Effects of the triazine herbicide, simetryn, on freshwater plankton communities in experimental ponds

The effects of a triazine herbicide, simetryn, on freshwater phyto- and zooplankton communities and water chemistry were investigated using outdoor experimental ponds. Simetryn was applied at a concentration of 0.1 or 1.0 mg l(-1) 18 days after the start of the experiment. Simetryn treatments reduced pH, dissolved oxygen concentration, chlorophyll a concentration and photosynthetic rate in the pond water, whereas they increased the dissolved nutrient concentration. In the phytoplankton community Volvocales and Cryptophyceae were clearly reduced by the herbicide application. In the 0.1 mg l(-1) simetryn-treated ponds, photosynthetic rate of the phytoplankton community recovered slightly about two weeks after the treatment, which coincided with recovery of centric diatoms and Cryptophyceae, whereas it was completely suppressed until the end of the experiment in the 1.0 mg l(-1) simetryn-treated ponds. The decrease in zooplankton density in the treated ponds was not due to the direct toxic effects of simetryn but to indirect effects resulting from reduced algal productivity which in turn reduced the food supply.     

Effects of the organophosphorus insecticide fenthion on phyto- and zooplankton communities in experimental ponds

The organophosphorus insecticide fenthion was applied to experimental ponds and its effects on phyto- and zooplankton communities were analysed. The sensitivity to fenthion differed among cladoceran and rotiferan species according to the following order: Daphnia galeata > Monia micrura > Bosmina fatalis >/= Polyarthra trigla > Keratella valga. The results suggest that large zooplankton species tend to be more sensitive to fenthion than small ones, large cladocerans more sensitive than small cladocerans, and cladocerans more sensitive than rotifers. The application of fenthion induced an increase in the density of rotifers and phytoplankton. This was a secondary effect of the chemical, which directly depressed cladoceran populations and consequently released rotifers and phytoplankton from competition with, and grazing by cladocerans.     

Experimental verification for real-time environmental emergency response system: WSPEEDI by European tracer experiment

Japan Atomic Energy Research Institute has developed an emergency response system WSPEEDI to forecast long-range atmospheric dispersions of radionuclides discharged into the atmosphere. The latest version of WSPEEDI consists of an atmospheric dynamic model MM5 for calculating meteorological fields and a particle random-walk model for atmospheric dispersion. The performance of WSPEEDI was evaluated by data obtained from a field tracer experiment over Europe (ETEX) in this paper. The model validation was done with respect to the following points: (1) the dependence of model accuracy on the temporal and spatial resolutions of the meteorological fields and (2) the superiority of an atmospheric dynamic model over a mass-consistent wind model. Regarding (1), it was shown that the calculation accuracy of the new version with high temporal resolution was improved, especially at the edge of the plume. Moreover, although the increase in horizontal spatial resolution of the old version had no substantial effect on the model performance, increase in horizontal resolution of the new version contributed to the significant improvement of the calculation accuracy. These results showed that the dynamically calculated meteorological field with the spatial resolution of the meso-β–γ scale greatly improved calculation accuracy.     

Effects of repeated application of carbaryl on zooplankton communities in experimental ponds with or without the predator Chaoborus

Zooplankton communities with or without Chaoborus larvae were established in outdoor experimental ponds, to which the insecticide carbaryl was applied repeatedly at 10 or 100 microg litre(-1). In the ponds without Chaoborus, Cladocera dominated, but the species composition differed among the treatments. A large-sized Cladocera Daphnia galeata dominated the controls, the medium-sized cladocerans Diaphanosoma brachyurum and Moina micrura became dominant in the low-dose treatment, and the small-sized Bosmina fatalis increased in the high-dose treatment. These results indicated differential sensitivity to carbaryl among the cladocerans, and that smaller Cladocera were more tolerant of the chemical than the larger one. In the ponds with Chaoborus, rotifers dominated the zooplankton, probably because Chaoborus released rotifers from competition with cladocerans and calanoid copepods, which were eliminated by the Chaoborus predation. No effects of low-dose carbaryl treatment were detected on zooplankton communities in the Chaoborus ponds. The dominance by rotifers, organisms tolerant to carbaryl, minimized the effects. Thus, Chaoborus altered the zooplankton community responses to the chemical application by changing community structure. Repeated application of high-dose carbaryl did affect the rotifer community, decreasing the dominance of Polyarthra trigla and increasing that of Keratella valga. These rotifer species may differ in their sensitivity to carbaryl.     

