Temporal and Spatial Dynamics of Estuarine Shoreline Change in the Albemarle-Pamlico Estuarine System,North Carolina,USA

Many shoreline studies rely on historical change rates determined from aerial imagery decades to over 50 years apart to predict shoreline position and determine setback distances for coastal structures. These studies may not illustrate the coastal impacts of short-duration but potentially high-impact storm events. In this study, shoreline change rates (SCRs) are quantified at five different sites ranging from marsh to sediment bank shorelines around the Albemarle-Pamlico estuarine system (APES) for a series of historical (decadal to 50-year) and short-term (bimonthly) time periods as well as for individual storm events. Long-term (historical) SCRs of approximately ?0.5 ± 0.07 m year^?1 are observed, consistent with previous work along estuarine shorelines in North Carolina. Short-term SCRs are highly variable, both spatially and temporally, and ranged from 15.8 ± 7.5 to ?19.3 ± 11.5 m year^?1 at one of the study sites. The influence of wave climate on the spatial and temporal variability of short-term erosion rates is investigated using meteorological observations and coupled hydrodynamic (Delft3D) and wave (SWAN) models. The models are applied to simulate hourly variability in the surface waves and water levels. The results indicate that in the fetch-limited APES, wind direction strongly influences the wave climate at the study sites. The wave height also has an influence on short-term SCRs as determined from the wave simulations for individual meteorological events, but no statistical correlation is found for wave height and SCRs over the long term. Despite the significantly higher rates of shoreline erosion over short time periods and from individual events like hurricanes, the cumulative impact over long time periods is low. Therefore, while the short-term response of these shorelines to episodic forcing should be taken into account in management plans, the long-term trends commonly used in ocean shoreline management can also be used to determine erosion setbacks on estuarine shorelines.     

Erosion and Flooding—Threats to Coastal Infrastructure in the Arctic: A Case Study from Herschel Island,Yukon Territory,Canada

Arctic coastal infrastructure and cultural and archeological sites are increasingly vulnerable to erosion and flooding due to amplified warming of the Arctic, sea level rise, lengthening of open water periods, and a predicted increase in frequency of major storms. Mitigating these hazards necessitates decision-making tools at an appropriate scale. The objectives of this paper are to provide such a tool by assessing potential erosion and flood hazards at Herschel Island, a UNESCO World Heritage candidate site. This study focused on Simpson Point and the adjacent coastal sections because of their archeological, historical, and cultural significance. Shoreline movement was analyzed using the Digital Shoreline Analysis System (DSAS) after digitizing shorelines from 1952, 1970, 2000, and 2011. For purposes of this analysis, the coast was divided in seven coastal reaches (CRs) reflecting different morphologies and/or exposures. Using linear regression rates obtained from these data, projections of shoreline position were made for 20 and 50 years into the future. Flood hazard was assessed using a least cost path analysis based on a high-resolution light detection and ranging (LiDAR) dataset and current Intergovernmental Panel on Climate Change sea level estimates. Widespread erosion characterizes the study area. The rate of shoreline movement in different periods of the study ranges from ?5.5 to 2.7 m·a^?1 (mean ?0.6 m·a^?1). Mean coastal retreat decreased from ?0.6 m·a^?1 to ?0.5 m·a^?1, for 1952–1970 and 1970–2000, respectively, and increased to ?1.3 m·a^?1 in the period 2000–2011. Ice-rich coastal sections most exposed to wave attack exhibited the highest rates of coastal retreat. The geohazard map combines shoreline projections and flood hazard analyses to show that most of the spit area has extreme or very high flood hazard potential, and some buildings are vulnerable to coastal erosion. This study demonstrates that transgressive forcing may provide ample sediment for the expansion of depositional landforms, while growing more susceptible to overwash and flooding.     

