Effect of sediment depth and sediment type on the survival of Vallisneria americana Michx grown from tubers

Sedimentation resulting from storms may have been one of the reasons for the elimination of submersed aquatic vegetation from the tidal Potomac River in the late 1930's. Laboratory studies were conducted to investigate the effects of different depths of overlying sediment and composition of sediment on the survival of Vallisneria americana Michx (wildcelery) grown from tubers. Survival of plants grown from tubers decreased significantly with increasing sediment depth. Survival of tubers declined from 90% or more when buried in 10 cm to no survival in greater than 25 cm of sediment. Survival with depth in sand was significantly lower than in silty clay.Field investigation determined that the majority of tubers in Vallisneria beds are distributed between 10 and 20 cm in depth in silty clay and between 5 and 15 cm in depth in sand. Based on the field distribution of tubers and on the percent survival of plants growing from tubers at each depth in the laboratory experiment, we suggest that the deposition of 10 cm or more of sediment by severe storms such as occurred in the 1930s could contribute to the loss of vegetation in the tidal Potomac River.     

The effects of grazers and light penetration on the survival of transplants of Vallisneria americana Michs in the tidal Potomac River,Maryland

Poor light penetration and grazing are among the factors potentially responsible for the lack of submersed aquatic macrophytes in the tidal Potomac River. Between 1980 and 1983, plugs, springs and tubers of Vallisneria americana Michx were transplanted from the oligohaline Potomac Estuary to six sites in the freshwater tidal Potomac River. Transplants made in 1980 and 1981 were generally successful only when protected by full exclosures which prevented grazing. Grazing resulted in the removal of whole plants or clipping off of plant leaves in unprotected plots. Plants protected in the first year were permanently established, despite the occurrence of grazing in subsequent years, at Elodea Cove and Rosier Bluff, where light penetration was high (average 1% light level was 1.6–1.7 m). Plants were not permanent;y established at Goose Island, where light penetration was lower (average 1% light level was 1.4 m) and grazing occurred, or Neabsco Bay where light penetration was very low (average 1% light level was 1.0 m) and grazing may not have occurred. In 1983, Secchi depth transparencies in the upper tidal river were improved significantly compared to 1978–1981. Both protected and unprotected transplants thrived in 1983.     

Vegetation and seed bank dynamics in a tidal freshwater marsh

Questions: What are the feedbacks among the seed bank, parent vegetation, and landscape structure that control plant species turnover in space and time in a tidal freshwater marsh? How can these feedbacks be used to better understand marsh community dynamics and to establish restoration practices that seek to restore vegetation diversity of this important and widely distributed ecosystem? Location: Potomac River, Virginia, United States (15 km south of Washington, DC). Methods: We sampled the seed bank and standing vegetation in a tidal freshwater marsh and explored similarities between seed bank and vegetation composition through space and time. We then investigated marsh surface elevation, distance to nearest tidal channels, and life history of component species as potential explanations for the observed vegetation patterns. Results: The composition of individual plots changed considerably from year to year; however, the composition at broader spatial (whole marsh) and temporal (4 years) scales was relatively stable. Species composition of the seed bank was dissimilar to both the previous and current year's standing vegetation, and similarity to standing vegetation was particularly low in plots dominated by annual species. Landscape structure and life history characteristics of individual species best explained the spatiotemporal variability in marsh vegetation. Conclusions: Restoration designs should be landscape‐dependent and explicitly incorporate spatially structured elements such as elevation gradients to maximize community diversity in reconstructed tidal freshwater marshes. Optimal designs include areas of high seed input, areas of high species turnover, as well as other areas of greater stability.     

