Toshio Iwakuma

Runoff properties and change in concentrations of agricultural pesticides in a river system during a rice cultivation period

Abstract Residues of 12 pesticides in water and sediment were surveyed in a river system during a rice cultivation period. The highest concentrations of pesticides were 36 μg l−1 for fenobucarb (carbamate insecticide), 11.3 μg l−1 for simetryn (herbicide) and 17.5 μg l−1 for iprobenfos (fungicide). The downstream flux was largest for iprobenfos, which showed a broader peak than the other pesticides. The runoff rates estimated from the ratio of the downstream flux of a pesticide to its retail amount was highest for fenobucarb and lowest for an organochlorine herbicide, chlornitrofen. These rates ranged over two orders according to the difference in the octanol-water partition coefficients or water solubility of the pesticides. Chlornitrofen was persistent in the stream sediment judging from the regression coefficient between concentrations in water and sediment.

Effects of permethrin on phytoplankton and zooplankton in an enclosure ecosystem in a pond

The insecticide permethrin (a synthetic pyrethroid) was applied into enclosures (1 m diameter and 3.5 m deep) placed in a pond. The chemical was rapidly removed from the water to the sediments. Daphnia rosea and its predator, Chaoborus flavicans were seriously affected by this application and disappeared from the enclosure. Acanthodiaptomus pacificus increased as Chaoborus decreased. Two species of Cladocera which had not been observed before the treatment, established their populations after 10 days, when Chaoborus had not recovered. Whereas Tropocyclops pracinus declined when permethrin was applied at 10 µg 1−1, the number of rotifer Keratella valga increased, suggesting a close relationship between these two species.Photosynthesis and phytoplankton were not significantly affected by permethrin, except for Ceratium hirundinella. The dominance of Dinobryon divergens continued in the treated enclosures, whereas other flagellate species, Scenedesmus and Nitzschia occurred during the second half of the experiments in the control enclosure and pond.

Studies on the biology of Chaoborus flavicans (Meigen) (Diptera : Chaoboridae) in a fish-free eutrophic pond, Japan

Population dynamics of Chaoborus flavicans larvae of various instars was studied from November 1986 to December 1987 in a eutrophic, fish-free pond, Japan. First and 2nd instar larvae were observed from late April to late October, indicating a reproductive period of about half a year. C. flavicans overwintered in the 4th instar larvae. In water column samples, total density of all instars was 680–23680 m-2, and pupal density 0–2600 m-2; larvae of the 1st, 2nd, and 3rd instars showed 5–6 density peaks in 1987, suggesting that 5–6 generations occur during a year (peaks of the 4th instar larvae were not clear, probably due to their longer development than those of younger instars). In sediment samples, no 1st and 2nd instar larvae were found, 3rd instar larvae were found occasionally but density of the 4th instar larvae was 280–18600 m-2, and pupal density varied between 0–502 m-2. Fouth instar larvae accumulated in sediment in the cold season and in the water column in the warm season; high temperature and low oxygen concentration were the most important factors limiting the distribution of larvae in the sediment in summer in the NIES pond. The dry weight of total C. flavicans larvae was 0.08–4.2 g m-2 in sediment samples and 24–599 µg l-1 (0.10–2.40 g m-2) in water column samples. Comparisons of maximum densities in the NIES pond in different years and in waters of different trophic status show that density is generally higher in eutrophic than in oligotrophic habitats.

Effects of insecticides on the benthic community in a model stream

Abstract The benthic community in an outdoor model stream was exposed to the organophosphorus insecticide temephos, at a concentration of 5 mg litre−1 for 30 min, or chlorphoxim at 2 mg litre−1 for 3 h. Three species of chironomids were predominant in this stream in the first experiment, together with Hydropsychodes brevilineata in the second experiment. Both insecticides killed the majority of the zoobenthos, and resulted in a bloom of epilithic algae. The extent of grazing pressure by chironomids on algae was assessed. The chironomid populations recovered quickly but the trichopteran larvae recovered slowly. There were some differences in the toxicity between the two insecticides. Paracyclops fimbriatus and Procladius sp. were affected only by chlorphoxim.

