Tidal dispersal of salt marsh insect larvae within the Westerschelde estuary

In the Westerschelde estuary, salt marshes are present as isolated patches fringing the estuary. In the present paper tidal transport of stem-boring larvae of Agapanthia villosoviridescens (Coleoptera) from salt marshes of the upper reaches of the Westerschelde estuary to marshes of the lower reaches is demonstrated. The evidence for the origin of the larvae is based on comparisons of growth and development characteristics of larvae found in flood debris belts and resident larval populations. These characteristics are different on the various salt marshes along the Westerschelde, probably as a result of estuarine gradients. Additional evidence for the larval origin comes from the plant composition of the flood debris. The occurrence of upward tidal transport is discussed. Considering the comparatively large area of salt marshes in the upper estuary, tidal dispersal of larvae probably will be dominated by transport in seaward direction. So far, very little is known on the role of tidal currents with regard to the exchange between salt marsh populations. The present results suggest that tidal transport may not only be important for dispersal of aquatic organisms in an estuary, but also for organisms inhabiting the semi-terrestrial estuarine salt marshes.     

Role of biotic interactions and physical factors in determining the distribution of marsh species along an estuarine salinity gradient

Understanding the mechanisms that shape plant distribution patterns is a major goal in ecology. We investigated the role of biotic interactions (competition and facilitation) and abiotic factors in creating horizontal plant zonation along salinity gradients in the Elbe estuary. We conducted reciprocal transplant experiments with four dominant species from salt and tidal freshwater marshes at two tidal elevations. Ten individuals of each species were transplanted as sods to the opposing marsh type and within their native marsh (two sites each). Transplants were placed at the centre of 9‐m² plots along a line parallel to the river bank. In order to disentangle abiotic and biotic influences, we set up plots with and without neighbouring vegetation, resulting in five replicates per site. Freshwater species (Bolboschoenus maritimus and Phragmites australis) transplanted to salt marshes performed poorly regardless of whether neighbouring vegetation was present or not, although 50–70% of the transplants did survive. Growth of Phragmites transplants was impaired also by competition in freshwater marshes. Salt marsh species (Spartina anglica and Puccinellia maritima) had extremely low biomass when transplanted to freshwater marshes and 80–100% died in the presence of neighbours. Without neighbours, biomass of salt marsh species in freshwater marshes was similar to or higher than that in salt marshes. Our results indicate that salt marsh species are precluded from freshwater marshes by competition, whereas freshwater species are excluded from salt marshes by physical stress. Thus, our study provides the first experimental evidence from a European estuary for the general theory that species boundaries along environmental gradients are determined by physical factors towards the harsh end and by competitive ability towards the benign end of the gradient. We generally found no significant impact of competition in salt marshes, indicating a shift in the importance of competition along the estuarine gradient.     

Spatial and temporal changes in salt marsh distribution in the Dee estuary,NW England,determined from aerial photographs

Vegetation changes in salt marsh communities of the Dee estuary, northwest England, were analysed with a combination of remote sensing techniques using data dating back to the 1950s. The distribution of communities in 1997 was classified using Airborne Thematic Mapper data and used to develop a methodology for the analysis of black and white photographs of the marsh. These methods were then applied retrogressively to a time sequence of monochrome photographs running from 1955 to 1975. At the apex of the salt marshes on the English shore of the Dee estuary, the marsh expanded dramatically to 1975, and consisted predominantly of pioneer and low marsh vegetation types. Between 1975 and 1997, however, there was only a slight increase in salt marsh area, but with an increase in mid and high marsh vegetation, replacing pioneer marsh. In a second area of the salt marsh on the English shore, a different pattern of salt marsh expansion was observed. The area occupied by marsh continued to increase right up to 1997, with extensive pioneer vegetation suggesting a process of continuing expansion. However, the pattern of marsh colonisation appeared to be different in 1997 compared to 1975. The significance of the changes in salt marsh distribution within the Dee estuary are discussed in relation to the historical pattern of salt marsh colonisation, the importance of Spartina anglica in the process and the implications for strategic management of the estuarine resources.     

