Historical descriptions of N. aperta habitat (Before Dams 1971–73).

The historical information from the 1971–73 research (Before Dams) is provided in Figs 1–12.

The importance of water flow on the infection rate of S. mekongi was highly evident during pre-impoundment baseline studies. Although specific measurement of flow was not part of these studies, shoreline areas with relatively lower flow and frequent contact by humans and animals at Khong Island, Laos were associated with higher infection rates of S. mekongi. Offshore areas in deeper water with moving currents are relatively free of human contact and the mixing, dilution, and lack of substrate for attachment for N. aperta may explain the absence of significant infection (Figs 11–12). Upatham [18] studied the Mekong River in Thailand and Laos and described the association of different water flow patterns on the bionomics of hydrobiid snails, including the alpha- and gamma-strains N. aperta. He reported higher densities of snails in areas with higher water flow, but data were not collected on the transmission of S. mekongi [18]. Site studies of favorable habitat for N. aperta at Khong Island prior to any changes in the natural flow regime of the Lower Mekong River indicate that the site reflected similar downstream water quality and sedimentation characteristics when compared to an upstream reference site at the village of Ban Dan, Thailand (Figs 2, 4, 5 and 10).

Water quality and water ion data from one monsoon cycle in the Mekong River prior to impoundment indicated a cyclic but stable habitat for snails, algae, and aquatic plants (Figs 4 and 5). Sediment data indicated lower levels of some parameters due to particles settling out with distance (Fig 10). Data from other studies of downstream stations in Vietnam showed similar trends in sediment transport and characteristics [19]. Statistically significant differences were noted in very fine sand, organic carbon, active iron, and Ca²⁺ (Fig 10. Studies of substrates favored by N. aperta indicate that they use hard substrates such as tree branches, leaves, metal, rocks, and aquatic plants [3] and that sand was seldom used by N. aperta (Figs 3, 9 and 10). Sediments are also a major source of nutrients supporting the primary producer community that provides substrate and food for N. aperta and other snail species of the Lower Mekong River. The microbial primary producer community on Khong Island had sixteen species that serve as food and substrate for N. aperta (Figs 11 and 12) and twenty-three species of dominant snails (Fig 12). The potential role of snail predation by fish, turtles, and other aquatic organisms is a topic that needs study in the N. aperta habitat areas. A study of the use of fish to control parasitic diseases, including schistosomiasis in Lake Malawi, identified the challenges involved in biological control. [20] Mekong River fish that are snail predators include (Yasuhikotakia morleti) [21] and Pangasius conchophilis. [22] The snail-eating turtle Malayemys subtrijuga in the Mekong River in Vietnam is known to be infected by 2 digenean species and could impact snail populations in different river areas. [23] The anticipated changes described in the updated N. aperta habitat descriptions could affect fish and turtle populations in the study area with impacts on snail predation.

Current description of potential N. aperta habitat in the LMR (After Dam surveys) of 2008–2010.

The MRC survey data used in the PBCM identified several basic similarities and differences in the habitat comparisons between 1971–73 and 2008–2010 [5,6]. Our examples of applying the PBCM to predict potentially suitable habitats for N. aperta and other snails compares data from recent surveys along with the historical baseline data from 1971–73. Recent reports of mainstream water quality were described as very good overall, but some areas with degraded habitat and water quality were reported. [5,6]. Although N. aperta was not reported from the surveys, one site surveyed at Done Ngiev, Laos was very similar to the Khong Island Laos transmission site with sand substrate, Homomonia riparia shrubs, and filamentous algae and diatoms. The PBCM identified the Done Ngiev site as a good match to Khong Island and would be a good site for more detailed follow-up habitat studies. Another similar site in Cambodia, at Kampi pool near the Lao border, had a sand substrate with some stones and a snail population of almost 400 Stenothyra sp., a common co-habitant of N. aperta. The PBCM also identified sites in Laos, Thailand, and Cambodia that are dissimilar to Khong Island and not good matches. Sites at Ban Xiengkok, Laos, Nakorn Panom, Thailand, and Se San, Cambodia had substrates dominated by rocks, gravel, and mud without much sand and generally low water quality and turbulent currents.

