A modeling study of the dynamics of pore water seepage from intertidal marsh sediments

A numerical boundary integral equation model has been used to simulate tidally driven transient variations in pore water seepage from salt marsh sediments into tidal channels and its subsequent recharge by tidal inundation. In general the results show that the maximum seepage discharge occurs at or near the intersection of the creek bank and the channel water surface. Over a tidal cycle typically two-thirds of the total seepage discharge occurs through the creek bank with only about a third discharging from the channel bottom. Of the creek-bank discharge up to a third occurs through the seepage face that develops above the tide line at tidal stages below mean tide. These results indicate that placement of seepage meters only on the channel bottom will not give samples or measures representative of the total seepage. Of the total recharge only about 5% occurs through the upper part of the creek bank with the remainder infiltrating vertically through the marsh platform during early stages of tidal submergence. For the platform recharge about 80% occurs within 3 m of the creek bank. Thus, most of the water that seeps out of marsh sediments is derived from sediments that lie within several meters of the creek bank and accordingly has had a relatively short residence (one to two years) in the marsh. Compared to the distal portion of the marsh this relatively rapid flushing may enhance the productivity of Spartina alterniflora in the creek-bank environment and control the differential generation of radium quartet isotopes.     

Seasonal distribution of bacteria in salt marsh sediments in North Carolina

The number and size of bacteria at four depths (0–1, 5–6, 10–11 and 20–21 cm) in a North Carolina salt marsh were minotored by direct counts for 13 months. The number of bacteria reached a maximum of about 1·4 × 10¹⁰ cells cm^?3 at the sediment surface in October, corresponding to the period of Spartina alterniflora die-back. Cell numbers were lowest and most consistent throughout the year at the 20 cm depth of sediment. Cell volumes averaged 0·2 μm³ at the marsh surface and decreased with depth. Mean standing crop of bacteria to a depth of 20 cm of sediment was about 14 g bacterial carbon m^?2. In surface sediments bacteria contribute up to 15% and algae up to 10% of total living microbial biomass as estimated by adenosine triphosphate (ATP). Bacteria were the major biomass component at sediment depths of 5, 10 and 20 cm. At all depths the microbial community contributes < 4% total organic carbon and < 8% of total nitrogen.     

Bacterial sulphate reduction in sediments of a european salt marsh: Acid-volatile and tin-reducible products

The products of ³⁵S-sulphate reduction by sedimentary bacteria were measured at two sites in a salt marsh on the east coast of England. Non-acid-volatile products were measured, after acid-volatile sulphide was removed, by their reduction to sulphide by digestion with tin. The proportion of the sulphate reduced to tin-reducible products varied between 25% in a salt marsh pan and 61% in creek sediment, over a 0–25 cm depth profile. There were also variations with depth at each site in the proportions of sulphate reduced to tin-reducible products. Further examination revealed differences in the proportions of sulphate which were reduced to free sulphide, acid-volatile sulphide, sulphur or pyrite at the two sites. The data suggest that previous work which did not measure non-acid-volatile products underestimated sulphate reduction rates by three-fold in the creek site, but by only one third in the pan.     

不同状态红树和盐沼植物样品的脂肪酸组成比较研究

本研究分析了九龙江口红树植物秋茄(Kandelia obovata)和桐花树(Aegiceras corniculatum),以及盐沼植物互花米草(Spartina alterniflora)和短叶茳芏(Cyperus malaccensis)不同状态(成熟、衰老和腐烂)叶片中脂肪酸组成及相对含量。结果共检测出20种脂肪酸,其中9种存在于所有样品中。样品中含量最高的饱和脂肪酸均为棕榈酸C16:0 (21.76%～39.55%),其次是硬脂酸C18:0 (4.98%～29.88%);不饱和脂肪酸主要有油酸C18:1n9 (2.04%～12.90%)、亚油酸C18:2n6 (8.25%～24.69%)和C20:1n9 (1.64%～40.45%)。红树植物和盐沼样品中的脂肪酸组成存在差异,C30:0为红树植物的特征脂肪酸。在植物衰老和腐烂的样品中的脂肪酸组成与新鲜成熟样品存在差异,C20:1n9在衰老的样品中出现明显的下降,而相比红树植物两种盐沼植物的C20:1n9含量则下降更快。研究结果表明,总体上湿地植物在衰老和腐烂后不饱和脂肪酸占脂肪酸的比例明显下降,而必需脂肪酸C18:2n6的水平则未出现升高。     