Living Shorelines Support Nearshore Benthic Communities in Upper and Lower Chesapeake Bay

Human population growth and sea-level rise are increasing the demand for protection of coastal property against shoreline erosion. Living shorelines are designed to provide shoreline protection and are constructed or reinforced using natural elements. While living shorelines are gaining popularity with homeowners, their ability to provide ecological services (e.g., habitat provision and trophic transfer) is not well understood, and information is needed to improve coastal and resource management decision-making. We examined benthic community responses to living shorelines in two case-study subestuaries of Chesapeake Bay using a before-after control-impact study design. At Windy Hill, a bulkhead was removed and replaced by three tombolos, sand fill, and native marsh vegetation. At Lynnhaven, 25 m of eroding marsh shoreline was stabilized with coir logs, sand fill, and native marsh vegetation. Communities of large (>?3 mm) infauna adjacent to living shorelines at both locations tended to increase in biomass by the end of the study period. Community compositions changed significantly following living shoreline construction at Windy Hill, reflecting a trend toward higher density and biomass of large bivalves at living shorelines compared to pre-construction. Increasing trends in density and biomass of clams and simultaneously decreasing density and decreasing trends in biomass of polychaetes suggest a transition toward stable infaunal communities at living shorelines over time, though longer-term studies are warranted.     

Net Primary Productivity of <Emphasis Type="Italic">Spartina densiflora</Emphasis> Brong in an SW Atlantic Coastal Salt Marsh

Coastal marshes are known as organic matter producers. The goal of this work is to study tiller demography, standing biomass, and net aerial primary productivity (NAPP) in a Spartina densiflora coastal wetland, using a method applied to permanent sample plots located at two sites differing in topographic location, a regularly flooded zone [relative low marsh (LM)] and an irregularly flooded one [relative high marsh (HM)]. Measurements were made every 2 months during the 2005–2007 period. The annual NAPP was estimated to be 2,599?±?705 gDW m^?2?year^?1 for the HM and 2,181?±?605 gDW m^?2?year^?1 and 602?±?154 gDW m^?2?year^?1 for the first and second period of the LM populations, respectively, showing a seasonal pattern reaching maximum values in summer. The reduced NAPP values of the LM sites in the second year was associated with an extremely high precipitation period related to the 2007–2008 El Niño event.     

The Declining Role of Organic Matter in New England Salt Marshes

The Northeast USA is experiencing severe impacts of a changing climate, including increased winter temperatures and accelerated relative sea level rise (RSLR). The sediment-poor, organic-rich nature of many Southern New England salt marshes makes them particularly vulnerable to these changes. In order to assess how marsh accretion has changed over time, we returned to Narragansett Bay, RI where salt marsh vertical accretion rates were documented almost 30 years ago. Using radionuclide tracers (²¹⁰Pb and ¹³⁷Cs), we observe no significant change in overall accretion rates (0.27–0.69 cm year^?1) compared to historical averages (0.24–0.60 cm year^?1), but we document a shift in how these marshes maintain elevation. Organic matter now plays a smaller role in contributing to vertical accretion across all study sites, declining by 22 % on average. We attribute this reduction to potentially higher decomposition rates fueled by higher water temperature. Inorganic matter also contributes less to accretion (declining by 44 % on average at marshes located more internal to the estuary), likely due to diminishing sediment supply in this region. With organic and inorganic solids accounting for less of the total accretion, several of the marshes are experiencing symptoms of swelling, with water and porespace contributing more towards accretion compared to historical values. Accretion rates (0.27–0.45 cm year^?1) at these organic-rich (>40 % sediment organic matter) marshes are predominantly lower than the current (30 years) rate of RSLR (0.41?±?0.07 cm year^?1). These results, combined with the increased rate of RSLR and the hardened shorelines inhibiting landward migration, call into question the long-term survivability of these marshes.     

