Lake In Japan Research Articles

Environmental Controls of Diffusive and Ebullitive Methane Emissions at a Subdaily Time Scale in the Littoral Zone of a Midlatitude Shallow Lake

AbstractEnvironmental controls on methane (CH4) emission from lakes are poorly understood at subdaily time scales due to a lack of continuous data, especially for ebullition. We used a novel technique to partition eddy covariance CH4 flux observed in the littoral zone of a midlatitude shallow lake in Japan and examined the environmental controls on diffusion and ebullitive CH4 flux separately at a subdaily time scale in different seasons. Using the high‐frequency data, we investigated how CH4 accumulation in the water and sediment layers alters the dynamics and environmental controls of fluxes. The contribution of ebullitive flux to total flux was 57% on average. Environmental controls of diffusive and ebullitive fluxes known in the literature were confirmed. We further found that the environmental controls were different in different seasons and suggested that additional consideration of CH4 accumulation could explain the variability. The transfer of accumulated dissolved CH4 from the bottom water layer to the surface in summer and the accumulation of dissolved CH4 under surface ice in winter were suggested to be important for explaining the variability of diffusive flux. In summer, a higher ebullitive flux tended to occur following triggers such as a decrease in hydrostatic pressure. In winter, the impact of triggers was not obvious, and a higher ebullitive flux tended to occur in the morning. We suggested that the low CH4 production rate in winter slowed the replenishment of bubbles in the sediment, negating the effect of triggers on ebullition.

Wind as a Main Driver of Spatial Variability of Surface Energy Balance Over a Shallow 102‐km2 Scale Lake: Lake Kasumigaura, Japan

AbstractLakes have often been treated as one‐dimensional entities for energy balance (EB) studies mostly based on point measurements. Therefore, our knowledge of the spatial variability of lake EB is quite limited. We created EB maps of Lake Kasumigaura, a 172‐km2 shallow lake in Japan, with a 90‐m horizontal resolution at a 3‐hr interval over 5 years based on spatially interpolated meteorological variables and water surface temperature, with turbulent fluxes estimated by the bulk equations. The results indicate that turbulent fluxes and total energy flux into water body G were spatially variable while radiative fluxes were more uniform. The spatial variability of turbulent fluxes averaged over a season, a year, and 5 years was mainly caused by wind speed difference; a longer fetch in downwind areas of the lake resulted in strong winds and higher turbulent fluxes. The spatial difference of turbulent fluxes and quasi‐uniform net radiation caused a total energy flux out of the water in the downwind area and a total energy flux into the lake in an upwind area. This spatial difference of G appeared to be compensated by heat transport from the upwind to downwind area through advection due to lake current.

Dual stable isotope characterization of excess methane in oxic waters of a mesotrophic lake

AbstractTo determine the source of excess methane in oxic, surface‐water columns often found in freshwater environments, we measured the in situ concentration and stable isotopic compositions (δ13C and δ2H) of methane in Lake Biwa, a mesotrophic lake in Japan. The values from the littoral zone and lake‐floor sediments were determined, besides those in the water column of the pelagic zone. Furthermore, we conducted incubation experiments to measure microbial oxidation rates and alterations in the isotopic signatures of methane. We found significant vertical and seasonal variations in both in situ concentrations and stable isotopic compositions of methane measured in the pelagic zone. We concluded that active microbial oxidation was primarily responsible for the variation in δ13C and δ2H values of methane in the pelagic water column. As a result, we defined a new indicator Δ(2,13) to characterize the sources of dissolved methane, in which variations in both δ13C and δ2H during methane oxidation had been corrected. The excess methane in oxic, surface‐water columns exhibited Δ(2,13) values similar to those in the littoral zone. We concluded that excess methane at the surface of the pelagic zone originated from the littoral zone via lateral transport. Anoxic near sediments and inflowing rivers were responsible for methane enrichment in water of the littoral zone and in the surface water columns of the pelagic zone.

