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.     

Temporal changes and spatial variation of soil oxygen consumption, nitrification and denitrification rates in a tidal salt marsh of the Lagoon of Venice, Italy

The aim of the present study was to investigate seasonal and spatial patterns of soil oxygen consumption, nitrification, denitrification and fluxes of dissolved inorganic nitrogen (DIN) in a tidal salt marsh of the Lagoon of Venice, Italy. In the salt marsh, intact soil cores including overlying water were collected monthly at high tide from April to October in salt marsh creeks and in areas covered by the dominant vegetation, Limonium serotinum. In May, cores were also collected in areas with vegetation dominated by Juncus maritimus and Halimione portulacoides. In laboratory incubations at in situ temperature in the dark, flux rates of oxygen and DIN were monitored in the overlying water of the intact cores. ¹⁵N-nitrate was added to the overlying water and nitrification and denitrification were measured using isotope-dilution and -pairing techniques. The results show that highest soil oxygen consumption coincided with the highest water temperature in June and July. The highest denitrification rates were recorded in spring and autumn coinciding with the highest nitrate concentrations. Soil oxygen consumption and nitrification rates differed between sampling sites, but denitrification rates were similar among the different vegetation types. The highest rates were recorded in areas covered with L. serotinum. Burrowing soil macrofauna enhanced oxygen consumption, nitrification and denitrification in April and May. The data presented in this study indicate high temporal as well as spatial variations in the flux of oxygen and DIN, and nitrogen transformations in the tidal salt marshes of the Venice lagoon during the growth season. The results identify the salt marshes of the Venice lagoon as being metabolically very active ecosystems with a high capacity to process nitrogen.     

潮沟系统水沙输运研究——以长江口崇明东滩为例

本研究以崇明东滩2015年4月实测潮间带水沙数据为基础,分析了潮沟、盐沼及光滩的水沙特征,重点研究了潮沟系统及邻近潮滩潮周期内悬沙通量情况。结果表明:（1）潮沟表层沉积物比潮滩细,二者平均中值粒径分别为21.7 μm和33.0 μm,悬沙粒径由海向陆逐渐变小;（2）大、小潮沟潮周期内潮流均以往复流为主,垂向平均流速分别为15.4 cm/s和34.6 cm/s;盐沼界和光滩则以旋转流为主,平均流速分别为11.3 cm/s和28.9 cm/s;（3）潮沟中的高悬沙浓度出现在涨潮初期,最大可达7.5 kg/m3,而潮滩高悬沙浓度则出现在潮落潮中期和高水位时刻;大、小潮沟和盐沼界站涨潮阶段平均悬沙浓度大于落潮阶段,光滩站则相反。潮沟悬沙主要来自邻近水域,而潮滩悬沙则与滩面表层沉积物密切相关;（4）潮沟在潮周期内净输沙方向均指向滩地,大潮沟潮周期单宽净输沙量可达4.0 t/m;盐沼界处垂直岸线和沿岸输沙强度相近,净输沙由海向陆,潮周期离岸输沙强度为1.0 t/m;光滩沿岸输沙强度远大于垂直岸线输沙,光滩净输沙由陆向海。研究揭示了潮间带潮沟系统的强供沙能力以及研究区域光滩冲蚀,盐沼植被带淤积的动力地貌过程。     

Sampling and computational design of nutrient flux from a southeastern U.S. saltmarsh

