Tidal wetlands, such as tidal marshes and mangroves, are hotspots for carbon sequestration. The preservation of organic matter (OM) is a critical process by which tidal wetlands exert influence over ...the global carbon cycle and at the same time gain elevation to keep pace with sea-level rise (SLR). The present study assessed the effects of temperature and relative sea level on the decomposition rate and stabilization of OM in tidal wetlands worldwide, utilizing commercially available standardized litter. While effects on decomposition rate per se were minor, we show strong negative effects of temperature and relative sea level on stabilization, as based on the fraction of labile, rapidly hydrolyzable OM that becomes stabilized during deployment. Across study sites, OM stabilization was 29 % lower in low, more frequently flooded vs. high, less frequently flooded zones. Stabilization declined by ∼ 75 % over the studied temperature gradient from 10.9 to 28.5 ∘C. Additionally, data from the Plum Island long-term ecological research site in Massachusetts, USA, show a pronounced reduction in OM stabilization by > 70 % in response to simulated coastal eutrophication, confirming the potentially high sensitivity of OM stabilization to global change. We therefore provide evidence that rising temperature, accelerated SLR, and coastal eutrophication may decrease the future capacity of tidal wetlands to sequester carbon by affecting the initial transformations of recent OM inputs to soil OM.
Abstract
Salt marshes occur globally across climatic and coastal settings, providing key linkages between terrestrial and marine ecosystems. However, salt marsh science lacks a unifying conceptual ...framework; consequently, historically well‐studied locations have been used as normative benchmarks. To allow for more effective comparisons across the diversity of salt marshes, we developed an integrative salt marsh conceptual framework. We review ecosystem‐relevant drivers from global to local spatial scales, integrate these multi‐scale settings into a framework, and provide guidance on applying the framework using specific variables on 11 global examples. Overall, this framework allows for appropriate comparison of study sites by accounting for global, coastal, inter‐, and intra‐system spatial settings unique to each salt marsh. We anticipate that incorporating this framework into salt marsh science will provide a mechanism to critically evaluate research questions and a foundation for effective quantitative studies that deepen our understanding of salt marsh function across spatial scales.
Coastal vegetation plays an important role for climate change mitigation. Compared with terrestrial ecosystems, coastal vegetation shows higher rates of atmospheric CO2 uptake and a more efficient ...retention of carbon (C) in sediments. Salt marshes present the highest values as C binders, although a global estimation of these values is still pending due to regional gaps in the records predominantly from the southern hemisphere. There are no clear patterns or dominant processes with enough evidence to account for the observed variability, suggesting that context dependent processes are likely greatest influencers on C storage. Salt marshes in the South West Atlantic (SWA) coast are densely populated by the intertidal burrowing and herbivore crab Neohelice (=Chasmagnathus) granulata. Many ecological processes related to C transformation occurring in these salt marshes are influenced by crab activities, either through bioturbation or via herbivory. We hypothesize that N. granulata could have a significant role in the capacity of SWA salt marshes to bind C. Reduction of plant biomass, increased aerobic decomposition in the sediment and facilitation of erosion are some of the multiple effects exerted by N. granulata that can directly and indirectly modify the capacity of salt marshes to bind C. Here, we compiled information available regarding C sequestration and accumulation in SWA coastal salt marshes and propose a hypothetical model including the mechanisms mediated by N. granulata that interfere the transformation paths of C in salt marshes. The data suggest that mechanisms that are top-down regulated, negatively affect C accumulation in the form of aboveground biomass especially in salt marshes dominated by Spartina alterniflora. While, mechanisms mediated by bioturbation can negatively (increasing oxygenation and thus facilitating aerobic degradation) affect as well as positively (increasing retention of macrodetritus) affect the accumulation of C, the latter being of greater magnitude in Spartina densiflora salt marshes.
Grasshopper herbivory can vary substantially among locations within a salt marsh or among marshes, but its variability along the marsh intertidal gradient (extending from the shoreline to the upland ...fringing forest) is not well reported. Previous papers have shown that grasshopper herbivory may affect nutrient processes in salt marsh ecosystems, but how such effects are tied up to the intensity of herbivory and how they vary spatially is poorly known. To help address these gaps, we evaluated whether grasshopper herbivory intensity and herbivore abundance together with other plant characteristics (such as total leaf length, plant live and dead biomass, plant nutrient content and plant nutrient standing stocks) varied along the intertidal gradient of two black needlerush marshes in the Northern Gulf of Mexico. Our results show that in one marsh grazing intensity decreased from the shoreline to the forest tree line, but in the other there was similar grazing intensity across the entire intertidal gradient. None of the measured plant characteristics followed the differences in herbivory found along the intertidal gradient and between salt marshes. We also found that, in the salt marsh with decreasing herbivory towards the upland edge, the combination of herbivory, plant nutrient content and plant nutrient standing stocks suggest two different functional zones along the intertidal gradient, one of nutrient availability and recycling near the shoreline and another one of nutrient inmobilization near the upland fringing forest. In concert, the results suggest that grasshopper herbivory intensity may vary along the intertidal gradient in some marshes, but not in others. In turn, spatial differences in herbivory along the intertidal gradient, if they occur, may influence nutrient processes, such as recycling and storage, leading to associated spatial differences in nutrient dynamics in the salt marsh.
