Coupled CH4 production and oxidation support CO2 supersaturation in a tropical flood pulse lake (Tonle Sap Lake, Cambodia).

Carbon dioxide (CO2) supersaturation in lakes and rivers worldwide is commonly attributed to terrestrial-aquatic transfers of organic and inorganic carbon (C) and subsequent, in situ aerobic respiration. Methane (CH4) production and oxidation also contribute CO2 to freshwaters, yet this remains largely unquantified. Flood pulse lakes and rivers in the tropics are hypothesized to receive large inputs of dissolved CO2 and CH4 from floodplains characterized by hypoxia and reducing conditions. We measured stable C isotopes of CO2 and CH4, aerobic respiration, and CH4 production and oxidation during two flood stages in Tonle Sap Lake (Cambodia) to determine whether dissolved CO2 in this tropical flood pulse ecosystem has a methanogenic origin. Mean CO2 supersaturation of 11,000 ± 9,000 µatm could not be explained by aerobic respiration alone. 13C depletion of dissolved CO2 relative to other sources of organic and inorganic C, together with corresponding 13C enrichment of CH4, suggested extensive CH4 oxidation. A stable isotope-mixing model shows that the oxidation of 13C depleted CH4 to CO2 contributes between 47 and 67% of dissolved CO2 in Tonle Sap Lake. 13C depletion of dissolved CO2 was correlated to independently measured rates of CH4 production and oxidation within the water column and underlying lake sediments. However, mass balance indicates that most of this CH4 production and oxidation occurs elsewhere, within inundated soils and other floodplain habitats. Seasonal inundation of floodplains is a common feature of tropical freshwaters, where high reported CO2 supersaturation and atmospheric emissions may be explained in part by coupled CH4 production and oxidation.

Evidence for population genetic structure in two exploited Mekong River fishes across a natural riverine barrier.

Impacts of urban development on aquatic populations are often complex and difficult to ascertain, but population genetic analysis has allowed researchers to monitor and estimate gene flow in the context of existing and future hydroelectric projects. The Lower Mekong Basin is undergoing rapid hydroelectric development with around 50 completed and under-construction dams and 95 planned dams. The authors investigated the baseline genetic diversity of two exploited migratory fishes, the mud carp Henicorhynchus lobatus (five locations), and the rat-faced pangasiid catfish, Helicophagus leptorhynchus (two locations), in the Lower Mekong Basin using the genomic double digest restriction site-associated DNA (ddRAD) sequencing method. In both species, fish sampled upstream of Khone Falls were differentiated from those collected at other sites, and Ne estimates at the site above the falls were lower than those at other sites. This was the first study to utilize thousands of RAD-generated single nucleotide polymorphisms to indicate that the Mekong's Khone Falls are a potential barrier to gene flow for these two moderately migratory species. The recent completion of the Don Sahong dam across one of the only channels for migratory fishes through Khone Falls may further exacerbate signatures of isolation and continue to disrupt the migration patterns of regionally vital food fishes. In addition, H. lobatus populations downstream of Khone Falls, including the 3S Basin and Tonle Sap system, displayed robust connectivity. Potential obstruction of migration pathways between these river systems resulting from future dam construction may limit dispersal, which has led to elevated inbreeding rates and even local extirpation in other fragmented riverine species.

Monitoring of tropical freshwater fish resources for sustainable use.

Tropical freshwater ecosystems are some of the world's most biodiverse and productive systems where determining what sustainable exploitation of inland fisheries looks like is particularly challenging. One of the greatest obstacles to sustainable management is collecting and using quality data on fish production and yield. The biodiversity and hydro-ecology of these systems often under open-access governance, add to the complexity of managing them. This paper describes an integrated citizen-science, earth observation, environmental DNA and independent survey approach to collecting fish and fisheries data, using the Cambodian Mekong as a case study.

Cryptic Lineages and a Population Dammed to Incipient Extinction? Insights into the Genetic Structure of a Mekong River Catfish.

An understanding of the genetic composition of populations across management boundaries is vital to developing successful strategies for sustaining biodiversity and food resources. This is especially important in ecosystems where habitat fragmentation has altered baseline patterns of gene flow, dividing natural populations into smaller subpopulations and increasing potential loss of genetic variation through genetic drift. River systems can be highly fragmented by dams built for flow regulation and hydropower. We used reduced-representation sequencing to examine genomic patterns in an exploited catfish, Hemibagrus spilopterus, in a hotspot of biodiversity and hydropower development-the Mekong River basin. Our results revealed the presence of 2 highly divergent coexisting genetic lineages which may be cryptic species. Within the lineage with the greatest sample sizes, pairwise FST values, principal component analysis, and a STRUCTURE analysis all suggest that long-distance migration is not common across the Lower Mekong Basin, even in areas where flood-pulse hydrology has limited genetic divergence. In tributaries, effective population size estimates were at least an order of magnitude lower than in the Mekong mainstream indicating these populations may be more vulnerable to perturbations such as human-induced fragmentation. Fish isolated upstream of several dams in one tributary exhibited particularly low genetic diversity, high amounts of relatedness, and a level of inbreeding (GIS = 0.51) that has been associated with inbreeding depression in other outcrossing species. Our results highlight the importance of assessing genetic structure and diversity in riverine fisheries populations across proposed dam development sites for the preservation of these critically important resources.