Salinity-induced increase of the hydraulic conductivity in the hyporheic zone of coastal wetlands

In coastal zones globally, salinization is rapidly taking place due to the combined effects of sea level rise, land subsidence, altered hydrology, and climate change. Although increased salinity levels are known to have a great impact on both biogeochemical and hydrological processes in aquatic sediments, only few studies have included both types of processes and their potential interactions. In the present paper, we used a controlled 3-year experimental mesocosm approach to test salinity induced interactions and discuss mechanisms explaining the observed hydrological changes. Surface water salinity was experimentally increased from 14 to 140 mmol Cl per L (0.9 and 9 PSU) by adding sea salt which increased pore water salinity but also increased sulfate reduction rates, leading to higher sulfide, and lower methane concentrations. By analyzing slug test data with different slug test analysis methods, we were able to show that hydraulic conductivity of the hyporheic zone increased 2.8 times by salinization. Based on our hydrological and biogeochemical measurements, we conclude that the combination of pore dilation and decreased methane production rates were major controls on the observed increase in hydraulic conductivity. The slug test analysis method comparison allowed to conclude that the adjusted Bouwer and Rice method results in the most reliable estimate of the hydraulic conductivity for hyporheic zones. Our work shows that both physical and biogeochemical processes are vital to explain and predict hydrological changes related to the salinization of hyporheic zones in coastal wetlands and provides a robust methodological approach for doing so

A modification of the constant-head permeameter to measure saturated hydraulic conductivity of highly permeable media

The saturated hydraulic conductivity (Ks) is a key characteristic of porous media, describing the rate of water flow through saturated porous media. It is an indispensable parameter in a broad range of simulation models that quantify saturated and/or unsaturated water flow. The constant-head permeameter test is a common laboratory method to determine Ks on undisturbed soil samples collected from the field. In this paper we show that the application of this conventional method may result in a biased Ks in the case of highly permeable media, such as the top layer of Sphagnum peat and gravel. Tubes in the conventional permeameter, that collect water under the sample, introduce a hydraulic head-dependent resistance for highly permeable media and result in an underestimation of Ks. We present a simple and low-budget alternative of the constant-head permeameter test that overcomes the disadvantages of conventional permeameters. The new method was successfully tested on intact highly permeable peatmoss collected from a northern peatland. • Conventional constant-head permeameters underestimate Ks of highly permeable media due to flow resistance in tubing systems • We developed the low-resistance permeameter to overcome this disadvantage. • Testing of the low-resistance permeameter demonstrated no systematic bias and successful application for highly permeable media

Data from: Can frequent precipitation moderate drought impact on peatmoss carbon uptake in northern peatlands?

Northern peatlands represent a large global carbon store that potentially can be destabilised by summer water table drawdown. Precipitation can moderate negative impacts of water table drawdown by rewetting peatmoss (Sphagnum spp.), the ecosystems’ key species. Yet, the frequency for such rewetting to be effective remains unknown. We experimentally assessed the importance of precipitation frequency for Sphagnum water supply and carbon uptake during a stepwise decrease in water tables in a growth chamber. CO2 exchange and the water balance were measured for intact cores of three peatmoss species (Sphagnum majus, S. balticum and S. fuscum) representative of three hydrologically distinct peatland microhabitats (hollow, lawn, hummock) and expected to differ in their water table-precipitation relationships. Precipitation contributed significantly to peatmoss water supply at deep water tables, demonstrating the importance of precipitation during drought. The ability to exploit transient resources was species-specific; S. fuscum carbon uptake increased linearly with precipitation frequency at deep water tables, whereas carbon uptake by S. balticum and S. majus was depressed at intermediate precipitation frequencies. Our results highlight an important role for precipitation on carbon uptake by peatmosses. Yet, the potential to moderate drought impact is species-specific and dependents on the temporal distribution of precipitation

High-resolution peat volume change in a northern peatland: Spatial variability, main drivers, and impact on ecohydrology

The depth of the groundwater table below the surface and its spatiotemporal variability are major controls on all major biogeophysical processes in northern peatlands, including ecohydrology, carbon balance, and greenhouse gas exchange. In these ecosystems, water table fluctuations are buffered by compression and expansion of peat. Controls on peat volume change and its spatial variability, however, remain elusive, hampering accurate assessment of climate change impact on functioning of peatlands. We therefore (1) analysed patterning of seasonal surface elevation change at high spatial resolution (0.5 m); (2) assessed its relationship with vegetation, geohydrology, and position within the peatland; and (3) quantified the consequences for peatland surface topography and ecohydrology. Changes in surface elevation were monitored using digital close-range photogrammetry along a transect in a northern peatland from after snowmelt up to midgrowing season (May–July). Surface elevation change was substantial and varied spatially from −0.062 to +0.012 m over the measurement period. Spatial patterns of peat volume change were correlated up to 40.8 m. Spatial variation of peat volume change was mainly controlled by changes in water table, and to a lesser extent to vegetation, with peat volume change magnitude increasing from lawn < hollow < flark. Our observations suggest that patchiness and vertical variability of peatland surface topography are a function of the groundwater table. In dry conditions, the variability of surface elevation increases and more localized groundwater flows may develop. Consequently, spatially variable peat volume change may enhance peatland water retention and thereby sustain carbon uptake during drought.</p