A sampling and computational approach for estimating nutrient fluxes from a salt marsh ecosystem is presented. Extensive and intensive sampling of tidal velocities, water depths, and nutrient concentrations was made synoptically across three tidal creeks, connecting a 34 km² South Carolina salt marsh with surrounding coastal waters. An estimate of nutrient exchange over each sampling period is based on measurements over four tidal cycles during a neap and spring tidal regime. The computation of instantaneous fluxes of NO₃^?NO₂^?, NH₄⁺, and o-PO₂^4? was based on the cross-multiplication of concentration, velocity, and integrated over the cross-sectional area of each tidal creek. The net flux of nutrients was estimated using a least-squares regression model which included periodic functions simulating tidal and diurnal cycles. This computational approach allows for a rigorous test of the statistical significance of the measured nutrient fluxes and a basis on which interpretations of the ecological significance of the exchange can be made.Tidal patterns in inorganic nutrient concentrations and the corresponding exchanges are presented for a spring time sampling. Nitrate-nitrite was exported consistently from the marsh to the coastal ocean with a mean value of 8.0 kg per tidal cycle for the neap sampling set and 15.6 kg per tidal cycle for the spring set. This corresponded to high concentrations of nitrate-nitrite (0.6 μM) on the ebb tide with low concentrations (0.1 μM) on the flood tide. Ammonia flux was variable and did not portray a consistent tidal concentration pattern. Concentrations ranged from 1 to 6 μM. Ammonia flux was exported to the coastal ocean only during the spring tidal set with a mean value of 114 kg per tidal cycle. Ortho-phosphate was also exported only on the spring tidal set with a mean flux of 40.0 kg per tidal cycle. A tidal concentration pattern of high concentrations (0.6 μM) on the ebb tide and low concentrations (0.05 μM) on the flood was consistent for ortho-phosphate during both neap and spring tidal sets.     

Nitrogen and phosphorus species in the coastal and shelf waters of Southeastern North Carolina, Mid-Atlantic U.S. coast

The diversity of small-scale wetlands, high salinity tidal creeks, salt marshes, estuaries, and a wide and shallow shelf with the Gulf Stream close to the break makes the coastal zone of south-eastern North Carolina (U.S.) a natural laboratory for the study of the cycling of nitrogen (N) and phosphorus (P) in coastal and shelf waters. We assessed the summer concentrations, forms, and ratios for each N (total dissolved N, nitrate + nitrite, ammonium and dissolved organic N) and P (total dissolved P, o-phosphate and dissolved organic P) pool as these nutrients travel from tidal creeks, salt marshes and two large estuaries to Long and Onslow Bays. Additionally, we measured ancillary physical (temperature, salinity and turbidity) and chemical (dissolved oxygen, chlorophyll a and pH) water properties. Highest concentrations of all individual N and P compounds were found in the upper parts of each tributary and were attributed to loads from agricultural and urban sources to the coastal watersheds, continuing downstream to receiving estuaries. In all areas, dissolved organic N and P species were predominant constituents of the total dissolved N and P pools (64–97% and 56–93%, respectively). The lower parts of estuaries and surface shelf waters were characterized by oceanic surface values, indicating removal of N and P downstream in all tributaries. The different watershed and hydrological characteristics also determined the different speciation of N and P pools in each estuary. Despite a high level of anthropogenic pressure on the uppermost coastal waters, there is self-regulation in this coastal ecosystem with respect to human perturbations; i.e. significant amounts of the N and P load are retained within estuarine and nearshore waters without reaching the shelf.     

The principal factors contributing to the flux of salt in a narrow, partially stratified estuary

Observations of the velocity and salinity structure of the Tees estuary were made at eight stations along the estuary axis between Victoria Bridge and the sea during the summer of 1975. The measurements were made on ten separate tidal periods covering neap and spring tides.The data were collected over a period of relatively low freshwater flows and the residual current was found to have a strong dependence on the Stokes drift. At the upstream stations, the residuals were more than an order of magnitude greater than the currents anticipated from the freshwater discharge. Although the mean stratification decreased as the tidal range increased, the vertical circulation was stronger on spring tides than on neaps. Vertical variations in the amplitude and phase of the tidal current results in a current which strengthens the vertical circulation. However, this effect only made a relatively small contribution to the observed vertical circulation.The relative contribution of the individual salt flux terms to the net upstream transport of salt varies along the estuary. As the estuary narrows, the contribution by the oscillatory terms dominates that from the shear in the steady state flow. Of these oscillatory terms, the correlation of velocity and salinity fluctuations plays a key rôle in the salt transport. The depth mean values make a greater contribution than deviations from the depth mean and the flux due to phase variations over depth is smaller than either of these. Since the Stokes drift is compensated by a down-stream steady state flow, it does not contribute to the tidal mean transport of salt.At the seaward end of the estuary, the salt fluxes due to the steady state vertical shear and the convariance of the tidal fluctuations act in a complementary way to counter the seaward transport of salt by the freshwater flow. With the possible exceptions of the wide or narrow reaches of the Tees, the longitudinal fluxes of salt due to transverse variations in velocity, salinity and depth and turbulent fluctuations are of secondary importance as contributors to the estuary salt budget.On both neap and spring tides, the computed total salt transports at the Newport and Victoria bridges did not match the values required for a salt balance with the corresponding freshwater flows. These fluxes were probably the cause of the observed downstream displacement of the tidal mean salinity distribution between neap and spring tides.     

