Hideshige Toda

Food source of riparian spiders analyzed by using stable isotope ratios

We analyzed the food source of riparian spiders in a middle reach of the Chikuma River, Japan, by using stable isotope ratios of carbon and nitrogen. The carbon and nitrogen isotope ratios of attached algae were higher than those of terrestrial plants, reflecting a large carbon isotope fractionation in terrestrial plants and a difference in nitrogen sources. The carbon isotope ratios of terrestrial insects were similar to those of the terrestrial plants, and the ratios of aquatic insects were scattered between those of the terrestrial plants and the attached algae. The carbon and nitrogen isotope ratios of spiders were intermediate between those of the terrestrial and aquatic insects. The two-source mixing model using the carbon isotope ratio showed that the web-building spiders utilized both the terrestrial and aquatic insects, with large contribution by the aquatic insects (54% on average with a maximum of 92% among spider’s taxa collected in each zone), in the riparian area in a middle reach of the Chikuma River. The large contribution of the aquatic insects was often observed for the spiders collected near river channel (<5 m) and for the horizontal web-building spiders collected across the riparian area. The relative contribution of the aquatic insects might be related with food availability (distance from river channel) and spider’s food preference reflected in their web types (horizontal vs. vertical). Our results showed that organic materials produced in the river channel, in the riparian area, and in the terrestrial area surrounding the riparian area were mixed at the carnivorous trophic level of riparian spiders.

Use of15N/14N rations to evaluate the food source of the mysid,Neomysis intermedia Czerniawsky, in a eutrophic lake in Japan

The stable isotope ratios of nitrogen were measured in the mysid,Neomysis intermedia, together with various biogenic materials in a eutrophic lake, Lake Kasumigaura, in Japan throughout a year of 1984/85. The mysid, particulate organic matter (POM, mostly phytoplankton), and zooplankton showed a clear seasonal change in δ15N with high values in spring and fall, but the surface bottom mud did not. A year to year variation as well as seasonal change in δ15N was found in the mysid. The annual averages of δ15N of each material collected in 1984/85 are as follows: surface bottom mud, 6.3‰ (range: 5.7–6.9‰); POM, 7.9‰ (5.8–11.8‰); large sized mysid, 11.6‰ (7.7–14.3‰); zooplankton, 12.5‰ (10.0–16.4‰); prawn, 13.2‰ (9.9–15.4‰); goby, 15.1‰ (13.8–16.7‰). The degree of15N enrichment by the mysid was determined as 3.2‰ by the laboratory rearing experiments. The apparent parallel relationship between the POM and the mysid in the temporal patterns of δ15N with about 3‰ difference suggests the POM (mostly phytoplankton) as a possible food source ofN. intermedia in this lake through the year.

Diffusion analysis of N2O cycling in a fertilized soil

The behavior of nitrous oxide (N2O) in fertilized soil was studied in terms of soil fluxes, the production rates at various depths and the turnover in soil. The diffusive losses of N2O to the atmosphere calculated from soil N2O profile compared favorably with the flux directly determined with a closed chamber technique. The estimate of N2O production rates at several depths demonstrated that the sites of N2O production was only near the soil surface. The calculated residence time of N2O in the entire soil column studied was only 1.4 hour during active emission period and less than 1 day even in the later period having trace N2O emission. The prolonged N2O emission observed after the active phase was due likely to a lasting N2O production rather than a supply from the soil N2O reservoir. The results suggested that most N2O in soil was emitted quite promptly to the atmosphere after its production. A minor role of soil as an N2O reservoir is emphasized from the viewpoint of the origin of groundwater N2O.

Abundance and life history ofNeomysis intermedia Czerniawsky in Lake Kasumigaura

AbstractWeekly observations ofNeomysis intermedia in Lake Kasumigaura showed two major peaks in abundance during spring and autumn (more than 104 individuals m−2) and minimum levels in summer and winter (less than 103 individuals m−2). Their increase in abundance followed a high egg ratio, suggesting that the increase in abundance was caused by a high reproductive rate. Major contributors to mysid population decreases include fish predation and commercial fisheries, and possible horizontal migration of the mysids. N. intermedia showed two types of life history in the lake. One type (overwintering generation) has a life span of about 6–7 months and produces about 27 eggs per brood. Another, appearing from spring to autumn, matures in 3–6 weeks at a smaller size, and produces 12 eggs per brood. The reproductive season ofN. intermedia was continuous from March through November.

Effects of Nitrogen Deposition on Nitrous Oxide Emissions from the Forest Floor

Increasing nitrogen deposition due to human activity might have a serious impact on ecosystem functions such as the nitrogen transformations conducted by microbes. We therefore focused on nitrous oxide (N2O) production as an indicator of soil microbial activity. The rates of N2O emission from the forest floor were measured every two weeks in two forest stands in the central part of Japan: a red pine stand at Kannondai and a deciduous stand at Yasato. Nitrogen deposition rates by throughfall were 30.6 kg N ha−1 y−1 at Kannondai and 15.7 at Yasato. The rates of N2O emission ranged from 0.5 to 14.2 µg N m−2 h−1 (mean 4.5) at Kannondai and from 0.2 to 7.0 µg N m−2 h−1 (mean 2.3) at Yasato. The N2O emission rate showed significant positive relationships with soil temperature and nitrogen deposition during the preceding two weeks. The annual emission rates of N2O were 0.38 kg N ha−1 y−1 at Kannondai and 0.20 at Yasato. As a the annual nitrogen deposition, these rates were 1.23% at Kannondai and 1.27% at Yasato.

