Seasonal and site-specific variability in terrigenous particulate organic carbon concentration in near-shore waters of Lake Biwa, Japan

Identifying sources of particulate organic matter (POM) is important for clarifying fundamental mechanisms by which lake food webs are sustained. We determined carbon and nitrogen stable isotope ratios of POM in near-shore waters of Lake Biwa, a large, meso-eutrophic lake in Japan, to estimate relative contributions of terrigenous particulate organic carbon (T-POC), plankton-derived POC (P-POC) and epilithon-derived POC (E-POC) to POC in near-shore waters. Samples were collected during different months (November, February, May and July) at 29 sites located near the mouth of tributary rivers with different discharge and catchment land use. The data revealed that POC mainly consisted of P-POC and T-POC, with relative contributions varying widely over season and among locations. E-POC generally contributed little to the near-shore POC. Path analyses revealed that the concentration of riverine POC whose isotopic signatures were similar to those of rice straws increased with a larger %paddy field area in the catchment of tributary rivers, which subsequently enhanced T-POC inputs to near-shore waters through riverine transportation. Furthermore, our results suggested that T-POC contribution was influenced, with a time lag, by wave-driven turbulence and shore topography, which appear to affect sedimentation and resuspension of T-POC

Seasonal changes of nutrient distributions along Selangor River, Malaysia

Water samples were taken six times along Selangor River between January 2014 and May 2015 in order to estimate the nutrient changes in different seasons. In the upstream, the nutrient concentrations were higher during the wet period compared to the dry due to the nutrient loading from the eroded soils. Aquaculture ponds could also contribute to the nutrient elevation. Nearly 10 % of the increased nutrients between two sampling stations could come from the aquaculture waste. The relatively high ammonia concentrations were observed in the lower part of Selangor River due to a significant input of ex-mining ponds water and one major arm river which flows through large towns. Our results showed the water quality of Selangor River is clean enough to be suitable for drinking water, but the further study of other parameters such as heavy metals is necessary to ensure the water quality of Selangor River

Natural 15 N Abundance of Plants and Soil N in a Temperate Coniferous Forest

Measurement of nitrogen isotopic composition (δ 15 N) of plants and soil nitrogen might allow the characteristics of N transformation in an ecosystem to be detected. We tested the measurement of δ 15 N for its ability to provide a picture of N dynamics at the ecosystem level by doing a simple comparison of δ 15 N between soil N pools and plants, and by using an existing model. δ 15 N of plants and soil N was measured together with foliar nitrate reductase activity (NRA) and the foliar NO 3 – pool at two sites with different nitrification rates in a temperature forest in Japan. δ 15 N of plants was similar to that of soil NO 3 – in the high-nitrification site. Because of high foliar NRA and the large foliar NO 3 – pool at this site, we concluded that plant δ 15 N indicated a great reliance of plants on soil NO 3 – there. However, many δ 15 N of soil N overlapped each other at the other site, and δ 15 N could not provide definitive evidence of the N source. The existing model was verified by measured δ 15 N of soil inorganic N and it explained the variations of plant δ 15 N between the two sites in the context of relative importance of nitrification, but more information about isotopic fractionations during plant N uptake is required for quantitative discussions about the plant N source. The model applied here can provide a basis to compare δ 15 N signatures from different ecosystems and to understand N dynamics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/41374/1/10021_2002_Article_132.pd

菌体の炭素・窒素安定同位体自然存在比とその生理生態学的意味

京都大学0048新制・課程博士博士(理学)甲第8198号理博第2220号新制||理||1173(附属図書館)UT51-2000-F102京都大学大学院理学研究科生物科学専攻(主査)教授 和田 英太郎, 教授 浅野 透, 助教授 村上 哲明学位規則第4条第1項該当Doctor of ScienceKyoto UniversityDA

Estimation of stream ecosystems by 13C and 15N natural abundances

Stream restoration has been conducted in the Shibetsu River in northern Japan, which aims at the recovery of its river-floodplain ecosystem altered dramatically in the last 40 years. Well-designed monitoring before and after restoration should be conducted to evaluate the effects of the stream restoration on its ecosystem. An analysis of stable isotope ratios (δ-values) is one of the most effective methods for monitoring material cycling in river ecosystems. Because product δ-values are roughly dependent upon their source δ-values, we can trace material flows by comparing the δ-values of various compounds. In the first section of this review, we introduced a basic concept of the δ-value analysis by using an analogy from various diffusion patterns of salt water. The second section described how to calculate δ-values and discussed the difference between the analyses of natural stable isotope ratios and the tracer analyses (another powerful tool for material flow analysis). The third section presented the theoretical basis of mechanisms which change the δ-values. A product δ-value becomes different from its source δ-value in two kinds of processes: isotope exchange equilibrium and kinetic isotope fractionation. The magnitude of this difference (Δδ-value: δproduct-δsource) tells us about the processes and activities of the reactions. The fourth section introduced specific examples from the carbon and nitrogen δ-values (δ13C, δ15N) of aquatic organisms to infer the degree of eutrophication and carbon recycling at ecosystem levels as well as to determine food web structures and photosynthetic activities at biological interaction levels. In the fifth section, we presented the result of our study indicating differences in food web structure between a main-stream channel and oxbow lake in the Shibetsu River watershed when their δ13C and δ15N values of diverse aquatic organisms were compared. The advantages and limitations of the stable isotope analysis in environmental assessment were also discussed

