Ecosystem boundary retreat due to human-induced pressure is a generally observed phenomenon. However, studies that document thresholds beyond which internal resistance mechanisms are overwhelmed are uncommon. Following the Deepwater Horizon (DWH) oil spill, field studies from a few sites suggested that oiling of salt marshes could lead to a biogeomorphic feedback where plant death resulted in increased marsh erosion. We tested for spatial generality of and thresholds in this effect across 103 salt marsh sites spanning ~430 kilometers of shoreline in coastal Louisiana, Alabama, and Mississippi, using data collected as part of the natural resource damage assessment (NRDA). Our analyses revealed a threshold for oil impacts on marsh edge erosion, with higher erosion rates occurring for ~1-2 years after the spill at sites with the highest amounts of plant stem oiling (90-100%). These results provide compelling evidence showing large-scale ecosystem loss following the Deepwater Horizon oil spill. More broadly, these findings provide rare empirical evidence identifying a geomorphologic threshold in the resistance of an ecosystem to increasing intensity of human-induced disturbance.
Microcystis population and microcystin (MC) dynamics were investigated in western Lake Erie coastal wetlands and downstream beach water. A three-dimensional (3-D) model was developed to quantify how Microcystis population size and structure affect MCs.
Salt marshes are valued for their ecosystem services, and their vulnerability is typically assessed through biotic and abiotic measurements at individual points on the landscape. However, lateral erosion can lead to rapid marsh loss as marshes build vertically. Marsh sediment budgets represent a spatially integrated measure of competing constructive and destructive forces: a sediment surplus may result in vertical growth and/or lateral expansion, while a sediment deficit may result in drowning and/or lateral contraction. Here we show that sediment budgets of eight microtidal marsh complexes consistently scale with areal unvegetated/vegetated marsh ratios (UVVR) suggesting these metrics are broadly applicable indicators of microtidal marsh vulnerability. All sites are exhibiting a sediment deficit, with half the sites having projected lifespans of less than 350 years at current rates of sea-level rise and sediment availability. These results demonstrate that open-water conversion and sediment deficits are holistic and sensitive indicators of salt marsh vulnerability.
Wetlands are the largest natural source of atmospheric methane. Here, we assess controls on methane flux using a database of approximately 19 000 instantaneous measurements from 71 wetland sites located across subtropical, temperate, and northern high latitude regions. Our analyses confirm general controls on wetland methane emissions from soil temperature, water table, and vegetation, but also show that these relationships are modified depending on wetland type (bog, fen, or swamp), region (subarctic to temperate), and disturbance. Fen methane flux was more sensitive to vegetation and less sensitive to temperature than bog or swamp fluxes. The optimal water table for methane flux was consistently below the peat surface in bogs, close to the peat surface in poor fens, and above the peat surface in rich fens. However, the largest flux in bogs occurred when dry 30-day averaged antecedent conditions were followed by wet conditions, while in fens and swamps, the largest flux occurred when both 30-day averaged antecedent and current conditions were wet. Drained wetlands exhibited distinct characteristics, e.g. the absence of large flux following wet and warm conditions, suggesting that the same functional relationships between methane flux and environmental conditions cannot be used across pristine and disturbed wetlands. Together, our results suggest that water table and temperature are dominant controls on methane flux in pristine bogs and swamps, while other processes, such as vascular transport in pristine fens, have the potential to partially override the effect of these controls in other wetland types. Because wetland types vary in methane emissions and have distinct controls, these ecosystems need to be considered separately to yield reliable estimates of global wetland methane release.
Drought has many consequences in the tidally dominatedSpartinasp. salt marshes of the southeastern US; including major dieback events, changes in sediment chemistry and obvious changes in the landscape. These coastal systems tend to be highly productive, yet many salt marshes are also nitrogen limited and depend on plant associated diazotrophs as their source of ‘new’ nitrogen. A 4-year study was conducted to investigate the structure and composition of the rhizosphere diazotroph assemblages associated with 5 distinct plant zones in one such salt marsh. A period of greatly restricted tidal inundation and precipitation, as well as two periods of drought (June-July 2004, and May 2007) occurred during the study. DGGE ofnifH PCR amplicons from rhizosphere samples, Principal Components Analysis of the resulting banding patterns, and unconstrained ordination analysis of taxonomic data and environmental parameters were conducted. Diazotroph assemblages were organized into 5 distinct groups (R² = 0.41,pvalue < 0.001) whose presence varied with the environmental conditions of the marsh. Diazotroph assemblage group detection differed during and after the drought event, indicating that persistent diazotrophs maintained populations that provided reduced supplies of new nitrogen for vegetation during the periods of drought.
