Pollen representation,source area,and basin size: Toward a unified theory of pollen analysis

The concepts of pollen source area and of production and dispersal biases in pollen representation are quantified by means of a simple theoretical model. Source areas and relative pollen representation are shown to depend on basin size according to functions that describe the amount of pollen remaining airborne at increasing distances from single pollen sources. The form of these functions is determined by physical processes. Standard formulas for elevated sources do not apply, but the integrated form of Sutton's equation for particle dispersal from a ground-level source gives useful approximations applicable to pollen transport over a forest canopy. Simulations using this equation yielded source areas that increased realistically with basin size, showed substantial differences between source areas for pollen grains with different deposition velocities, and predicted that lighter pollen grains should become better represented with increasing basin size. All of these predictions are qualitatively consistent with present knowledge of the characteristics of pollen assemblages in different depositional environments. The model further allows parameters that can be estimated by statistical calibration methods to be predicted from underlying physical quantities. This extension suggests procedures for testing the theory with quantitative data on surface pollen and forest composition. Preliminary results showed reasonable agreement between estimated and predicted values of dispersal indices for the most abundant taxa in pollen spectra from the northern midwestern United States.     

Nekton community change along estuarine salinity gradients: Can salinity zones be defined?

Organisms tend to inhabit predictable portions of estuaries along salinity gradients between the ocean inlets (salinity > 35 psu) and the freshwater tributaries (salinity = 0). Previous studies have suggested that the continuous change in biological community structure along this gradient is relatively rapid at certain salinities. This is the basis for estuarine salinity zonation schemes similar to the classic Venice System (i.e., 0–0.5, 0.5–5, 5–18, 18–30, 30–40, > 40). An extensive database (n > 16,000 samples) of frequency of occurrence of nekton was used to assess evidence for estuarine salinity zones in two southwest Florida estuaries: Tampa Bay and Charlotte Harbor. Rapid change in nekton community structure occurred at each end of the estuarine salinity gradient, with comparatively slow (but steady) change in between. There was little strong evidence for estuarine salinity zones at anything other than low salinities (0.1–1). As previously suggested by other authors, estuaries may be regarded as ecoclines, because they form areas of relatively slow but progressive ecological change. The ends of the estuarine salinity gradient appear to be ecotones (areas of rapid change) at the interfaces with adjacent freshwater and marine habitats. This study highlights the rapid change that occurs in nekton community structure at low salinities, which is of relevance to those managing freshwater inflow to estuaries.     

Benthic metabolism and nutrient cycling along an estuarine salinity gradient

Benthic metabolism and nutrient exchange across the sediment-water interface were examined over an annual cycle at four sites along a freshwater to marine transect in the Parker River-Plum Island Sound estuary in northeastern Massachusetts, U.S. Sediment organic carbon content was highest at the freshwater site (10.3%) and decreased along the salinity gradient to 0.2% in the sandy sediments at the marine end of the estuary. C:N ratios were highest in the mid estuary (23:1) and lowest near the sea (11:1). Chlorophyll a in the surface sediments was high along the entire length of the estuary (39–57 mg chlorophyll a m⁻²) but especially so in the sandy marine sediments (172 mg chlorophyll a m⁻²). Chlorophyll a to phaeophytin ratios suggested most chlorophyll is detrital, except at the sandy marine site. Porewater sulfide values varied seasonally and between sites, reflecting both changes in sulfate availability as overlying water salinity changed and sediment metabolism. Patterns of sediment redox potential followed those of sulfide. Porewater profiles of inorganic N and P reflected strong seasonal patterns in remineralization, accumulation, and release. Highest porewater NH₄ ⁺ values were found in upper and mid estuarine sediments, occasionally exceeding 1 mM N. Porewater nitrate was frequently absent, except in the sandy marine sediments where concentrations of 8 μM were often observed. Annual average respiration was lowest at the marine site (13 mmol O₂ m⁻² d⁻¹ and 21 mmol TCO₂ m⁻² d⁻¹) and highest in the mid estuary (130 mmol O₂ m⁻² d⁻¹ and 170 mmol TCO₂ m⁻² d⁻¹) where clam densities were also high. N₂O and CH₄ fluxes were low at all stations throughout the year: Over the course, of a year, sediments varied from being sources to sinks of dissolved organic C and N, with the overall spatial pattern related closely to sediment organic content. There was little correlation between PO₄ ³⁻ flux and metabolism, which we attribute to geochemical processes. At the two sites having the lowest salinities, PO₄ ³⁻ flux was directed into the sediments. On average, between 22% and 32% of total system metabolism was attributable to the benthos. The mid estuary site was an exception, as benthic metabolism accounted for 95% of the total, which is attributable to high densities of filter-feeding clams. Benthic remineralization supplied from less than 1% to over 190% of the N requirements and 0% to 21% of the P requirements of primary producers in this system. Estimates of denitrification calculated from stoichiometry of C and N fluxes ranged from 0% for the upper and mid estuary site to 35% for the freshwater site to 100% of sediment organic N remineralization at the marine site. We hypothesize that low values in the upper and mid estuary are attributable to enhanced NH₄ ⁺ fluxes during summer due to desorption of exchangeable ammonium from rising porewater salinity. NH₄ ⁺ desorption during summer may be a mechanism that maintains high rates of pelagic primary production at a time of low inorganic N inputs from the watershed.     

