Takayuki Hanazato

Pesticide effects on freshwater zooplankton: an ecological perspective

The effects of pesticides on zooplankton are reviewed and their ecological significance is discussed. Toxicity is shown to vary depending on animal species, genotype, life stage, and size at birth. Natural stresses such as food shortage, oxygen depletion and odors of potential predators can also affect toxicity. Populations in the growth phase are vulnerable to pesticides but have the potential to recover rapidly from the damage. Pesticides may affect the population dynamics by controlling individual survival and reproduction, and by altering the sex ratio. Furthermore, toxic chemicals may control predation risk by changing swimming behavior and body morphology, and this in turn influences the population dynamics. Many zooplankton display morphological and behavioral responses to predators when exposed to their odor-producing chemicals. However, pesticides induce a maladaptive response to predator odor, and this poses an ecological risk. The following patterns are recognized as effects of pesticides at the community and ecosystem levels: (1) induction of dominance by small species; (2) an increase of species richness and diversity; and (3) elongation of the food chain and reduction of energy transfer efficiency from primary producers to top predators.

Zooplankton community structure driven by vertebrate and invertebrate predators

SummaryA zooplankton community was established in outdoor experimental ponds, into which a vertebrate predator (topmouth gudgeon: Pseudorasbora parva) and/or an invertebrate predator (phantom midge larva: Chaoborus flavicans) were introduced and their predation effects on the zooplankton community structure were evaluated. In the ponds which had Chaoborus but not fish, small- and medium-sized cladocerans and calanoid copepods were eliminated while rotifers became abundant. A large-sized cladoceran Daphnia longispina, whose juveniles had high helmets and long tailspines as anti-predator devices, escaped from Chaoborus predation and increased. In the ponds which had fish but not Chaoborus, the large-sized Daphnia was selectively predated by the fish while small-and medium-sized cladocerans and calanoid copepods predominated. In the ponds containing both Chaoborus and fish, the fish reduced the late instar larvae (III and IV) of Chaoborus but increased the early instar larvae (I and II). Small- and large-sized cladocerans were scarcely found. The former might have been eliminated by predation of the early instar larvae of Chaoborus, while the latter was probably predated by fish. Consequently, the medium-sized cladocerans, which may have succeeded in escaping from both types of predator, appeared abundantly. The results suggest that various combinations of vertebrate and invertebrate predators are able to drive various kinds of zooplankton community structure.

Response of a zooplankton community to insecticide application in experimental ponds: a review and the implications of the effects of chemicals on the structure and functioning of freshwater communities

A review is presented of experimental studies in outdoor experimental ponds to investigate the effects of various insecticide exposures on natural zooplankton communities. Large zooplankton species, which generally are superior to small zooplankton species in competition, are also more sensitive to insecticides. Relatively low insecticide concentrations, which damage only large taxa (Daphnia), may affect the population dynamics of other zooplankton indirectly through altered competitive relationships. The effects of insecticide on the zooplankton community are also influenced by factors such as temperature, chemical properties (e.g. degradation rate), population trends among the organisms, community structure (presence or absence of predators), and timing of the chemical application. These factors modify interrelationships between organisms and, therefore, control the recovery process of the zooplankton community following insecticide impacts. Results to date suggest that insecticide stress decreases the average size of the organisms, reduces energy transfer efficiency, elongates the food chain and sometimes increases species richness.

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 Odums 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 Odums 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 Odums predictions.

Influence of time of application of an insecticide on recovery patterns of a zooplankton community in experimental ponds.

A Zooplankton community was established in out-door concrete ponds to which an insecticide, carbaryl (0.5 mg/L final concentration), was applied at different times relative to the population trend. The chemical application markedly reduced the cladoceran and copepod populations, but not rotifer population. After the treatments,Bosmina fatalis recovered earlier thanDaphnia spp. and was predominant until recovery of theDaphnia. The reappearance ofDaphnia was gradually delayed when the treatment was carried out at later times. Thus, the treatment induced the predominance ofBosmina, and the period whenBosmina predominated was extended when the carbaryl applications were delayed. The recovery ofDaphnia was probably retarded by the decline in water temperature, which decreased steadily during most of the experimental period. When carbaryl was applied during the increasing phase of theKeratella valga population, the population increased still further in density. When the population was exposed to the chemical during its decreasing phase, it did not recover even when competitors disappeared. Thus, applications of the insecticide at different times induced different recovery patterns of the Zooplankton community in the ponds.

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.

Evaluation of Microcystis as food for zooplankton in a eutrophic lake

Four experiments were conducted to evaluate Microcystis as food for zooplankton in Lake Kasumigaura, and the following results were obtained. (1) Moina micrura (Cladocera) showed little growth and no reproduction when the animal was reared with Microcystis cultured in the laboratory. The animal did not grow nor reproduce well when Chlorella was mixed with Microcystis as food. (2) Moina micrura assimilated Microcystis much less than Chlorella when the animal fed on single species of Microcystis or a mixture with Chlorella. (3) Microcystis collected from Lake Kasumigaura could not be utilized by Moina micrura even though the colonies were broken up into edible sizes. However, the alga turned into utilizable food when it was decomposed. (4) No inhibitors of Moina micrura population growth could be found in the non-filtered water of Lake Kasumigaura where Microcystis was blooming heavily. Decomposed Microcystis seemed to be utilized by zooplankton as an important food source in Lake Kasumigaura.

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.

Significance of a low oxygen layer for a Daphnia population in Lake Yunoko, Japan

Population dynamics and vertical migration of Daphnia longispina in Lake Yunoko were studied. The Daphnia population was small in spring and early summer, probably because of high predation pressure by fish. The population grew in midsummer, when thermal stratification developed and the dissolved oxygen became very low in the deeper layer of the hypolimnion. In this season, adults of D. longispina concentrated in the daytime near the lake bottom, where fish were absent because of the anoxic conditions, but ascended at night to the upper layer of the hypolimnion, where food was most abundant. The low oxygen layer near the bottom kept out the predators and protected Daphnia from predation, and consequently contributed to the built-up of its population. However, the low oxygen layer was unfavorable for reproduction of Daphnia, as reflected in the low egg ratio and high percentage of males in the population. The population decreased in the fall, when thermal stratification disappeared and predation pressure seemed to increase.

Growth analysis of Daphnia early juvenile stages as an alternative method to test the chronic effect of chemicals

It is recognized that the mature size and clutch size of the cladoceran Daphnia are strongly affected by the size at birth and growth rate during the early juvenile stages. Individuals at the neonatal stage are most sensitive to environmental stress such as toxic chemicals, and therefore such stress may indirectly control the reproductivity of Daphnia by directly affecting the growth of neonates. These facts suggest that growth rate during the juvenile stages would be a good indicator of the effect of toxic chemicals on Daphnias reproduction, which is the parameter usually determined in chronic toxicity tests. In the present paper, I propose a new short-term test that involved analysis of Daphnia growth at the early juvenile stages.