•Grasshopper herbivory varies along the intertidal gradient in some marshes.•Herbivory variation along the marsh could enhance nutrient recycling zones.•Plant characteristics and nutrient content do not follow the herbivory patterns found.
Just as some species are used as model systems in organismal biology (e.g., physiology, genetics), many ecosystems are commonly used as model systems in ecology. Salt marshes, for instance, are great ...models to perform manipulative field experiments, and thus, were historically used to understand the drivers of community and ecosystem function. Decades of experimental work, indeed, made a strong contribution to community ecology as a discipline, but most of the emerged hypotheses and models were grounded in a few sites. When studies from new sites came onboard, looking to enlarge generalities, their results challenged the prevailing ideas. Here, we review more than 25 years of intense experimentation in South West Atlantic salt marshes, which helped not only to increase the knowledge about salt marsh functioning, but also to expand this knowledge beyond salt marshes helping to refine community and ecosystem function theory. We show that results coming from SW Atlantic marshes significantly contribute to understand 1) the separate and interactive effect of biotic and abiotic stress for species distribution and even for ecosystem stability, 2) the integrated role of species that can function as ecosystem engineers and as consumers, 3) the balance between stochastic and deterministic forces as drivers of community structure and 4) the regulation of cross-ecosystem fluxes. Nevertheless, we believe SW Atlantic salt marshes still have a lot more to offer, not only as conceptual models that help satisfy our intellectual curiosity, but also as key ecosystems that provide valuable benefits to our societies.
Questions
Coastal vegetated systems are known to play a fundamental role in climate change mitigation as a result of their efficiency sequestering and storing atmospheric CO2. While most of the work ...evaluating carbon sequestration capacity has focused on global change factors that can affect carbon release from plant litter decomposition through changes in (large‐scale) environmental conditions, less is known about the possible effects of the loss (or replacement) of dominant species. We hypothesized that dominant marsh plants can influence decomposition not only through changes in litter quality but also through changes in (microscale) soil environmental conditions such as humidity, soil temperature or solar radiation.
Location
We performed a field manipulative experiment in a southwestern (SW) Atlantic salt marsh in Argentina.
Methods
We simulate a selective disturbance (i.e., removal of the dominant grass species Spartina densiflora) thus allowing removal plots to develop an alternative plant community. To evaluate the effect of the dominant grass species on litter decomposition, in an experiment we performed a litterbag approach three years after the establishment of the removal plots.
Results
Results showed that the presence of S. densiflora significantly decreased litter decomposition directly by producing less labile litter, but also by effects that seem to be related to its structure as standing dominant vegetation. The experimental removal of S. densiflora led to an alternative plant community, formed by otherwise subordinate species, which is less densely packed, allowing higher radiation incidence on the soil and elevated midday soil temperature.
Conclusions
Our results suggest that salt marsh litter decomposition, and thus C sequestration, is determined in part by the identity of the dominant plant, not only because of the quality of the produced litter but also as a consequence of the vegetation structure . Changes in species diversity, above all changes in the dominant species in these coastal systems, could have large impacts on the carbon turnover and mitigation capacity of these ecosystems.
Here we show that litter decomposition in a salt marsh is strongly linked to the dominant plant species as a consequence of both litter quality and its standing vegetation structure. Our findings highlight the importance of the dominant species for ecosystem functioning and the potential threat that the loss of these species may have for valuable marsh ecological services such as C sequestration.
Herbivory is a common process in salt marshes. However, the direct impact of marsh herbivory on nutrient cycling in this ecosystem is poorly understood. Using a ¹⁵N enrichment mesocosm study, we ...quantified nitrogen (N) cycling in sediment and plants of black needlerush (Juncus roemerianus) salt marshes, facilitated by litter decomposition and litter plus grasshopper feces decomposition. We found 15 times more ¹⁵N recovery in sediment with grasshopper herbivory compared to sediment with no grasshopper herbivory. In plants, even though we found three times and a half larger ¹⁵N recovery with grasshopper herbivory, we did not find significant differences. Thus, herbivory can enhance N cycling in black needlerush salt marshes sediments and elevate the role of these salt marshes as nutrient sinks.
