Tidal Marsh Restoration Design Affects Feedbacks Between Inundation and Elevation Change

Tidal marsh (re)creation on formerly embanked land is increasingly executed along estuaries and coasts in Europe and the USA, either by restoring complete or by reduced tidal exchange. Ecosystem functioning and services are largely affected by the hydro-geomorphologic development of these areas. For natural marshes, the latter is known to be steered by feedbacks between tidal inundation and sediment accretion, allowing marshes to reach and maintain an equilibrium elevation relative to the mean sea level. However, for marsh restoration sites, these feedbacks may be disturbed depending on the restoration design. This was investigated by comparing the inundation-elevation change feedbacks in a natural versus restoration site with reduced tidal exchange in the Scheldt estuary (Belgium). This study analyzes long-term (9 years) datasets on elevation change and tidal inundation properties to disentangle the different mechanisms behind this elevation-inundation feedback. Moreover, subsequent changes in sediment properties that may affect this feedback were explored. In the restoration area with reduced tidal exchange, we found a different elevation-inundation feedback than on natural marshes, which is a positive feedback on initially high sites (i.e., sediment accretion leads to increasing inundation, hence causing accelerating sediment accretion rates) and a gradual silting up of the whole area. Furthermore, there is evidence for the presence of a relict consolidated sediment layer. Consequently, shallow subsidence is less likely to occur. Although short-term ecological development of the tidal marsh was not impeded, long-term habitat development may be affected by the differences in hydro-geomorphological interactions. An increase of inundation frequency on the initially high sites may cause inhibition of habitat succession or even reversed succession. Over time, the climax state of the restoration area may be different compared to natural marshes. Moreover, sediment-related ecosystem services, such as nutrient and carbon burial, may be positively influenced because of continuing sedimentation, although flood water storage potential will decrease with increasing elevation. Depending on the restoration goals, ecosystem trajectories and delivery of ecosystem services can be controlled by adaptive management of the tidal volume entering the restoration area.     

Fertilization Effects on Elevation Change and Belowground Carbon Balance in a Long Island Sound Tidal Marsh

We report the results of a 5-year fertilization experiment in a central Long Island Sound salt marsh, aimed at understanding the impacts of high nutrient loads on marsh elevational processes. Fertilization with nitrogen led to some significant changes in marsh processes, specifically increases in aboveground primary production and in CO₂ fluxes from the soil. However, neither nitrogen nor phosphorus fertilization led to elevation loss (relative to controls), reduced soil carbon, or a decrease in belowground primary production, all of which have been proposed as links between elevated nutrient loads and marsh drowning. Our data suggest that high nutrient levels increase gross carbon loss from the sediment, but that this is compensated for by other processes, leading to no net deleterious effect of nutrient loading on carbon storage or on marsh stability with respect to sea level rise.     

Ecosystem Responses of a Tidal Freshwater Marsh Experiencing Saltwater Intrusion and Altered Hydrology

Tidal freshwater marshes exist in a dynamic environment where plant productivity, subsurface biogeochemical processes, and soil elevation respond to hydrological fluctuations over tidal to multi-decadal time scales. The objective of this study was to determine ecosystem responses to elevated salinity and increased water inputs, which are likely as sea level rise accelerates and saltwater intrudes into freshwater habitats. Since June 2008, in situ manipulations in a Zizaniopsis miliacea (giant cutgrass)-dominated tidal freshwater marsh in South Carolina have raised porewater salinities from freshwater to oligohaline levels and/or subtly increased the amount of water flowing through the system. Ecosystem-level fluxes of CO₂ and CH₄ have been measured to quantify rates of production and respiration. During the first 20 months of the experiment, the major impact of elevated salinity was a depression of plant productivity, whereas increasing freshwater inputs had a greater effect on rates of ecosystem CO₂ emissions, primarily due to changes in soil processes. Net ecosystem production, the balance between gross ecosystem production and ecosystem respiration, decreased by 55% due to elevated salinity, increased by 75% when freshwater inputs were increased, and did not change when salinity and hydrology were both manipulated. These changes in net ecosystem production may impact the ability of marshes to keep up with rising sea levels since the accumulation of organic matter is critical in allowing tidal freshwater marshes to build soil volume. Thus, it is necessary to have regional-scale predictions of saltwater intrusion and water level changes relative to the marsh surface in order to accurately forecast the long-term sustainability of tidal freshwater marshes to future environmental change.     

