Acute and chronic effects of colchicine on the freshwater cladoceran Daphnia magna were examined, based on a 48-h acute immobilization test and a 21-day reproduction test with female neonates, respectively. The 48-h EC₅₀ value of colchicine to neonates was 24 mg L^-1. In the reproduction tests, NOEC was 0.2 mg L^-1 and more than 0.5 mg L^-1 of colchicine showed a significant effect on embryogenesis and stopped embryo development. In the following embryo developmental inhibition test, embryos burst within 48–72 hours after oviposition in the brood chamber, when embryos within 5 hours after oviposition were exposed to colchicine at 1 mg L^-1. Colchicine inhibited the early embryogenesis, but did not affect the gastrulation or organogenesis. As a result of nuclear staining test, the embryos exposed to colchicine at 1 mg L^-1 continuously in the ovary showed normal cleavage until at least 4 hours after oviposition, but the specific fluorescence intensities of nuclei had almost disappeared at 24 hours after oviposition. These results indicate that 1) oogenesis in ovaries of D. magna is not inhibited by colchicine, 2) 1 mg L^-1 of colchicine inhibits the cell division of embryos at 24 hours after oviposition although its inhibition effect is delayed, 3) more than 0.5 mg L^-1 of colchicine inhibits early embryogenesis, and 4) 0.5–1 mg L^-1 of colchicine causes bursting of embryos. The latter two results are remarkable, but its mechanism remains unresolved in the present study.

Cell counts of marine phytoplankton in formaldehyde, formaldehyde–glutaraldehyde, and acid Lugol’s iodine fixatives were carried out for natural seawater samples, which were stored at room- and cooled-storage temperature for 3, 6, and 12 months and were concentrated with the Utermöhl method. The number of species in the taxonomic groups identified for the 3 and 6 months samples of 0.1–1％ formaldehyde increased, mainly due to diatoms at the cooled storage temperature. The loss rates of the dominant diatoms (Leptocylindrus danicus and Pseudo-nitzschia complex) in the acid 1％ Lugol’s iodine fixatives under the room temperature storage for 3 months was almost the same (87–100％) as those in the cooled temperature storage, but were 1.5–2 times as high as those in 0.1％ formaldehyde or the mixtures of 0.1％ formaldehyde and 0.025％ glutaraldehyde in cooled temperature storage. Cooled-storage temperature could decrease the loss rate of diatoms in 0.01–1％ formaldehyde. The abundance of the Pseudo-nitzschia seriata complex in 0.01％formaldehyde-0.025％glutaraldehyde was lower than that in 0.1％formaldehyde-0.025％glutaraldehyde (P < 0.03). The loss rate of the cell numbers of diatoms (Leptocylindrus danicus and Pseudo-nitzschia complex) in 0.1％formaldehyde (0.1％formaldehyde-0.025％glutaraldehyde) at cooled storage after 12 months was as low as 19％ (25％) on average. The loss rates of Gephyrocapsa oceanica in formaldehyde fixatives for 3 months were insensitive to the storage temperature, the concentrations of formaldehyde, and the addition of glutaraldehyde. The adoption of 0.1％ formaldehyde and cooled storage could keep the preserved phytoplankton in good condition and even minimize risk management of chemically hazardous materials as well as improve working environments for counting.

Bioassays with aquatic organisms, including phytoplankton and zooplankton, have been used worldwide for evaluating the toxicity and environmental risks of anthropogenic chemicals, for example, agrochemicals. Risk assessment of agrochemicals based on bioassays has the following characteristics; (1) estimation from effects on some representative species to risk to whole ecosystem, (2) species in very wide taxonomic groups of aquatic (algae, crustaceans, fishes, insects, etc.) and terrestrial (mammals, avian, insects, earthworms, soil organisms and plants) organisms, should be evaluated, (3) risk assessment consists of exposure assessment and ecotoxicology study (hazard assessment), and is a relative assessment using the ratio of endpoint (for example, EC₅₀) and PEC (Predicted Environmental Concentration), (4) tiered approach from laboratory data to realistic field data. Major plankton species used for bioassays are the freshwater green alga, Pseudokirchneriella subcapitata (Korshikov) Hindak, the marine diatom, Skeletonema costatum (Greville) Cleve and the freshwater diatom, Navicula pelliculosa (Kützing), on which a growth inhibition tests have been, conducted. For zooplankton, the freshwater cladoceran, Daphnia magna Straus, and the saltwater mysid, Americamysis bahia (Molenock) have been used for acute and reproduction studies. Test methodologies for risk assessment in freshwater aquatic organisms are well established and outdoor mesocosm studies have become a realistic tool. Ecotoxicology using plankton bioassays and risk assessment has two major issues, (A) development of test method for evaluation of growth including metamorphosis and sexual reproduction, and (B) basic physiological and pharmacological research for neuro, endocrine and metabolism systems in zooplankton test species. Both issues are very important for the progress of ecotoxicology in the future.