Daily and inter-tidal variations of Fe,Mn and Hg in the water column of a contaminated salt marsh: Halophytes effect

It has been shown that salt marshes may function as efficient sinks for contaminants, namely for mercury. At the rhizo-sediment Hg may be associated with Mn and Fe oxyhydroxides, precipitated as sulphides or incorporated into organic matter. However, to our knowledge, in situ studies have not focused on the related processes at a daily or tidal cycle scales. Thus, the present work aims to study the effect of a common salt marsh halophyte in temperate latitudes (Sarcocornia perennis) on dissolved Fe, Mn and Hg concentrations in the water column. The in situ approach was carried out at a mercury-contaminated salt marsh and at the adjacent non-vegetated area (distance ≤ 4 m), covering two consecutive tidal cycles in order to include the photosynthetic active period and the night processes. During high tide no daily or spatial effects were observed on the concentrations of Mn, Fe and Hg in the water column, due to the dilution effect of the incoming seawater. During low tide the concentrations of Mn, Fe and Hg were significantly higher in the overlaying water column of the salt marsh. At S. perennis mats the concentration of dissolved total Hg was significantly related with the concentration of Mn (r = 0.459, p = 0.028, n = 23), but not with that of Fe (r = 0.367, p = 0.085, n = 23) while no significant relations were found at the adjacent non-vegetated sediments.     

The process of sedimentation on the surface of a salt marsh

An unditched salt marsh-creek drainage basin (Holland Glade Marsh, Lewes, Delaware) has a sedimentation rate of 0·5 cm year^?1. During normal, storm-free conditions, the creek carries negligible amounts of sand and coarse silt. Of the material in the waters flooding the marsh surface, over 80% disappears from the floodwaters within 12 m of the creek. About one-half of the lost material is theoretically too fine to settle, even if flow were not turbulent; however, sediment found on Spartina stems can account for the loss.The quantity of suspended sediment that does reach the back marsh during these normal tides is inadequate to maintain the marsh surface against local sea level rise. This suspended sediment is also much finer than the deposited sediments. Additionally, remote sections of low marsh, sections flooded by only the highest spring tides, have 15–30 cm of highly inorganic marsh muds.This evidence indicates that normal tidal flooding does not produce sedimentation in Holland Glade. Study of the effects of two severe storms, of a frequency of once per year, suggests that such storms can deposit sufficient sediment to maintain the marsh.The actual deposition of fine-grained sediments (fine silt and clay) appears to result primarily from biological trapping rather than from settling. In addition, this study proposes that the total sedimentation on mature marshes results from a balance between tidal and storm sedimentation. Storms will control sediment supply and movement on micro- and meso-tidal marshes, and will have less influence on macro-tidal marshes.     