Anaerobic Metabolism in Tidal Freshwater Wetlands: II. Effects of Plant Removal on Archaeal Microbial Communities

The interaction of plant and microbial communities are known to influence the dynamics of methane emission in wetlands. Plant manipulations were conducted in an organic rich (JB-organic) and a mineral rich (JB-mineral) site in a tidal freshwater wetland to determine if plant removal impacted archaeal populations. In concert, a suite of process-based measurements also determined the effects of plant removal on rates of methanogenesis and Fe-reduction. The microbial populations were analyzed with clone libraries of the SSU ribosomal RNA (rRNA) gene from selected plots, and terminal restriction length polymorphism (tRFLP) of the SSU rRNA and the methyl-coenzyme M reductase (mcrA) gene. Overall, methanogenesis dominated anaerobic carbon mineralization at both sites during the most active growing season. A total of 114 SSU rRNA clones from four different plots revealed a diversity of Euryarchaeota including representatives of the Methanomicrobiales, Methanosarcinales and Thermoplasmatales. The clone libraries were dominated by the Thaumarchaeota, accounting for 65 % of clones, although their diversity was low. A total of 112 tRFLP profiles were generated from 56 samples from 25 subplots; the patterns for both SSU rRNA and mcrA showed little variation between sites, either with plant treatment or with the growing season. Overall these results suggest that wetland soil archaeal populations were resilient to changes in the associated surface plant communities. The work also revealed the presence of novel, mesophilic Thaumarchaeota of unknown metabolic function.     

Sustainability of a Tidal Freshwater Marsh Exposed to a Long-term Hydrologic Barrier and Sea Level Rise

A 115-year-old railroad levee bisecting a tidal freshwater marsh perpendicular to the Patuxent River (Maryland) channel has created a northern, upstream marsh and a southern, downstream marsh. The main purpose of this study was to determine how this levee may affect the ability of the marsh system to gain elevation and to determine the levee’s impact on the marsh’s long-term sustainability to local relative sea level rise (RSLR). Previously unpublished data from 1989 to 1992 showed that suspended solids and short-term sediment deposition were greater in the south marsh compared to the north marsh; wetland surface elevation change data (1999 to 2009) showed significantly higher elevation gain in the south marsh compared to the north (6?±?2 vs. 0?±?2 mm year^?1, respectively). However, marsh surface accretion (2007 to 2009) showed no significant differences between north and south marshes (23?±?8 and 26?±?7 mm year^?1, respectively), and showed that shallow subsidence was an important process in both marshes. A strong seasonal effect was evident for both accretion and elevation change, with significant gains during the growing season and elevation loss during the non-growing season. Sediment transport, deposition and accretion decreased along the intertidal gradient, although no clear patterns in elevation change were recorded. Given the range in local RSLR rates in the Chesapeake Bay (2.9 to 5.8 mm year^?1), only the south marsh is keeping pace with sea level at the present time. Although one would expect the north marsh to benefit from high accretion of abundant riverine sediments, these results suggest that long-term elevation gain is a more nuanced process involving more than riverine sediments. Overall, other factors such as infrequent episodic coastal events may be important in allowing the south marsh to keep pace with sea level rise. Finally, caution should be exercised when using data sets spanning only a couple of years to estimate wetland sustainability as they may not be representative of long-term cumulative effects. Two years of data do not seem to be enough to establish long-term elevation change rates at Jug Bay, but instead a decadal time frame is more appropriate.     