The ebb and flood of Silica: Quantifying dissolved and biogenic silica fluxes from a temperate salt marsh

Salt marshes are widely studied due to the broad range of ecosystem services they provide including serving as crucial wildlife habitat and as hotspots for biogeochemical cycling. Nutrients such as nitrogen (N), phosphorus (P), and carbon (C) are well studied in these systems. However, salt marshes may also be important environments for the cycling of another key nutrient, silica (Si). Found at the land–sea interface, these systems are silica replete with large stocks in plant biomass, sediments, and porewater, and therefore, have the potential to play a substantial role in the transformation and export of silica to coastal waters. In an effort to better understand this role, we measured the fluxes of dissolved (DSi) and biogenic (BSi) silica into and out of two tidal creeks in a temperate, North American (Rowley, Massachusetts, USA) salt marsh. One of the creeks has been fertilized from May to September for six years allowing us to examine the impacts of nutrient addition on silica dynamics within the marsh. High-resolution sampling in July 2010 showed no significant differences in Si concentrations between the fertilized and reference creeks with dissolved silica ranging from 0.5 to 108 μM and biogenic from 2.0 to 56 μM. Net fluxes indicated that the marsh is a point source of dissolved silica to the estuary in the summer with a net flux of approximately 169 mol h⁻¹, demonstrating that this system exports DSi on the same magnitude as some nearby, mid-sized rivers. If these findings hold true for all salt marshes, then these already valuable regions are contributing yet another ecosystem service that has been previously overlooked; by exporting DSi to coastal receiving waters, salt marshes are actively providing this important nutrient for coastal primary productivity.     

C25 and C30 biogenic alkenes in sediments and detritus of a New England salt marsh

As part of a geochemical study of C₂₅ and C₃₀ biogenic alkenes in estuarine environments, distributions of these compounds in detritus and sediments collected from a New England salt marsh (Round Swamp on Conanicut Island in Narragansett Bay, Rhode Island) have been determined. The alkene assemblages detected, consisting primarily of four acyclic C₂₅ dienes and trienes and a C₃₀ bicyclic diene, qualitatively resemble those previously reported for other sediments in which anoxic conditions were prevalent. These similarities exist despite significant differences in the principal sources of sedimentary organic matter, suggesting that the occurrence of these specific alkenes is more likely associated with an in situ process common to anoxic environments than with a direct input from a specific source. Size fractionation (> 840 μm and < 840 μm to 1·2 μm) of marsh detritus revealed that the larger size fraction, consisting primarily of decaying Spartina debris, contains significant amounts of alkenes. This result, together with alkene subsurface profiles which show high surface concentrations decreasing to near-background levels by 20 cm, suggest that anaerobic bacteria are mediating in situ production of these compounds. Previous studies of bacterial hydrocarbons have not reported the presence of these C₂₅ and C₃₀ alkenes, although similar compounds have been isolated from several species of methanogenic bacteria. However, attempts to induce alkene synthesis by decomposing Spartina anaerobically in the laboratory were unsuccessful. In light of this result, the exact source of alkenes in marsh sediments remains uncertain. The absence from marsh sediments of other C₂₅ alkenes whose sedimentary distributions had been previously correlated with the presence of marine (planktonic) organic matter implies the existence of different origins for structurally related constituents of this hydrocarbon series.     