Leachable particulate iron in the Columbia River,estuary, and near-field plume

This study examines the distribution of leachable particulate iron (Fe) in the Columbia River, estuary, and near-field plume. Surface samples were collected during late spring and summer of 2004–2006 as part of four River Influence on Shelf Ecosystems (RISE) cruises. Tidal amplitude and river flow are the primary factors influencing the estuary leachable particulate Fe concentrations, with greater values during high flow and/or spring tides. Near the mouth of the estuary, leachable particulate Fe [defined as the particulate Fe solubilized with a 25% acetic acid (pH 2) leach containing a weak reducing agent to reduce Fe oxyhydroxides and a short heating step to access intracellular Fe] averaged 770 nM during either spring tide or high flow, compared to 320 nM during neap tide, low flow conditions. In the near-field Columbia River plume, elevated leachable particulate Fe concentrations occur during spring tides and/or higher river flow, with resuspended shelf sediment as an additional source to the plume during periods of coastal upwelling and spring tides. Near-field plume concentrations of leachable particulate Fe (at a salinity of 20) averaged 660 nM during either spring tide or high flow, compared to 300 nM during neap tide, low flow conditions. Regardless of tidal amplitude and river flow, leachable particulate Fe concentrations in both the river/estuary and near-field plume are consistently one to two orders of magnitude greater than dissolved Fe concentrations. The Columbia River is an important source of reactive Fe to the productive coastal waters off Oregon and Washington, and leachable particulate Fe is available for solubilization following biological drawdown of the dissolved phase. Elevated leachable Fe concentrations allow coastal waters influenced by the Columbia River plume to remain Fe-replete and support phytoplankton production during the spring and summer seasons.     

The effects of the reed,Phragmites australis (Trin.), on substratum grain-size distribution in a salt marsh

The effects of the reed,Phragmites australis (Trin.), growing in a brackish water lagoon, were studied in relation to the grain-size distribution of the substratum. At the salt marshes near the lagoon, the upper soils from the surface to a depth of 20 cm contained much silt-clay. These fine particles were found to be transferred from the river and fish ponds near the lagoon, and to be deposited when the tidal rhythm changed, that is, when the water current stopped. In addition, the fine particles, which were deposited on the bottom of the lagoon adjacent to the marshes, became resuspended as a result of wind-caused wave action, and then were transported and redeposited in the salt marshes at the flood tide. Since the reeds further reduced the water current caused by the waves and tide, the reeds were thought to promote redeposition of the resuspended matter. In other words, the reeds were considered to protect deposited and redeposited particles such as silt and clay from resuspension as a result of wave action by reducing the effects of waves and wind. These processes suggested that silt-clay will become abundant in the substratum of the salt marsh adjacent to the lagoon.     

Nutrient flux in the Rhode River: Tidal exchange of nutrients by brackish marshes

Tidal exchanges of nitrogen, phosphorus, and organic carbon by a high and a low elevation marsh in the Rhode River estuary were measured throughout the year. Both marshes tended to import particulate matter and export dissolved matter, although they differed in the fluxes of certain nutrients. Compared with tidal exchanges, bulk precipitation was a major source of ammonia and nitrate and a minor source of other nutrients. There was a net retention of nutrients by the portion of the Rhode River that included both marshes and a mudflat. However, the marshes accounted for only 10% of the phosphorus retention and 1% of the nitrogen retention while they released organic carbon amounting to 20% of the retention. This suggests that the mudflat acted as a sink for nutrients. The primary role of the marshes seems to be transformation of particulate to dissolved nutrients rather than nutrient retention or release.     

Estuarine salt conservation: a Lagrangian approach

Understanding tidal and mean flow transport mechanisms that maintain the estuarine salt balance against seaward transport by river flow is one of the fundamental questions of estuarine oceanography. Previous salt transport studies have failed to resolve this problem for two principal methodological reasons, in addition to the inherent variability of estuaries. First, salt transport expansions used to represent the salt balance have included a large number of terms that could not be related to any theory of estuarine circulation and whose physical meaning was thus unclear. Second, it has proven difficult to measure small mean velocities in the presence of much larger tidal variations. A new analysis method that expresses Lagrangian scalar property transport in terms of time and space variations of Eulerian variables is much simpler than expansions previously used and yields new insights into estuarine transport processes. This approach (like previous expansions) is valid for narrow systems in which cross-channel transport processes are weak and the ratio of tidal amplitude to mean depth is small. A surface boundary condition is used to eliminate one class of troublesome terms. The equivalence of the tidal-cycle average transport of tidal variations in salinity and the Stokes transport of mean salinity is then employed to show that the vertically integrated tidal flow plays no role in the overall salt balance. That is, seaward transport of salt caused by the river flow can only be balanced by landward transport resulting from correlations between shear and stratification in the mean, tidal and overtide flows. In a hypothetical inviscid estuary, tide and overtide flows would be vertically uniform, and outward fluvial salt transport could only be balanced by shear and stratification in the mean gravitational circulation. In a more realistic example with strong friction, the gravitational circulation would be severely damped, and inward transport could only be accomplished by correlations of shear and stratification in the tide and overtide flows.     

