结果显示鄱阳湖北部自2001年以来随着船只数目的增加水体透明度显著下降。结合对船只最终目的地以及船只分布与水体透明度变化一致性等方面的分析,可以确认采砂是引起此区域水体透明度下降的主要原因。夏季频繁发生的长江江水倒灌推动鄱阳湖北部的浑浊水体向南移动,从而扩大采砂的影响范围至鄱阳湖中部,甚至南部。采砂繁荣了地方经济,同时也对鄱阳湖生态系统具有重大的负面影响。平衡经济发展与生态保护之间的关系,将因采砂而造成的负面影响下降到最低限度,使鄱阳湖的各种生态功能全面发挥是当务之急。     

Effects of dredging operations on soft bottom macrofauna in a harbor in the Patos Lagoon estuarine region of southern Brazil.

An evaluation was made of the effects of dredging on the soft bottom community in the channel of the Rio Grande harbor in the Patos Lagoon estuarine region of southern Brazil. During four seasonal cruises, samples were collected from nine biological stations, one of which was located outside the dredged area. Three macrobenthic samples were collected on each cruise from each station, using a 0.08 m2 van Veen grab. A cluster analysis was applied to group summer and autumn cruise stations before the dredging period, revealing larger numbers of species (35-36 spp.) and higher densities of organisms. The station located outside the dredging area was always included in this group, regardless of the sampling period or conditions of salinity. Another group comprised the winter and spring stations during the dredging period. This group was characterized by stations with the fewest species (20-18 spp.) and the lowest and most variable organism densities. The efficient strategies of resilience of the dominant estuarine species may minimize the effects of dredging on the biota through the rapid recolonization of the soft bottom of the harbor channel.     

Spatial Patterns in Water Quality Changes during Dredging in Tropical Environments

Dredging poses a potential risk to tropical ecosystems, especially in turbidity-sensitive environments such as coral reefs, filter feeding communities and seagrasses. There is little detailed observational time-series data on the spatial effects of dredging on turbidity and light and defining likely footprints is a fundamental task for impact prediction, the EIA process, and for designing monitoring projects when dredging is underway. It is also important for public perception of risks associated with dredging. Using an extensive collection of in situ water quality data (73 sites) from three recent large scale capital dredging programs in Australia, and which included extensive pre-dredging baseline data, we describe relationships with distance from dredging for a range of water quality metrics. Using a criterion to define a zone of potential impact of where the water quality value exceeds the 80^th percentile of the baseline value for turbidity-based metrics or the 20^th percentile for the light based metrics, effects were observed predominantly up to three km from dredging, but in one instance up to nearly 20 km. This upper (~20 km) limit was unusual and caused by a local oceanographic feature of consistent unidirectional flow during the project. Water quality loggers were located along the principal axis of this flow (from 200 m to 30 km) and provided the opportunity to develop a matrix of exposure based on running means calculated across multiple time periods (from hours to one month) and distance from the dredging, and summarized across a broad range of percentile values. This information can be used to more formally develop water quality thresholds for benthic organisms, such as corals, filter-feeders (e.g. sponges) and seagrasses in future laboratory- and field-based studies using environmentally realistic and relevant exposure scenarios, that may be used to further refine distance based analyses of impact, potentially further reducing the size of the dredging footprint.     

The Effects of Hydraulic Dredging to Control Oyster Drills on Benthic Macrofauna of Oyster Grounds in Delaware Bay,New Jersey

This study describes the extent and nature of the effects of hydraulic dredging to control oyster drills (Urosalpinx cinerea and Eupleura caudata, Family Muricidae, Order Neogastropoda) on benthic macrofauna and sediments of the oyster grounds in Delaware Bay, New Jersey. The immediate effects of hydraulic dredging were reductions in numbers of species as well as in total numbers of animals on the three oyster grounds selected. However, oyster drills were most affected. Benthic populations have recovered three to ten months after dredging. The sediments of the dredged grounds can be described as muddy sands. Immediately after dredging, additional mud was brought up from subsurface layers which reduced the median grain size on Ground 154 test plot. On Ground 515 test plot, however, there was a slight loss in the mud which increased the median grain size.     