Evolutionary History and Diversity of Unionoid Mussels (Mollusca: Bivalvia) in the Japanese Archipelago

The evolutionary history and diversity of unionoid mussels in East Asia need to be clarified and would shed light on the formation process of the unique fauna of Japan. Unionoid mussels (Mollusca: Bivalvia) are unique models for understanding the process by which organisms have diversified before and after the formation of the Japanese archipelago. Unionoid mussels have poor dispersal ability, so it is thought that they would have been strongly influenced by the archipelago’s formation. Therefore, the speciation and diversification processes of mussels before and after the archipelago’s formation were investigated by analyzing the nuclear and mitochondrial DNA of a wide range of species, particularly those inhabiting East Asia. The evolutionary history and divergence time of these mussels were examined. Unionoid mussels were found to have higher endemicity than other freshwater organisms. Although most of the endemic unionoid mussels of Japan are likely to have diverged before the formation of the Japanese archipelago, some other Japanese unionoid mussel species, including species endemic to Lake Biwa, an ancient lake in Japan, potentially diverged after the Japanese archipelago began to separate from the continent. This suggest that adaptation to the unique habitat of the ancient lake has caused diversification in the mussels endemic to it.

Draft Genome Sequence of the Microcystin-Degrading Bacterium Novosphingobium sp. Strain MD-1.

This report describes the whole-genome sequence of a microcystin-degrading bacterium, Novosphingobium sp. strain MD-1, isolated from a lake in Japan. The Novosphingobium sp. strain MD-1 genome had a total length of 4,617,766 bp. Moreover, strain MD-1 showed a conserved microcystin-degrading gene cluster (mlrA to mlrF), similar to Sphingopyxis sp. strain C-1.

Molecular-level comparison of dissolved organic matter in 11 major lakes in Japan by Orbitrap mass spectrometry

AbstractDissolved organic matter (DOM) causes organic pollution in lakes, resulting in the occurrence of off-flavour etc. when lake water is used as a drinking water source. In this study, DOM in 11 major lakes in Japan was characterised by high-resolution Orbitrap mass spectrometry. Molecular formulas were assigned to 845–1,451 components per sample. Among them, 555 components were commonly found in all lakes. The DOM compositions of the 11 lakes were clustered into four groups. Correlation analysis could extract specific components whose relative intensities were associated with water quality indices such as specific ultraviolet absorbance, specific dissolved chemical oxygen demand (DCODMn), and specific trihalomethane formation potential (R = 0.80–0.93, p < 0.05). Although further molecular structural analyses of DOM components are necessary, these results could be informative for exploring key candidates related to specific water quality issues. Pre-treatment of samples with permanganate oxidation was applied to screen components which could contribute to DCODMn. DCODMn components accounted for only 7–30% of total peak intensities, indicating the limited performance of permanganate oxidation. Pre-treatment by permanganate coupled with Orbitrap MS revealed that components with higher molecular weight, higher oxygen-to-carbon ratios (O/C), and lower hydrogen-to-carbon ratios (H/C) could be responsible for DCODMn.

A novel Enterostomula (Platyhelminthes, Prolecithophora) species from two brackish lakes in Japan.

BackgroundThe genus Enterostomula Reisinger, 1926 belongs to the family Pseudostomidae and comprises generally small and often conspicuously coloured species living on hard bottoms, in gravel and amongst algae. The Pseudostomidae comprises approximately 44 known species from Europe as well as North and South America. Previously, only one species, Allostoma durum, had been recorded in Japan. Known Enterostomula species are predominantly found in marine and brackish habitats.New informationI collected seaweed and sand samples from two brackish lakes near the coast of Shimane Prefecture, Japan and isolated turbellarians from them. The animals were observed as both living and preserved. Here, I describe a novel Enterostomula species with two dorsal black bands and a thick bursal wall.

Application of Orbitrap mass spectrometry to investigate seasonal variations of dissolved organic matter composition in a eutrophic lake in Japan

Orbitrap mass spectrometry can reveal the composition of DOM and the fraction responsible for CODMn in an eutrophic lake.