Shimada [24] described the high susceptibility of the gamma strain of N. aperta to S. mekongi from Khong Island, Laos. A 2017 survey from Khong Island noted the presence of more than 5500 N. aperta from the same shoreline areas studied in 1972, with some snails testing positive for S. mekongi [25]. More recently, Kumagai [26] developed a loop-mediated isothermal amplification (LAMP) assay for S. mekongi and evaluated the utility of the assay for detecting DNA of S. mekongi in snails and human stool and snail samples in endemic areas on Khon Island, Laos. The LAMP assay results were used to develop a risk map for monitoring S. mekongi and preventing epidemics. Although these surveys [25,26] did not include the habitat characteristics, the results strongly suggest that some of the favorable N. aperta/S mekongi habitats outlined in the Before Dam descriptions remain in place at Khong Island. A follow-up detailed study of the Khong Island site could provide new information to update the PBCM. The PBCM will benefit from additional updates that provide information on current environmental trends, particularly the impacts of climate change and dam construction and operation.

Models characterizing general impacts from climate change and dams in the study area.

Models describing climate change impacts on the LMR can be useful in updating information in the PBCM. Some model-based projections of flood risks in the Lancang-Mekong River continuum predict more frequent and bigger flood events in the future in response to climate change. Through reservoir regulation, flood risks can be mitigated particularly in the upper basin where large reservoirs are located. Models indicate that the combined effects of climate change and reservoir regulation can reduce risks in the upper basin, but the increased flood magnitude and frequency, from climate change, are still dominant in the lower basin [8]. Recent reports from the Sixth phase of Coupled Inter-comparison Project (CMIP6 models), including the Standardized Precipitation Index (SPI) and Standardized Precipitation-Evapotranspiration Index (SPEI) data, predict that seasonal impacts of droughts in the Lower Mekong River Basin (LMRB) will include wetter rainy season and drier non-rainy season conditions. The SPEI shows that the middle reaches of the LMR will experience the highest levels of drought duration and intensity [27].

Reservoirs across the Lancang River reduced the annual average flow by 5% at Chiang Saen during 2008–2016, but their impacts on flow were undetectable downstream at Vientiane Laos. This flow data highlights the importance of site-specific descriptions like that provided by the PBCM since the After Dams data was collected during this period. The streamflow changes noted downstream of Mukdahan in southern Laos, Cambodia, and Vietnam were attributed to local reservoirs and climate change, with the latter changing the magnitude and frequency of floods by up to 14% in Cambodia and 45% in South Vietnam [28]. Current data on annual surface water flow in the LMR indicate a general downward trend during 1960–2010, with no clear trend after 2010. However, while most studies indicate a decreasing trend, a few studies reported the opposite with increasing flow; this may be the result of different data and methods applied in each study [28].

Hydropower development, irrigation expansion, and global climate change were examined in a study of the LMR [29]. The authors suggested studies, including transboundary collaboration in order to address sustainable development goals, could help to meet planned sustainability objectives. Other studies [30,31], using the year 2040 development scenarios, showed that hydropower and irrigation development predicted limited impacts on annual flows compared to the early development period. Predicted climate change can reduce the streamflow in June between years 2040 to 2080 for some scenarios and increase the streamflow in October in the same period. Furthermore, the streamflow showed significant increases in the long-term during September-November and December-February but showed decreases during March-May and June-August. Hydropower decreased the streamflow in the wet season by 7% but increased in the dry season by 29%. The combined impacts of all driving factors caused substantial flow reductions during the first half of the wet season by—42%, but increased the flow in the dry season by 40% and 14% compared to the early development and current scenario, respectively [29].

A recent study [32] described an integrated approach with mixed methods and assessed future sustainability challenges in social, hydrological, and ecological dimensions using a case study from the Lower Mekong basin. The study area included the combined basin of the Se Kong, Se San, and Sre Pok (3S) Rivers that deliver approximately 20% of the flow to the Mekong River system. The results suggested that future river flows in the 3S river system could move closer to natural conditions during the dry season, with increased floods during the wet season, and require a shift in the current dam operations, from maintaining minimum flows to reducing flood hazards [32].

Potential impacts specific to N. aperta/S. mekongi habitat.

The projected changes in flow in the Lancang- Mekong continuum will lead to major impacts on sediment transport and characteristics along different river channel areas and dam reservoir areas. The changes are both local and site-specific. The proposed PBCM framework can help characterize major determinants of the sediment architecture associated with the observed favorable habitat for N. aperta (Figs 3 and 10–12).Snail habitat sites along the lower Mekong River can be described as patchy or mosaics habitats that display high intra-and inter-site variability in substrate type, nutrient availability, and primary producer organisms that regulate food sources and water quality [33]. This was evident in the 2008–2010 studies completed by [2,3].