Will lowering estuarine salinity increase Gulf of Mexico oyster landings?

Previous studies provide conflicting opinions on whether lower than average salinities in Gulf of Mexico (GOM) estuaries are likely to increase or decrease oyster harvests (Crassostrea virginica), which represented 69% and 54% of the United States oyster landings by weight, and dockside value, respectively, in 2003. The present study examined a 54-yr record (1950–2003) of oyster harvests and river discharge in five major estuaries in GOM states (Florida, Alabama, Mississippi, Louisiana, and Texas). Oyster landings were inversely related to freshwater inflow. Peaks in landings, 21 of 23 in West Florida, Alabama, Mississippi, and Texas combined, were coincidental with lows in river discharge from the major rivers in the estuaries. Lows in landings in these states (17 of 19) coincided with peaks in discharge of the major rivers feeding their estuaries. Landings in Breton Sound, Louisiana, were also inversely related to river discharge. The only exception to this pattern was for landings in the Plaquemines Parish, Louisiana, part of the Breton Sound estuary, where there were higher landings following increased Mississippi River discharge. The Bonnet Carré spillway, completed in 1931, diverts flood waters from the Mississippi River to Lake Pontchartrain, and it has been opened to reduce flood heights in 1937, 1950, 1973, 1975, 1979, 1983, and 1997. Twenty-five of 28 times after the spillway was opened, oyster landings in Mississippi were lower than in the other four states. The inverse relationship between freshwater inflow and oyster landings suggests that the proposed Bonnet Carré Freshwater Project, designed to reduce estuarine salinity, cannot be justified on the basis of anticipated higher oyster yields in Mississippi or Louisiana. Manipulating estuarine salinity in the GOM should be done within the context of the whole estuary and not just part of the estuary.     

Distribution and community structure of estuarine copepods

Distribution, abundance, and community structure were studied over a 30 month period in the planktonic copepod community of the estuaries near Beaufort, North Carolina. Many of the copepod species showed a demersal distribution during the day and entered the surface waters at night. Several species were largely confined to vegetated littoral areas during the day. The copepod community showed consistent trends of seasonal abundance and succession of dominant species which differed greatly from those found by previous workers, whose methods were inadequate to sample quantitatively the small, demersal copepods which dominated the community. Copepod abundances were higher than found in previous studies and were correlated with water temperature. Species composition changed from a winter community dominated byCentropages spp., to a spring community dominated byAcartia tonsa, to a summer community jointly dominated byParacalanus crassirostris andOithona spp. Copepods were much more important grazers in these estuaries than previous studies had concluded.     

A unified theory for the formation of galactic structures

A new theory for the formation of the main structures of galaxies is proposed: these structures are viewed as low-frequency normal modes in disks consisting of precessing stellar orbits. Mathematically, the theory is based on an integral equation in the form of a classical eigenvalue problem, with the eigenvalues being equal to the angular velocities Ω_p of the modes. Analysis of the general properties of the master integral equation (without finding concrete solutions) shows that it admits two types of solutions: barlike and spiral. The numerical algorithms are discussed and particular solutions of the integral equation are presented. If resonance interaction can be neglected, the bar mode represents a neutral perturbation of the disk. This mode can be amplified by the effect of the long-range gravitational field of the mode on stars located in the vicinity of the corotation and outer-Lindblad resonances. Spiral perturbations are waves with zero total angular momentum, and spiral modes are excited at the inner-Lindblad resonance. The approach proposed is compared to currently accepted mechanisms for the formation of galactic structures. In particular, Toomre's application of the swing amplification mechanism to explain the formation of global modes is critically discussed. In addition, we show that it is not correct to simulate the real stellar velocity dispersion in a galaxy using softened gravity.     