Evaluation of Water Management Effectiveness by a Multi-Dimensional Approach

As water is utilized in various ways across the whole spectrum of human life, better understanding of water management effectiveness and sharing of information and experience are crucial for achieving sustainability from a water perspective. However, water management effectiveness in the long term has been insufficiently studied due partially to the lack of long term data and methodology for long term assessment. The present study presents a case study, dealing with the environmental restoration of a once listed as the worst lake in Japan for 27 years through a water diversion project. It highlights the need and importance of evaluating water management practice from both chemical and biological dimensions for its long-term effectiveness. By combining field surveys of phytoplankton communities and E. coli at multiple locations in the lake in different seasons and data analysis of long term change of chemical parameters of water quality, it revealed that although the diversion project led to a shift in phytoplankton community structure from being bluegreen algae dominated to diatom dominated, the eutrophic level remains high and the diatom-dominated phytoplankton community structure is probably the factor hindering the true recovery of the ecosystem in the lake. Wording differently, the diversion operation alleviated the symptom of eutrophication but did not solve the problem. A message form the present study is that such a multi-dimensional assessment approach allows the explanation of water management effectiveness in ecological relevance and identification of the direction for further improvement work. Therefore, it also serves as a call for the further development of multi-dimensional assessment approaches.

In-Flight Radiometric Calibration of Compact Infrared Camera (CIRC) Instruments Onboard ALOS-2 Satellite and International Space Station

The Compact Infrared Camera (CIRC) instruments onboard the Advanced Land Observing Satellite-2 (ALOS-2) and the Calorimetric Electron Telescope (CALET) attached to the International Space Station are satellite-borne 2D-array thermal infrared cameras for technical demonstrations in fields such as forest fire monitoring, volcano monitoring, and heat island analysis. Since they have the characteristics of low cost and low power consumption and have no onboard calibrator such as a blackbody or shutter, in-flight calibration should be performed by vicarious calibration (VC) and cross-calibration (CC). In this study, we determined the recalibration coefficients for both of the CIRC instruments as a function of time based on VC experiments in Lake Kasumigaura (Japan) and Railroad Valley Playa (USA), VC with telemetry data from three lakes in Japan and the USA, and CC with imagers onboard two geostationary satellites (MTSAT-2 and Himawari-8). As a result, the derived recalibration coefficients improved the accuracy of the ground-testing-based radiance remarkably in both of the CIRC instruments, suggesting that the recalibrated radiance can satisfy the target accuracy of CIRC, given as 2 K at 300 K. These coefficients, as a function of time, will be applied to all CIRC images by reprocessing planned in the near future.

Characteristics of declining dissolved oxygen concentrations in Lakes Kasumigaura and Kitaura, two shallow polymictic eutrophic lakes in Japan

AbstractTo elucidate the patterns and characteristics of dissolved oxygen (DO) decline near bottom in shallow polymictic eutrophic lakes, we conducted a statistical analysis of monthly measurements for 12 years at 14 stations and hourly observation for 8 years at two stations in Lakes Kasumigaura and Kitaura. Results indicated that DO decline (hereafter, <2 mg/L) was negligible at stations with a depth of <5 m. Thus, we consider this depth to be critical depth for studying DO decline conditions in these lakes. The collected data revealed that DO decline events lasting more than 2 days occurred a few times a year. During these events, wind blew weakly (usually, <2 m/s), and diurnal stratification was formed by solar radiation. The averaged DO declining rate leading up to these events was 4.3 (±2.7) mg L−1 d−1, suggesting that within a few days, DO can fall into a declined state. The occurrence probability of DO declined events in the respective summer increased significantly with the proportion of weak wind hours (<2 m/s). Significant correlations between wind velocity and DO change rate (1 hr difference in DO) supported the importance of wind on DO changes in polymictic lakes. The influence of mean air temperature on DO decline events was insignificant.

Widespread Dominance of Kinetoplastids and Unexpected Presence of Diplonemids in Deep Freshwater Lakes.