Tidal sands as biogeochemical reactors

Sandy sediments of continental shelves and most beaches are often thought of as geochemical deserts because they are usually poor in organic matter and other reactive substances. The present study focuses on analyses of dissolved biogenic compounds of surface seawater and pore waters of Aquitanian coastal beach sediments. To quantitatively assess the biogeochemical reactions, we collected pore waters at low tide on tidal cross-shore transects unaffected by freshwater inputs. We recorded temperature, salinity, oxygen saturation state, and nutrient concentrations. These parameters were compared to the values recorded in the seawater entering the interstitial environment during floods. Cross-shore topography and position of piezometric level at low tide were obtained from kinematics GPS records. Residence time of pore waters was estimated by a tracer approach, using dissolved silica concentration and kinetics estimate of quartz dissolution with seawater. Kinetics parameters were based on dissolved silica concentration monitoring during 20-day incubations of sediment with seawater. We found that seawater that entered the sediment during flood tides remained up to seven tidal cycles within the interstitial environment. Oxygen saturation of seawater was close to 100%, whereas it was as low as 80% in pore waters. Concentrations of dissolved nutrients were higher in pore waters than in seawater. These results suggest that aerobic respiration occurred in the sands. We propose that mineralised organic matter originated from planktonic material that infiltrated the sediment with water during flood tides. Therefore, the sandy tidal sediment of the Aquitanian coast is a biogeochemical reactor that promotes or accelerates remineralisation of coastal pelagic primary production. Mass balance calculations suggest that this single process supplies about 37 kmol of nitrate and 1.9 kmol of dissolved inorganic phosphorus (DIP) to the 250-km long Aquitanian coast during each semi-diurnal tidal cycle. It represents about 1.5% of nitrate and 5% of DIP supplied by the nearest estuary.     

Tidal exchange of zooplankton between Lough Hyne and the adjacent coast

Plankton samples collected in November 2002, February, May and August 2003 were used to examine seasonal variation in tidal exchange of zooplankton biomass, abundance and species composition between Lough Hyne Marine Nature Reserve and the adjacent Atlantic coast. Micro- to mesozooplankton were collected by pump over 24-h sampling periods during spring and neap tides from the narrow channel connecting the semi-enclosed water body to the Atlantic. Sample biomass (dry weight) and total zooplankton abundance peaked in the summer and were lowest in winter, showing a positive relationship with temperature. Zooplankton biomass, total abundance and numbers of holo- and meroplankton revealed import during some diel cycles and export in others. However, the tidal import of these planktonic components was generally dominant, especially during May. The greatest import of numbers of holoplankters and meroplanktonic larvae occurred during May and August, respectively. There was no significant variation in sample biomass between periods of light and dark, but some variation in zooplankton abundance could be explained by this diel periodicity. Significant differences in sample assemblage composition between flood and ebb tide samples were always observed, except during winter neap tides. There was a net import of the copepods Temora longicornis and Oithona helgolandica and the larval stages of Mytilus edulis during spring and summer. Proceraea cornuta and Capitellid trochophores were imported during winter, and a hydrozoan of the genus Obelia during the spring spring tides. Seasonal export from the lough was shown by Pseudopolydora pulchra larvae (autumn and spring), Serpulid trochophores (autumn) and veligers of the bivalve Anomia ephippium (summer). It is suggested that the direction of tidal exchange of meroplanktonic taxa is related to the distribution of the adult populations. Copepod naupliar stages dominated the assemblages except during May spring tides when the copepod Pseudocalanus elongatus made up over 22% of the abundance. The general import of micro- to mesozooplankton may, in part, explain the higher densities of this size-class of zooplankton within the semi-enclosed system of Lough Hyne.     