Effects of dredging on critical ecological processes for marine invertebrates,seagrasses and macroalgae,and the potential for management with environmental windows using Western Australia as a case study

Dredging can have significant impacts on benthic marine organisms through mechanisms such as sedimentation and reduction in light availability as a result of increased suspension of sediments. Phototrophic marine organisms and those with limited mobility are particularly at risk from the effects of dredging. The potential impacts of dredging on benthic species depend on biological processes including feeding mechanism, mobility, life history characteristics (LHCs), stage of development and environmental conditions. Environmental windows (EWs) are a management technique in which dredging activities are permitted during specific periods throughout the year; avoiding periods of increased vulnerability for particular organisms in specific locations. In this review we identify these critical ecological processes for temperate and tropical marine benthic organisms; and examine if EWs could be used to mitigate dredging impacts using Western Australia (WA) as a case study. We examined LHCs for a range of marine taxa and identified, where possible, their vulnerability to dredging. Large gaps in knowledge exist for the timing of LHCs for major species of marine invertebrates, seagrasses and macroalgae, increasing uncertainty around their vulnerability to an increase in suspended sediments or light attenuation. We conclude that there is currently insufficient scientific basis to justify the adoption of generic EWs for dredging operations in WA for any group of organisms other than corals and possibly for temperate seagrasses. This is due to; 1) the temporal and spatial variation in the timing of known critical life history stages of different species; and 2) our current level of knowledge and understanding of the critical life history stages and characteristics for most taxa and for most areas being largely inadequate to justify any meaningful EW selection. As such, we suggest that EWs are only considered on a case-by-case basis to protect ecologically or economically important species for which sufficient location-specific information is available, with consideration of probable exposures associated with a given mode of dredging.     

鄱阳湖采砂南移扩大影响范围——多源遥感的证据

采砂是一项具有巨大生态影响的经济活动.利用多源遥感影像描述鄱阳湖中部的采砂分布,分析其对悬浮泥沙浓度的影响.2011年7月28日的Landsat Thematic Mapper (TM)5用于采砂相关船只的识别,2009-2011年7-8月的Terra卫星中分辨率成像光谱仪(MODIS)影像用于悬浮泥沙浓度的反演,(Before-after-control-impact,BACI)方法用于采砂影响评价.Landsat TM 5影像解译结果发现鄱阳湖中部的两个采砂区和90余艘船只,MODIS Terra反演结果显示在采砂区及其下游区域悬浮泥沙浓度剧增,BACI评价结果揭示采砂是导致此区域悬浮泥沙浓度增加的主要因素.平衡经济发展与生态保护之间的关系,将因采砂而造成的负面影响降到最低,使鄱阳湖的各种生态功能全面发挥是紧要的.     

Comparison of the Long-Term Risks of Removal and In Situ Management of Contaminated Sediments in the Fox River

The long-term potential risks of environmental dredging vs. in situ contaminated sediment management practices are discussed and compared for the Lower Fox River, Wisconsin. The risks are identified as being largely associated with the residual sediment contamination associated with either approach. The integral of the surface area-weighted average contaminant concentration in surface sediment is proposed as a metric to compare these risks. Capping is shown to exhibit significantly reduced exposure and risk relative to the dredging scenarios, even if potential undetected erosion of 5% of the cap is considered. Even with the improbable event of undetected failure of 25% of the cap, the exposure and risk associated with capping is approximately equal to or below all dredging scenarios. A preference for dredging due to the perception that it eliminates the long-term risk of in situ capping is not supported by this analysis. Although strictly applicable only to the Lower Fox River, the results suggest sitespecific analyses must be conducted to determine which sediment management approaches minimize the potential for long-term exposure and risk.     

Temporal Patterns in Seawater Quality from Dredging in Tropical Environments

Maintenance and capital dredging represents a potential risk to tropical environments, especially in turbidity-sensitive environments such as coral reefs. There is little detailed, published observational time-series data that quantifies how dredging affects seawater quality conditions temporally and spatially. This information is needed to test realistic exposure scenarios to better understand the seawater-quality implications of dredging and ultimately to better predict and manage impacts of future projects. Using data from three recent major capital dredging programs in North Western Australia, the extent and duration of natural (baseline) and dredging-related turbidity events are described over periods ranging from hours to weeks. Very close to dredging i.e. <500 m distance, a characteristic features of these particular case studies was high temporal variability. Over several hours suspended sediment concentrations (SSCs) can range from 100–500 mg L-1. Less turbid conditions (10–80 mg L-1) can persist over several days but over longer periods (weeks to months) averages were <10 mg L-1. During turbidity events all benthic light was sometimes extinguished, even in the shallow reefal environment, however a much more common feature was very low light ‘caliginous’ or daytime twilight periods. Compared to pre-dredging conditions, dredging increased the intensity, duration and frequency of the turbidity events by 10-, 5- and 3-fold respectively (at sites <500 m from dredging). However, when averaged across the entire dredging period of 80–180 weeks, turbidity values only increased by 2–3 fold above pre-dredging levels. Similarly, the upper percentile values (e.g., P99, P95) of seawater quality parameters can be highly elevated over short periods, but converge to values only marginally above baseline states over longer periods. Dredging in these studies altered the overall probability density distribution, increasing the frequency of extreme values. As such, attempts to understand the potential biological impacts must consider impacts across telescoping-time frames and changes to extreme conditions in addition to comparing central tendency (mean/median). An analysis technique to capture the entire range of likely conditions over time-frames from hours to weeks is described using a running means/percentile approach.     

