Modelling effects of water regulation on the population viability of threatened amphibians

The regulation of river systems alters hydrodynamics and often reduces lateral connectivity between river channels and floodplains. For taxa such as frogs that rely on floodplain wetlands to complete their lifecycle, decreasing inundation frequency can reduce recruitment and increase the probability of local extinction. We virtually reconstructed the inundation patterns of wetlands under natural and regulated flow conditions and built stochastic population models to quantify the probability of local extinction under different inundation scenarios. Specifically, we explored the interplay of inundation frequency, habitat size, and successive dry years on the local extinction probability of the threatened southern bell frog Litoria raniformis in the Murray River floodplains of South Australia. We hypothesised that the changes in wetland inundation resulting from river regulation are driving the decline of L. raniformis in this system. Since river regulation began in the 1920s, the inundation frequency of many reliable breeding habitats has decreased to a point where they no longer support local populations. Increasing successive dry years drives the probability of local extinction, particularly in smaller wetlands. Larger wetlands and those with more frequent average inundation are less susceptible to these effects. Synthesis and Applications. Although the availability of suitable habitats has reduced, environmental water provision is a promising tool to mitigate the negative impacts of river regulation on amphibian populations. Our modelling approach can be used to prioritise the delivery of environmental water (through pumping or the operation of flow-regulating structures) to minimise the probability of local extinction in L. raniformis and potentially many other frog species. By quantifying the extinction risk of amphibian populations, we can strategically manage environmental water to reduce successive catastrophic breeding failures and increase the probability of persistence.

Increasing inundation frequencies enhance the stochastic process and network complexity of soil archaeal community in coastal wetlands

Coastal wetlands are experiencing frequent flooding because of global climate changes, such as the rising sea level. Despite the key role of archaea in soil biogeochemical cycles, the assembly processes and co-occurrence patterns of archaeal communities in coastal wetlands in response to increasing inundation frequencies remain elusive. Herein, we established an in situ mesocosm with an inundation frequency gradient to investigate the response of soil archaeal community toward increasing inundation frequencies in monocultures of Spartina alterniflora (SA) and a mangrove species, Kandelia obovata (KO). Both neutral community model and null model analyses suggested that stochastic processes are dominant in governing the archaeal community assembly, and that the stochastic processes are enhanced with increasing inundation frequencies. Increasing inundation frequencies significantly increased the community niche width. Moreover, archaeal community in SA soil displayed lower niche overlap and higher stochasticity than in KO soil. Co-occurrence network analysis revealed that the network complexity increases with increase in the inundation frequencies. Soil water content is the most decisive factor influencing the archaeal communities. Overall, we found that increasing inundation frequencies enhance the stochastic processes and network complexity of the soil archaeal community in coastal wetlands. This study could enhance our understanding on the response of soil archaeal communities in coastal wetlands toward global change. Importance Coastal wetlands, subjected to regular disturbances by periodic tidal, are highly productive and important in the regulation of climate change. However, the assembly mechanisms and co-occurrence patterns of soil archaeal communities in coastal areas remain poorly known, especially for their responses to increasing inundation frequencies. In this study, we aim at unravelling these uncertainties by studying typical estuarine ecosystems in southern China. We show that increasing inundation frequencies enhance the stochastic processes and network complexity of the soil archaeal community. This study offers a new path for an improved understanding of archaeal community assembly and species coexistence in coastal environments, with a special focus on the role of inundation frequency.

