Artificial coastal dykes have been widely built to prevent erosion, or to accelerate sedimentation processes in saltmarshes. To investigate the sediment dynamic changes induced by construction of dykes, three observation stations were set around a dyke within an expanding Scirpus mariqueter saltmarsh, in southern Hangzhou Bay. The hydrodynamics of the study area was dominated by tidal currents. The presence of dyke, in combination with the adjacent tidal creek, altered the hydrodynamic boundary conditions, thereby affecting the flow field. When the tidal flows approaching the dyke, the speed was accelerated at the head of the dyke and then decreased at the rear, due to the narrowing effect of the dyke head. Turbulence intensity around the dyke showed a similar pattern to that of the tidal flows. The net sediment input behind the dyke is approximately 2.3 times of that at the dyke head, illustrating sediment deposition in the back region of the dyke. As a result, flows entrained the sediments and then deposited behind the dyke to cause a local accretion to promote saltmarsh vegetation succession. The construction of dykes should be taken into account for future coastal wetland management.

沉积物的活动性是指在一定时段内海底沉积物处于运动状态的时间占总时间的百分比。从全世界范围来看,长江是流域建坝蓄水拦沙导致入海沉积物通量锐减的典型案例。在此背景下,长江口及其邻近海域的沉积物活动性不仅关系到长江水下三角洲

Waves, currents, and related sediment transport are important factors driving the development of saltmarshes. Separating the effects of waves and currents accurately from simple field observations is a technical challenge in shallow water environments with limited inundation such as saltmarshes. In this paper, the estimation method of both wave and current information was studied mainly through the data obtained by a solely used field ADV (Acoustic Doppler Velocimeter, Vector). Phase and pressure inversion wave methods were both used to estimate wave parameters, which were then compared to synchronous observation by wave loggers at the front of a saltmarsh. Our results show that ADV is able to achieve simultaneous observations of tidal currents and waves independently. The pressure inversion wave method (r(2) similar to 0.996) is more effective than the phase method (r(2) similar to 0.876) in estimating wave energy when comparing with wave logger results. However, the former is more restricted by submergence depth and duration in field, while the latter provides cut-off frequencies for the pressure inversion wave method. Both methods can be combined to best estimate wave parameters from field ADV data. Further, hydrodynamic observation on a Scirpus mariqueter patch at the front of a saltmarsh was used as an application to indicate the importance of obtaining both wave and current information from field data. The patch was found to mainly reduce the advection of tidal currents, but it slightly increases wave energy. Therefore, it is necessary to consider the different effects of plants on tidal currents and waves simultaneously in field observations. This study confirms that ADV and associated analysis can detect waves and currents at reliable accuracy at the marsh edge, which is vital in assessing the long-term resilience of marshes to sea level rise and increased storm severity.

Continentalcollision zones have been proposed as primary sites of net continental crustal growth. Therefore, studies on syn-collisional granitoids with mafic magmatic enclaves (MMEs) are essential for testing this hypothesis. The Baojishan (BJS) and Qumushan (QMS) syn-collisional plutons in the North Qilian Orogen (NQO) on the northern margin of the Tibetan Plateau have abundant MMEs in sharp contact with host granitoids, sharing similar constituent minerals but with higher modal abundances of mafic minerals in MMEs. The QMS host granitoids have high Sr/Y and La/Yb ratios, showing adakitic compositions, which are differentfrom the BJS granitoids. Based on bulkrock compositions and zircon U-Pb age-dating, recent studies on these two plutons proposed that MMEs represent cumulates crystallized early from the same magmatic system as their host granitoids, and their parental melts are best understood as andesitic magmas produced by partial melting of the underthrusting upper ocean crust upon collision with some terrigenous sediments under amphibolite facies. Here, we focus on the trace-element geochemistry of the constituent mineral phases of both MMEs and their host granitoids of the QMS and BJS plutons. Weshow that different mineral phases preferentially host different trace elements; for example, most rare earth elements (REEs and Y) reside in titanite (only found in the QMS pluton), amphibole, apatite, epidote and zircon (mostly heavy-REEs); and high-field-strength elements (HFSEs) reside in biotite, titanite, amphibole and zircon. Based on the mineral chemical data, we show that for these two plutons, MMEs are of similar cumulate origin, crystallized from primitive andesitic melts in the early stage of granitoid magmatism. The primitive andesitic melts for these syn-collisional granitoids are most likely produced by the partial melting of the oceanic crust, supporting the hypothesis of continental crustal growth considering the syn-collisional granitoids represent juvenile continental crust. As evidenced by distinct mineral compositions, the two plutons have different parental magma compositions, for example higher TiO2 content and higher Sr/Y and La/Yb ratios in the QMS parental magmas, a signature best understood as being inherited from the source. The higher TiO2 content of the parental magma for the QMS pluton leads to the common presence of titanite in the QMS pluton (absent in the BJS pluton), crystallization of which in turn controls the trace-element (REE, Y, Nb, Ta and others) systematics in the residual melts towards an adakitic signature. Therefore, parental magmas with high TiO2 content and high Sr/Y and La/Yb ratios, as well as their further fractionation of titanite, are important factors in the development of adakitic compositions, as represented by the QMS host granitoids. This model offers a new perspective on the petrogenesis of adakitic rocks. The present study further demonstrates that, in general, mineral chemistry holds essential information for revealing the petrogenesis of granitoid rocks.