Variability in Concentrations and Fluxes of Methane in the Indian Estuaries

In order to examine the fluxes of methane (CH₄) from the Indian estuaries, measurements were carried out by collecting samples from 26 estuaries along the Indian coast during high discharge (wet) and low water discharge (dry) periods. The CH₄ concentrations in the estuaries located along the west coast of India were significantly higher (113?±?40 nM) compared to the east coast of India (27?±?6 nM) during wet and dry periods (88?±?15 and 63?±?12 nM, respectively). Supersaturation of CH₄ was observed in the Indian estuaries during both periods ((0.18 to 22.3?×?10³ %). The concentrations of CH₄ showed inverse relation with salinity indicating that freshwater is a significant source. Spatial variations in CH₄ saturation were associated with the organic matter load suggesting that its decomposition may be another source in the Indian estuaries. Fluxes of CH₄ ranged from 0.01 to 298 μmol m^?2 day^?1 (mean 13.4?±?5 μmol m^?2 day^?1) which is ~30 times lower compared to European estuaries (414 μmol m^?2 day^?1). The annual emission from Indian estuaries, including Pulicat and Adyar, amounted to 0.39?×?10¹⁰ g CH₄?year^?1 with the surface area of 0.027?×?10⁶ km² which is significantly lower than that in European estuaries (2.7?±?6.8?×?10¹⁰ g CH₄?year^?1 with the surface area of 0.03?×?10⁶ km²). This study suggests that Indian estuaries are a weak source for atmospheric CH₄ than European estuaries and such low fluxes were attributed to low residence time of water and low decomposition of organic matter within the estuary. The CH₄ fluxes from the Indian estuaries are higher than those from Indian mangroves (0.01?×?10¹⁰ g CH₄?year^?1) but lower than those from Indian inland waters (210?×?10¹⁰ g CH₄?year^?1).     

Integration of geospatial technologies with RUSLE for analysis of land use/cover change impact on soil erosion: case study in Rib watershed,north-western highland Ethiopia

In recent times, soil erosion interlocked with land use and land cover (LULC) changes has become one of the most important environmental issues in developing countries. Evaluation of this complex interaction between LULC change and soil erosion is indispensable in land use planning and conservation works. This paper analysed the impact of LULC change on soil erosion in the north-western highland Ethiopia over the period 1986–2016. Rib watershed, the area with dynamic LULC change and severe soil erosion problem, was selected as a case study site. Integrated approach that combined geospatial technologies with revised universal soil loss equation model was utilized to evaluate the spatio-temporal dynamics of soil loss over the study period. Pixel-based overlay of soil erosion intensity maps with LULC maps was carried out to understand the change in soil loss due to LULC change. Results showed that the annual soil loss in the study area varied from 0 to 236.5 t ha^?1 year^?1 (tons per hectare per year) in 1986 and 0–807 t ha^?1 year^?1 in 2016. The average annual soil loss for the entire watershed was estimated about 40 t ha^?1 year^?1 in 1986 comparing with 68 t ha^?1 year^?1 in 2016, a formidable increase. Soil erosion potential that was estimated to exceed the average soil loss tolerance level increased from 34.5% in 1986 to 66.8% in 2016. Expansion of agricultural land at the expense of grassland and shrubland was the most detrimental factor for severe soil erosion in the watershed. The most noticeable change in soil erosion intensity was observed from cropland with mean annual soil loss amount increased to 41.38 t ha^?1 year^?1 in 2016 from 26.60 in 1986. Moreover, the most successive erosion problems were detected in eastern, south-eastern and northern parts of the watershed. Therefore, the results of this study can help identify the soil erosion hot spots and conservation priority areas at local and regional levels.     

Spatio-Temporal Patterns of Submerged Macrophytes in Three Hydrologically Altered Mediterranean Coastal Lagoons