Shifting nutrient limitation and eutrophication effects in marsh vegetation across estuarine salinity gradients

In light of widespread coastal eutrophication, identifying which nutrients limit vegetation and the community consequences when limitation is relaxed is critical to maintaining the health of estuarine marshes. Studies in temperate salt marshes have generally identified nitrogen (N) as the primary limiting nutrient for marsh vegetation, but the limiting nutrient in low salinity tidal marshes is unknown. I use a 3-yr nutrient addition experiment in mid elevation,Spartina patens dominated marshes that vary in salinity along two estuaries in southern Maine to examine variation in nutrient effects. Nutrient limitation shifted across estuarine salinity gradients; salt and brackish marsh vegetation was N limited, while oligohaline marsh vegetation was co-limited by N and phosphorus (P). Plant tissue analysis ofS. patens showed plants in the highest salinity marshes had the greatest percent N, despite N limitation, suggesting that N limitation in salt marshes is partially driven by a high demand for N to aid in salinity tolerance. Fertilization had little effect on species composition in monospecificS. patents stands of salt and brackish marshes, but N+P treatments in species-rich oligohaline marshes significantly altered community composition, favoring dominance by high aboveground producing plants. Eutrophication by both N and P has the potential to greatly reduce the characteristic high diversity of oligohaline marshes. Inputs of both nutrients in coastal watersheds must be managed to protect the diversity and functioning of the full range of estuarine marshes.     

Changes in metabolic rates under fluctuating salinity regimes for two subtidal estuarine habitats

The metabolic rate of individual habitats can differ significantly in their contribution to the total system productivity of estuaries. Changing environmental conditions such as those created by tidal exchange can frequently alter these rates. In an effort to quantify these rate responses, metabolic rates were measured for macroalgal and sediment habitats at different salinities. Microcosms representing the two habitats were incubated at three salinity ranges (high: 25 to 31‰; moderate: 12 to 18‰; and low: 0 to 4‰) and production and respiration rates were estimated. The production rates for both habitats were proportional to the salinity of the water in the incubation, with the lowest metabolic rates associated with the lowest salinity. Average macroalgal habitat net production rates were 879 mg O₂ m^?2 h^?1, 609 mg O₂ m^?2 h^?1, and 451 mg O₂ m^?2 h^?1 at high, moderate, and low salinity treatments, respectively, and the dark respiration rates were ?401 mg O₂ m^?2 h^?1, ?341 mg O₂ m^?2 h^?1, and ?333 mg O₂ m^?2 h^?1. Average sediment habitat net production rates were 60 mg O₂ m^?2 h^?1, 13 mg O₂ m^?2 h^?1 and 10 mg O₂ m^?2 h^?1 and the respiration rates were ?114 mg O₂ m^?2 h^?1, ?55 mg O₂ m^?2 h^?1, and ?31 mg O₂ m^?2 h^?1 at high, moderate, and low salinity treatments. The larger contribution of macroalgal habitats to system metabolism may account for observed diurnal changes in water column oxygen levels in some estuaries. Macroalgal production rates explained 83% of the increase in water column oxygen levels during daylight hours and macroalgal respiration rates explained 65% of the decline in oxygen levels during the night. The contribution of macroalgal metabolism to the system can be influenced by even short-term changes in water column salinity. Environmental processes that alter salinity levels on hourly time scales may moderate the effect of macroalgal metabolism on oxygen levels.     

Differential effects of salinity changes on two estuarine fishes,Leiostomus xanthurus andMicropogonias undulatus

Salinity fluctuation has been proposed as an important determinant of estuarine fish distribution. To test this idea, we compared distribution, behavioral preference and physiological sensitivity of two juvenile estuarine fishes, spot (Leiostomus xanthurus) and croaker (Micropogonias undulatus), with respect to salinity change. In field collections, spot: croaker ratios were positively correlated with salinity variation. Subsequent behavioral observations revealed that croaker tend to cross a 10‰ salinity gradient less often than spot. We proposed that energetic costs of salinity adaptation may be higher for croaker, resulting in the observed avoidance behavior. Oxygen consumption rates over rapid salinity fluctuations showed no significant differences in metabolic response between species, although there was some indication that sensitivity changes with fish size. Apparently, juvenile spot and croaker are well-equipped to withstand extreme changes in salinity. We conclude that environmental factors correlated with salinity change may be responsible for distribution differences between these two abundant species.     