The environmental consequences of climatic change on British salt marsh vegetation

The present relationship between sea level and the zonation of salt marsh vegetation is discussed in terms of the salt marshes of the Essex and Kent coasts. These marshes are already decreasing in area as a result of a number of different environmental pressures, including the sinking of the land relative to the sea, at a rate of about 3 mm per year, the result of isostatic adjustment following the last glaciation. Because most British salt marshes are backed by a sea wall the marshes can not respond to rising sea levels by migrating landwards, thus increasing the impact of sea level change. In view of this and of the importance of salt marshes as protection for the sea walls themselves, a conceptual model has been developed, of the likely impact of climate change and the resulting sea level rise, on British salt marsh vegetation. The basis of this approach is the assumption that a rise in sea level will cause the drowning of certain existing vegetation zones and their subsequent replacement by new vegetation types appropriate to the changed sea level. Estimates have been made of the likely impact of rises in sea level of 0.5, 1.0 and 1.5 metres on the five major vegetation zones identified in East Anglia. The validity of this approach is discussed, together with the likely additive effect of present degenerative changes observed in the Essex salt marshes. It is estimated that over the next 60 years a sea level rise of only 0.5 m, when existing degeneration is taken to account, would cause a loss of over 40% of the present area of salt marsh in Essex and probably also in Kent. These losses would mainly effect the higher salt marsh vegetation zones which would be replaced by pioneer communities. These predictions would be greatly magnified by larger rises in sea level. The wider ecological implication of these changes and some possible remedial measures are considered. These predictions are discussed in relation to the situation in the rest of East Anglia and for Britain as a whole.     

Tagus estuary salt marshes feedback to sea level rise over a 40-year period: Insights from the application of geochemical indices

Sea level rise (SLR) has been evaluated using data acquired from two Tagus estuary salt marshes. Sediment accumulation rates over a 40-year study period were determined using ¹³⁷Cs along with an evaluation of several geochemical indices and ratios as proxies of the mechisms underlying these SAR variations. Correlating SLR data from 1963 to 2001 with the sediment accretion rates (SARs) an inverse pattern of interaction was observed, with lower SAR associated to periods of higher mean sea level (MSL) heights. This pointed out to an erosion effect of the salt marsh during higher tidal flooding. Although SLR apparently slows down SAR, it still presents a positive balance with SLR, similar to that identified in most mesotidal estuaries. The geochemical analysis of sediments and chemical alteration index (CAI) also suggest that the major processes inherent to the SAR vary inversely, being mostly based by physical disturbances. Geochemical ratio-based indices showed that both salt marshes presented enhanced high-energy transport driven inputs of sediments, although in Pancas salt marsh there is a slight evidence of chemical weathering of the sediments. Anthropogenic contamination of the sediments by heavy metals was identified and has been decreasing from 1963 to 2001, mostly linked to a marked reduction of industrial activities in some areas surrounding the Tagus estuary, rather than the sedimentary history of the estuary.     

The issue of outwelling in the Guadiana River estuary (Portugal): some findings and research suggestions in the context of recent evidence

The “Outwelling Theory” states that salt marshes play a major role in exporting production to adjacent estuarine and coastal ecosystems. However, it has been found that some marshes act as net importers instead of net exporters of organic matter and nutrients. Once we include mangroves and refine the analysis to comprehend bacterioplankton, organic and stable isotope tracers, the picture became, more complex, making room for a revival of the outwelling idea. The exchanges between the Castro Marim salt marsh and the main estuary were tentatively established determining periodically, in a selected cross-section, the concentrations of TSS, FSS, VSS, NH₄, NO₂, NO₃, N_Kjeldhal, SiO₄, PO₄, TDP, Chlorophyll a and Pheopigments, measuring their fluxes along tidal cycles and computing the corresponding budgets. Apparently, the sedimentary behaviour of the marsh will be close to equilibrium during the period of study. However, it will import mainly inert matter and export mainly organic matter in the same period. Moreover, extrapolating these results to the entire Guadiana salt marshes, the exchanges of sediment do not seem to be significant. Particularly, the marshes will not trap a significantly amount sediment transported by the main river (0.5%). It also seems to follow, that in a general way, the Guadiana salt marshes might have a more significant role than was anticipated in the system economy of OM and nutrients and their outwelling to coastal waters, assuring outputs that could amount to something like 6% of the river load of N, 1.2% of the river load of P, and 20-57% of the river load of TOC, for an average year, and 42% of the river load of N and 35% of the river load of P in a dry year. These findings suggest that a more detailed investigation, over an extended period of time, is certainly worthwhile.     