Surface distributions of copepods in relation to regional upwellings around the Izu Islands in summer of 1988

Surface distributions of zooplankton were surveyed with simultaneous measurements of temperature, salinity, chlorophyll and nitrate, around the Izu Islands in July 1988. Several phytoplankton patches, which had developed in the upwelled waters, were encountered around the islands. Zooplankters collected with a 20µm mesh mainly consisted of copepods including eggs, nauplii, copepodites and adults.Paracalanus parvus dominated among the copepod females. High concentrations of all copepod stages were associated with the phytoplankton patches. The increase in reproductive activity of female copepods was considered as a possible process to form the copepod patches associated with the phytoplankton patches.

Aquatic subsidies transport anthropogenic nitrogen to riparian spiders

Stable nitrogen isotopic composition (δ15N) of aquatic biota increases with anthropogenic N inputs such as sewage and livestock waste downstream. Increase in δ15N of riparian spiders downstream may reflect the anthropogenic pollution exposure through predation on aquatic insects. A two-source mixing model based on stable carbon isotopic composition showed the greatest dependence on aquatic insects (84%) by horizontal web-building spiders, followed by intermediate (48%) and low (31%) dependence by cursorial and vertical web-building spiders, respectively. The spider body size was negatively correlated with the dietary proportion of aquatic insects and spider δ15N. The aquatic subsidies transported anthropogenic N to smaller riparian spiders downstream. This transport of anthropogenic N was regulated by spiders guild designation and body size.

Relating body size to the role of aquatic subsidies for the riparian spider Nephila clavata

We examined the relationship between body size of the riparian spider Nephila clavata and the contribution of allochthonous (aquatic insects) and autochthonous (terrestrial insects) sources to its diet using stable isotope analysis. During the study period from July to September, the body size of the females increased remarkably (about 60-fold) but that of males remained small. The biomass of both aquatic and terrestrial insects trapped on the spider webs increased with spider size, with the biomass of the former ranging between 30 and 70% of that of the terrestrial insects. The average relative contribution of aquatic insects to the diet of the spiders, calculated from δ13C values, was 40–50% in spiders in the early juvenile and juvenile stages, 35% in adult males and 4% in adult females. There was a significant negative relationship between the relative contribution of aquatic insects and body size of the female spiders. We conclude that aquatic insects might be an important seasonal dietary subsidy for small spiders and that these allochthonous subsidies may facilitate the growth of riparian spiders, which may in turn enable the spiders to feed on larger prey.

Influences of nitrogen and phosphorus addition on polyphenol oxidase activity in a forested Andisol

Increased atmospheric N deposition could suppress plant litter decomposition, due to the P limitation for soil microorganisms in Japanese forested Andisols with a high P sorption capacity. To explore this possibility, we used a laboratory incubation experiment to study the influence of N addition on β-d-glucosidase and polyphenol oxidase activities, which are important for cellulose and lignin degradation, respectively, in an Andisol with larch (Larix kaempferi) leaf litter. The addition of N increased the β-d-glucosidase activity, whereas it decreased the polyphenol oxidase activity in the soil. However, the addition of both N and P increased the polyphenol oxidase activity in the soil, suggesting the possibility of; (1) an inferior competitive ability of polyphenol oxidase-producing microorganisms under nutrient-rich conditions and; of (2) their P limitation through competition in the Andisol.

Soil properties affecting phosphorus forms and phosphatase activities in Japanese forest soils: Soil microorganisms may be limited by phosphorus

Abstract To explore a possible phosphorus limitation of soil microbial processes, we fractionated phosphorus in Japanese forest soils (10 Inceptisols, three allophanic Andisols, seven nonallophanic Andisols, and one Spodosol) by sequential extraction into inorganic P (Pi) and organic P (Po) in H2O, 0.5 M NaHCO3, 0.1 M NaOH, 1 M HCl and conc. HCl fractions, and total P in residual fractions. NaOH-Pi and NaOH-Po fractions were the largest P components in all soil types. Apart from H2O-Pi, NaOH-Pi, and NaOH-Po, P concentration in each fraction did not differ significantly among soil types. Concentrations of P in fractions H2O-Pi, H2O-Po, 0.1 M NaOH-Pi, 0.1 M NaOH-Po, and residual P were correlated with active Al, but not with active Fe, indicating a more significant contribution of Al in controlling P forms in the soils. The proportion of available P (H2O + NaHCO3 − P) to total P was negatively affected by active Al and Fe contents and by pH in the soils. High phosphomonoesterase and phosphodiesterase are known to be indicators of low soil P availability, and both activities were higher in soils with low available P in this study, suggesting that microorganisms of these forest soils may be P limited.