Benthic phosphorus regeneration in the Potomac River Estuary

The flux of dissolved reactive phosphate from Potomac riverine and estuarine sediments is controlled by processes occurring at the water-sediment interface and within surficial sediment.In situ benthic fluxes (0.1 to 2.0 mmoles m⁻² day⁻¹) are generally five to ten times higher than calculated diffusive fluxes (0.020 to 0.30 mmoles m⁻² day⁻¹). The discrepancy between the two flux estimates is greatest in the transition zone (river mile 50 to 70) and is attributd to macrofaunal irrigation. Bothin situ and diffusive fluxes of dissolved reactive phosphate from Potomac tidal river sediments are low while those from anoxic lower estuarine sediments are high. The net accumulation rate of phosphorus in benthic sediment exhibits an inverse pattern. Thus a large fraction of phosphorus is retained by Potomac tidal river sediments, which contain a surficial oxidized layer and oligochaete worms tolerant of low oxygen conditions, and a large fraction of phosphorus is released from anoxic lower estuary sediments. Tidal river sediment pore waters are in equilibrium with amorphous Fe (OH)₃ while lower estuary pore waters are significantly undersaturated with respect to this phase. Benthic regeneration of dissolved reactive phosphorus is sufficient to supply all the phosphorus requirements for net primary production in the lower tidal river and transition-zone waters of the Potomac River Estuary. Benthic regeneration supplies approximately 25% as much phosphorus as inputs from sewage treatment plants and 10% of all phosphorus inputs to the tidal Potomac River. When all available point source phosphorus data are put into a steady-state conservation of mass model and reasonable coefficients for uptake of dissolved phosphorus, remineralization of particulate phosphorus, and sedimentation of particulate phosphorus are used in the model, a reasonably accurate simulation of dissolved and particulate phosphorus in the water column is obtained for the summer of 1980.     

武汉月湖水生植被重建的实践与启示

通过生物操纵、浮水植物和植物浮岛原位净化、岸边人工湿地净化和科学种植等措施,2－3年内,在长江中游地区、面积为0.66km2、水质为劣Ⅴ类的重富营养湖泊武汉月湖重建了以菹草和伊乐藻为优势种的沉水植物先锋群落,2005年春季的沉水植物盖度达到45％,提高了透明度。利用水位低、透明度相对高的冬季种植伊乐藻、菹草等适合冬春季生长的沉水植物是建立沉水植物先锋群落的一个有效途径。重污染水体过低的透明度是重建沉水植被的主要限制因子,在低水位或较高透明度时期,水生植物可以耐受相对较高的污染负荷,并且在有条件降低水位时,会大大提高重建沉水植被的成功率。重建并保持长期稳定的水生植被必须截污,并把营养盐浓度削减到一定范围内。水生植被重建初期是最脆弱的时期,水位、污染负荷等生态因子的较大波动很容易破坏植物群落,在这一时期要尽量保持关键因子的稳定,注意控制浮萍的扩张,使形成的生态系统逐步走向成熟。清除鱼类对提高水的透明度有重要作用,但必须伴随着水生植被的恢复或重建才能维持长久的良好效果。     

三峡库区消涨带植物群落的数量分析

运用双向指示种分析法(TWINSPAN)和除趋势对应分析法(DCA)对三峡库区消涨带植物群落进行了数量分类和排序分析。TWINSPAN将122个样方分成19类,可归为4个植被型,分类结果反映了植物群落与环境梯度间的关系,并在DCA分布图上得到较好的验证。DCA分布图的对角线基本反映了水淹时间长短和土壤湿度高低,表明水淹时间和土壤湿度是该区域植物群落组成和空间分布的主要限制性影响因子。对消涨带不同部位植物群落的物种组成和分布规律的分析为筛选耐水淹物种、进行三峡水库消涨带的植被恢复提供了物种参考。     

水体悬浮物浓度对大型沉水植物黑藻反射光谱的影响

沉水植物的恢复是改善富营养化水体和重建水生生态系统的有效措施．应用遥感技术可以实时、大面积监测沉水植物的分布和生长情况,而水体中的悬浮物等物质影响沉水植物的光谱特征,直接影响对湖泊、河流遥感影像中沉水植物的准确解译．本研究利用地物光谱仪在室外实测了不同水体悬浮物浓度条件下沉水植物黑藻群落的光谱反射率．结果表明,在400～900nm波段,水体中悬浮物浓度越高,沉水植物的光谱反射率越大．水体悬浮物浓度与黑藻群落光谱反射率在400～900nm均为显著相关（P〈0．05）,在450～900nm为极显著相关（P〈0．01）．在蓝、绿、红和近红外4个波段,将水体悬浮物浓度与其光谱反射率进行回归分析,均得到了较好的线性关系．利用本研究结果对富含悬浮物水体中沉水植物的反射光谱进行修正,可为准确、快速和大尺度遥感监测沉水植物的分布和动态提供科学依据．     

Phylogeography and post-glacial colonization patterns of the rainbow darter, Etheostoma caeruleum (Teleostei: Percidae)