High site fidelity and low site connectivity in temperate salt marsh fish populations: a stable isotope approach

Adult and juvenile fish utilise salt marshes for food and shelter at high tide, moving into adjacent sublittoral regions during low tide. Understanding whether there are high levels of site fidelity for different species of coastal fish has important implications for habitat conservation and the design of marine protected areas. We hypothesised that common salt marsh fish species would demonstrate a high site fidelity, resulting in minimal inter-marsh connectivity. Carbon (¹³C) and nitrogen (¹⁵N) stable isotope ratios of larvae and juveniles of five common salt marsh fish (Atherina presbyter, Chelon labrosus, Clupea harengus, Dicentrarchus labrax, Pomatoschistus microps), seven types of primary producer and seven secondary consumer food sources were sampled in five salt marshes within two estuary complexes along the coast of south-east England. Significant differences in ¹³C and ¹⁵N signatures between salt marshes indicated distinct sub-populations utilising the area of estuary around each salt marsh, and limited connectivity, even within the same estuary complex. ¹⁵N ratios were responsible for the majority of inter-marsh differences for each species and showed similar site-specific patterns in ratios in primary producers, secondary consumers and fish. Fish diets (derived from isotope mixing models) varied between species but were mostly consistent between marsh sites, indicating that dietary shifts were not the source of variability of the inter-marsh isotopic signatures within species. These results demonstrate that for some common coastal fish species, high levels of site fidelity result in individual salt marshes operating as discrete habitats for fish assemblages.     

Impacts of Seawalls on Saltmarsh Plant Communities in the Great Bay Estuary,New Hampshire USA

Seawalls are often built along naturally dynamic coastlines, including the upland edge of salt marshes, in order to prevent erosion or to extend properties seaward. The impacts of seawalls on fringing salt marshes were studied at five pairs of walled and natural marshes in the Great Bay Estuary of New Hampshire, USA. Marsh plant species and communities showed no difference in front of walls when compared with similar elevations at paired controls. However, seawalls eliminated the vegetative transition zone at the upper border. Not only did the plant community of the transition zone have high plant diversity relative to the low marsh, but it varied greatly from site to site in the estuary. The effects of seawall presence on other marsh processes, including sediment movement, wrack accumulation, groundwater flow, and vegetation distribution and growth, were examined. Although no statistically significant effects of seawalls were found, variation in the indicators of these processes were largely controlled by wave exposure, site-specific geomorphology and land use, and distance of the sampling station from the upland. Trends indicated there was more sediment movement close to seawalls at high energy sites and less fine grain sediment near seawalls. Both trends are consistent with an increase in energy from wave reflection. The distribution of seawalls bordering salt marshes was mapped for Great and Little Bays and their rivers. Throughout the study area, 3.54% of the marshes were bounded by shoreline armoring (5876 m of seawalls along 165.8 km of marsh shoreline). Localized areas with high population densities had up to 43% of marshes bounded by seawalls. Coastal managers should consider limiting seawall construction to preserve plant diversity at the upper borders of salt marshes and prevent marsh habitat loss due to transgression associated with sea level rise.     

