安定同位体を用いた、シロアリ-微生物共生系の栄養生態学的研究

京都大学0048新制・課程博士博士(理学)甲第6698号理博第1833号新制||理||998(附属図書館)UT51-97-H82京都大学大学院理学研究科動物学専攻(主査)教授 安部 琢哉, 教授 和田 英太郎, 教授 東 正彦学位規則第4条第1項該当Doctor of ScienceKyoto UniversityDFA

Reconstruction of the extinct Ezo wolf's diet

On Hokkaido, Japan, the Ezo wolf (Canis lupus hattai), an apex predator, became extinct at the end of the 19th century owing to human activities. Top predators often have an important role in their ecosystems, yet we have no scientific information on the feeding habits of the Ezo wolf. We performed carbon and nitrogen stable isotope analysis and radiocarbon dating of specimens of the wolf (n = 7) and its prey species and estimated the components of the wolves' diet using an isotope mixing model. Radiocarbon dating suggested that most of the wolves examined came from different populations or generations. The mean stable isotope ratios of the wolves were −19.5 ‰ (± 1.9 ‰ SD) for δ13C and 8.7 ‰ (± 2.6 ‰ SD) for δ15N. The discrimination-corrected isotopic ratios of five of the seven wolves were almost the same as those of Sika deer at the same sites. In contrast, those of two wolves had clearly higher isotopic values than those of deer, suggesting that these wolves depended partly on marine prey such as salmon and marine mammals. Thus, Ezo wolves had similar ecological roles to Canadian grey wolves, and were a second subspecies shown to have fed on a marine diet, in addition to the "coastal wolves" of British Columbia

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

Is the Relationship between Body Size and Trophic Niche Position Time-Invariant in a Predatory Fish? First Stable Isotope Evidence

Characterizing relationships between individual body size and trophic niche position is essential for understanding how population and food-web dynamics are mediated by size-dependent trophic interactions. However, whether (and how) intraspecific size-trophic relationships (i.e., trophic ontogeny pattern at the population level) vary with time remains poorly understood. Using archival specimens of a freshwater predatory fish Gymnogobius isaza (Tanaka 1916) from Lake Biwa, Japan, we assembled a long-term (>40 years) time-series of the size-dependence of trophic niche position by examining nitrogen stable isotope ratios (δ15N) of the fish specimens. The size-dependence of trophic niche position was defined as the slope of the relationship between δ15N and log body size. Our analyses showed that the slope was significantly positive in about 60% of years and null in other years, changing through time. This is the first quantitative (i.e., stable isotope) evidence of long-term variability in the size-trophic relationship in a predatory fish. This finding had implications for the fish trophic dynamics, despite that about 60% of the yearly values were not statistically different from the long-term average. We proposed hypotheses for the underlying mechanism of the time-varying size-trophic relationship

Nitrogen and oxygen isotope measurements of nitrate to survey the sources and transformation of nitrogen loads in rivers

This paper reviews the studies on evaluation of river environments in terms of water pollution, ecosystem disturbances, excess nutrient (nitrogen) loads, and developments in the isotopic measurements of nitrate and present an update and future perspectives regarding the application of nitrate isotopes to river nutrient assessments. Then, we present the advantages of simultaneous measurement of the nitrogen and oxygen isotopes of nitrate in streamwaters.Dual isotope measurement has recently been used to identify the sources and paths of nitrogen loading in several stream systems. The most recently developed high-resolution denitrifier method is a promising tool with which to investigate the detailed spatial and temporal variation and mechanisms of nitrogen loading and transformation in rivers

Utility of Nd isotope ratio as a tracer of marine animals : regional variation in coastal seas and causal factors

Isotopic compositions of animal tissue are an intrinsic marker commonly used to trace animal origins and migrations; however, few isotopes are effective for this purpose in marine environments, especially on a local scale. The isotope ratio of the lanthanoid element neodymium (Nd) is a promising tracer for coastal animal migrations. Neodymium derives from the same geologic materials as strontium, well known as an isotopic tracer (87Sr/86Sr) for terrestrial and anadromous animals. The advantage of the Nd isotope ratio (143Nd/144Nd, expressed as εNd) is that it varies greatly in the ocean according to the geology of the neighboring continents, whereas oceanic 87Sr/86Sr is highly uniform. This study explored the utility of the Nd isotope ratio as a marine tracer by investigating the variation of εNd preserved in tissues of coastal species, and the causes of that variation, in a region of northeastern Japan where the bedrock geology is highly variable. We measured εNd and 87Sr/86Sr in seawater, river water, and soft tissues of sedentary suspension feeders: the mussels Mytilus galloprovincialis and Mytilus coruscus and the oyster Crassostrea gigas. We also measured concentrations of three lanthanoids (La, Ce, and Pr) in shellfish bodies to determine whether the Nd in shellfish tissue was derived from solution in seawater or from suspended particulates. The εNd values in shellfish tissue varied regionally (−6 to +1), matching the ambient seawater, whereas all 87Sr/86Sr values were homogeneous and typical of seawater (0.7091–0.7092). The seawater εNd values were in turn correlated with those in the adjacent rivers, linking shellfish εNd to the geology of river catchments. The depletion of Ce compared to La and Pr (negative Ce anomaly) suggested that the Nd in shellfish was derived from the dissolved phase in seawater. Our results indicate that the distinct Nd isotope ratio derived from local geology is imprinted, through seawater, on the soft tissues of shellfish. This result underscores the potential of εNd as a tracer of coastal marine animals

Magnesium and zinc stable isotopes as a new tool to understand Mg and Zn sources in stream food webs