Salt Marsh Elevation Drives Root Microbial Composition of the Native Invasive Grass Elytrigia atherica

Elytrigia atherica is a native invasive plant species whose expansion on salt marshes is attributed to genotypic and phenotypic adaptations to non-ideal environmental conditions, forming two ecotypes. It is unknown how E. atherica–microbiome interactions are contributing to its adaptation. Here we investigated the effect of sea-water flooding frequency and associated soil (a)biotic conditions on plant traits and root-associated microbial community composition and potential functions of two E. atherica ecotypes. We observed higher endomycorrhizal colonization in high-elevation ecotypes (HE, low inundation frequency), whereas low-elevation ecotypes (LE, high inundation frequency) had higher specific leaf area. Similarly, rhizosphere and endosphere bacterial communities grouped according to ecotypes. Soil ammonium content and elevation explained rhizosphere bacterial composition. Around 60% the endosphere amplicon sequence variants (ASVs) were also found in soil and around 30% of the ASVs were ecotype-specific. The endosphere of HE-ecotype harbored more unique sequences than the LE-ecotype, the latter being abundant in halophylic bacterial species. The composition of the endosphere may explain salinity and drought tolerance in relation to the local environmental needs of each ecotype. Overall, these results suggest that E. atherica is flexible in its association with soil bacteria and ecotype-specific dissimilar, which may enhance its competitive strength in salt marshes.

Does Ecological Water Replenishment Help Prevent a Large Wetland from Further Deterioration? Results from the Zhalong Nature Reserve, China

Ecological water replenishment (EWR) has been increasingly applied to the restoration and maintenance of wetland hydrological conditions across China since the beginning of the 21st century. However, little is known about whether EWR projects help protect and/or restore wetland ecohydrology. As one of the earliest and longest-running EWR projects in China, water has been released from the Nenjiang River into the Zhalong wetland since 2001. It is important to examine the ecohydrological effects of this EWR project. In this study, long time series remote sensing data were used to extract the water area, inundation frequency, and normalized difference vegetation index (NDVI) to explore how eco-hydrological conditions changed during the pre- (1984–2000) and post-EWR (2001–2018) periods in the Zhalong wetland. Results show that the inundation area decreased due to the reduced surface water inflow during the pre-EWR period. Similarly, monthly vegetation NDVI in the growing season generally exhibited a decreasing and an increasing trend during the pre- and post-EWR periods, respectively. In the post-EWR period, NDVI increased by 19%, 73%, 45%, 28%, 13% for the months of May through September, respectively. Due to EWR, vegetation growth in areas with low inundation frequency was better than in areas with high inundation frequency. We found that the EWR project, runoff, and precipitation contributed 25%, 11%, and 64% to changes in the NDVI, respectively, and 46%, 37%, and 17% to changes in inundation area, respectively. These results indicate that the EWR project has improved hydrological conditions in the Zhalong wetland. For further maximum benefits of EWR in the Zhalong wetlands, we suggest that implementing similar eco-hydrological projects in the future should focus on flood pulse management to increase the inundation area, improve hydrological connectivity, and create new habitats.

Assessing historic water extents in rapidly changing lakes: a hybrid remote sensing classification approach

The empirical attribution of rapid hydrologic change presents a unique data availability challenge in terms of establishing baseline prior conditions. On the one hand, one cannot go back in time to collect the necessary in situ data if it were not serendipitously collected when the change was taking place. On the other hand, modern satellite monitoring missions are often too recent to capture changes that are ancient enough to provide sufficient observations for adequate statistical inference. In that context, the four decades of continuous global high resolution monitoring enabled by the Landsat missions are an unrivaled source of information to study hydrologic change globally. However, extracting the relevant time series information in a systematic way across Landsat missions remains a monumental challenge. Cloud masking and inconsistent image quality often complicate the automatized interpretation of optical imagery. Focusing on the monitoring of lake water extents, we address this challenge by coupling supervised and unsupervised image classification techniques. Unsupervised classification is first used to detect water on unmasked (cloudless and high quality) pixels. Classification results are then compiled across images to estimate the inundation frequency of each pixel, hinging on the assumption that different pixels will be masked at different times. Inundation frequency is then leveraged to infer the inundation status of masked pixels on individual images through supervised classification. Applied to a representative set of global and rapidly changing lakes, the approach successfully captured water extent fluctuations obtained from in situ gauges (when applicable), or from other Landsat missions during overlapping time periods.