Species zonation in Corroios salt marsh in the Tagus estuary (Portugal) and its dynamics in the past fifty years

The objective of this study was to understand the main factors controlling salt marsh plant species structure and dynamics. So, we determined plant cover and composition across a permanent transect, 450 m long and 1 m wide, defined in 1951 in Corroios salt marsh, in the Tagus estuary (Portugal) and we characterized the physicochemical variables every 50 m along this transect. Based on those results we discuss the dynamic and evolution of salt marsh vegetation during the last 50 years comparing former and recent data. The results showed that differences in salinity and flooding were determinant factors in plant species composition and distribution along the studied transect. In addition, long-term variations of these factors as a consequence of vertical accretion and sea level rise seem to be responsible for the evolution in plant structure and vegetation zonation patterns, during the last 50 years in the Tagus estuary salt marshes.     

Restoration of the Sieperda Tidal Marsh in the Scheldt Estuary, The Netherlands

Because of land reclamation, reinforcement of dikes, and the deepening of shipping channels, large areas of tidal marshes have been removed or eroded from the Scheldt estuary during the last two centuries. Tidal wetland restoration contributes toward compensating this loss of habitat. Not all restoration projects are meticulously planned, however; some are forced by nature. During a severe storm in 1990, a dike was breached in the brackish part of the Scheldt estuary and returned tidal influence to the Sieperda polder. In the 10 years since the dike breach, the former polder has changed into a brackish tidal marsh. Here we report on the geomorphologic and ecological developments that have taken place in the marsh. Tidal intrusion into the former polder turned crop fields into mudflats and changed pastures into salty marsh vegetation. The digging of a new creek improved marsh hydrology and enhanced tidal intrusion further into the marsh. Macrofauna typical of estuarine mudflats established rapidly in the developing marsh. Vegetation succession took place rapidly. Within 5 years, large areas of mudflats became covered with marsh vegetation. Birds characteristic of salt marshes were observed breeding or seen foraging in the marsh. The number of wading birds declined as areas of mudflat became overgrown. It is demonstrated that tidal flow is the engine to tidal marsh restoration. Tidal influence caused geomorphologic changes, which directed ecological developments in the former polder.     

Short-term sedimentation in Tagus estuary,Portugal: the influence of salt marsh plants

Short-term sediment deposition was studied at four salt marsh areas in the Tagus estuary. In areas covered with Sarcocornia perennis, Sarcocornia fruticosa, Halimione portulacoides and Spartina maritima and also in the non-vegetated areas, sedimentation was measured as the monthly accumulation of sediments on nylon filters anchored on the soil surface, from August 2000 to May 2001. Our experiments were used also to determine the influence of the different plant species in vertical accretion rates. Short-term sedimentation rates (from 2.8 to 272.3 g m⁻² d⁻¹) did show significant differences when the four salt marshes studied in the Tagus estuary were compared to each others. Salt marshes closer to the sediment sources had higher sedimentation rates. Our results suggest that the salt marsh type and surface cover may provide small-scale variations in sedimentation and also that sediment deposition values do change according to the position of the different plant species within the salt marsh. Sedimentation is an essential factor in salt marsh vertical accretion studies and our investigation may provide support to help forecast the adaptative response of the Tagus estuary wetlands to future sea level rise.     

Is the salt marsh vegetation a determining factor in the sedimentation processes?

Many authors have referred to the important role of vegetation in the consolidation of salt marsh sediments, but experiments previously carried out by us have shown results that do not always agree with these statements. In other words, the type of salt marsh surface coverage is not the main factor that contributes to the consolidation of sediments. To test this hypothesis different Portuguese salt marsh stations (species/unvegetated areas) from two sites, Tagus estuary (Corroios and Pancas) and Ria de Aveiro (Barra and Verdemilho), were compared to evaluate their influence on suspended matter deposition on the salt marsh surface. A short-term sedimentation study was performed within stands of Spartina maritima, Halimione portulacoides, Sarcocornia perennis subsp. perennis and unvegetated areas, by analysing the deposition of sediment material on nylon filters anchored to the marsh surface. Numerical results obtained from hydrodynamic models coupled to a Lagrangean module implemented for the Ria de Aveiro and the Tagus Estuary, namely the root-mean square velocity (V _rms) and residual velocity of tides, were also used. Average sedimentation rates (mean value between the different surface cover in a salt marsh) showed a seasonal trend more or less defined but with significantly different values between sites and salt marshes. Sedimentation rates varied between marshes: there are significant differences between Pancas and the other three marshes, but only significant differences in sedimentation rates between Spartina and Sarcocornia. Despite the important role of vegetation in the consolidation of salt marsh sediments, our results suggest that, the position of stations and related abiotic conditions in the salt marshes are determining factors of variation to take into account in the studies related with the stabilization and survival of salt marshes facing sea level rise. Handling editor: P. Viaroli     