Aim To examine the effects of historical climate change and drainage isolation on the distribution of mitochondrial DNA cytochrome b genetic variation within the rainbow darter, Etheostoma caeruleum (Percidae: Etheostomatinae). Location Eastern North American streams including tributaries to the Mississippi River, Great Lakes, Potomac River and Hudson Bay drainages. Methods Parsimony analyses, Bayesian analyses and haplotype networks of mitochondrial DNA sequences. Results Four major clades were recovered from sampled populations of E. caeruleum. Three of four clades are distributed in the western portion of the species’ range (primarily west of the Mississippi River). Samples from this region do not form a monophyletic group, and sequences often vary greatly between samples from adjacent stream systems (up to 7.2% divergence). A basal clade includes samples from the White River system in the Ozark Highlands. The northern Ozarks–upper Midwest clade includes samples from Missouri River tributaries and the upper Midwest (Hudson Bay, upper Mississippi River, and western Lake Michigan drainage). The eastern clade is composed of individuals from the Ohio River, Great Lakes and Potomac River. The Mississippi River corridor clade includes samples from middle and lower Mississippi River tributaries. Main conclusions The four major clades of E. caeruleum are deep allopatric lineages with well‐defined boundaries and have additional phylogeographical structure within each clade. The Ozark Highlands have the greatest levels of diversity relative to distributional area, with marked cytochrome b subdivisions between adjacent stream systems. Samples from previously glaciated areas do not have a subset of the cytochrome b diversity found in unglaciated areas, but four separate source areas are identified based on phylogenetic analyses. Dispersal into previously glaciated areas followed several known glacial outlets and, based on sequence divergence between populations, may have occurred during different glacial or interglacial stages. The disjunct distribution and cytochrome b pattern of E. caeruleum in the Mississippi River corridor clade is consistent with late Pleistocene and Recent changes in the course and characteristics of the middle and lower Mississippi River. Phylogeographical boundaries between clades of E. caeruleum correspond to independent sources of biogeographical information and provide insight into historical stream drainage relationships, post‐glacial colonization and drainage isolation patterns.     

Blooms of cyanobacteria on the potomac river

Blooms of cyanobacteria have appeared on the Potomac River near Washington, DC in years of drought and low river volume. The location of the bloom may be related to tidal activity. In 1983, the bloom of Microcystis aeruginosa used ammonia as its nitrogen source and contained low levels of toxic peptides. Cells collected from this bloom proved to be homogeneous and were an excellent source material for the isolation of proteins involved in photosynthesis.     

Development of 11 polymorphic microsatellite markers in a macrophyte of conservation concern, Vallisneria americana Michaux (Hydrocharitaceae)

Vallisneria americana Michaux (wild celery) is currently a target of submersed aquatic vegetation restoration efforts in the Chesapeake Bay watershed. To aid these efforts, we have developed 11 polymorphic microsatellite markers to assess the distribution and degree of genetic diversity in both restored and naturally occurring populations in the Chesapeake Bay. In 59 individuals from two populations, we detected two to 10 total alleles per locus. Observed heterozygosity ranged from 0.125 to 0.929, and two loci exhibited significant deviations from Hardy-Weinberg equilibrium in at least one of the populations assayed.     

Spatial and temporal dynamics of floating and drift-line seeds at a tidal freshwater marsh on the Potomac River, USA

The dispersal of seeds through hydrochory can be an important driver of community dynamics and play an integral role in the colonization of restored wetlands. We assessed sources of seeds on the shoreline and in adjacent waters of the Potomac River to Dyke Marsh in Virginia. Drift-line samples were taken at 40 random points four times during 2005–2006, and water surface net trawling took place on the Potomac River from 2003–2005 using six, 200-m transects around the perimeter of the marsh. Seed supply through hydrochory and species richness was low at Dyke Marsh when compared to other regional tidal freshwater marshes. We discovered distinct temporal patterns, where high species richness and seed density were found in the fall for the water trawl samples but in the spring for the drift-line samples. High fall dispersal in the water trawls may exist owing to peak plant senescence and seed release, while high seed germination in the spring along shorelines may arise due to sufficient cold stratification of marsh seeds.     