祁连山海北高寒湿地气候变化及植被演替分析

分析了近40a海北高寒湿地区域气候变化特征,以及近期湿地退化和植被演替的情况.结果表明:祁连山海北地区自1957年以来年平均气温以0.157℃·10a^-1的倾向率升高,年降水量约以1859mm·10a^-1的倾向率递减,年平均地温比同期气温的增加更为迅速,表现出海北地区气候及土壤性状均向干暖化趋势发展,特别是土壤干暖化程度尤为明显.由于人类活动加剧影响,超载过牧,原生植被遭受破坏,草场退化严重,地表潜在蒸散力加大.深层的多年冻土退化,冻胀草丘坍塌,导致湿地植被发生变化,使沼泽化草甸向典型草甸演替.不同年度调查结果表明,高寒湿地植被在气候干暖化趋势的加剧影响下,植物群落组成发生变异,物种多样性、生态优势度均比湿地原生植被的物种有增多的趋势.原生适应寒冷、潮湿生境的藏嵩草为主的草甸植被类型逐渐退化,有些物种甚至消失,而被那些寒冷湿中生为主的典型草甸类型所替代.组成植物群落的湿中生种类减少,中生种类(如线叶嵩草)大量增加,群落盖度相对降低,群落生产量大幅度下降.     

Anammox in Tidal Marsh Sediments: The Role of Salinity,Nitrogen Loading,and Marsh Vegetation

Anammox bacteria are widespread in the marine environment, but studies of anammox in marshes and other wetlands are still scarce. In this study, the role of anammox in nitrogen removal from marsh sediments was surveyed in four vegetation types characteristic of New England marshes and in unvegetated tidal creeks. The sites spanned a salinity gradient from 0 to 20 psu. The impact of nitrogen loading on the role of anammox in marsh sediments was studied in a marsh fertilization experiment and in marshes with high nitrogen loading entering through ground water. In all locations, nitrogen removal through anammox was low compared to denitrification, with anammox accounting for less than 3% of the total N₂ production. The highest relative importance of anammox was found in the sediments of freshwater-dominated marshes, where anammox approached 3%, whereas anammox was of lesser importance in saline marsh sediments. Increased nitrogen loading, in the form of nitrate from natural or artificial sources, did not impact the relative importance of anammox, which remained low in all the nitrogen enriched locations (<1%).     

Magnitude and Patterns of Change in Submerged Aquatic Vegetation of the Tidal Freshwater Hudson River

Three aerial photography inventories were used to examine change in submerged aquatic vegetation (SAV) in the tidal freshwater Hudson River over the interval 1997 to 2007. Overall, there was about a 30 % net decline in SAV coverage although there were also many individual areas of expansion. The invasive water chestnut (Trapa natans) did not change appreciably in net cover over the interval, and there was replacement of SAV by water chestnut along with slightly fewer cases of SAV replacing the exotic. A fine-scale (100 m by 100 m quadrats) analysis showed that about 30 % of quadrats that supported vegetation changed by more than 10 % in plant cover and overall SAV was quite dynamic. SAV in the Hudson is limited by light which is in turn controlled by suspended sediment. SAV was rarely found at depths >1 m below low water, and interannual differences in clarity affected the ability of SAV beds to maintain locally supersaturated levels of dissolved oxygen. We found that location within the River channel (proximity to shore) influenced the magnitude and variability in change in SAV between census periods. The physical nature of the adjacent shoreline also affected the magnitude of change with greater declines in cover in areas next to hard-engineered shore types. SAV in the Hudson is highly dynamic, apparently quite resilient, and the control of light by suspended sediment rather than phytoplankton growth offers a contrast to eutrophication-influenced changes in other estuaries. Management and protection of SAV habitat must recognize the highly variable nature of plant cover and that absence in any particular year does not preclude future appearance of submerged plants at that location.     