中国滨海盐沼湿地碳收支与碳循环过程研究进展

滨海盐沼湿地由于其较高的初级生产力和较缓慢的有机质降解速率而成为缓解全球变暖的有效蓝色碳汇,近年来引起全球范围内的热切关注.我国滨海盐沼湿地分布较广,国内学者对滨海盐沼湿地碳循环及碳收支研究取得了一定进展,深入研究滨海盐沼湿地碳循环有助于对全球碳循环及全球变化的理解,并为利用滨海湿地进行碳的增汇减排提供科学依据.主要从我国滨海盐沼湿地碳循环主要观测方法、碳收支与碳循环过程及特点、碳库的组成与影响因素、气态碳的输入输出、潮汐作用对其碳收支的影响这5个方面出发,对国内的滨海盐沼湿地碳循环与碳收支的研究进展进行了归纳总结,并对今后的研究方向给出如下建议:(1)加强滨海盐沼湿地土壤碳库在深度上和广度上的研究;(2)标准化滨海盐沼湿地碳储量、碳通量的量化方法和观测技术;(3)在研究尺度上要宏观、微观并重,同时加强长期原位监测湿地碳通量的变化与室内模拟研究;(4)量化在潮汐影响下滨海盐沼湿地碳与邻近生态系统之间的横向交换通量.只有对我国滨海盐沼湿地碳库收支进行更准确的评估和长期的碳库动态变化监测,方可进一步认识我国盐沼湿地对全球气候变化的影响及其反馈作用,这对于预测全球变化及制定湿地碳储备功能的提升策略具有重要的意义.     

Salt marshes along the coast of The Netherlands

The area of salt marshes does no longer increase. The recent erosion coincides with a rise in MHT-level in the last 25 years. Despite the decrease in area, sedimentation continues, especially in the lower salt marsh, which acts as a sink of nitrogen. Assimilation and mineralization of nitrogen are in balance in most plant communities along the gradient from lower to higher salt marshes. Mineralization of nitrogen increases towards the higher salt marsh, whereas the above-ground production and the mean nitrogen content of plants decrease. There is a positive correlation between quality of food plants in salt marshes and breeding success of Brent geese in the arctic tundra. Sedimentation on mainland salt marshes can compensate for the expected sea level rise. This is not the case for island salt marshes, if the relative sea level rise is more than 0.5–1.0 cm yr⁻¹. The natural succession on salt marshes results in an accumulation of organic material, which is related to the dominance of single plant species. It is not clear to which extent this process is enhanced by eutrophication from acid deposition and seawater. Human exploitation of unprotected salt marshes is old and heavy in the system of mound settlements. Reclamation rates by dikes in the last centuries were higher than the rate of area increase. Grazing by cattle as a management practice results in both a higher plant species-richness and community diversity than abandoning; hay-making is intermediate, but shows less structural diversity than grazing with low stocking density. The invertebrate fauna is favoured by a short period of abandoning, but eventually characteristic salt marsh invertebrates are replaced by inland species. Many bird species prefer grazed salt marshes. The final section gives some perspectives. Provided that no further embankments take place the optimal nature management option for plants and animals is a vegetation pattern, which includes areas with a low canopy (grazed) and areas with a tall canopy.     