Kinetoplastid flagellates are generally abundant in the deep sea and recently they were even found to be dominant in the hypolimnion of a deep freshwater lake. Therefore, to understand the distribution of kinetoplastids in deep freshwater lakes, we have collected vertical samples from five lakes in Japan. The abundance of kinetoplastids was enumerated by Catalyzed Reporter Deposition-Fluorescence in situ Hybridization, and the diversity was determined by 18S amplicon sequencing using universal eukaryote and kinetoplastid-specific primers. Kinetoplastids were abundant in the deep waters of all the lakes, contributing up to 53.6% of total nanoeukaryotes. Despite this significant contribution, kinetoplastids remain undetected by amplicon sequencing using universal primers that are widely used in eukaryotic diversity studies. However, they were detected with specific primers, and the communities were characterized by both ubiquitous and lake-specific unique OTUs. Oligotyping of a ubiquitous and dominant OTU revealed the presence of lake-specific sequence types (oligotypes). Remarkably, we also detected diplonemids (a sister group of kinetoplastids and considered to be specific in the marine habitat) using kinetoplastid-specific primers, showing their presence in freshwaters. Underestimation of kinetoplastids and diplonemids using universal primers indicates that euglenozoan flagellates are overlooked in diversity studies worldwide. The present study highlighted the importance of kinetoplastids in the hypolimnion of deep lakes, thereby indicating their role in material cycling in deep waters.

Nuisance growth of macrophytes in lakes in Japan and overseas.

Nuisance growth of macrophytes in lakes in Japan and overseas.

Decrease in bacterial production over the past three decades in the north basin of Lake Biwa, Japan

In Lake Biwa, the largest lake in Japan, external pollutant loads have decreased since the 1980s, leading to improved water quality, such as reduction in biochemical oxygen demand (BOD) and PO43− concentrations. We examined long-term variation of bacterial production (BP) under these environmental changes from 2016–2017 in the north basin of Lake Biwa. BP was estimated by measuring the incorporation of stable isotope 15N-labeled deoxyadenosine (15N-dA) from June 2016 and December 2017 and compared with measurements from 1986 and 1997–1998. In 1986, BP was measured by following 3H-labeled thymidine (3H-TdR) incorporation and in 1997–1998 by tracking bacterial abundance in incubations and calculating specific growth rates (μ). To allow direct comparison of 15N-dA and 3H-TdR incorporation rates, we determined a conversion factor. To estimate μ in 2016–2017, we determined a factor for converting 15N-dA incorporation to cell number increase. In 2016–2017, the 15N-dA incorporation rate ranged from 0.13 to 30.7 pmol l−1 h−1 and μ ranged from 0.016 to 0.70 day−1. BP values from 3H-TdR incorporation rates in 1986 and μ in 1997–1998 were 4.6 and 2.1 times BP values in 2016–2017, respectively, confirming the decrease in BP over the past 3 decades in Lake Biwa. Water quality data showed only low decrease rates for BOD and total phosphorus concentration from the 1980s, whereas the rate of decrease for PO43− concentrations was equivalent to that of BP. BP and decomposition of organic matter are known to be strongly P-limited in Lake Biwa. Our results suggest that the decrease in BP can be explained by a reduction in readily bioavailable PO43−. Organic phosphorus can also be an important P-source for BP under conditions with very low PO43− concentrations (nM), and changes in the bioavailability of organic phosphorus might have also regulated BP dynamics.

