Hidetake Hayashi

A Stable Isotope Study on Seasonal Food Web Dynamics in a Eutrophic Lake

Phytoplankton, zooplankton, benthic invertebrates, and pond smelt were collected in a eutrophic lake (Lake Suwa, Japan) from spring to autumn in 1986 and 1987. Their stable carbon and nitrogen isotope ratios were analyzed together with conventional examination of the gut contents of pond smelt. Temporal variations in isotope rations were similar among the organisms other than benthic animals. The @d15N values of diet for several animals in the pelagic food web were estimated, assuming a stepwise enrichment in 15N with increasing trophic level. A calanoid copepod, Eodiaptomus japonicus, seemed to feed on the predominant phytoplankton, including Microcystis spp., while Microcystis did not seem to be important as an organic source for other zooplankton and pond smelt. The temporal variation of the isotope ratios of pond smelt was well explained by visual gut content analysis. It is suggested that intensive measurements of carbon and nitrogen isotope ratios of each species serve to effectively assess the yearly dynamics of the food web structure in eutrophic lakes when productivity and species composition of plankton change seasonally.

Identification and estimation of microcystins in freshwater mussels

Accumulation of microcystins mainly produced by cyanobacteria Microcystis was investigated for freshwater mussels and fishes collected from a lake where heavy blooms of Microcystis occurred every year. The identification of microcystins was performed by HPLC equipped with a frit FAB mass spectrometer. Microcystins LR and RR were identified in the mussels Unio douglasiae and Anadonta woodiana, whereas no microcystin was identified by the present method in fishes, such as Cyprinus carpio, Carassius carassius, and Hypomesius transpacificus.

Estimation of feeding history by measuring carbon and nitrogen stable isotope ratios in hair of Asiatic black bears

Abstract To investigate feeding habits, carbon and nitrogen stable isotope ratios were measured in hair sampled from Asiatic black bears (Ursus thibetanus) inhabiting an alpine area, including the Northern Japanese Alps and the periphery of villages in Nagano Prefecture, in central Japan. Asiatic black bears in the Northern Japanese Alps are subject to little human influence, but in rural areas human encounters with bears seeking food in cornfields and garbage have become an issue that needs to be resolved. We investigated the feeding habits of bears by analyzing the isotopic changes along the entire length of hair samples. Rural bears, including nuisance bears, showed slightly higher δ15N and δ13C values than alpine bears, suggesting that rural nuisance bears may have greater access to anthropogenic food resources than their alpine counterparts. Hair samples were further examined by growth section analysis (GSA), in which sectioned samples from the root to the tip are used for isotopic analysis, to estimate feeding history during hair growth. Hairs of alpine bears exhibited low δ15N and δ13C values from the root to the tip, and the deviation was small. In contrast, hairs of rural bears, particularly of nuisance bears, showed large deviations in isotope values. One bear captured in a cornfield showed high δ13C values at its hair root. Another bear that thrived on garbage showed high δ15N and δ13C values at its hair root, similar to those of Japanese people. One captured bear, assumed to be part of nuisance activities, showed low δ15N and δ13C values from hair root to tip, suggesting that captured bears are not always the ones causing damage. By comparing and classifying GSA fluctuation patterns, we estimated the dependence of nuisance bears on human-related food sources. We expect these methods to provide relevant information for bear conservation and management programs.

ROLE OF ENCYSTMENT AND EXCYSTMENT OF PERIDINIUM BIPES F. OCCULATUM (DINOPHYCEAE) IN FRESH WATER RED TIDES IN LAKE KIZAKI, JAPAN1

The encystment flux of Peridinium bipes f. occulatum (Dinophyceae) was investigated with sediment traps from 1968 to 1990 in Lake Kazki. Cysts of P. bipes were formed throughout the blooms, Encystment flux of P. bipes in the pelagic zone was usually lower than those at shallow sites, and the density of P. bipes cysts in lake sediment was higher in the shallow region than in the pelagic zone. However, in the shallower region, The concentration of P. bipes cysts varied widely, possibly due to high rates of encystment and excystment.

Relationships between Asiatic black bear kills and depredation costs in Nagano Prefecture, Japan

Abstract Over 1,000 Asiatic black bears (Ursus thibetanus) are killed each year in Japan to control depredation activity. Our objective was to determine if killing bears reduces depredation costs. We focused our study on Nagano Prefecture, where 2,562 nuisance bears were reported killed and where reported depredation cost exceeded ¥1,430 million between 1979 and 1999. We used mixed models with repeated measures to determine if annual depredation costs were associated with the number of bears killed. Our data set included 15 years (1985–99) of kill and cost data for 122 municipal jurisdictions within 10 regions. We performed analyses at the regional level based on combined harvest and nuisance kill data, and at the municipal level based only on nuisance kill data. We classified the number of kills into 3 classes (low, medium, high). Analyses were repeated using prior-year kills to examine whether a possible time-lag existed. Annual depredation costs were positively associated with the kill data at the regional level (F = 5.51; 2, 72.3 df; P = 0.006) during the same year. However, we observed no association based on prior-year kill data (F = 0.96; 2, 65.1 df; P = 0.390), suggesting that depredation costs and bear kills are a function of nuisance bear numbers rather than reflecting a causal relationship between the 2 measures. Nuisance bear numbers may in turn be affected either by the availability of natural foods or by general population trends. At the municipal level, depredation costs were not associated with the number of nuisance bears killed during the same year (F = 1.36; 2, 466 df; P = 0.258) or the prior year (F = 0.42; 2, 459 df; P = 0.656). Our results suggest that systematically killing Asiatic bears may not be an effective tool for mitigating nuisance costs. In municipalities where nuisance costs remain high, we recommend that alternative methods be tested for their efficacy in mitigating costs. Such methods may include public education, changing or removing financial incentives to kill bears, changing crop rotations to crops that are not attractive to bears in risk areas, promoting natural food production, using electric fences, and applying aversive conditioning techniques.