黄河三角洲滨海湿地芦苇、碱蓬混生群落空间点格局分析

为了解黄河三角洲滨海湿地芦苇、碱蓬群落内的种群相互关系、多样的生态策略, 通过重复取样的调查方法, 运用多种汇总统计函数分析了黄河入海口处以黄河河道为基轴向北延伸至海边的三个不同区域的芦苇和碱蓬混生种群的种群结构和空间点格局。结果表明: 黄河边碱蓬和芦苇在全部尺度上趋于聚集分布; 中部滩涂湿地碱蓬在2-35 cm尺度上呈现聚集分布, 在1cm以及36-45cm尺度上呈现随机分布, 在其他尺度呈现均匀分布,芦苇在3-4 cm以及15 cm、20 cm尺度上呈现聚集分布, 其他尺度上呈现随机分布; 近海区域碱蓬在全部尺度上呈现随机分布, 芦苇在0-16 cm、21-25 cm以及44-46 cm尺度上呈现随机分布, 在其他尺度呈现聚集分布。黄河边芦苇和碱蓬种群种间关系趋于负相关, 种群之间相互排斥, 在中部滩涂湿地和近海区域两种群无关联, 仅在近海区域28-31 cm尺度出现正相关关系。黄河边两种植物的点格局分布以及种间关系与其他两个区域存在明显差异,需进一步开展相关实验深入了解黄河三角洲滨海湿地环境因素对芦苇、碱蓬混生群落点格局的影响。     

Large‐Scale Zostera marina (eelgrass) Restoration in Chesapeake Bay,Maryland, USA. Part II: A Comparison of Restoration Methods in the Patuxent and Potomac Rivers

In response to systemic losses of submerged aquatic vegetation (SAV) in the Chesapeake Bay (east coast of North America), the U.S. Environmental Protection Agency's (EPA) Chesapeake Bay Program (CBP) and Maryland Department of Natural Resources (MD DNR) have considered SAV restoration a critical component in Bay restoration programs. In 2003, the CBP created the “Strategy to Accelerate the Protection and Restoration of Submerged Aquatic Vegetation in the Chesapeake Bay” in an effort to increase SAV area. As part of this strategy, large‐scale eelgrass (Zostera marina) restoration efforts were initiated in the Patuxent and Potomac Rivers in Maryland. From 2004 to 2007, nearly 4 million Z. marina seeds were dispersed over 10 ha on the Patuxent River and almost 9 million seeds over 16 ha on the Potomac River. Z. marina seedling establishment was consistent throughout the project (<4%); however, restored eelgrass survival was highly dependent on restoration site. Restoration locations on the Patuxent River experienced initial Z. marina seedling germination, but no long‐term plant survival. Restored Z. marina on the Potomac River has persisted and expanded, both vegetatively and sexually, beyond initial seeding areas. Healthy Z. marina beds now cover approximately five acres of the Potomac River bottom for the first time in decades. The differential success of Z. marina restoration efforts in the two rivers is evidence for the necessity of carefully considering site‐specific characteristics when using large‐scale seeding methods to achieve successful SAV restoration.     

Estimating biomass of submersed vegetation using a simple rake sampling technique

We evaluated the use of a simple rake sampling technique for predicting the biomass of submersed aquatic vegetation. Vegetation sampled from impounded areas of the Mississippi River using a rake sampling technique, was compared with vegetation harvested from 0.33-m² quadrats. The resulting data were used to model the relationship between rake indices and vegetation biomass (total and for individual species). We constructed linear regression models using log-transformed biomass data for sites sampled in 1999 and 2000. Data collected in 2001 were used to validate the resulting models. The coefficient of determination (R ²) for predicting total biomass was 0.82 and ranged from 0.59 (Potamogeton pectinatus) to 0.89 (Ceratophyllum demersum) for individual species. Application of the model to estimate total submersed aquatic vegetation is illustrated using data collected independent of this study. The accuracy and precision of the models tested indicate that the rake method data may be used to predict total vegetation biomass and biomass of selected species; however, the method should be tested in other regions, in other plant communities, and on other species. Handling editor: S. M. Thomaz     

The application of the ecotone concept in defining nutrient management requirements for the upper Potomac River basin

The utility of the ecotone concept in nutrient management studies of the Potomac River basin, a large USA watershed, is examined. The MAB hypothesis suggesting that the influence of an ecotone or adjacent system is proportional to the length and scope of the interfaces (Naiman et al., 1989) is the major focus of this paper.The land-riverine ecotone appears to have a major influence on the total nitrogen balance and river export flux of nitrogen for the upper Potomac River basin. A method of converting conventional areal flux units into linear flux rates is suggested.     