玛曲气候变化对生态环境的影响

通过对温度、降水等气候要素的分析,发现玛曲草原的气候向暖干化方向发展.依据黄河玛曲段年径流量、产流量以及冻土最大深度的变化和牧草产量、牲畜数量等经济指标,初步探讨了玛曲生态恶化的原因.结果表明:气候变化和人类不合理的活动是玛曲草原生态恶化的主要原因.气候变化造成蒸发增加,冻土变浅,黄河玛曲段年产流量急剧减少,牧草产量下降.同时,过度放牧等人类活动使牛羊肉产量的大幅度上升,加速了草场的退化.     

Effects of Increased Salinity and Inundation on Inorganic Nitrogen Exchange and Phosphorus Sorption by Tidal Freshwater Floodplain Forest Soils, Georgia (USA)

We investigated the effects of increasing salinity and inundation on inorganic N exchange and P sorption/precipitation in soils of tidal freshwater floodplain forests (TFFF) of coastal Georgia, USA. Our objectives were to better understand how sea level rise, increasing inundation, and saltwater intrusion will affect the ability of TFFFs to retain nitrogen (N) and phosphorus (P). We collected soil cores (0–5 cm) from three TFFFs that do not currently experience saltwater intrusion and from one TFFF currently experiencing saltwater intrusion and measured NH₄-N exchange and PO₄-P removal over five simulated 6-h tidal cycles using nutrient-enriched freshwater (30 μM NH₄-N and 5 μM PO₄-P). In a second experiment, we exposed soil cores to three salinities (0, 2, and 5) and two inundation depths (5 and 10 cm) using the same nutrient enrichment. When flooded with nutrient-enriched freshwater, soils from the three TFFFs that do not experience saltwater intrusion removed inorganic N and P in amounts ranging from 5.2 to 10.7 and 2.3 to 4.4 mg/m², respectively, and the TFFF soils experiencing saltwater intrusion removed 2.1 to 3.8 mg P/m². However, TFFF soils experiencing saltwater intrusion released inorganic N to the water column in amounts ranging from 7.1 to 67.5 mg/m². In the second experiment, soils from TFFFs not experiencing saltwater intrusion released NH₄-N to the water column when exposed to 2 and 5 salinity, and the amount of N released increased with salinity and number of tidal cycles. In contrast, the same TFFF soils sorbed two and three times more PO₄-P when exposed to 2 and 5 salinity than when exposed to 0 salinity. P removal on a mass basis was greater under 10 cm of inundation, but the efficiency of removal was greater under the 5 cm flooding depth. Our findings suggest that saltwater intrusion caused by sea level rise will promote N release into the water column through organic matter mineralization and/or ion exchange and may promote P sorption, or precipitation of P with metal cations. In addition, release of N and resulting increased N/P could exacerbate eutrophication of estuaries in the future.     

Anaerobic Metabolism in Tidal Freshwater Wetlands: I. Plant Removal Effects on Iron Reduction and Methanogenesis

For energetic reasons, iron reduction suppresses methanogenesis in tidal freshwater wetlands; however, when iron reduction is limited by iron oxide availability, methanogenesis dominates anaerobic carbon mineralization. Plants can mediate this microbial competition by releasing oxygen into the rhizosphere and supplying oxidized iron for iron reducers. We utilized a plant removal experiment in two wetland sites to test the hypothesis that, in the absence of plants, rates of iron reduction would be diminished, allowing methanogenesis to dominate anaerobic metabolism. In both sites, methanogenesis was the primary anaerobic mineralization pathway, with iron reduction dominating only early and late in the growing season in the site with a less organic soil. These patterns were not influenced by the presence of plants, demonstrating that plants were not a key control of microbial metabolism. Instead, we suggest that site conditions, including soil chemistry, and temperature are important controls of the pathways of anaerobic metabolism.     