长江口盐沼滩面演化的沉积和地球化学特征

选择长江口盐沼典型高程部位(高潮滩上部、中潮滩下部、光滩)柱状样,采用小间隔(1 cm)取样,通过粒度、颗粒有机碳(POC)含量与碳稳定同位素组成(δ13C)分析,研究盐沼滩面发育的沉积和地球化学特征,结果表明,盐沼沉积物的粒度参数、POC含量与δ13C值随深度变化明显;在盐沼典型高程部位沉积物的颗粒组成与地球化学特征差异显著,有明显的滩面趋势,这与滩面的规律性演化密切相关.柱样有限深度内有明显的相对淤积期与冲刷作用相对较强时期的交替,表明长江口盐沼滩面发育过程并不是线性的.盐沼沉积物黏粒含量对POC含量和δ13C值的垂向变化及滩面趋势影响显著,表明流域来水来沙明显制约河口盐沼的发育.对于沉积动力环境复杂、冲淤过程频繁、厚度为毫米级的砂质纹层和泥质纹层发育的河口盐沼,在沉积和地球化学研究中采用小间隔取样的效果明显,有助于揭示盐沼发育过程的更多信息,为年际滩面冲淤演变研究提供新途径.     

Sources and preservation of organic matter in Plum Island salt marsh sediments (MA, USA): long-chain n-alkanes and stable carbon isotope compositions

Elemental (TOC, TN, C/N) and stable carbon isotopic (δ¹³C) compositions and n-alkane (nC_16–38) concentrations were measured for Spartina alterniflora, a C₄ marsh grass, Typha latifolia, a C₃ marsh grass, and three sediment cores collected from middle and upper estuarine sites from the Plum Island salt marshes. Our results indicated that the organic matter preserved in the sediments was highly affected by the marsh plants that dominated the sampling sites. δ¹³C values of organic matter preserved in the upper fresh water site sediment were more negative (−23.0±0.3‰) as affected by the C₃ plants than the values of organic matter preserved in the sediments of middle (−18.9±0.8‰) and mud flat sites (−19.4±0.1‰) as influenced mainly by the C₄ marsh plants. The distribution of n-alkanes measured in all sediments showed similar patterns as those determined in the marsh grasses S. alterniflora and T. latifolia, and nC₂₁ to nC₃₃ long-chain n-alkanes were the major compounds determined in all sediment samples. The strong odd-to-even carbon numbered n-alkane predominance was found in all three sediments and nC₂₉ was the most abundant homologue in all samples measured. Both δ¹³C compositions of organic matter and n-alkane distributions in these sediments indicate that the marsh plants could contribute significant amount of organic matter preserved in Plum Island salt marsh sediments. This suggests that salt marshes play an important role in the cycling of nutrients and organic carbon in the estuary and adjacent coastal waters.     

长江口北支盐水倒灌的数值模型研究

20世纪80年代以来,国内对长江口盐水入侵进行了大量系统专门的研究,对长江口的盐水入侵规律有了一个比较基本的认识.但是,长江口特别是在南支及南北港的盐度变化规律极其复杂,主要有盐度的周日变化峰值与潮流变化关系不尽协调,落潮时盐度反而最大;盐度的纵向分布上游比下游高;盐度的半月变化峰值一般发生在小潮和寻常潮期间,而且各测点盐度峰值的发生时间不尽一致[1,2]等等.其中北支盐水向南支倒灌是引起长江口盐度变化异常复杂的主要原因[1-5].北支倒灌在长江口盐度变化中扮演了重要的角色,掌握北支倒灌是认识长江口盐水入侵规律特别是南支和南北港盐度变化规律的重要内容,同时也是长江口淡水咨源利用的一个重要方面.     

渤海天文-风暴潮数值模拟和一种多年一遇极值水位的计算方法

在渤海选用了82个强天气过程,利用三维模式模拟了海区的天文-风暴潮,模式经实测资料检验,获得了较满意的模拟结果。根据渤海沿岸主要验潮站观测年极值高(低)水位和年极值风增(减)水所得到的多年一遇高(低)水位和多年一遇风增(减)水,以及天文最高(最低)潮位,建立了由多年一遇风增(减)水和天文最高(最低)潮位的线性组合计算多年一遇高(低)水位的计算公式,并以此公式推算了渤海海区5个典型地区的多年一遇高(低)水位,供海洋工程设计时使用。