Rice cultivation in the Ebro Delta (Catalonia, Spain) has inverted the natural hydrological cycles of coastal lagoons and decreased water salinities for over 150 years. Adjustments in the water management practices—in terms of source and amount of freshwater inputs—have resulted in changes in the diversity, distribution and productivity of submerged angiosperms. Between the 1970s and late 1980s, a massive decline of the aquatic vegetation occurred in the Encanyissada–Clot and Tancada lagoons, but little information on the status is available after the recovery of macrophytes in the 1990s. Here, we evaluate the influence of salinity regimes resulting from current water management practices on the composition, distribution, seasonal abundance and flowering rates of submersed macrophytes, as well as on the occurrence of epiphyte and drift macroalgae blooms in three coastal lagoons. Our results show that Ruppia cirrhosa is the dominant species in the Encanyissada lagoon (185.97?±?29.74 g?DW?m^?2?year^?1; 12–27?‰ salinity) and the only plant species found in the Tancada lagoon (53.26?±?10.94 g?DW?m²?year^?1; 16–28?‰ salinity). Flowering of R. cirrhosa (up to 1,011?±?121 flowers?m^?2) was only observed within the Encanyissada and suggests that mesohaline summer conditions may favor these events. In contrast, low salinities in Clot lagoon (～3–12?‰) favor the development of Potamogeton pectinatus (130.53?±?13.79 g?DW?m²?year^?1) with intersperse R. cirrhosa (8.58?±?1.71 g?DW?m^?2) and mixed stands of P. pectinatus and Najas marina (up to ～57 g?DW?m^?2?year^?1) in some reduced areas. The peak biomasses observed during the study are 88 to 95 % lower than maximum values reported in the literature at similar salinities, and there is also little or no recovery in some areas compared to last reports more than 20 years ago. The main management actions to restore the natural diversity and productivity of submersed angiosperms, such as the recovering of the seagrass Zostera noltii, should be the increase of salinity during the period of rice cultivation, by reducing freshwater inputs and increasing flushing connections with the bays.     

Temporal Changes in Seawater Carbonate Chemistry and Carbon Export from a Southern California Estuary

Estuaries are important subcomponents of the coastal ocean, but knowledge about the temporal and spatial variability of their carbonate chemistry, as well as their contribution to coastal and global carbon fluxes, are limited. In the present study, we measured the temporal and spatial variability of biogeochemical parameters in a saltmarsh estuary in Southern California, the San Dieguito Lagoon (SDL). We also estimated the flux of dissolved inorganic carbon (DIC) and total organic carbon (TOC) to the adjacent coastal ocean over diel and seasonal timescales. The combined net flux of DIC and TOC (F_DIC?+?TOC) to the ocean during outgoing tides ranged from ??1.8±0.5?×?10³ to 9.5±0.7?×?10³?mol C h^?1 during baseline conditions. Based on these fluxes, a rough estimate of the net annual export of DIC and TOC totaled 10±4?×?10⁶?mol C year^?1. Following a major rain event (36 mm rain in 3 days), F_DIC?+?TOC increased and reached values as high as 29.0 ±?0.7?×?10³?mol C h^?1. Assuming a hypothetical scenario of three similar storm events in a year, our annual net flux estimate more than doubled to 25 ±?4?×?10⁶?mol C year^?1. These findings highlight the importance of assessing coastal carbon fluxes on different timescales and incorporating event scale variations in these assessments. Furthermore, for most of the observations elevated levels of total alkalinity (TA) and pH were observed at the estuary mouth relative to the coastal ocean. This suggests that SDL partly buffers against acidification of adjacent coastal surface waters, although the spatial extent of this buffering is likely small.     

Organic carbon stocks and erosion in the soils of Guangdong,South China

Soil organic carbon (SOC) storage and erosion in South China at the regional scale in the past decades remains far from being understood. This paper calculated the SOC density, storage and erosion in 14 soil classes in Guangdong Province, South China, based on statistical data from the soil survey and soil erosion survey of Guangdong, which was performed in the 1990s. The purpose of this study is to understand the relationships between soil classes and SOC erosion at the regional scale. The results indicated that the SOC density in the soils of Guangdong varied from 12.7 to 144.9 Mg ha^?1 over the entire profile and from 12.6 to 68.4 Mg ha^?1 in the top 20-cm soil layer. The average area-weighted SOC density in the topsoil (0–20 cm) and the entire profile was 32 ± 3 and 86 ± 4 Mg ha^?1, respectively. The total SOC storage was 1.27 ± 0.06 Pg, with 35.6 % (0.46 ± 0.04 Pg) located in the topsoil. The average area-weighted strength of the SOC erosion in the 1990s was 20.6 ± 0.8 Mg km^?2 year^?1. The results indicated that SOC erosion was strongly related to soil class.     