Large amounts of tidally accumulated detritus (i.e., wrack) are an important source of disturbance affecting different abiotic and biotic characteristics in salt marshes, which could in turn affect ...the macrofauna assemblage. The purpose of this study was to evaluate the importance of wrack disturbance in Southwest Atlantic (SWA) salt marshes and its effects on the surface-active arthropod assemblage under different environmental contexts. By sampling the most important SWA salt marshes (from 36°19 'S to 41°01 'S), we found that wrack is a widespread disturbance in this region, present in all the salt marshes and periods sampled. However, the biomass and type of wrack (Spartina alterniflora vs. S. densiflora) vary according to the species that dominates each salt marsh. At two of these sites (Bahia Blanca (BB), 38°59 'S and San Clemente (SC), 36°19 'S), chosen because they represent the two salt marsh types in the SWA region (dominated by Spartina alterniflora or S. densiflora), we performed a field experiment by manipulating the presence and absence of wrack and conducting field samplings of sediment organic matter content and water content. We found that wrack affects surface arthropod assemblage but that this effect was not consistent for the different salt marshes: in BB, it changed the surface-active arthropod assemblage (shifted towards more detritivorous taxa) and increased the number of total individuals but had no effect on the number of species or diversity. At SC, wrack had no effect on any of the parameters evaluated. We suggest that the type of wrack in each salt marsh modulates the amount of organic matter content in the sediment: BB had wrack of better nutritional quality (dominated by S. alterniflora) and in turn had greater organic matter content in the sediment of wrack zones than in no-wrack zones, while in SC (dominated by S. densiflora), there is no differences between the two zones. We also suggest that depending on the original surface-active arthropod assemblage, those modifications will either favor (BB) or not favor (SC) wrack colonization by the surface-active arthropod assemblage. Moreover, considering that SWA wrack has different compositions and that the biomass differs among the different salt marshes, we expect wrack effects in the SWA, and probably in other regions, to be site-specific.
Salt marsh zonation patterns generate different abiotic and biotic conditions that can accentuate species inherent differences in primary production and biomass. In South West Atlantic marshes, there ...are two Spartina species: Spartina alterniflora in the low intertidal and Spartina densiflora in the high intertidal. These two species are generally found in all marshes but with different dominance: In some marshes, the S. densiflora zone occupies higher extents, and in others, the S. alterniflora zone is the one that prevails. We found through field sampling that, in six studied marshes, there is greater S. densiflora live and total (i.e., dead+live) aboveground biomass (g m⁻²) in the marshes dominated by S. densiflora than in the ones dominated by S. alterniflora. Spartina alterniflora had similar aboveground biomass in the six marshes, regardless of the dominance of each species. When comparing the two Spartina species within each marsh, S. densiflora had greater live and total biomass in the marshes it dominates. In the marshes dominated by S. alterniflora, both species had similar live and total biomass. In all marshes, there was greater dead S. densiflora biomass. A multivariate analysis using selected abiotic factors (i.e., salinity, latitude, and tidal amplitude) showed that S. alterniflora aboveground biomass patterns are mainly correlated with salinity, while S. densiflora live biomass is mainly correlated with salinity and latitude, dead biomass with salinity and tidal amplitude, and total biomass with salinity alone. We conclude that in S. densiflora dominated marshes, the main processes of that species zone (i.e., nutrient accumulation) will be accentuated because of its higher biomass. We also conclude that climatic conditions, in combination with specific Spartina biotic and ambient abiotic parameters, can affect marsh ecological functions.
Biological invasions in marine and coastal systems may produce new trophic and nontrophic interactions influencing the structure of the invaded community. In the intertidal salt marshes of ...Samborombón Bay (36°19′20″S, 56°46′26″W; mouth of La Plata River; Argentina), there is a new non-indigenous oyster species, Crassostrea sp., which settles on the dominant smooth cordgrass Spartina alterniflora. Here, we analyzed if the oyster affects S. alterniflora. Sampling showed that density of live plant was similar across intertidal levels, but there were higher density of dead plant stems at low intertidal levels. This pattern coincides with higher density and larger shell size of Crassostrea sp. at the low intertidal where oysters are attached to the basal part of the plant stems. An experiment manipulating oysters attached to S. alterniflora stems and oyster mimics shows that Crassostrea sp. can indeed increase mortality of S. alterniflora. The negative effect of bivalves on plant could be because several oysters settle around the Spartina stem, and by growing during the year, strangle the plants increasing their mortality rate. Together, all these evidences strongly suggest that these non-indigenous oysters can control the lower intertidal level of plant distribution in this system.
•New non-indigenous oyster settles on the smooth cordgrass Spartina alterniflora.•Oyster affects mortality of S. alterniflora.•The negative effect of bivalves on plant is higher at the lower intertidal level.
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