Inundation Of Sites Research Articles

Unique stable isotope signatures of large cyclonic events as a tracer of soil moisture dynamics in the semiarid subtropics

Evaporative flux from soils in arid and semi-arid climates can be very high and may substantially reduce soil moisture retained between infrequent rainfall events. Direct measurement of the evaporative losses from soils is technically challenging. However, environmental tracers such as water stable hydrogen and oxygen isotope composition can be used to calculate evaporation rates if the initial signature of the infiltrating rainwater is distinct from the signature of residual soil moisture. Large tropical cyclones typically result in rainfall events of large volume and very negative δ18O signatures that are significantly lower than the signatures of more frequent and smaller rainfall events. These very negative stable isotope signatures are retained in the soil and can be used to understand the depth of water infiltration, retention and subsequent rate of evaporation from the soil. At our study site in dry subtropical northwest Australia, we repeatedly sampled rainwater and soil moisture prior to, during and after tropical Cyclones Heidi and Lua in 2012. Site inundation from Cyclone Heidi (rainfall 213 mm, δ18O −17.6‰) replenished soil moisture in the unsaturated zone for several months, completely replacing soil moisture down to depths of ~3.5 m and contributing to groundwater recharge. The transient momentary evaporative losses from wet soil at the time of sampling varied between 0.21 and 0.60 mm × day−1 (equivalent to 76 to 220 mm × yr−1 recalculated as an annual rate). During the prolonged dry period between cyclones, evaporative losses decreased to between 8 and 30 mm × yr−1. Mean long-term groundwater recharge for the study site was low (<6 mm × yr−1). Recharge is primarily driven by infrequent but high-volume cyclones that are an important source of soil moisture and an essential water source for vegetation in this semi-arid environment. However, variation in lithology, position in the landscape and time since the last inundation contribute to highly heterogeneous patterns of δ18O in the vadose zone, which complicates upscaling observations from a local to a regional scale model of evaporative demand.

Sediment structure and physicochemical changes following tidal inundation at a large open coast managed realignment site

Managed realignment (MR) schemes are being implemented to compensate for the loss of intertidal saltmarsh habitats by breaching flood defences and inundating the formerly defended coastal hinterland. However, studies have shown that MR sites have lower biodiversity than anticipated, which has been linked with anoxia and poor drainage resulting from compaction and the collapse of sediment pore space caused by the site's former terrestrial land use. Despite this proposed link between biodiversity and soil structure, the evolution of the sediment sub-surface following site inundation has rarely been examined, particularly over the early stages of the terrestrial to marine or estuarine transition. This paper presents a novel combination of broad- and intensive-scale analysis of the sub-surface evolution of the Medmerry Managed Realignment Site (West Sussex, UK) in the three years following site inundation. Repeated broad-scale sediment physiochemical datasets are analysed to assess the early changes in the sediment subsurface and the preservation of the former terrestrial surface, comparing four locations of different former land uses. Additionally, for two of these locations, high-intensity 3D-computed X-ray microtomography and Itrax micro-X-ray fluorescence spectrometry analyses are presented. Results provide new data on differences in sediment properties and structure related to the former land use, indicating that increased agricultural activity leads to increased compaction and reduced porosity. The presence of anoxic conditions, indicative of poor hydrological connectivity between the terrestrial and post-inundation intertidal sediment facies, was only detected at one site. This site has experienced the highest rate of accretion over the terrestrial surface (ca. 7 cm over 36 months), suggesting that poor drainage is caused by the interaction (or lack of) between sediment facies rather than the former land use. This has significant implications for the design of future MR sites in terms of preparing sites, their anticipated evolution, and the delivery of ecosystem services.

Longitudinal survey on the distribution of Biomphalaria sudanica and B. choanomophala in Mwanza region, on the shores of Lake Victoria, Tanzania: implications for schistosomiasis transmission and control