Non‐traditional stable isotopes of metals were recently shown as new dietary tracers in terrestrial and marine mammals. Whether these metal stable isotopes can be used to understand feeding habits in stream food webs is not known yet. In this study, we explored the potential of stable isotopes of essential Mg (δ26Mg) and Zn (δ66Zn) as a new tool in stream ecology. For this purpose, we determined δ26Mg and δ66Zn values of stream organisms and their potential metal sources in upper and lower reaches of two streams in the Lake Biwa catchment, Central Japan. Our goals were (1) to explore variations in δ26Mg and δ66Zn across organisms of different feeding habits and (2) to understand Mg and Zn sources to stream organisms. Overall, δ26Mg and δ66Zn values of organisms were neither related to each other, nor to δ13C and δ15N values, indicating different elemental sources and factors controlling isotopic fractionation depending on element and taxa. Low δ26Mg values in filter‐feeding caddisfly larvae and small gobies indicated aqueous Mg uptake. Higher δ26Mg values in leaf‐shredding crane fly and grazing mayfly larvae suggested Mg isotopic fractionation during Mg uptake from the diet. While the δ26Mg values of stonefly nymphs reflected those of caddisfly larvae as a potential prey, the highest δ26Mg values found in dobsonfly nymphs can be explained by 26Mg enrichment during maturing. δ66Zn values of caddisfly and mayfly larvae indicated Zn was a mixture of aqueous and dietary available Zn, while higher δ66Zn values in crane fly larvae pointed to Zn isotopic fractionation during Zn uptake from plant litter. δ66Zn values in stonefly and dobsonfly nymphs were often in the range of those of caddisfly larvae as their prey, while dragonfly nymphs and small goby were depleted in 66Zn relative to their dietary Zn sources. We conclude that δ26Mg is a promising indicator to assess Mg sources in stream ecology depending on taxa, while the use of δ66Zn is limited due to the complexity in Zn sources

Magnesium and zinc stable isotopes as a new tool to understand Mg and Zn sources in stream food webs

Non-traditional stable isotopes of metals were recently shown as new dietary tracers in terrestrial and marine mammals. Whether these metal stable isotopes can be used to understand feeding habits in stream food webs is not known yet. In this study, we explored the potential of stable isotopes of essential Mg (δ²⁶Mg) and Zn (δ⁶⁶Zn) as a new tool in stream ecology. For this purpose, we determined δ²⁶Mg and δ⁶⁶Zn values of stream organisms and their potential metal sources in upper and lower reaches of two streams in the Lake Biwa catchment, Central Japan. Our goals were (1) to explore variations in δ²⁶Mg and δ⁶⁶Zn across organisms of different feeding habits and (2) to understand Mg and Zn sources to stream organisms. Overall, δ²⁶Mg and δ⁶⁶Zn values of organisms were neither related to each other, nor to δ¹³C and δ¹⁵N values, indicating different elemental sources and factors controlling isotopic fractionation depending on element and taxa. Low δ²⁶Mg values in filter-feeding caddisfly larvae and small gobies indicated aqueous Mg uptake. Higher δ²⁶Mg values in leaf-shredding crane fly and grazing mayfly larvae suggested Mg isotopic fractionation during Mg uptake from the diet. While the δ²⁶Mg values of stonefly nymphs reflected those of caddisfly larvae as a potential prey, the highest δ²⁶Mg values found in dobsonfly nymphs can be explained by ²⁶Mg enrichment during maturing. δ⁶⁶Zn values of caddisfly and mayfly larvae indicated Zn was a mixture of aqueous and dietary available Zn, while higher δ⁶⁶Zn values in crane fly larvae pointed to Zn isotopic fractionation during Zn uptake from plant litter. δ⁶⁶Zn values in stonefly and dobsonfly nymphs were often in the range of those of caddisfly larvae as their prey, while dragonfly nymphs and small goby were depleted in ⁶⁶Zn relative to their dietary Zn sources. We conclude that δ²⁶Mg is a promising indicator to assess Mg sources in stream ecology depending on taxa, while the use of δ⁶⁶Zn is limited due to the complexity in Zn sources

Investigation of inter-annual variation in the feeding habits of Japanese sardine (Sardinops melanostictus) and mackerels (Scomber spp.) in the Western North Pacific based on bulk and amino acid stable isotopes

Inter-annual variation in the feeding habits and food sources of Japanese sardine and mackerel at age-0 and age-1+ caught in the Kuroshio-Oyashio transition zone of the Western North Pacific were investigated based on analyses of bulk stable isotopes (δ13C, δ15N) and amino acid nitrogen isotopes (δ15NAA). Differences in δ13C and δ15N between Japanese sardine and mackerel were small for age-0, and inter-annual variation trends were similar, suggesting they depend on similar food sources in the same food web at this age. In contrast, inter-annual variation in δ13C and δ15N were significantly different between both species at age-1+, and both δ15N of phenylalanine (δ15NPhe: an indicator of nitrogen source) and trophic position estimated from δ15NAA (TPAA) were higher in mackerel, suggesting that the two species depend on distinct food webs as they age. Inter-annual variations in δ15NPhe were considered to have different causes for the two species; differences in food web structure due to the degree of southward intrusion of the Oyashio Current for Japanese sardine, compared to a shift in migration area and depth for mackerel. Furthermore, competition for food due to the recent increases in the population densities of both fishes appeared to be reflected in increased TPAA of mackerel. Although they are caught in the same region, the mechanism of variation in food sources differs because of differences in migration area, depth, and feeding habits. Differences in the feeding habits of Japanese sardine and mackerel may affect trophic status and spawning characteristics, potentially leading to different shifts in stock abundances