Iron Reduction Along an Inundation Gradient in a Tidal Sedge (<Emphasis Type="Italic">Cyperus malaccensis</Emphasis>) Marsh: the Rates,Pathways, and Contributions to Anaerobic Organic Matter Mineralization

Incubation experiments were adopted to characterize the rates and pathways of iron reduction and the contributions to anaerobic organic matter mineralization in the upper 0–5 cm of sediments along a landscape-scale inundation gradient in tidal marsh sediments in the Min River Estuary, Southeast China. Similar sediment characteristics, single-species vegetation, varied biomass and bioturbation, distinct porewater pH, redox potential, and electrical conductivity values have resulted in a unique ecogeochemical zonation along the inundation gradient. Decreases in solid-phase Fe(III) and increases in nonsulfidic Fe(II) and iron sulfide were observed in a seaward direction. Porewater Fe²⁺ was only detected in the upland area. High rates of iron reduction were observed in incubation jars, with significant accumulations of nonsulfidic Fe(II), moderate accumulations of iron sulfides, and negligible accumulations of porewater Fe²⁺. Most of the iron reduction was microbially mediated rather than coupled to reduced sulfides. Microbial iron reduction accounted for 20–89 % of the anaerobic organic matter mineralization along the inundation gradient. The rate and dominance of microbial iron reduction generally decreased in a seaward direction. The contributions of microbial iron reduction to anaerobic organic matter mineralization depended on the concentrations of bioavailable Fe(III), the spatial distribution of which was significantly related to tidal inundation. Our results clearly showed that microbial iron reduction in the upper sediments along the gradient is highly dependent on spatial scales controlled primarily by tidal inundation.     

Tropicalization of the Northern Gulf of Mexico: Impacts of Salt Marsh Transition to Black Mangrove Dominance on Faunal Communities

The tropically associated black mangrove (Avicennia germinans) is expanding into salt marshes of the northern Gulf of Mexico (nGOM). This species has colonized temperate systems dominated by smooth cordgrass (Spartina alterniflora) in Texas, Louisiana, Florida and, most recently, Mississippi. To date, little is known about the habitat value of black mangroves for juvenile fish and invertebrates. Here we compare benthic epifauna, infauna, and nekton use of Spartina-dominated, Avicennia-dominated, and mixed Spartina and black mangrove habitats in two areas with varying densities and ages of black mangroves. Faunal samples and sediment cores were collected monthly from April to October in 2012 and 2013 from Horn Island, MS, and twice yearly in the Chandeleur Islands, LA. Multivariate analysis suggested benthic epifauna communities differed significantly between study location and among habitat types, with a significant interaction between the two fixed factors. Differences in mangrove and marsh community composition were greater at the Chandeleurs than at Horn Island, perhaps because of the distinct mangrove/marsh ecotone and the high density and age of mangroves there. Infaunal abundances were significantly higher at Horn Island, with tanaids acting as the main driver of differences between study locations. We predict that if black mangroves continue to increase in abundance in the northern GOM, estuarine faunal community composition could shift substantially because black mangroves typically colonize shorelines at higher elevations than smooth cordgrass, resulting in habitats of differing complexity and flooding duration.     

土地利用和气候变化对王家桥小流域径流的影响

以三峡库区内典型小流域王家桥为例,利用SWAT模型模拟土地利用和气候变化及两者共同作用对径流的影响。结果表明:扩大耕地、退耕还林、发展经济林的土地利用情景下,年均径流量较基准年变化率分别为15.13%、-13.99%、23.22%,退耕还林能有效调节和减少流域径流量;浓度路径为RCP2.6、RCP4.5、RCP8.5的气候变化情景下,径流量变化率分别为7.13%、7.78%、8.91%,径流量随未来温室气体和硫酸盐气溶胶排放情况增大而增加;两者综合情景下年均径流量均增加,2030年左右增幅较显著;对影响径流的因素进行方差分析,发现土地利用变化对径流的影响比气候变化的影响更显著。在未来气候变化背景下,可通过合理配置流域的土地利用类型,实现对流域水量平衡的调节。     

气候变化对新疆玛纳斯河流域水文水资源的影响

考虑积雪和降水不均等特点,对流域进行分带处理,提出和建立了包含积雪融雪结构的水文评价模型。利用该模型求得未来不同气候变化情况下的月均流量过程,分析气候变化对玛纳斯河流域水文水资源的影响。结果表明:若气温升高2℃,降水减少20%,则夏季径流减少52.59%,冬季径流减少1.77%,年均径流减少46.87%。     