Influence of persistence period of an insecticide on recovery patterns of a zooplankton community in experimental ponds

An insecticide, carbaryl, was applied singly or repeatedly to experimental ponds in order to control the residue of the chemical, and the subsequent changes in the zooplankton community were investigated. In ponds where a single application of carbaryl, which degraded rapidly in the water, was made, cladocerans were reduced, but recovered soon and suppressed rotifers through competition. On the other hand, in ponds receiving repeated chemical applications, the treatment suppressed cladocerans for longer and induced the occurrence of abundant rotifers. The rotifer abundance after the treatment seemed to depend on the persistence period of the chemical. From these findings it can be hypothesized that applications of chemicals which have different degradation rates induce different zooplankton community structures.     

Zooplankton community responses to chemical stressors: a comparison of results from acidification and pesticide contamination research

The response of freshwater zooplankton communities to two chemical stressors, acidification and pesticide contamination, were investigated in a review of published research results. The objective was to test Odum's predictions (Odum, 1985) that in response to stress, both the average body size of organisms and their efficiency in utilizing resources are reduced. Acidification and pesticide contamination were both found to favor dominance by small cladorecans and rotifers, the smallest zooplankton taxa. This finding was consistent with Odum's predictions, however, there were exceptions to the trend. The dominance of small taxa may be due to rapid reproductive rates, physiological tolerance, development with few transitions through sensitive stages (eg. post-molting), or to the great richness of small species. Regardless of the mechanism, there is evidence that when acidification and pesticide contamination result in small zooplankton dominance, the efficiency of carbon and energy transfer from algae to zooplankton is reduced. This finding is also consistent with Odum's predictions.     

Nutrient dynamics and the eutrophication of shallow lakes Kasumigaura (Japan), Donghu (PR China), and Okeechobee (USA)

We compared the nutrient dynamics of three lakes that have been heavily influenced by point and non-point source pollution and other human activities. The lakes, located in Japan (Lake Kasumigaura), People's Republic of China (Lake Donghu), and the USA (Lake Okeechobee), all are relatively large (> 30 km2), very shallow (< 4 m mean depth), and eutrophic. In all three lakes we found strong interactions among the sediments, water column, and human activities. Important processes affecting nutrient dynamics included nitrogen fixation, light limitation due to resuspended sediments, and intense grazing on algae by cultured fish. As a result of these complex interactions, simple empirical models developed to predict in-lake responses of total phosphorus and algal biomass to external nutrient loads must be used with caution. While published models may provide 'good' results, in terms of model output matching actual data, this may not be due to accurate representation of lake processes in the models. The variable nutrient dynamics that we observed among the three study lakes appears to be typical for shallow lake systems. This indicates that a greater reliance on lake-specific research may be required for effective management, and a lesser role of inter-lake generalization than is possible for deeper, dimictic lake systems. Furthermore, accurate predictions of management impacts in shallow eutrophic lakes may require the use of relatively complex deterministic modeling tools.     

Evaluation of a long-range lagrangian dispersion model with ETEX

Performance of a Lagrangian dispersion model was examined in connection with its dependency on the boundary layer modelling and the input data resolution. The European Tracer Experiment (ETEX) data were used as reference. According to the sensitivity analysis of the model performance, the long-range dispersion model with the sparse input data was not noticeably different from that with the finer resolution data. The assumption of the prescribed constant mixing depth did not largely degrade the prediction results as compared with the simulation results with the temporally changing boundary layer. It is, therefore, concluded that the model is practical, considering the limited input data in the operational mode. However, it was also pointed out that the parameterization for the horizontal and vertical diffusion processes used in the present model enhanced the growth of plume. The improvement of input data resolution in time and space caused further dispersion of tracer deterministically. These resulted in the underestimation of the maximum concentration and the unfocussed concentration distribution map although the mean concentration was predicted fairly well.     

Effects of a carbamate insecticide, carbaryl, on the summer phyto- and zooplankton communities in ponds

A carbamate insecticide, carbaryl (1-naphthyl-N-methylcarbamate), was applied in concrete ponds and the effects on plankton communities were studied. In a control pond, Cladocera declined following the increase in the density of inedible algae after a cladoceran peak. Once the density of Cladocera became low, Chaoborus larvae suppressed the increase of Cladocera and consequently supported the rotifer dominance in the zooplankton community by their selective predation on cladocerans. In a treated pond, the plankton community and its succession were similar to those in the control pond until the chemical application. 1 ppm of carbaryl killed all zooplankton and Chaoborus larvae. Cladocera reappeared soon and increased rapidly due to the absence of Chaoborus larvae. Consequently, rotifer populations were suppressed. Thus, the chemical application altered the dominance of rotifers to that of cladocerans. The same phenomenon was observed again after the second chemical application 12 days after. Although apparent direct effects of the chemical application on phytoplankton were not found, the phytoplankton community structure changed following the changes in the zooplankton density.