Transport of salt and suspended sediments in a curving channel of a coastal plain estuary: Satilla River, GA

This study describes the transport of salt and suspended sediment in a curving reach of a shallow mesotidal coastal plain estuary. Circulation data revealed a subtidal upstream bottom flow during neap tide, indicating the presence of a gravitational circulation mode throughout the channel. During spring tide, landward bottom flow weakened considerably at the upstream end of the channel and changed to seaward in the middle and downstream areas of the reach, suggesting the importance of tidal pumping. Salt flux near-bottom was landward at both ends of the channel during neap tide. At spring, however, the salt flux diverged along the bottom of the thalweg suggesting that tidal pumping caused a transfer of salt vertically and laterally into the intertidal zone. Thus, landward flux of salt is maintained even in the presence of subtidal seaward flow along the bottom at the downstream end of the channel.Landward bottom stress is greater than seaward stress, preferentially transporting suspended sediments upstream. Compared with salt, however, the weight of the suspended sediments causes less upward transfer of sediments into the intertidal zone. Flood flow carried more suspended sediments landward at the upstream end compared with the downstream end. We speculate that secondary flow in the curving channel picks up increasing amounts of suspended sediments along the sides during flood and adds them to the axial flow in the thalweg. Since the landward flow along the bottom of the thalweg weakens and even reverses during spring tide, there appears to be a complex re-circulation system for sediments re-suspended in curving channels that complicates the picture of a net transport of sediments landward.     

江苏海岸带盐沼潮滩在小尺度下的沉积变化过程

在小尺度范围内,潮滩沉积的研究一般都是基于几个站位或断面进行短时日的调查和监测来获取数据,或多或少地存在数据的连续性不足等问题。基于此,本文在潮滩地貌典型的江苏平原海岸选取了约160 000 m²的盐沼潮滩作为研究对象,在多年的野外连续观测下,分析了人类围垦活动对盐沼潮滩沉积过程及变化的影响。结果表明：人类围垦活动（如堤坝建设等）对潮滩沉积和盐沼植被覆盖的变化有重要影响。围垦堤坝修建之前,研究区向陆一侧为盐沼植物（现为养殖池）,向海一侧为光滩,岸线形态比较平直。在2006年围垦堤坝修建后,研究区变为“凹岸”型的人工岸线,沉积环境变得更有利于泥沙的淤积。根据2007-2012年对滩面高程的野外观测,在风暴潮的影响下,潮滩最大淤积率高达23 cm/a。盐沼植被在泥沙快速淤积的基础上不断发育生长,盐沼植物覆盖面积不断扩张。同时外部的沙嘴也在不断增长,潮汐风浪侵蚀携带沙嘴外部泥沙进入盐沼滩地,盐沼植物在泥沙的覆盖下,出现萎缩退化,覆盖面积减少,但是对潮滩上的泥沙而言还是不断堆积增厚。柱状样岩芯的粒度垂向剖面特征也记录了盐沼潮滩沉积环境的变化。小尺度下盐沼潮滩沉积表现出的阶段性变化规律,揭示出合理的围垦活动有利于堤坝前的潮滩淤积和盐沼植被的扩张。     

Observations on the ecological importance of salt marshes in the Cumberland Basin,a macrotidal estuary in the Bay of Fundy