武汉莲花湖水生植被重建的实践与启示

通过截污、控制养鱼、控藻和科学种植等措施,2~3年内,在长江中游地区水质为劣Ⅴ类的重富营养湖泊武汉小莲花湖重建了以伊乐藻(Elodea nuttallii)为优势种的沉水植物先锋群落,2004和2005年春季的沉水植被盖度分别达到98%和95%,抑制了水华的发生,改善了水质。利用冬季水位低、透明度相对高的时机种植伊乐藻、菹草(Potamogeton crispu)等这些适合冬春季生长的沉水植物是建立沉水植物先锋群落的一个有效途径。由于以伊乐藻或菹草为优势种的先锋群落不稳定,必须配备丰富物种、优化结构、增加群落稳定性等后续措施。水生植被重建初期是最脆弱的时期,某些生态因子的较大波动很容易破坏植物群落,在这一时期要尽量保持关键因子的稳定,如禁止养鱼、防止污染负荷突然增加、稳定水位是必需的。     

The structure of population genetic diversity in <Emphasis Type="Italic">Vallisneria</Emphasis><Emphasis Type="Italic">americana</Emphasis> in the Chesapeake Bay: implications for restoration

Submersed aquatic macrophyte beds provide important ecosystem services, yet their distribution and extent has declined worldwide in aquatic ecosystems. Effective restoration of these habitats will require, among other factors, reintroduction of genetically diverse source material that can withstand short- and long-term environmental fluctuations in environmental conditions. We examined patterns of genetic diversity in Vallisneria americana because it is a cosmopolitan freshwater submersed aquatic macrophyte and is commonly used for restoring freshwater habitats. We sampled 26 naturally occurring populations of V. americana in the Chesapeake Bay estuary and its tributaries and found that the majority of populations have high genotypic diversity and are not highly inbred. Fourteen of the populations had high allelic and genotypic diversity and could serve as source sites for restoration material. However, substantial geographic structuring of genetic diversity suggests that caution should be used in moving propagules to locations distant from their source. In particular, we suggest that propagules at least be limited within four primary geographic areas that correspond to freshwater tidal and non-tidal, oligohaline, and seasonally mesohaline areas of the Chesapeake Bay.     

Individual growth and reproductive behavior in a newly established population of northern snakehead (<Emphasis Type="Italic">Channa argus</Emphasis>), Potomac River,USA

Northern snakehead (Channa argus) were first found in the Potomac River in 2004. In 2007, we documented feeding and reproductive behavior to better understand how this species is performing in this novel environment. From April to October, we used electrofishing surveys to collect data on growth, condition, and gonad weight of adult fish. Growth rates of young were measured on a daily basis for several weeks. Mean length-at-age for Potomac River northern snakehead was lower than for fish from China, Russia, and Uzbekistan. Fish condition was above average during spring and fall, but below average in summer. Below-average condition corresponded to periods of high spawning activity. Gonadosomatic index indicated that females began spawning at the end of April and continued through August. Peak spawning occurred at the beginning of June when average temperatures reached 26°C. Larval fish growth rate, after the transition to exogenous feeding, was 2.3 (SD ± 0.7) mm (total length, TL) per day. Although Potomac River northern snakehead exhibited lower overall growth rates when compared to other populations, these fish demonstrated plasticity in timing of reproduction and rapid larval growth rates. Such life history characteristics likely contribute to the success of northern snakehead in its new environment and limit managers’ options for significant control of its invasion.     

Relationship between the hydrological conditions and the distribution of vegetation communities within the Poyang Lake National Nature Reserve,China

Hydrological characteristics have been recognized as major driving forces for wetland vegetation. The water cycle and hydrological processes of wetland are increasingly influenced by the ongoing climate change and more intensive human activities, which may in turn affect the distribution and structure of vegetation communities. Poyang Lake, located on the south bank of the lower reach of Yangtze River, receives inflows from five tributaries and discharges to the Yangtze River. The unique hydrological conditions of the Poyang Lake wetland create abundant wetland vegetation communities. As a major national hydraulic project, the Three Gorges Dam across the Yangtze River has changed the water regime of Poyang Lake and hence may affect the vegetation distribution. This work aims to investigate the influences of hydrological properties on vegetation structure at broad spatial and temporal scales. Histograms and sensitivity index are used to link the hydrological processes with the vegetation distribution across the Poyang Lake National Nature Reserve. The results show that different vegetation communities react differently to the hydrological conditions. Specifically, certain communities, e.g. Carex and Eremochloa ophiuroides, are able to survive a wide variety of mean water depth and percent time inundated, while others, like Carex–Polygonum criopolitanum, are found to be relatively sensitive to hydrological conditions. It is suggested that this work provides a new insight for evaluating the impact of hydro-engineering projects on vegetation communities and wetland vegetation restoration.