Sediment architecture and water quality are major determinants of the aquatic food web components supporting N. aperta populations, particularly macrophytes, microorganisms, and macroinvertebrates. The habitat for N. aperta on Khong Island described in the PBCM is conditioned by sediment architecture and particle size distribution. Sediment is a strong determinant of nutrient type, quantity, and availability to support the primary producer base of the food web. Sediments and other substrates (4) are an important source of food and attachment for snails including N. aperta (Figs 3 and 6–10).6Muntz [34] studied the decomposition of organic matter above and below dams in the USA. The results can be used to better understand dam impacts using the PBCM. Muntz found decomposition rates were significantly greater in the impoundments than in one free-flowing section of the river but did not find a significant relationship between decomposition and oxygen or temperature. The average temperature was slightly warmer in the impoundments, suggesting a stronger effect of temperature than oxygen on decomposition.

Current studies in China indicate similar general results with pre- and post-impoundment studies of hydrological impacts on sediment transport in the Han River Basin in China that produce a significant quantity of sediment loss downstream [35]. Sediment descriptions in the PCBM indicate a general decrease in sediment in the Khong Island, Laos area compared to upstream at Ban Dan, Thailand. Estimates of sediment transport in the Lancang-Mekong River system are highly variable but reflect conditions prevailing prior to the ongoing construction of a cascade of seven dams on the Lancang, that will trap an estimated 83% of the Lancang basin sediment when complete [36]. Thus, almost half of the total natural sediment load of the Mekong will be lost in the reservoirs of the Lancang, and the sediment load of the Lower Mekong River will consist largely of sediment derived from sources within the Lower Mekong River basin itself. These sediment losses could lead to site-specific spates of hungry water [37] in areas described in the PBCM causing habitat changes for N. aperta due to scouring and bank and channel erosion [38]. The important role of paleogeography and paleoclimate on speciation and endemism in isolated populations of Schistosoma vectors of the genus Bulinus in Africa has been described [39]. Snail vectors can be sensitive to changes in the ecological communities in which they are embedded. Macrophyte communities provide attachment substrate and nutrients for snails in littoral habitats of the LMR as described in the PBCM (Figs 11 and 12). Hydro-morphological alterations influenced the functional diversity and composition of macrophyte communities in the Danube River [40]. Macrophytes were reported to modulate the detritus cycle and change the nutrient supply in S. mansoni snail vector habitats [41]. Qualitative and quantitative changes in nutrients can result in significant changes in habitat structure. The enhanced release, export, and transport of diffuse nutrients from watershed forest litter in the Lancang River basin, during climate change, have been reported using remote sensing data, experimental parameters, and the SWAT model [42].

Tomczyk [43] compared the points closest to and downstream of a Hydro Powerplant (HP) in Poland and noted an increase in concentrations of most heavy metals. Although data on heavy metal contaminants were not included in the PBCM, future studies should include habitat contamination in the areas with water quality degradation. Fine-grained fractions dominate the clays and sandy clays upstream of the hydropower plants, while sands, sandy clay loams, or sandy clays are dominated downstream. The dominance of fine-grained fractions upstream of the HPs favors the accumulation of metals due to the high sorption capacity of these fractions [43]. When comparing the analyzed groups of points, the average pH values are arranged in the following sequence: upstream HP (U) > downstream HP (D). EC has opposite values than pH (i.e., U < D) EC is greater downstream. In the 1972 pre-impoundment studies, the same trend in pH and EC was noted between Ban Dan and Khong Island. The Tomcyzk study may support sustainable sediment management options for use with the PBCM and allows for the development of recommendations related to the rational management of HPs.

Shi [44] developed a niche model using machine learning algorithms to predict the probability of suitable snail habitats for the S. japonicum vector Oncomelania hupensis in China. The six greatest contributors to predicting potential O. hupensis distribution included silt content (13.13%), clay content (10.21%), population density (8.16%), annual accumulated temperatures of ≥0°C (8.12%), night-time lights (7.67%), and average annual precipitation (7.23%). The Before Dams description in the PBCM shows somewhat similar trends in habitat for N. aperta regarding the percent sand, silt, and clay at Ban Dan and Khong Island (Fig 10).