Using remote sensing to quantify fishing effort and predict shorebird conflicts in an intertidal fishery

Accurate estimates of fishing effort are necessary in order to assess interactions with the wider ecosystem and for defining and implementing appropriate management. In intertidal and inshore fisheries in which vessel monitoring systems (VMS) or logbook programmes may not be implemented, quantifying the distribution and intensity of fishing can be difficult. The most obvious effects of bottom-contact fishing are often physical changes to the habitat, such as scarring of the sediment following dredging or trawling. We explored the potential of applying remote sensing techniques to aerial imagery collected by an unmanned aerial vehicle, or drone, in an area of intertidal mud flat (0.52 km²) in Poole Harbour, UK, where shellfish dredging is widely carried out and conflicts between commercial fishing interests and the conservation of internationally important shorebird populations are a concern. Image classification and image texture analysis were performed on imagery collected during the open dredge season in November 2015, in order to calculate measures of fishing intensity across three areas of the harbour subject to different management measures. We found a significant correlation between results of the image texture analysis and official sightings records collected during the dredging season, indicating that this method most accurately quantified dredging disturbance. The relationship between shorebird densities and food intake rates and the results of this analysis method were then investigated to assess the potential for using remotely sensed measures of fishing effort to assess responses of overwintering shorebird populations to intertidal shellfish dredging. Our work highlights the application of such methods, providing a low-cost tool for quantifying fishing effort and predicting wildlife conflicts.     

Versatile habitat conditioning by damselfish cultivating turf algae on giant clams

In this paper, a monitoring and modelling concept for ecological optimized harbour dredging and fine sediment disposal in large rivers is presented. According to the concept, first a preliminary assessment should be performed previous to the dredging and dumping procedure to derive knowledge about the current status in hydrodynamics, morphology and instream habitat quality. During the performance of the maintenance work, a high-resolution monitoring program has to be organized to measure flow velocities, the suspended sediment concentrations and the extent of the occurring plume. These data can then be compared with natural suspended sediment conditions and serve as input data for numerical sediment transport modelling. Furthermore, bathymetric surveys and biotic sampling enable the detection of possible effects of dredging and disposal in the post-dumping stage. Based on sediment transport modelling approaches, short- to mid-term developments of the sediment plume can be predicted with an additional and final habitat evaluation at the end of the project. This concept was applied and optimized during the maintenance work at the case study winter harbour Linz at the Danube River. The findings of the presented study highlight the necessity of integrated monitoring and modelling approaches for harbour dredging especially in large river systems.     

Spatial heterogeneity of spring phytoplankton in a large tropical reservoir: could mass effect homogenize the heterogeneity by species sorting?

Dredging Austrian Danube harbours becomes necessary to keep the navigation channel free from bed sediment and to insure safe navigation, especially after major floods. In most cases, the excavated substrate is deposited in the free-flowing river section in the vicinity of the mouth of a harbour. The effects of harbour dredging on the biota need to be studied to minimize possible outcomes that may endanger the ecological status of the river system in order to implement the aims of the European Water Framework Directive. This study presents the first quantitative information on the colonisation of benthic invertebrates with respect to Danube harbours. One sampling at Linz Harbour was conducted before dredging and four samplings were carried out after dredging. Four transects were documented in the affected river section. The study used an air-lift sampler and a grab sampler to investigate if and to what extent the benthic invertebrate community is disturbed. It found that the sediment dredging heavily affected the benthic invertebrates (decline of 82% of biomass), while the dumping of the material had no effects on the benthic Danube biota. As a rough estimate (conferring to a simple trend line analysis) the harbour bottom fauna would recover after 235 days.     