Use of legacy nitrogen as a resource: Unfertilized lotus fields contribute to water quality improvement and biodiversity conservation

Managing legacy nitrogen (N) that has accumulated over decades of intensive agriculture is necessary to balance agricultural production and environmental conservation. Using legacy N as a resource can reduce extant legacy N. The uplands of Japan's Lake Inba watershed are dominated by agroecosystems, and spring waters in the lowlands contain high NO3-N concentrations. We focused on an unfertilized, commercial lotus (Nelumbo nucifera) farm where paddy fields are irrigated with spring waters. We hypothesized that the lotus field could reduce the export of legacy N and provide habitat for fish (co-benefit). Dating of spring waters using sulfur hexafluoride revealed that the residence time of water at the study site was 12.5 years and suggested that the spring waters contained legacy N. We found large reductions of NO3-N and total nitrogen (TN) concentrations in water that passed through the lotus fields. A meta-analysis revealed that the reductions of NO3-N concentrations were highest in unfertilized lotus fields, and reductions of TN concentrations were higher in unfertilized paddy and lotus fields than in fertilized fields. The N removal by harvesting lotus roots 38.7 kg N/ha/year was smaller than the literature-based rate of N removal by denitrification. We used environmental DNA metabarcoding to identify the fish fauna in the lotus fields and an adjacent stream. The native fish richness was a little lower in the lotus fields than in the stream, but the presence of two endangered fish species in the lotus field suggested a moderate biodiversity conservation function. Our case study could be a good example of a nature-based solution to harness ecosystem functions to reduce legacy N

Heat Waves Can Cause Hypoxia in Shallow Lakes

Abstract We assessed how warm air temperatures, high solar radiation, and weak wind speeds might induce hypoxia in a shallow lake during a heat wave. We simulated bottom‐water dissolved oxygen concentrations and compared concentrations in 2022 with the average for the previous 30 years. We found that hypoxia was most sensitive to wind speeds. When the wind speed was low, convection was insufficient to prevent hypoxia, but there was no hypoxia if the wind speed equaled the average speed during the previous 30 years. However, if solar radiation and air temperatures equaled the respective averages during the previous 30 years, hypoxia did not occur, even if wind speeds were low. We conclude that the combined effects of weak winds and either high solar radiation or air temperatures induced hypoxia during the heat wave of 2022

Microdiversity and phylogeographic diversification of bacterioplankton in pelagic freshwater systems revealed through long-read amplicon sequencing

[Background] Freshwater ecosystems are inhabited by members of cosmopolitan bacterioplankton lineages despite the disconnected nature of these habitats. The lineages are delineated based on > 97% 16S rRNA gene sequence similarity, but their intra-lineage microdiversity and phylogeography, which are key to understanding the eco-evolutional processes behind their ubiquity, remain unresolved. Here, we applied long-read amplicon sequencing targeting nearly full-length 16S rRNA genes and the adjacent ribosomal internal transcribed spacer sequences to reveal the intra-lineage diversities of pelagic bacterioplankton assemblages in 11 deep freshwater lakes in Japan and Europe. [Results] Our single nucleotide-resolved analysis, which was validated using shotgun metagenomic sequencing, uncovered 7–101 amplicon sequence variants for each of the 11 predominant bacterial lineages and demonstrated sympatric, allopatric, and temporal microdiversities that could not be resolved through conventional approaches. Clusters of samples with similar intra-lineage population compositions were identified, which consistently supported genetic isolation between Japan and Europe. At a regional scale (up to hundreds of kilometers), dispersal between lakes was unlikely to be a limiting factor, and environmental factors or genetic drift were potential determinants of population composition. The extent of microdiversification varied among lineages, suggesting that highly diversified lineages (e.g., Iluma-A2 and acI-A1) achieve their ubiquity by containing a consortium of genotypes specific to each habitat, while less diversified lineages (e.g., CL500-11) may be ubiquitous due to a small number of widespread genotypes. The lowest extent of intra-lineage diversification was observed among the dominant hypolimnion-specific lineage (CL500-11), suggesting that their dispersal among lakes is not limited despite the hypolimnion being a more isolated habitat than the epilimnion. [Conclusions] Our novel approach complemented the limited resolution of short-read amplicon sequencing and limited sensitivity of the metagenome assembly-based approach, and highlighted the complex ecological processes underlying the ubiquity of freshwater bacterioplankton lineages. To fully exploit the performance of the method, its relatively low read throughput is the major bottleneck to be overcome in the future