Reports describing the Lower Mekong Basin indicate that overall water quality was good up into the 2000s [1,4]. Recent studies indicate that localized degradation has occurred over the past decade near Vientiane City; the Sekong, Sesan, and Srepok Rivers, the Tonle Sap Lake system, and the Mekong Delta [45,46]. Water quality degradation likely corresponds to flow alteration, erosion, sediment trapping, and point and nonpoint wastewater. Habitat updates in the PBCM can be clarified using this type of data at specific sites.

A study of the relationship between climatic, hydrological, and water quality parameters of the LMR flowing through four different countries (Thailand, Cambodia, Lao PDR, and Vietnam) indicated that DO, pH, conductivity, Ca, Mg, Na, K, alkalinity, Cl, SO4²− and Si had fair to strong negative correlations with all hydrological parameters. TSS, alkalinity, and conductivity were proposed as sensitive water quality parameters for monitoring the impacts of changing climate in the lower Mekong River [47]. Descriptions of these water quality parameters in the Before and After Dams information in the PBCM can be useful in future monitoring programs.

Current reports describing the overall mainstream water quality and ion analyses in the Lower Mekong basin near Khong Island cite good to excellent conditions like the monsoon cycle and diel cycle data reported in the PBCM Before Dams studies of 1972–73 (Fig 1–10)]; however, site-specific data were not collected as part of these surveys and other reports [45,46] cite water quality and habitat deterioration at some sites near Khong Island. A river study in France reported that dams did not disturb the seasonality of major anions but did modify silica and phosphorus concentration-flow relationships, especially during low flow. Such changes in the dynamics of river-water composition may affect downstream biological communities [48] and may occur in the LMR.

Three types of Green House Gases (GHGs), carbon dioxide, methane, and nitrous oxide emitted from reservoirs have an important but often ignored impact on climate change and need to be studied in detail in the LMRB. The biogeochemical cycle of carbon in the aquatic environment is understood to be the main mechanism for generating reservoir-based GHG emissions [49]. Future updates of the PBCM should include current data on GHGs.

The impact of temperature change on infections of S. japonicum in China was studied using a growing degree-days simulation model. The results predicted changes in transmission patterns and altered frequency and transmission dynamics of S. japonicum with climate change [50].

Kalinda [51] used a synthesis of 19 published articles and indicated that temperature rise may alter the distribution, and optimal conditions for breeding, growth, and survival of intermediate host snails, which may eventually increase the spread and/or transmission of schistosomiasis. Other studies in rivers and lakes have reported that transmission may be high at 15–19°C. and 20-25°C. respectively [52]. Both temperature effects studies can be used to enhance the habitat descriptions over time in the PBCM. Liu and Xu [53] studied a unique form of flow regulation (e-flow) called hydropeaking that introduces frequent, short-duration, artificial flow events to river systems in China. Hydropeaking is one form of the sporadic flows affecting habitat structure in the After Dams effects noted in the PBCM. They reported that hydropeaking drove plant species distribution, assemblage structure, and species richness-biomass relationships. Hydropeaking also generated complex species-specific effects on plant biomass patterns. They concluded that hydropeaking is likely a major driver of plant community assembly in hydropeaking-affected rivers.

Twenty sites were sampled along the length of the Mekong River from northern Thailand to Lao PDR, Cambodia, and southern Vietnam. Sites included lower Mekong River shorelines and major tributaries. Data were collected on four groups of organisms considered suitable for biomonitoring studies: benthic diatoms, zooplankton, littoral macroinvertebrates, and benthic macroinvertebrates [5,6]. Data from selected lower Mekong River sites in Laos, Thailand, and Cambodia are included in the PBCM, and results indicate that all groups were diverse and abundant in the Mekong River system with considerable variability in abundance and composition from site to site, with the mosaic conditions as described in [15,33]. In another study, thirty-two sites were assessed and classified as to their ‘ecological health’ using indicator characteristics, including physical and chemical measures, diatoms, and littoral macroinvertebrates. These sites were then classified as excellent, good, moderate, or poor. Ten sites were moderate and one site was poor, with the remaining sites classified as good to excellent [6].

Limpanont et al. [54] reported a new habitat site for the γ- strain of N. aperta (with molecular confirmation of identity) from Nong Khai Province, where it occurred in a habitat novel for this species—under paving slabs instead of under natural bed rocks as noted in [4] and PBCM data descriptions. The new site had algal aufwuchs and was located on the islet in the middle of the Mekong River. The new location is approximately four hundred km upstream from the nearest previously known site for this species.