The life history with seasonal migration of the lacustrine shrimpPalaemon paucidensin an ancient lake in Japan

AbstractThe lacustrine shrimpPalaemon paucidensundertakes seasonal migration between shallow waters in spring and summer to deep waters in autumn and winter in Lake Biwa in Japan. Previous studies hypothesized that the migration to deep waters was for hibernation. The findings of this study oppose this hypothesis. We showed thatP. paucidensis physiologically active in deep waters during winter; that the carbon stable isotope ratio indicated that the species forages in winter and that there was a difference in food sources between summer and winter; that lipid content was highest in January; and that gene expression activity (as measured by theRNA/DNAratio) remained constant throughout the year. Thus,P. paucidensindividuals that migrate to the bottom are likely to forage in winter, but do not hibernate. Nutrients gained in winter were not reflected in individual growth but may have been allocated to gonadal growth and the production of gametes, suggesting that winter foraging by this species in deep sites contributes to obtain resources for reproductive investment. In addition, we found that both small individuals with no reproductive experience and relatively large females with reproductive experience overwintered, indicating thatP. paucidenshas a life‐cycle that is longer than 1 yr in Lake Biwa.

Metaepigenomic analysis reveals the unexplored diversity of DNA methylation in an environmental prokaryotic community

DNA methylation plays important roles in prokaryotes, and their genomic landscapes—prokaryotic epigenomes—have recently begun to be disclosed. However, our knowledge of prokaryotic methylation systems is focused on those of culturable microbes, which are rare in nature. Here, we used single-molecule real-time and circular consensus sequencing techniques to reveal the ‘metaepigenomes’ of a microbial community in the largest lake in Japan, Lake Biwa. We reconstructed 19 draft genomes from diverse bacterial and archaeal groups, most of which are yet to be cultured. The analysis of DNA chemical modifications in those genomes revealed 22 methylated motifs, nine of which were novel. We identified methyltransferase genes likely responsible for methylation of the novel motifs, and confirmed the catalytic specificities of four of them via transformation experiments using synthetic genes. Our study highlights metaepigenomics as a powerful approach for identification of the vast unexplored variety of prokaryotic DNA methylation systems in nature.

Long-term observations of the vertical distributions of mineral elements and phosphorus dynamics in sediments in a shallow eutrophic lake in Japan

PurposeTo explore the mechanisms in the deposition and release of phosphorus (P) in the sediment of a shallow eutrophic lake using preserved samples, we investigated the vertical and temporal changes in P, manganese (Mn), sulfur (S), iron (Fe), aluminum (Al), calcium (Ca), and magnesium (Mg) in the sediment samples and the phosphate in the sediment pore water samples over a period of 6 years.Materials and methodsThe upper 15 cm of sediment from Lake Kasumigaura in Japan was collected monthly from 2003 to 2008 from the center of the lake. Sediment cores were divided into seven depth segments and were acid-digested for an elemental analysis via inductively coupled plasma atomic emission spectroscopy. Phosphate concentrations of the sediment pore water were determined using the molybdenum blue method. A multiple regression analysis was conducted by setting the P content as the response variable and Mn, S, Fe, Al, Ca, and Mg as explanatory variables.Results and discussionThe results of the multiple regression analysis demonstrated that P co-precipitates with Fe and Al oxides and accumulates on the sediment surface. The vertical distributions of Mn and S suggest that Mn reduction occurs within the 0–1-cm-depth layer of the sediment and that iron sulfide is actively formed in the 6–10-cm-depth layer of the sediment. These findings imply that the layer in which ferric oxides are reduced to ferrous ions is present near the 1–6-cm-depth layer of the sediment. This layer corresponds to the layer in which the maximum phosphate concentration of the sediment pore water often occurred (the 2–6-cm-depth layer). These results indicate that vertical distributions of mineral elements are useful for assessing P dynamics in sediments.ConclusionsThe lake sediments record the dynamics of P in the sediment. Our analytical approach using long-term observation data demonstrated that the accumulation and release of P associated with a change in the redox state can be assessed based on the vertical distributions of mineral elements in the lake sediments.