水土压力统一计算理论的提出和发展

地下结构上水土压力的合算与分算既关系到基坑围护的设计,又关系到地下结构的抗拔桩设置。现场数据表明,砂性土采用水土压力分算是合理的,但黏性土采用水土压力合算是否合理一直是学术界与工程界争论的焦点问题。为解决争议,笔者提出了可以统一水土压力合算与分算的方法,作为一个逻辑上自洽的理论,实现了水土分算与合算之间的过渡,为水土合算提供了理论基础。回顾该理论的提出过程,对其合理性做了进一步说明,分析了该理论的学术价值和发展前景。     

基于半隐式积分方案统一强度理论ANSYS二次开发

目前,岩土工程中常用的塑性屈服准则多为局部不可导的分段函数,在隐式有限元求解中均采用显式向前欧拉法更新应力和Newton-Raphson迭代法求解方程。但显式向前欧拉法容易导致应力偏离屈服面和计算不收敛,而Newton-Raphson迭代法需频繁求导,对于分段函数而言这些方法均不利于求解。为此,提出了采用半隐式向后欧拉法和免导数的Steffensen迭代法联合更新应力与一致切线模量的方案。根据上述方案,利用ANSYS提供的User Mat模块编写了基于统一强度理论与拉破坏复合屈服准则的理想塑性模型。用自定义的本构模型模拟深埋圆形巷道弹塑性开挖过程,数值模拟结果与理论解很好地吻合,验证了模型代码和所提出方案的正确性和可行性。采用半隐式向后欧拉法与Steffensen迭代法相结合的手段,可简化应力与一致切线模量的求解,该方法避免了应力偏离屈服面和求解流动函数偏导,算法简便,易于推广应用。     

基于统一强度理论深埋圆形隧道围岩的剪胀分析

基于连续介质理论中岩体的剪胀角与围压和塑性剪切应变密切相关,隧洞周边岩体的应力状态因开挖卸荷而发生应力重分布,迫使其围压由原地应力逐渐衰减,塑性剪切应变不断增加,引起剪胀效应呈非线性变化。首先,基于统一强度理论和非关联流动法则,将潜在塑性区围岩按等围压释放划分为若干同心圆,提出了考虑中间主应力和非线性剪胀性的有限差分法,计算应变软化围岩的力学问题,并以实例验证其正确性。其次,通过参数分析,研究塑性区内岩体的剪胀角受中间主应力、临界软化系数和支护力的影响规律。研究结果表明,中间主应力主要影响剪胀角的峰值,随着中间主应力效应增加,剪胀峰值增加;临界软化系数主要影响剪胀角的变化率,随着临界软化系数的增加,剪胀角变化缓慢;中间主应力和临界软化系数共同影响塑性区剪胀角的变化;随着支护力的增加,洞壁处的剪胀角增加;双剪强度理论计算的位移值较小,应谨慎采用,同时采用Mohr-Coulomb强度准则时可以适当考虑围岩的承载潜力。     

Toward exploratory analysis of diversity unified across fields of study: an information visualization approach

The study of the diversity of multivariate objects shares common characteristics and goals across disciplines, including ecology and organizational management. Nevertheless, subject-matter experts have adopted somewhat separate diversity concepts and analysis techniques, limiting the potential for sharing and comparing across disciplines. Moreover, while large and complex diversity data may benefit from exploratory data analysis, most of the existing techniques emphasize confirmatory analysis based on statistical metrics and models. This work aims to bridge these gaps. First, by cross comparing the analyses of species diversity, microbial diversity, and workgroup diversity, we introduce a framework of diversity concerns aligned across the three areas. The alignment framework is validated and refined by feedback from subject-matter experts. Then, guided by the framework and theoretical information visualization and visual analytics principles (as distinguished from scientific visualization), we propose a unified taxonomy of common analytical tasks for exploration of diversity.     