The Cumberland Basin, a 118 km² estuary at the head of the Bay of Fundy which has an average tidal range of about 11m, contains large tracts of salt marsh (15% of the area below highest high water). Low marsh (below about 0·9 m above mean high water) is composed almost exclusively of Spartina alterniflora while the vegetation on high marsh is more diverse but dominated by Spartina patens. Because of its higher elevation, high marsh is flooded infrequently for short periods by only extreme high tides. Low marsh is inundated much more frequently by water as much as 4m deep for periods as long as 4 h per tide. Temporal variability in the occurrence of extreme tides influences the flooding frequency of high marsh for any given month and year. Using a modification of Smalley's method, the mean annual net aerial primary production (NAPP) of low and high marsh is estimated to be 272 and 172 g C m^?2, respectively. Vegetation turnover times average 1·0 and 2·0 y for low and high marsh, respectively. Because of abundant tidal energy, much of the low marsh production appears to be exported and distributed widely about the estuary. Since high levels of turbidity suppress phytoplankton production, salt marshes produce approximately half of the carbon fixed photosynthetically in the Cumberland Basin. It is concluded that salt marshes play a major ecological role in the Cumberland Basin.     

台湾海峡夏季与冬季的水体及热盐通量观测

利用在台湾海峡附近的下放式声学多普勒流速剖面仪(Lowered Acoustic Doppler Current Profiler,LADCP)观测资料和温盐观测资料,通过对连续站的两个季节观测进行正压和斜压潮流分析从而去除潮流得到准定常流,并在此基础上计算了南海和东海之间通过台湾海峡输运的水体及热盐通量。结果表明:台湾海峡大部分海域是半日潮海区(正规半日潮及不正规半日潮海区),半日潮主要分量为太阴半日分潮M2;台湾海峡的水体输运及热盐通量呈现明显的季节变化:夏季台湾海峡内表现为一支东北流向的海流,即台湾海峡暖流,存在3.3 Sv(1Sv=106 m3/s)的东北向水体输运,冬季东北季风较强,西南方向的海流加强,混合层可达到底部,存在1.8 Sv的东北向水体输运。与此对应的热盐通量分别为:夏季热通量为0.34×1015 W,盐通量为118.6×109 g/s;冬季热通量为0.14×1015 W,盐通量为72.9×109 g/s。该结果对台湾海峡通量的研究给出了一个直接观测的准确值,并为相关的数值研究提供了参考。     

Mercury fluxes between an impacted coastal lagoon and the Atlantic Ocean

The objective was to estimate the seasonal and inter-tidal variability of dissolved and particulate mercury fluxes between the Ria de Aveiro (Portugal) and the Atlantic Ocean. The mercury fluxes were estimated by means of a two-dimensional vertically integrated hydrodynamic model. Results showed that the particulate fraction plays an important role in the transport of mercury while the contribution of the seston fraction to the transport of mercury was always <0.5%. During spring tides, in summer, about 2% of mercury transported in seston was present in an organic form. The mass balance for the mercury fractions revealed that the mercury export to the Atlantic Ocean varied with season and tidal regime, mainly in terms of the relative importance of the dissolved and particulate fractions. An approximate range of values for the annual mass balance between 42 and 77 kg shows that the export of dissolved and particulate mercury makes little impact on the near shore region of the Atlantic Ocean and that the recovery of the lagoon from mercury contamination is likely to remain a long-term issue.     

长江口盐沼湿地底栖微藻的分布特征及其对有机质产出的贡献

2006年6月(夏季)和12月(冬季)分别对长江口九段沙湿地潮滩底栖微藻的分布状况进行了调查,估算了湿地内各主要初级生产者的生产力并进行了比较。结果显示分布于该盐沼湿地的底栖微藻在光照条件下主要聚集于沉积物表层,形成其生物量随深度增加而迅速减少的垂直分布模式,并随滩面植被盖度从大到小的顺序呈现为:光滩、藨草区、米草区≈芦苇区的水平分布特征。九段沙湿地中单位面积上底栖微藻的生物量远高于上覆水中浮游植物的生物量,其单位面积生产力也显著高于后者,表明底栖微藻在源自单胞藻的湿地有机产出中占有主导地位。九段沙湿地中底栖微藻总生物量远低于地表维管植物,其生物量仅占潮滩初级生产者总生物量的约0.41%,所产出的有机质却达到潮滩总初级生产力约16.53%,意味着这种高营养、易摄取的有机产出有能力成为维持该湿地生态系统的重要食物来源之一。