Spatial early warning signals for impending regime shifts: A practical framework for application in real‐world landscapes

Prediction of ecosystem response to global environmental change is a pressing scientific challenge of major societal relevance. Many ecosystems display nonlinear responses to environmental change, and may even undergo practically irreversible ‘regime shifts’ that initiate ecosystem collapse. Recently, early warning signals based on spatiotemporal metrics have been proposed for the identification of impending regime shifts. The rapidly increasing availability of remotely sensed data provides excellent opportunities to apply such model‐based spatial early warning signals in the real world, to assess ecosystem resilience and identify impending regime shifts induced by global change. Such information would allow land‐managers and policy makers to interfere and avoid catastrophic shifts, but also to induce regime shifts that move ecosystems to a desired state. Here, we show that the application of spatial early warning signals in real‐world landscapes presents unique and unexpected challenges, and may result in misleading conclusions when employed without careful consideration of the spatial data and processes at hand. We identify key practical and theoretical issues and provide guidelines for applying spatial early warning signals in heterogeneous, real‐world landscapes based on literature review and examples from real‐world data. Major identified issues include (1) spatial heterogeneity in real‐world landscapes may enhance reversibility of regime shifts and boost landscape‐level resilience to environmental change (2) ecosystem states are often difficult to define, while these definitions have great impact on spatial early warning signals and (3) spatial environmental variability and socio‐economic factors may affect spatial patterns, spatial early warning signals and associated regime shift predictions. We propose a novel framework, shifting from an ecosystem perspective towards a landscape approach. The framework can be used to identify conditions under which resilience assessment with spatial remotely sensed data may be successful, to support well‐informed application of spatial early warning signals, and to improve predictions of ecosystem responses to global environmental change

All relevant datasets are included in the zip file

The zip file contains 5 files: 1) Precipitation dependence data 2) Carbon fluxes 3) Volumetric water content vs carbon fluxes 4) Volumetric water content vs photosystem II efficiency and 5) Recovery: carbon fluxes at wet and rewetted treatment. See for detailed file description in the ReadMe fil

Emerging forest–peatland bistability and resilience of European peatland carbon stores

Northern peatlands store large amounts of carbon. Observations indicate that forests and peatlands in northern biomes can be alternative stable states for a range of landscape settings. Climatic and hydrological changes may reduce the resilience of peatlands and forests, induce persistent shifts between these states, and release the carbon stored in peatlands. Here, we present a dynamic simulation model constrained and validated by a wide set of observations to quantify how feedbacks in water and carbon cycling control resilience of both peatlands and forests in northern landscapes. Our results show that 34% of Europe (area) has a climate that can currently sustain existing rainwater-fed peatlands (raised bogs). However, raised bog initiation and restoration by water conservation measures after the original peat soil has disappeared is only possible in 10% of Europe where the climate allows raised bogs to initiate and outcompete forests. Moreover, in another 10% of Europe, existing raised bogs (concerning ∼20% of the European raised bogs) are already affected by ongoing climate change. Here, forests may overgrow peatlands, which could potentially release in the order of 4% (∼24 Pg carbon) of the European soil organic carbon pool. Our study demonstrates quantitatively that preserving and restoring peatlands requires looking beyond peatland-specific processes and taking into account wider landscape-scale feedbacks with forest ecosystems

Emerging forest–peatland bistability and resilience of European peatland carbon stores

Northern peatlands store large amounts of carbon. Observations indicate that forests and peatlands in northern biomes can be alternative stable states for a range of landscape settings. Climatic and hydrological changes may reduce the resilience of peatlands and forests, induce persistent shifts between these states, and release the carbon stored in peatlands. Here, we present a dynamic simulation model constrained and validated by a wide set of observations to quantify how feedbacks in water and carbon cycling control resilience of both peatlands and forests in northern landscapes. Our results show that 34% of Europe (area) has a climate that can currently sustain existing rainwater-fed peatlands (raised bogs). However, raised bog initiation and restoration by water conservation measures after the original peat soil has disappeared is only possible in 10% of Europe where the climate allows raised bogs to initiate and outcompete forests. Moreover, in another 10% of Europe, existing raised bogs (concerning ∼20% of the European raised bogs) are already affected by ongoing climate change. Here, forests may overgrow peatlands, which could potentially release in the order of 4% (∼24 Pg carbon) of the European soil organic carbon pool. Our study demonstrates quantitatively that preserving and restoring peatlands requires looking beyond peatland-specific processes and taking into account wider landscape-scale feedbacks with forest ecosystems