BackgroundSchistosomiasis is hyper-endemic in the Lake Victoria basin; with intestinal schistosomiasis plaguing communities adjacent to the lake, where the intermediate host snails live. The two intermediate host species of Schistosoma mansoni in the Mwanza region are Biomphalaria sudanica, found on the banks of the lakes, and B. choanomphala, found in the lake itself. There are few longitudinal surveys documenting changing abundance and differential transmission patterns of these Biomphalaria snails across seasons and years. We undertook 15 field surveys at 26 sites over four years to determine the parameters that influence Biomphalaria abundance, presence of S. mansoni-shedding snails and impact of schistosomiasis treatment interventions on transmission potential in the Mwanza region.ResultsStatistical analysis revealed seasonal difference in the abundance of B. sudanica with the highest number of snails found in the dry season (Kruskal-Wallis χ2 = 37.231, df = 3, P < 0.0001). Water measurements were not associated with B. sudanica abundance; however, high levels of rainfall did have a negative effect on B. sudanica [coefficient effect -0.1405, 95% CI (-0.2666, -0.0144)] and B. choanomphala abundance [coefficient effect -0.4388, 95% CI (-0.8546, -0.0231)] potentially due to inundation of sites “diluting” the snails and influencing collection outcome. Biomphalaria sudanica snails were found at all sites whereas B. choanomphala were far more focal and only found in certain sites. Shedding Biomphalaria did not show any variation between dry and rainy seasons; however, a decrease in shedding snails was observed in year 4 of the study.ConclusionsBiomphalaria sudanica is uniformly present in the Mwanza region whereas B. choanomphala is far more focal. Seasonality plays a role for B. sudanica abundance, likely due to its habitat preference on the banks of the lake, but not for B. choanomphala. The decrease in shedding Biomphalaria abundance in Year 4 could be linked to ongoing schistosomiasis treatment efforts in the neighbouring human populations. The highest number of shedding Biomphalaria was observed at sites with high levels of human movement. Prioritising snail control at such sites could greatly reduce transmission in these high-risk areas.

Vegetation and Tree Species Patterns near the Northern Terminus of the Southern Floodplain Forest

The bottomland forest on Horseshoe Lake Island, located on the Mississippi alluvial plain in Alexander County, Illinois in comprised of 2 stands, one relatively undisturbed and the other which is recovering from disturbance in the late 1800s or early 1900s. In both stands vegetational structure, gradient relationships, diversity and size—class characteristics were studied. In the old—growth stand, 35 soil—site variables were measured or estimated for interpretation of vegetational patterns and species distributions. Both direct and indirect gradient analyses were evaluated for use in floodplain forest studies and ultimately the indirect approach with multiple regression interpretation was used to construct a gradient model of the vegetation. Basal area and density of the old—growth and floristic composition of both stands are typical of much of the original Southern Floodplain Forest. The coenocline was divided into 3 segments, mesic, transition and wet with the first and last corresponding to the ridge and hardwood bottoms described by Braun (1950). The important species on the ridge bottoms are Acer saccharum, Asimina triloba, Liquidambar styraciflua, Ulumus rubra, Quercus rubra and Tilia americana and are associated with well—drained soils and infrequent flooding. The transitional segment is dominated by Asimina triloba, Liquidambar styraciflua, Quercus michauxii, Quercus muhlenbergii and Ulmus americana with Quercus pagodaefolia and Quercus shumardii as associates on moderately heavy and poorly drained soils with intermediate flooding. The hardwood bottom sites are dominated by Acer rubrum, Fraxinus pennsylvanica, Liquidambar styraciflua and Ulmus americana and may be flooded for several months each year and have heavy textured, poorly drained soils. Multiple regression analysis with ridge regression revealed that Fraxinus americana and Acer saccharum have wider tolerances to flooding and poor aeration than previously reported. Importance Values of several species were related to duration and depth of flooding, soil mottling and soil texture indicating that distributions are affected by a site—inundation, soil drainage—aeration complex. Diversity of the tree stratum is comparable with that found in some of the mixed mesophytic forests elsewhere. Due to competition from Asimina triloba and Aesculus discolor and possibly some environmental limitations due to the northern location of the stands, the shrub—sapling stratum is depauperate. Along the coenocline from mesic to wet sites density and equitability decrease while basal area increases. Size—class analysis indicates that both the negative power and negative exponential functions fit the size—class distribution for the 2 stands but that neither is entirely appropriate. The old—growth stand is characterized by 3 groups of individuals each with different mortality rates which give a plateaued deviation from each model. The secondary stand is more balanced and does not exhibit a well developed plateau. A comparison of individual species reveals that various regeneration patterns are present which range from uneven—aged (sized) in the case of Acer saccharum to even—aged for Fagus grandifolia, Quercus rubra and Ulmus rubra. The plateaued pattern in the old—growth stand does not reflect past disturbance but apparently is a characteristic of this forest. The principal mode of regeneration in this stand is gap—phase replacement.