Influence of Shoreline Stabilization Structures on the Nearshore Sedimentary Environment in Mesohaline Chesapeake Bay

Shorelines around many estuaries and coastal embayments are rapidly eroding (approximately several meters/year), with more rapid erosion rates expected in the future due to natural and anthropogenic stressors. In response, a variety of techniques have been used to stabilize shorelines, but there are limited quantitative, long-term data available about their effects on the sedimentary environment immediately adjacent to them (i.e., the nearshore). This study evaluated changes in sediment characteristics (mud and organic content) and accumulation rates associated with installation of breakwaters, riprap, and living shorelines with (“hybrid”) and without (“soft”) a structural component. ²¹⁰Pb (half-life 22.3 years) geochronologies were used to identify horizons in core profiles that corresponded to years when structures were built. Sites with naturally eroding shorelines (i.e., no structures) were used as a control group at which any sedimentary changes represent broad environmental trends, in contrast to changes at the protected sites that also include the influence of structures. Observations were placed within the context of modeled wave climate, shoreline-erosion rates, land use, dominant sediment source, and the apparent effect on submersed aquatic vegetation (SAV) inhabiting the nearshore sedimentary environment. The main conclusion of this study is that there was no “one size fits all” answer to anticipated impacts of structures on nearshore sedimentary environments. Instead, specific changes associated with structures depended on individual site characteristics, but could be predicted with multiple linear regression models that included structure type, shoreline-erosion rate, dominant sediment source, and land use. Riprap or breakwater installation had either positive or no obvious impact on SAV at six of seven sites but negatively impacted SAV at one riprapped site. No obvious impacts on SAV were observed at living shoreline sites.     

Mapping Shoreline Variability of Two Barrier Island Segments Along the Florida Coast

Recent projections of global climate change necessitate improved methodologies that quantify shoreline variability. Updated analyses of shoreline movement provide important information that can aid and inform likely intervention policies. This paper uses the Analyzing Moving Boundaries Using R (AMBUR) technique to evaluate shoreline change trends over the time period 1856 to 2015. Special emphasis was placed on recent rates of change, during the 1994 to 2015 period of active storm conditions. Small segments, on the order of tens of kilometers, along two sandy barrier island regions on Florida’s Gulf and Atlantic coasts were chosen for this study. The overall average rate of change over the 159-year period along Little St. George Island was ??0.62?±?0.12 m/year, with approximately 65% of shoreline segments eroding and 35% advancing. During periods of storm clustering (1994–2015), retreat rates along portions of this Gulf coast barrier accelerated to ??5.49?±?1.4 m/year. Along the northern portion of Merritt Island on Florida’s Atlantic coast, the overall mean rate of change was 0.22?±?0.08 m/year, indicative of a shoreline in a state of relative dynamic equilibrium. In direct contrast with the Gulf coast shoreline segment, the majority of transects (65%) evaluated along the oceanfront of Merritt Island over the long term displayed a seaward advance. Results indicate that episodes of clustered storm activity with fairly quick return intervals generally produce dramatic morphological alteration of the coast and can delay natural beach recovery. Additionally, the data show that tidal inlet dynamics, shoreline orientation, along with engineering projects, act over a variety of spatial and temporal scales to influence shoreline evolution. Further, the trends of shoreline movement observed in this study indicate that nearshore bathymetry—the presence of shoals—wields some influence on the behavior of local segments of the shoreline.     

Unprecedented rates of landslide and surface erosion along a newly constructed road in Yunnan, China

Field measurements conducted 4 years after the construction of a new portion of the Weixi?CShangri-La road in Yunnan, China, reveal that unprecedented rates of mass wasting occurred along the road with much of this sediment directly impacting the headwaters of the Mekong River. Landslide erosion (including dry ravel) exceeded 33,000 t ha^?1 year^?1 along the most severely eroded sections of the road and averaged more than 9,600 t ha^?1 year^?1 along the surveyed 23.5 km of road; these values are the highest ever reported for road-related landslides. While surface erosion was only about 7% of the total erosion from the road, it is still more than an order of magnitude higher than typical surface erosion rates from disturbed lands in Southeast Asia. Combined landslide and surface erosion from this road delivered an estimated 19 times more sediment to the river than the remaining 99.6% of the contributing catchment. These sediment inputs are aggrading local channels, promoting downstream sediment transport, degrading aquatic habitat, and creating the possibility for a future debris flood or hyperconcentrated flow.     