Development and N2-Fixing Activity of the Benthic Microbial Community in Transplanted Spartina alterniflora Marshes in North Carolina

Growth and maturation of transplanted salt marshes is often limited by the availability of nitrogen (N). We examined the role of N₂-fixing benthic microbial assemblages (microalgae and associated bacteria) in two restored marshes (1-year-old and 6-year-old marsh) and a natural salt marsh in the Newport River Estuary, North Carolina. Benthic N₂ fixation (nitrogenase activity, NA), chlorophyll a (Chl a ) concentration, Spartina alterniflora (smooth cordgrass) stem counts, and sediment organic matter content were determined in the three marshes. Significant differences were observed between sites for both Chl a and NA. The 1-year-old marsh always exhibited the highest levels of NA and Chl a . Sediment organic matter content was lowest in the 1-year-old marsh (∼2%), intermediate in the 6-year-old marsh (∼5%), and highest in the natural marsh (∼10%). Carbon and nitrogen analyses were also performed on the 1-year-old marsh sediments, which were depleted in N. A positive correlation was observed between surface sediment N and Chl a . Remineralized, microbially derived N may provide growth-limiting inorganic N to Spartina transplants. N₂-fixing microbial assemblages in the 1-year-old marsh may also be an important food source for marsh infauna. Benthic N₂-fixing microbial assemblages play a key role in the N economy of restored salt marshes.     

Nitrogen sequestration capacity of two salt marshes from the Tagus estuary

The role of salt marshes as nitrogen sink is examined taking into consideration the seasonal variation of above and belowground biomass of Spartina martima and Halimione portulacoides in two marshes from Tagus estuary, Pancas and Corroios, and the degradation rates of belowground litter. Total nitrogen was determined in plant components, decomposing litter and sediment. Biomass was higher in Corroios, the saltier marsh, with 7190 g m⁻² y⁻¹ dw of S. maritima and 6593 g m⁻² y⁻¹ dw of H. portulacoides and the belowground component contributed to 96% and 90% of total biomass, respectively. In the other marsh, Pancas, belowground biomass contributed to 56% and 76% of total biomass for S. maritima and H. portulacoides, respectively. Litterbag experiment showed that between 25% and 50% of nitrogen is lost within the first month and remained relatively constant in the next four months. Slower decomposition is observed in sediments with higher nitrogen concentration (max. 0.7% N in the saltier marsh). Higher concentrations of N were found in the sediment upper layers. Considering the sediment-root system, most of the nitrogen is stored in the sediment compartment and only about 1–4% of the total N was found in the roots. Considering these results, Tagus salt marshes act as a sink for nitrogen.     

Estuaries as Filters: The Role of Tidal Marshes in Trace Metal Removal

Flux calculations demonstrate that many estuaries are natural filters for trace metals. Yet, the underlying processes are poorly investigated. In the present study, it was hypothesized that intertidal marshes contribute significantly to the contaminant filter function of estuaries. Trace metal concentrations and sediment characteristics were measured along a transect from the subtidal, over an intertidal flat and marsh to a restored marsh with controlled reduced tide. Metal concentrations in the intertidal and restored marsh were found to be a factor two to five higher than values in the subtidal and intertidal flat sediments. High metal concentrations and high accretion rates indicate a high metal accumulation capacity of the intertidal marshes. Overbank sedimentation in the tidal marshes of the entire estuary was calculated to remove 25% to 50% of the riverine metal influx, even though marshes comprise less than 8% of the total surface of the estuary. In addition, the large-scale implementation of planned tidal marsh restoration projects was estimated to almost double the trace metal storage capacity of the present natural tidal marshes in the estuary.     