生物地貌学研究生物过程和地貌过程之间的双向交互作用,是一门新兴的交叉学科。早期的生物地貌学关注陆地生态系统,近期海岸带成为生物地貌学研究的热点地区,尤其是滨海湿地（如盐沼、红树林）成为研究生物地貌学过程和机理的重要区域。本文回顾了生物地貌学研究的概念、历史发展和方法,选取滨海湿地作为生物地貌学的主要研究对象,并就其研究方向、重要因素以及核心机制展开综述。最后对生物地貌学在海岸带生态系统修复工作中的应用进行梳理,分析了生物地貌学在红树林修复、滨海盐沼湿地修复和互花米草入侵防控上的应用展望。

A saltmarsh has developed rapidly on the mudflat of Andong Shoal, in southern Hangzhou Bay, over the last decade since embankment. The saltmarsh vegetation changes are driven by both sediment dynamic conditions and the competition between the exotic species Spartina alterniflora (S. alterniflora) and the native species Scirpus mariqueter (S. mariqueter). This study attempted to investigate large-scale spatial variations in the exotic and native species, by analyzing and interpreting a time series (2016–2018) of high-resolution (less than 1 m) remote sensing images. The total area of the saltmarsh increased at a rate of 1.07 km²/year, due to the accretion of the whole tidal flat. The spatial patterns revealed a new bimodal pattern for S. alterniflora invasion. S. alterniflora expanded over the upper to middle saltmarsh at a rate of 1.68 km²/year. However, the S. alterniflora patches at the seaward edge expanded at a negative rate of −0.005 km²/year, indicating a different pattern in competition: S. alterniflora had more advantages in the upper saltmarsh, while S. mariqueter had more advantages in the pioneer zone with low elevation. Consequently, S. alterniflora mainly established new habitats by invading S. mariqueter in the middle saltmarsh, but S. mariqueter occupied the bare mudflat for tradeoff. Our results aindicated that the interspecific competition result between S. alterniflora and S. mariqueter depended on elevation, and low elevation might create a favorable environment for S. mariqueter to win the competition with exotic species. This finding could be applied to future invasive species control and saltmarsh management.
© 2020 Hohai University

Current rehabilitation efforts to restore degrading mangroves mainly include afforestation with nursery grown plants. Hydrodynamic processes associated with sediment deposition and erosion play major roles in determining seedling establishment success. However, only a few detailed reports of the influence of seedlings on tidal flows are available. In situ measurements were conducted in this study to compare tidal flows between a restored mangrove flat covered by seedlings and a bare mudflat, located within Yueqing Bay, southeast China. High frequency 3D flow velocity variations and vegetation attributes were recorded on comparative sites to determine the effects of vegetation characteristics on flows at a small-scale. The vertical velocity profile displayed an S-shape distribution within the seedlings that was closely related to the vertical distribution of vegetation biomass. However, the turbulent kinetic energy within the seedlings varied minimally in the vertical direction, except for an abrupt increase at the top of the canopy. Comparing the mangrove seedling site and the bare mudflat site, the horizontal flow magnitude in the mangrove seedlings was reduced only limitedly from the mudflat, while the turbulence kinetic energy density had been noticeably reduced. Thus, mangrove seedlings were found to mainly modify the vertical velocity profile and the turbulent energy magnitude in terms of their influences on tidal flows. These findings are able to provide more detailed mechanisms for biogeomorphological modeling and further apply into mangrove restoration.

Limited studies have investigated the modification of tidal currents by mangrove trees. In particular, the impacts of mangrove trees on a vertical velocity profile remain unclear. An automatic system is developed to observe the vertical velocity profiles within a mangrove forest composed of artificially introduced Kandelia obovata located on the Ximen Island, the southern Zhejiang Province. The results reveal low flow velocities throughout the vertical profile within the mangrove forest. The vertical profile shows an overall decrease of the velocity with an increasing height above the bed. This pattern is due to the vertical increase of the drag force by the trunks and tree canopies. In addition, the turbulent energy density also varies vertically, corresponding to the vertical structure of the mangrove trees. In comparison with the vertical structure of flows within bare mudflats (semi-logarithmic) and salt marshes (nearly J-shape), the mangrove trees are shown to have a considerable impact on the vertical velocity profile in different ways.