Ditches as species‐rich secondary habitats and refuge for meadow species in agricultural marsh grasslands

Questions

Can drainage ditches in agricultural marsh grassland provide a suitable habitat for the persistence of fen meadow species? How does the ditch margin vegetation develop as a function of regular dredging? Is ornithologically oriented management also beneficial for plant biodiversity?

Location

Riparian marshes, Eider‐Treene‐Sorge lowland, Schleswig‐Holstein, Germany.

Methods

We performed vegetation surveys of drainage ditches along with their water body, slope and margin structures annually for 3 years. The data were analysed with respect to date and means of ditch dredging. In addition, we recorded vegetation of the surrounding agricultural grassland, measured nutrient status of the soil and the water body and sampled seed bank of the ditch slopes. We used ANOVA and multivariate methods to describe the development of the ditch vegetation and the persistence of target meadow species.

Results

Vegetation re‐development of ditch margins proceeds quite rapidly after disturbance from dredging. Dominance of mudbank species was observed only in the first year, followed by an increase of reed species and reduction of phytodiversity. Target species of wet meadow communities reach highest abundance in the second and third year and build a significant seed bank before being suppressed by reeds.

Conclusions

In heavily eutrophicated, intensively used marsh grassland, regularly disturbed ditch margins are important secondary habitats for pioneer and subdominant wetland species, which have nearly disappeared in a larger area. Current management cycles of ditch dredging every 3–4 years comply with the successional development, allowing the mudbank and wet meadow species to persist in the vegetation and seed bank. In contrast to the frequency, the form of dredging (ditch profile), which is crucial for bird protection, plays a minor role for plants. We recommend moderate disturbance (mowing of ditch margins) to suppress strong competitors in the years between dredging for additional support to target plant species.     

The negative effect of dredging and dumping on shorebirds at a coastal wetland in northern Spain

Dredging and/or dumping actions at coastal environments are a common phenomenon worldwide. The re-working of dumped sediments from their disposal sites to places of great ecological value can have a very strong impact on the ecosystems through deep changes over the communities and the trophic web. Using a relevant dredging-dumping episode carried out in 2003 at Urdaibai, one the chief estuary areas in northern Iberia, we tested the consequence of this action on the subsequent use of the zone by shorebirds. The surface sediment characteristics before and after the dredging and dumping actions were also compared. The dredging at Urdaibai showed a negative effect on bird abundance in three out of the eight species tested overall (dunlin, grey plover, common ringed plover). Highest-ranked models supported a decrease in their population sizes two years after the event. In this scenario, local authorities should be appealed to take dredging and dumping effects into account in order to improve the estuary management.     

Effects of ditch management on caddisfly, dragonfly and amphibian larvae in intensively farmed peat areas

Conservation of natural values within farming practice is growing rapidly within the Netherlands. The focus is primarily on terrestrial flora and fauna such as the vegetation in ditch banks and meadow birds. Knowledge needed to enhance biodiversity in ditches is limited. Therefore, a field study was set up to determine the effects of dredging, ditch cleaning and nutrient supply in the adjacent fields on caddisfly, dragonfly and amphibian larvae in the ditches in a peat area.Two-hundred forty ditches spread over 84 dairy farms were selected to determine the individual effect of several management aspects. Generalised linear modelling was used as a tool to detect the most relevant aspects and to obtain quantitative relations with the chance of the larvae being present.Dredging had an impact on the presence of all larvae types. The type of dredging machine, the dredging period, the water depth and the frequency of dredging can influence the presence of the larvae. The presence of caddisfly larvae was also affected by the cleaning machine and period and by the P supply in the adjacent field. The presence of amphibian larvae was also affected by the cleaning period.Measures that will enhance the presence of the larvae are formulated. Options for water boards and other government authorities to stimulate farmers to take these measures are given.     

Species composition and ecologo-phytocenotic spectra of vascular plants, mosses, and lichens on gold-mining sites in the taiga zone of the lower Amur River basin, the Russian Far East

Two-hundred-three species of vascular plants, 50 species of bryophyte and 41 species of lichens have been found on dredging and hydraulic gold-mining sites in the taiga zone of the lower Amur River basin. The main features of the anthropogenic relief and vegetation have been described. Dredging and hydraulic dumps are supposed to have the biggest similarity in the species composition and set of ecologo-phytocenotic elements, while the biggest difference is observed between dredging and hydraulic dumps on the one hand and pockets and settling pits, on the other. According to the data obtained in different regions, the species diversity of vascular plants growing on dredging and hydraulic gold-mining sites is quite rich.