Geospatial assessment of soil erosion intensity and sediment yield: a case study of Potohar Region,Pakistan

Estimation of spatial extent of soil erosion, one of the most serious forms of land degradation, is critical because soil erosion has serious implications on soil fertility, water ecosystem, crop productivity and landscape beauty. The primary objective of the current study was to assess and map the soil erosion intensity and sedimentation yield of Potohar region of Pakistan. Potohar is the rainfed region with truncated and complex topography lying at the top of the Indus Basin, the world’s largest irrigation networks of canals and barrages. Spatially explicit Revised Universal Soil Loss Equation (RUSLE) Model integrated with Remote Sensing-GIS techniques was used for detecting/mapping of erosion prone areas and quantification of soil losses. The results show that the Potohar region is highly susceptible to soil erosion with an average annual soil loss of 19 tons ha^?1 year^?1 of which the maximum erosion (70–208 tons ha^?1 year^?1) was near the river channels and hilly areas. The sediment yield due to the erosion is as high as 148 tons ha^?1 year^?1 with an average of 4.3 tons ha^?1 year^?1. It was found that 2.06% of the total area falls under severe soil erosion, 13.34% under high erosion, 15.35% under moderate soil erosion while 69.25% of the area lies in the low (tolerable) soil erosion. Chakwal and Jhelum districts of the region are seriously affected by erosion owing to their topography and soil properties. The information generated in this study is a step forward towards proper planning and implementation of strategies to control the erosion and for protection of natural resources. It is, hence, necessary that suitable water harvesting structures be made to control water to prevent soil erosion and provision of water in the lean season in this region. Tree plantation and other erosion control practices such as strip cropping can also minimize soil erosion in this region.     

Assessing Nitrogen Dynamics Throughout the Estuarine Landscape

Assessing nitrogen dynamics in the estuarine landscape is challenging given the unique effects of individual habitats on nitrogen dynamics. We measured net N₂ fluxes, sediment oxygen demand, and fluxes of ammonium and nitrate seasonally from five major estuarine habitats: salt marshes, seagrass beds (SAV), oyster reefs, and intertidal and subtidal flats. Net N₂ fluxes ranged from 332?±?116 μmol?N-N₂?m^?2?h^?1 from oyster reef sediments in the summer to ?67?±?4 μmol?N-N₂?m^?2?h^?1 from SAV in the winter. Oyster reef sediments had the highest rate of N₂ production of all habitats. Dissimilatory nitrate reduction to ammonium (DNRA) was measured during the summer and winter. DNRA was low during the winter and ranged from 4.5?±?3.0 in subtidal flats to 104?±?34 μmol?¹⁵NH ₄ ⁺ ?m^?2?h^?1 in oyster reefs during the summer. Annual denitrification, accounting for seasonal differences in inundation and light, ranged from 161.1?±?19.2 mmol?N-N₂?m^?2?year^?1 for marsh sediments to 509.9?±?122.7 mmol?N-N₂?m^?2?year^?1 for SAV sediments. Given the current habitat distribution in our study system, an estimated 28.3?×?10⁶?mol of N are removed per year or 76 % of estimated watershed nitrogen load. These results indicate that changes in the area and distribution of habitats in the estuarine landscape will impact ecosystem function and services.     

Evaluating Light Availability,Seagrass Biomass,and Productivity Using Hyperspectral Airborne Remote Sensing in Saint Joseph’s Bay,Florida