Sulfate reduction in profundal sediments in Lake Kizaki, Japan

The sulfate reduction rate was measured for almost four years in the profundal sediments of Lake Kizaki, a mesotrophic lake in central Japan. The rate was generally highest in the surface layer and decreased with depth. Seasonally, sulfate reduction tended to be high in spring and summer, and then to decrease until the end of stratification (December) in spite of a constant in situ temperature of around 6 °C, although fluctuations were found in every year. The rate also fluctuated greatly according to year. The maximum rate of sulfate reduction was 0.33 mmol m−2 d−1 in May, 1990, and the minimum was 0.004 mmol m−2 d−1 in March, 1993. These relatively low rates, compared with those reported for freshwater sediments, seem to be due to low concentrations of sulfate in the sediments (5–23 µmol l−1 in the surface layer). The rate was highly correlated with the concentration of sulfate in the sediments. The addition of sulfate stimulated sulfate reduction in all sediment samples tested, but adding lactate did not. Therefore, sulfate reduction should be limited mainly by the supply of sulfate. Measurements of sulfate reduction rates at different concentrations of added sulfate revealed a low concentration of half-saturation constant as low as 12 µmol l−1.

Role of bottom sediments in sustaining plankton production in a Lake ecosystem — Experimental demonstration using enclosed water bodies in a shallow eutrophic Lake

Using large enclosures set in Lake Suwa, experimental studies were conducted to examine the effects of bottom sediments on plankton production in a natural lake ecosystem. Successive changes in biotic and abiotic components in two types of enclosure with and without bottom sediments were surveyed throughout a one-month period in the early fall of 1985. Remarkable differences were found between the two types of enclosure with regard to the time courses of abundance of chlorophyll, PON, PP, TN, TP and zooplankton as well as those of POC/PON and POC/Chl ratios and primary production rate in terms of mg N m−2 day−1. Quantitative examination of the major causes of these differences revealed that the release of nitrogen and phosphorus from the bottom sediments was significantly effective for sustaining the active production of phytoplankton cells with a high nitrogen and phosphorus content, subsequently resulting in active zooplankton growth.

Impact of whitefish on an enclosure ecosystem in a shallow eutrophic lake: changes in nutrient concentrations, phytoplankton and zoobenthos

Large bag-type (75 m3) and tube-type (105 m3) enclosures were set up in the shallow eutrophic Lake Suwa and were each stocked with exotic planktivorous whitefish (Coregonus lavaretus maraena). The release of whitefish caused the increase in nutrient concentration in the tube-type enclosure whereas no such increase was observed in the bag-type enclosure. Bottom sediment seemed to be an important source of chironomid food for whitefish. The proportion of phytoplankton measuring<10µm and 20–40µm, which respectively corresponded toOchromonas spp. andCryptomonas sp., were lower in the fish enclosures than in the control, which might have been caused by high grazing pressure by rotifers. The predation by whitefish might have affected the species composition of phytoplankton through reducing copepod predation on rotifers, not through reducing the densities of cladocerans which directly feed on phytoplankton as many investigators have reported. The phytoplankton biomass was not affected much by the release of fish. Possible reasons are that the increase in density of rotifers reduced the biomass of available phytoplankton and also that inedible Cyanophyceae were in the decreasing phase of their seasonal succession and could not increase successfully in spite of elevated nutrient levels.

Impact of whitefish on an enclosure ecosystem in a shallow eutrophic lake: selective feeding of fish and predation effects on the zooplankton communities

Bag-type enclosures (75 m3) with bottom sheets and tube-type enclosures (105 m3) open to the bottom sediment were stocked with exotic whitefish (Coregonus lavaretus maraena) to study their predation effects on the plankton community. The fish fed mainly on adult chironomids during the period of their emergence (earlier part of the experimental period). Thereafter, the food preference was shifted to larvae of chironomids and crustacean zooplankters. The predation effects on the plankton community were not evident in the bag-type enclosures where zooplankton densities were consistently low. The fish reduced the crustacean populations composed ofBosmina fatalis, B. longirostris andCyclops vicinus in the tube-type enclosures where the prey density was high (above ca. 50 individuals 1−1). The results suggested that the intensity of predation depended on the prey density. Rotifers increased in the fish enclosure, probably becauseCoregonus reduced the predation pressure byCyclops vicinus on rotifers and allowed the latter to increase. In the fish enclosures, no marked changes in species composition were observed. Zooplankton predated by the fish seemed to be distributed near the walls of the enclosures. Problems of enclosure experiments for examining the effects of fish predation on pelagic zooplankton communities are discussed.

Significance of dissolved organic matter in a mire pool ecosystem

In most clearwater lakes, zooplankton production depends on phytoplankton production and thus the zooplankton biomass is often correlated with the phytoplankton biomass. In many humic lakes, however, the zooplankton biomass is high in comparison with the phytoplankton biomass, suggesting that allochthonous organic inputs support the herbivore production (SALONEN & HAMMAR 1986, HESSEN 1985, HESSEN et al. 1989). Peadands have many humic water bodies. We studied the pool ecosystem in a peadand, Ozegahara Mire, where enclosure experiments were performed. The experimental results have suggested the significance of allochthonous organic inputs in maintaining the pool ecosystem. These results are presented and discussed in the present paper. Ozegahara Mire is situated at about l ,400 m above sea level on mountains in central Japan. The mire has more than l ,500 small pools, some of which are colored brown with a high content of dissolved organic matter (KANAl 1982).