Wetlands are the single largest natural source of atmospheric methane (CH4), a greenhouse gas, and occur extensively in the northern hemisphere. Large discrepancies remain between “bottom-up” and “top-down” estimates of northern CH4emissions. To explore whether these discrepancies are due to poor representation of non-growing season CH4emissions, we synthesized non-growing season and annual CH4flux measurements from temperate, boreal, and tundra wetlands and uplands. Median non-growing season wetland emissions ranged from 0.9 g m-2in bogs to 5.2 g m-2in marshes and were dependent on moisture, vegetation, and permafrost. Annual wetland emissions ranged from 0.9 g m-2y-1in tundra bogs to 78 g m-2y-1in temperate marshes. Uplands varied from CH4sinks to CH4sources with a median annual flux of 0.0 ± 0.2 g m-2y-1. The measured fraction of annual CH4emissions during the non-growing season (observed: 13 to 47%) was significantly larger than was predicted by two process-based model ensembles, especially between 40-60º N (modeled: 4 to 17%). Constraining the model ensembles with the measured non-growing fraction increased total non-growing season and annual CH4emissions. Using this constraint, the modeled non-growing season wetland CH4flux from >40° north was 6.1 ± 1.5 Tg y-1, three times greater than the non-growing season emissions of the unconstrained model ensemble. The annual wetland CH4flux was 37 ± 7 Tg y-1from the data-constrained model ensemble, 25% larger than the unconstrained ensemble. Considering non-growing season processes is critical for accurately estimating CH4emissions from high latitude ecosystems, and necessary for constraining the role of wetland emissions in a warming climate. This article is protected by copyright. All rights reserved.
Two 454 shotgun metagenomes were sequenced from hypersaline soil samples collected in the Odiel salt marsh area in Huelva, southwestern Spain. Analysis of contigs and 16S rRNA-related sequences showed thatHalobacteria,Balneolaeota, andBacteroideteswere the dominant groups.RhodothermaeotaandNanohaloarchaeotawere also abundant.
Climate change and other anthropogenic stressors are converging on coastal ecosystems worldwide. Understanding how these stressors interact to affect ecosystem structure and function has immediate implications for coastal planning, however few studies quantify stressor interactions. We examined past and potential future interactions between two leading stressors on New England salt marshes: sea-level rise and marsh crab (Sesarma reticulatum) grazing driven low marsh die-off. Geospatial analyses reveal that crab-driven die-off has led to an order of magnitude more marsh loss than sea-level rise between 2005 and 2013. However, field transplant experimental results suggest that sea-level rise will facilitate crab expansion into higher elevation marsh platforms by inundating and gradually softening now-tough high marsh peat, exposing large areas to crab-driven die-off. Taking interactive effects of marsh softening and concomitant overgrazing into account, we estimate that even modest levels of sea-level rise will lead to levels of salt marsh habitat loss that are 3x greater than the additive effects of sea-level rise and crab-driven die-off would predict. These findings highlight the importance of multiple stressor studies in enhancing mechanistic understanding of ecosystem vulnerabilities to future stress scenarios and encourage managers to focus on ameliorating local stressors to break detrimental synergisms, reduce future ecosystem loss, and enhance ecosystem resilience to global change.
The unique environment of a 4m-thick, free-floating peat island within the Posta Fibreno lake (Central Italy) was analyzed using DNA-based techniques to assess bacterial and fungal community members identity and abundance. Two depths were sampled at 41 and 279 cm from the surface, the former corresponding to an emerged portion of Sphagnum residues accumulated less than 30 yrs ago, and the latter mainly consisting of silty peat belonging to the deeply submerged part of the island, dating back to 1520-1660 AD. The corresponding communities were very diverse, each of them dominated by a different member of the Delta-proteobacteria class for prokaryotes. Among Eukaryotes, Ascomycota prevailed in the shallow layer while Basidiomycota were abundant in the deep sample. The identity of taxa partitioning between acidic surface layer and neutral core is very reminiscent of the differences reported between bogs and fens respectively, supporting the view of Posta Fibreno as a relic transitional floating mire. Moreover, some microbial taxa show an unusual concurrent species convergence between this sub-Mediterranean site and far Nordic or circumpolar environments. This study represents the first report describing the biotic assemblages of such a peculiar environment, and provides some insights into the possible mechanisms of its evolution.
Although transport of oxygen via the aerenchyma tissue and subsequent oxygen loss across root surfaces is well-documented for salt marsh grasses, only few studies have measured the oxygenation of sediment surrounding roots and rhizomes. In this study, sediment oxygenation was assessed in situ in rhizospheres of the intertidal salt marsh grass, Spartina anglica - an invading species, vigorously spreading in many wetlands around the world. The rhizospheres of two populations of S. anglica with differing plant morphology growing in different sediment types were investigated in situ using a novel multifiber optode system with 100 oxygen probes. No oxygen was detected inside the rhizospheres at any depth in either location, indicating a limited impact of plant-mediated sediment oxygenation on the bulk anoxic sediment. Subsequent planar optode studies imaging the oxygen content around the roots substantiated these findings showing that sediment oxygenation was present in both locations, but it was confined only to the immediate vicinity of the root tips. The size of the oxic zones surrounding the root tips differed between sediment-types: in S. anglica growing in permeable sandy sediment, oxic root zones extended 1.5mm away from the roots surface compared to only 0.4mm in muddy tidal flat deposit, which had a substantially higher oxygen demand. The oxygen concentration inside the oxic root zones remained stable during continuous light and air-exposure of the aboveground biomass. In comparison, sediment oxygenation generated by burrowing infauna (Hediste diversicolor) showed to be markedly more temporally variability, reaching anoxic conditions multiple times during a 5-h period.