Growth and leaf life-span of a floating-leaved plant, Trapa natans L., as influenced by nitrogen flux

Abstract Trapa natans L., cultured in tanks under varying nitrogen flux, was examined with particular reference to the change in foliage structure. Leaf initiation rate, leaf loss rate and leaf life-span were independent of nitrogen flux and ranged from 140 to 250 mg N m−2 day−1. In contrast, leaf and rosette size were positively correlated with nitrogen flux. The maximum biomass was estimated to be 487 g dry weight m−2 in the most nitrogen-enriched tank. With increasing nitrogen flux, proportions of petiole and reproductive parts (fruit) in the total biomass increased, while those of stem and root decreased. Net production of T. natans was also positively correlated with nitrogen flux and ranged from 5 to 14 g dry weight m−2 day−1 for all experimental tanks, excluding a tank with algal competition.

Emergence of Chironomidae from the shallow eutrophic Lake Kasumigaura, Japan

Seasonal chironomid emergence was monitored in the shallow eutrophic Lake Kasumigaura and 18 species were collected with a battery-operated light trap fixed on a floating stage and with surface emergence traps. During October–December, samples in the light trap comprised exclusively Tokunagayusurika akamusi (Tokunaga) and small numbers of one or two other species. T. akamusi, Procladuis (Holotanypus) culiciformis (L.), and Chironomus plumosus (L.) constituted 91.6% of the annual catch of chironomid adults. The predominance of T. akamusi (75.3 % of chironomid catch) and the high ratio (13) of T. akamusi to C. plumosus was more marked in this lake than other Japanese eutrophic lakes. Glyptotendipes tokunagai Sasa and Dicrotendipes pelochloris (Kieffer) were also caught abundantly with the light trap, but not with surface traps, indicating these were littoral species. The dry weight of emerging adults during May–December 1982 was 2.87 g m−2, of which 1.92 gm−2 (67%) was T. akamusi and 0.67 gm−2 (23%) C. plumosus and 0.23 g m−2 (8%) Clinotanypus sugiyamai Tokunaga and 0.03 gm−2 (1%) P. (H.) culiciformis. The weight of emerging Tanypodinae was much higher than the annual mean larval biomass or estimated larval production, which have been due to underestimating the population density using an Ekman-Birge dredge. High numbers of individuals and species of chironomids were caught during April–July, presumably as a result of the high food supply for chironomid communities.

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.

Effects of temephos on zooplankton communities in enclosures in a shallow eutrophic lake

An organophosphorus insecticide, temephos, was applied to large-volume (105 m3) enclosures set up in a shallow eutrophic lake. Application of the chemical at a target concentration of 500 microg litre(-1) eliminated almost all zooplankters. No recovery of cladocerans was evident at the termination of the experiment (47th day after the treatment). Copepods showed a slight recovery after having been absent for 26 days in one enclosure and 40 days in another. The residual chemical remaining in the water until the final day may have suppressed the recovery of the crustacean zooplankters. The rotifer community was reconstructed 16-20 days after the treatment. However, the species composition of this community differed from that of the rotifer community in the control enclosures. Rotifer species might therefore show differences in susceptibility to temephos.

Behaviour of pesticides in Lake Kasumugaura, Japan

Abstract Pesticides such as oxadiazon, isoprothiolane, diazinon, IBP, simetryne and benthiocarb, from surface runoff from rice paddy fields to Lake Kasumigaura, Japan were examined in order to determine seasonal changes in their concentrations in the lake water. The highest concentrations were observed near the river mouths during May to August, soon after pesticide application; hence they are transported mainly by surface runoff. Although these pesticides have been reported to be labile, concentrations of some of them, dispersed with the movement of the water, did not decrease rapidly in the natural aquatic environment. The highest concentrations, in 1985, of oxadiazon, isoprothiolane, diazinon, IBP, simetryne, benthiocarb and CSB were 0.66, 0.84, 0.16, 3.24, 3.23, 2.15 and 1.13 ppb, respectively. The relative order of persistence and mobility of pesticides in the lake water was estimated as: isoprothiolane, CSB, simetryne > IBP > diazinon ⪢ oxadiazon > benthiocarb.

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.