1960-2005年南四湖流域气候变化趋势及其突变分析

利用1960-2005年南四湖流域22个气象台站逐月气温、降水量、 20 cm蒸发皿蒸发量资料, 分析了该流域近46 a来的气候变化趋势及其突变情况. 结果表明: 南四湖流域春季、冬季及年平均气温呈显著的上升趋势;四季及年降水量变化趋势不显著;四季及年蒸发皿蒸发量均呈显著的下降趋势, 且夏、春两季降幅大于秋、冬两季. 年平均气温在1985年发生了增温突变, 年降水量没有发生突变;年蒸发皿蒸发量在1969年和1985年发生年际突变. 利用单个环境因子的变化来解释蒸发皿蒸发量变化会产生偏颇.     

Atmospheric Circulation and Inland Flooding in Twentieth Century North Carolina,USA: Implications for Climate Change Impacts?

Widespread inland floods for 20th century North Carolina, USA were defined from stream flow records as events where flow was more than one standard deviation above the mean annual peak for at least two contiguous drainage basins simultaneously. Thirty-one events were identified. One snowmelt flood was detected. For the others, synoptic causes were identified from precipitation and circulation data. Eight events were directly related to hurricanes. Each required a precursor storm, often another hurricane, to provide sufficient precipitation to overcome the dry soils and low stream flows of the autumnal hurricane season. The decadal frequencies of these floods were poorly correlated with the total number of hurricanes, with no hurricane floods between 1955 and 1999 despite frequent hurricanes. Further, most events involved slow-moving decaying systems, not intense ones. An increase in hurricane intensity, often suggested as a consequence of climate change, may lead to fewer floods. The other floods were produced by either extra-tropical storms or squall lines, and precursor systems were also needed. These floods were common in the first and last three decades of the century, virtually absent in the middle four. This corresponded to a small dip in the total number of cyclones, and to periods of rising temperature statewide. This suggests a future increase in North Carolina floods as global temperatures increase. However, the synoptic causes of the relationship are not clear, and detailed quantitative analyses of recent events are required.     

Acute Effects of Drought on Emergent and Aquatic Communities in a Brackish Marsh

Plants and animals in brackish marshes are adapted to live within a wide, yet finite, range of conditions. Events that shift the environmental state beyond that range can dramatically alter habitats and, potentially, the numerous ecosystem services they provide. A prolonged exceptional drought in Texas (October 2010–January 2012) provided a unique opportunity to understand how brackish wetland habitats respond to an extreme environmental event. We examined marshes in the Lower Neches Wildlife Management Area (Texas, USA) that fell within the drought affected area, including restored areas and an adjacent reference marsh. To test our hypothesis that the brackish marsh community would be sensitive to drought conditions, we quantified emergent plant and submerged aquatic vegetation (SAV) and animal (invertebrates, fish) characteristics in summer 2010 and 2011. In spite of its severity, the exceptional drought of 2011 did not have a negative impact on emergent plant communities: biomass, stem density, and chlorophyll a concentrations were the same in pre-drought and drought years in all restored and reference areas. In contrast, SAV biomass was reduced by up to 100 % in the drought year. Some fish and invertebrate densities were also reduced by an order of magnitude or more, possibly due to the loss of SAV. Aquatic faunal species composition was markedly different in the drought year, largely due to the loss of the hydrobiid snail Probythinella protera and the gain of some marine species, including Gulf menhaden (Brevoortia patronus), brown shrimp (Farfantepenaeus aztecus), and white shrimp (Litopenaeus setiferus). By altering aquatic the plant and animal community, this drought event may subsequently reduce trophic support for higher consumers, or contribute to a decline in water quality. Restoration monitoring programs that only focus on relatively stress-resistant, emergent wetland plant communities may underestimate the sensitivity of these ecosystems to extreme environmental events like droughts.