Seagrasses provide a number of critical ecosystem services, including habitat for numerous species, sediment stabilization, and shoreline protection. Ariel photography is a useful tool to estimate the areal extent of seagrasses, but recent innovations in radiometrically calibrated sensors and algorithm development have allowed identification of benthic types and retrieval of absolute density. This study demonstrates the quantitative ability of a high spatial resolution (1 m) airborne hyperspectral sensor (3.2 nm bandwidth) in the complex coastal waters of Saint Joseph’s Bay (SJB). Several benthic types were distinguished, including submerged and floating aquatic vegetation, benthic red algae, bare sand, and optically deep water. A total of 23.6 km² of benthic vegetation was detected, indicating no dramatic change in vegetation area over the past 30 years. SJB supported high seagrass density at depths shallower than 2 m with an average leaf area index of 2.0?±?0.6 m² m^?2. Annual seagrass production in the bay was 13,570 t C year^?1 and represented 41 % of total marine primary production. The effects of coarser spatial resolution were investigated and found to reduce biomass retrievals, underestimate productivity, and alter patch size statistics. Although data requirements for this approach are considerable, water column optical modeling may reduce the in situ requirements and facilitate the transition of this technique to routine monitoring efforts. The ability to quantify not just areal extent but also productivity of a seagrass meadow in optically complex coastal waters can provide information on the capacity of these environments to support marine food webs.     

Phosphorus biological cycle in the different Suaeda salsa marshes of the Yellow River estuary, China

Much uncertainty exists in the phosphorus (P) cycle in the marshes of the intertidal zone. This study explored the P cycling in the two Suaeda salsa marshes [middle S. salsa marsh (MSM) and low S. salsa marsh (LSM)] of the Yellow River estuary during April 2008 to November 2009. Results showed seasonal fluctuations and vertical distributions of P in different S. salsa marsh soils, and variations in P content in different parts of plants due to water and salinity status. The N/P ratios of the different S. salsa were 9.87 ± 1.23 and 15.73 ± 1.77, respectively, indicating that plant growth in MSM was limited by N, while that in LSM was limited by both N and P. The S. salsa litter in MSM released P to the environment throughout the year, while that in LSM immobilized P from the environment at all times. The P absorption coefficients of S. salsa in MSM and LSM were very low (0.0010 and 0.0001, respectively), while the biological cycle coefficients were high (0.739 and 0.812, respectively). The P turnovers among compartments of MSM and LSM showed that the uptake amounts of roots were 0.4275 and 0.0469 g m^?2 year^?1 and the values of aboveground parts were 1.1702 and 0.1833 g m^?2 year^?1, the re-translocation quantities from aboveground parts to roots were 0.8544 and 0.1452 g m^?2 year^?1, the translocation amounts from roots to soil were 0.0137 and 0.0012 g m^?2 year^?1, the translocation quantities from aboveground living bodies to litter were 0.3157 and 0.0381 g m^?2 year^?1, and the annual return quantities from litter to soil were less than 0.0626 and ?0.0728 g m^?2 year^?1 (minus represented immobilization), respectively. P was an important limiting factor in S. salsa marshes, especially in LSM. S. salsa was seemingly well adapted to the low-nutrient condition and the vulnerable habitat, and the nutrient enrichment due to the import of N and P from the Yellow River estuary would be a potential threat to the S. salsa marshes.     

The Impact of Sediment and Carbon Fluxes on the Biogeochemistry of Methane and Sulfur in Littoral Baltic Sea Sediments (Himmerfjärden, Sweden)

Three sediment stations in Himmerfjärden estuary (Baltic Sea, Sweden) were sampled in May 2009 and June 2010 to test how low salinity (5–7 ‰), high primary productivity partially induced by nutrient input from an upstream waste water treatment plant, and high overall sedimentation rates impact the sedimentary cycling of methane and sulfur. Rates of sediment accumulation determined using ²¹⁰Pb_excess and ¹³⁷Cs were very high (0.65–0.95 cm?year^?1), as were the corresponding rates of organic matter accumulation (8.9–9.5 mol C?m^?2?year^?1) at all three sites. Dissolved sulfate penetrated <20 cm below the sediment surface. Although measured rates of bicarbonate methanogenesis integrated over 1 m depth were low (0.96–1.09 mol?m^?2?year^?1), methane concentrations increased to >2 mmol?L^?1 below the sulfate–methane transition. A steep gradient of methane through the entire sulfate zone led to upward (diffusive and bio-irrigative) fluxes of 0.32 to 0.78 mol?m^?2?year^?1 methane to the sediment–water interface. Areal rates of sulfate reduction (1.46–1.92 mol?m^?2?year^?1) integrated over the upper 0–14 cm of sediment appeared to be limited by the restricted diffusive supply of sulfate, low bio-irrigation (α?=?2.8–3.1 year^?1), and limited residence time of the sedimentary organic carbon in the sulfate zone. A large fraction of reduced sulfur as pyrite and organic-bound sulfur was buried and thus escaped reoxidation in the surface sediment. The presence of ferrous iron in the pore water (with concentrations up to 110 μM) suggests that iron reduction plays an important role in surface sediments, as well as in sediment layers deep below the sulfate–methane transition. We conclude that high rates of sediment accumulation and shallow sulfate penetration are the master variables for biogeochemistry of methane and sulfur cycling; in particular, they may significantly allow for release of methane into the water column in the Himmerfjärden estuary.     