基于统一强度理论的柱形孔扩张问题研究

考虑了岩土材料的软化和剪胀特性,利用空间轴对称问题的统一强度理论分析了柱形孔扩张问题,求出了岩土材料在柱形孔扩张时的应力场、应变场、位移场和最终扩张压力的统一解表达式。对蒋明镜-沈珠江扩孔公式从不同的方面作了论述。通过算例分析了岩土材料的软化和剪胀特性以及统一强度理论参数对应力场、位移场和扩张压力的影响。已有的关于该问题的Tresca解和Mohr-Coulomb解均为本文的一个特例。     

基于统一强度理论的灰土挤密桩应力分析

采用统一强度理论对灰土挤密桩应力进行研究,分析了灰土挤密桩挤密成孔过程及土体处于弹塑性状态时孔周应力情况,得出了灰土挤密桩成孔过程的弹性解及孔壁屈服时的弹性极限荷载,并求出了孔壁均布内压与塑性挤密半径的关系式。该解考虑了中间主应力及拉压性能差异的影响,Mohr-Coulomb解是特例。其结果对于全面研究灰土挤密桩的综合作用效用,进一步节约材料有一定的参考价值。     

基于统一强度理论的冻结壁弹塑性应力分析

将冻结壁等效为弹性模量、黏聚力呈抛物线分布的功能梯度材料(FGM)厚壁圆筒,基于统一强度理论,考虑中间主应力的影响,推导得到冻结壁的弹性极限荷载、弹塑性应力场及塑性极限荷载的解析解。将所得结果与均质冻结壁进行比较,并分析强度理论参数对各个解析解的影响。研究结果表明:考虑冻结壁的FGM特性,应力分布较均质冻结壁有一定的变化,环向应力由线性分布变为抛物线型分布;环向应力的最大值不再出现在冻结壁的内壁或外壁,而是出现在冻结壁厚度的中部;与均质冻结壁相比,FGM冻结壁的弹性极限荷载较大,但塑性极限荷载却较小;FGM冻结壁的弹性极限荷载、塑性极限荷载、径向应力及环向应力均随强度理论参数的增大而增大。所得结果可为冻结壁的设计提供一定的理论参考。     

基于非线性统一强度理论下的边坡稳定性极限平衡分析

在边坡稳定性分析中,常采用的强度准则一般较少考虑中主应力对边坡破坏效应的影响,然而岩体的实际应力状况表明中主应力存在,且中主应力对边坡岩体的强度潜能发挥起着重要作用。基于考虑中主应力的统一强度理论下的Hoek-Brown(简称H-B)准则,通过对滑动面上的应力进行假设,并采用泰勒级数简化统一强度理论下的H-B准则表达式,然后根据滑动体满足所有静力平衡条件求解滑动面上的应力,进而得到边坡的安全系数。当中主应力影响系数b=0时(即为基本H-B强度准则),与等效的线性Mohr-Coulomb(简称M-C)强度准则下的瑞典法1、简化Bishop法、Spencer法和Morgenstern-Price(简称M-P)法以及基于统一强度理论下的瑞典法2进行算例对比分析,验证了方法的可行性。同时,以边坡的坡高H、坡角β和b为变量进行参数分析,研究表明:b增大(即中主应力对边坡的破坏效应影响越大)时,计算所得的安全系数增大,由此表明基本H-B强度准则下分析所得的边坡稳定性偏于安全。另外,由于提出的方法能够满足滑动体的所有静力平衡条件,且对未考虑条间作用力影响所得滑动面上的初始正应力进行了修正,故此方法较瑞典法2所得结果要大且更为严格。     

钱塘江河口一维盐度动床预报模型及应用

钱塘江河口为强涌潮、高含沙量、河床冲淤剧烈的河口,其盐度输移时空变化受河床冲淤的反馈影响十分显著。建立了考虑河床冲淤变化的一维盐度动床数学模型,耦合求解水沙运动、河床冲淤及盐度输移过程,数值计算方法采用守恒性较好的有限体积法。验证结果表明:河床冲淤对氯度的影响非常显著,动床模型的结果与实测基本吻合,在长历时盐度预报中采用动床模型是必要的。应用该模型分析了钱塘江河口咸水入侵对上游建库、治江缩窄工程等人类活动的响应,探讨了杭州城市供水水源保证率。结果表明,新安江水库、河口治理缩窄工程对改善河口淡水资源利用、保障杭州市供水安全显著;供水保证率要达95%以上,需采取上游水库泄水调度和新建备用水库等措施。