A protocol for the analysis of aquatic biodiversity by multiple β diversities

Species assemblage can be fairly unstable in aquatic ecosystems because of strong environmental constraints. Measuring β diversity is one of the most useful tools for assessing community dynamics and differentiations among communities. Whereas many β diversity measurements have been proposed for these issues, we present here a unifying framework based on the properties of Hill number, q, to unravel the complex changes occurring in dynamic communities. Using ideal numerical examples of an aquatic community, the sensitivity of β diversity with q = 0 ∼ 2 was examined. Whereas β diversity for q = 0 has a higher sensitivity to rare species drift (rapid exchanges or migration of many rare species) than do others, β diversity for q = 2 adequately responds to the variation or changes of dominant species. A scatterplot based on these two β diversity measures is proposed for the detailed examination of aquatic community dynamics. We show that the scatterplot is able to describe the monthly dynamics of dominant and rare species in the phytoplankton assemblage of a freshwater lake in Japan.

Seasonal changes in the biomass of floating leaved plant, Trapa spp., and its relation with a leaf beetle, Galerucella nipponensis, in Lake Inba, Japan

Trapa spp. dominate many shallow eutrophic lakes in Japan, which must affect the nutrient dynamics in lakes. Trapa spp. are utilized by several animals, in particular the leaf beetle, Galerucella. To quantify the trophic links and material transfers between Trapa spp. and G. nipponensis, their seasonal dynamics, the amount of leaves being consumed, and carbon and nitrogen stable isotope ratios (δ13C and δ15N) in Lake Inba were tracked. Trapa spp. began to increase in May, reached their highest biomass in September, and disappeared in October. G. nipponensis exhibited continuous reproduction from May until October. Grazing by G. nipponensis did not seem to affect the abundance of Trapa, as the average percentage of Trapa leaf consumed was 3%. Rather, G. nipponensis was influenced by Trapa, as δ13C and δ15N of Trapa spp. and G. nipponensis showed very similar seasonal fluctuations. The difference in δ15N was 3–4‰, which indicates that Trapa leaves were a main food source for G. nipponensis and a trophic linkage exists between them. The estimated nitrogen and phosphorus amounts in Trapa corresponded to 1% of nitrogen and 18% of phosphorus in the entire lake. If G. nipponensis migrate and die in a terrestrial environment, G. nipponensis can transport certain nutrients from lake to terrestrial environments. The nutrients from Trapa spp. are mostly regenerated, except for that biomass transported to terrestrial ecosystems by humans.

Bottom-up linkages between primary production, zooplankton, and fish in a shallow, hypereutrophic lake.

Nutrient supply is a key bottom-up control of phytoplankton primary production in lake ecosystems. Top-down control via grazing pressure by zooplankton also constrains primary production and primary production may simultaneously affect zooplankton. Few studies have addressed these bidirectional interactions. We used convergent cross-mapping (CCM), a numerical test of causal associations, to quantify the presence and direction of the causal relationships among environmental variables (light availability, surface water temperature, NO3 -N, and PO4 -P), phytoplankton community composition, primary production, and the abundances of five functional zooplankton groups (large cladocerans, small cladocerans, rotifers, calanoids, and cyclopoids) in Lake Kasumigaura, a shallow, hypereutrophic lake in Japan. CCM suggested that primary production was causally influenced by NO3 -N and phytoplankton community composition; there was no detectable evidence of a causal effect of zooplankton on primary production. Our results also suggest that rotifers and cyclopoids were forced by primary production, and cyclopoids were further influenced by rotifers. However, our CCM suggested that primary production was weakly influenced by rotifers (i.e., bidirectional interaction). These findings may suggest complex linkages between nutrients, primary production, and rotifers and cyclopoids, a pattern that has not been previously detected or has been neglected. We used linear regression analysis to examine the relationships between the zooplankton community and pond smelt (Hypomesus nipponensis), the most abundant planktivore and the most important commercial fish species in Lake Kasumigaura. The relative abundance of pond smelt was significantly and positively correlated with the abundances of rotifers and cyclopoids, which were causally influenced by primary production. This finding suggests that bottom-up linkages between nutrient, primary production, and zooplankton abundance might be a key mechanism supporting high planktivore abundance in eutrophic lakes. Because increases in primary production and cyanobacteria blooms are likely to occur simultaneously in hypereutrophic lakes, our study highlights the need for ecosystem management to resolve the conflict between good water quality and high fishery production.