Contrasting Decadal-Scale Changes in Elevation and Vegetation in Two Long Island Sound Salt Marshes

Northeastern US salt marshes face multiple co-stressors, including accelerating rates of relative sea level rise (RSLR), elevated nutrient inputs, and low sediment supplies. In order to evaluate how marsh surface elevations respond to such factors, we used surface elevation tables (SETs) and surface elevation pins to measure changes in marsh surface elevation in two eastern Long Island Sound salt marshes, Barn Island and Mamacoke marshes. We compare marsh elevation change at these two systems with recent rates of RSLR and find evidence of differences between the two sites; Barn Island is maintaining its historic rate of elevation gain (2.3?±?0.24 mm year^?1 from 2003 to 2013) and is no longer keeping pace with RSLR, while Mamacoke shows evidence of a recent increase in rates (4.2?±?0.52 mm year^?1 from 1994 to 2014) to maintain its elevation relative to sea level. In addition to data on short-term elevation responses at these marshes, both sites have unusually long and detailed data on historic vegetation species composition extending back more than half a century. Over this study period, vegetation patterns track elevation change relative to sea levels, with the Barn Island plant community shifting towards those plants that are found at lower elevations and the Mamacoke vegetation patterns showing little change in plant composition. We hypothesize that the apparent contrasting trend in marsh elevation at the sites is due to differences in sediment availability, salinity, and elevation capital. Together, these two systems provide critical insight into the relationships between marsh elevation, high marsh plant community, and changing hydroperiods. Our results highlight that not all marshes in Southern New England may be responding to accelerated rates of RSLR in the same manner.     

Beach erosion under rising sea‐level modulated by coastal geomorphology and sediment availability on carbonate reef‐fringed island coasts

This study addresses gaps in understanding the relative roles of sea‐level change, coastal geomorphology and sediment availability in driving beach erosion at the scale of individual beaches. Patterns of historical shoreline change are examined for spatial relationships to geomorphology and for temporal relationships to late‐Holocene and modern sea‐level change. The study area shoreline on the north‐east coast of Oahu, Hawaii, is characterized by a series of kilometre‐long beaches with repeated headland‐embayed morphology fronted by a carbonate fringing reef. The beaches are the seaward edge of a carbonate sand‐rich coastal strand plain, a common morphological setting in tectonically stable tropical island coasts. Multiple lines of geological evidence indicate that the strand plain prograded atop a fringing reef platform during a period of late‐Holocene sea‐level fall. Analysis of historical shoreline changes indicates an overall trend of erosion (shoreline recession) along headland sections of beach and an overall trend of stable to accreting beaches along adjoining embayed sections. Eighty‐eight per cent of headland beaches eroded over the past century at an average rate of ?0·12 ± 0·03 m yr^?1. In contrast, 56% of embayed beaches accreted at an average rate of 0·04 ± 0·03 m yr^?1. Given over a century of global (and local) sea‐level rise, the data indicate that embayed beaches are showing remarkable resiliency. The pattern of headland beach erosion and stable to accreting embayments suggests a shift from accretion to erosion particular to the headland beaches with the initiation of modern sea‐level rise. These results emphasize the need to account for localized variations in beach erosion related to geomorphology and alongshore sediment transport in attempting to forecast future shoreline change under increasing sea‐level rise.