Changes in area,geomorphology and sediment nature of salt marshes in the Oosterschelde estuary (SW Netherlands) due to tidal changes

As a result of the construction of a storm-surge barrier across the mouth of the Oosterschelde (SW Netherlands) in 1987, the tidal range and mean high water level in the estuary have been reduced permanently to about 88% of their original values. During the final stage of construction (1985–1987) the tidal range and mean high water level were reduced even further for more than 18 months, by up to about 65% of their original values. This paper describes the consequences of these reductions for some abiotic aspects of the salt marshes.Strong ripening of the soil, especially in basins of the middle high salt marshes, resulted in the soils in these basins having more or less the dry nature of levees. This may cause moisture deficits for the vegetation during dry periods locally, and may lead locally to acidification of the soil as a result of oxidation of pyrite.Erosion of the edges of the salt marshes has increased in many places since 1986, both due to lowering of the surface level of the foreland, causing wave action to affect the marsh cliff more strongly than before, and weakening of cliff strength as a consequence of desiccation of the salt marsh soil and subsequent withering of plants and plant roots. In addition, the gradual salt marsh gradients have decreased on a large scale, as a consequence of increased wave attack and frost damage to Spartina. Finally, also due to desiccation and plant withering, levees have degraded and eroded, forming shoulders in the creeks.Settling, especially in the basins, has steepened and narrowed the height gradients between basin and levee.     

Distribution and speciation of phosphorus along a salinity gradient in intertidal marsh sediments

We examined forms of solid phosphorus fractions in intertidal marsh sediments along a salinity (0–22%.) gradient in a river-dominated estuary and in a marine-dominated salt marsh with insignificant freshwater input. Freshwater marsh sediments had the highest ratio of organic N:P of between 28:1 and 47:1 mol:mol, compared to 211 to 311 molmol in the saltmarshes, which is consistent with a trend toward P-limitation of primary production in freshwater and N-limitation in salt marshes. However, total P concentration, 24.7±11.1mol P g dw^–1 (±1 SD) averaged over the upper meter of sediment, was greatest in the freshwater marsh where bioavailablity of P is apparently limited. In the freshwater marsh the greatest fraction of total P (24–51%.) was associated with humic acids, while the importance of humic-P decreased with increasing salinity to 1–23%. in the salt marshes. Inorganic P contributed considerably less to total sediment P in the freshwater marsh (15–40%.) than in the salt marshes (33–85%.). In reduced sediments at all sites, phosphate bound to aluminum oxides and clays was an important inorganic P pool irrespective of salinity. Inorganic P associated with ferric iron [Fe(III)] phases was most abundant in surface sediments of freshwater and brackish marshes, while Ca-bound P dominated inorganic P pools in the salt marshes. Thus, our results showed that particle-bound P in marsh sediments exhibited changes in chemical association along the salinity gradient of an estuarine system, which is a likely consequence of changes in ionic strength and the availability of iron and calcium.     

The Contribution of Mangrove Expansion to Salt Marsh Loss on the Texas Gulf Coast

Landscape-level shifts in plant species distribution and abundance can fundamentally change the ecology of an ecosystem. Such shifts are occurring within mangrove-marsh ecotones, where over the last few decades, relatively mild winters have led to mangrove expansion into areas previously occupied by salt marsh plants. On the Texas (USA) coast of the western Gulf of Mexico, most cases of mangrove expansion have been documented within specific bays or watersheds. Based on this body of relatively small-scale work and broader global patterns of mangrove expansion, we hypothesized that there has been a recent regional-level displacement of salt marshes by mangroves. We classified Landsat-5 Thematic Mapper images using artificial neural networks to quantify black mangrove (Avicennia germinans) expansion and salt marsh (Spartina alterniflora and other grass and forb species) loss over 20 years across the entire Texas coast. Between 1990 and 2010, mangrove area grew by 16.1 km², a 74% increase. Concurrently, salt marsh area decreased by 77.8 km², a 24% net loss. Only 6% of that loss was attributable to mangrove expansion; most salt marsh was lost due to conversion to tidal flats or water, likely a result of relative sea level rise. Our research confirmed that mangroves are expanding and, in some instances, displacing salt marshes at certain locations. However, this shift is not widespread when analyzed at a larger, regional level. Rather, local, relative sea level rise was indirectly implicated as another important driver causing regional-level salt marsh loss. Climate change is expected to accelerate both sea level rise and mangrove expansion; these mechanisms are likely to interact synergistically and contribute to salt marsh loss.