Sublethal impacts

In acute toxicity tests, the test organisms are exposed for a relatively short period in relation to their generation time. The acute effects recorded with such tests usually concern the survival of the test species, regardless of how mortality is evoked. In chronic tests, the test organisms are exposed for a significant part of their life cycle or the entire life cycle of one to several generations. The (sub)chronic effects primarily concern sublethal impacts such as reduced reproduction or growth, altered behaviour or development, which ultimately may also lead to increased mortality. 

Elucidation of mechanisms and modes of toxicity for organotin compounds, especially those involving sublethal chronic exposures and cellular and subcellular impacts (WHO 1980 Reuhl and Cranmer 1984 Hall and Pinkney 1985 Thompson et al. 1985 Vrijhof 1985). 

Sanchez, F.A.A. and M. de Ariz. 1997. Carbofuran acute and sublethal toxicity on freshwater algae and fish. Pages 743-764 in P.N. Cheremisinoff (ed.). Ecological Issues and Environmental Impact Assessment. Gulf Publishing, Houston, TX. 

Jackson, F. L. C. (1994). Bioanthropological impact of chronic exposure to sublethal cyanides from cassava in Africa. ActaeHorticulturae 375,295-319. 

Some responses, such as mortality, are irreversible. However, many sublethal responses may be reversible, such as the impact of the photosynthesis-inhibiting herbicide linuron on macrophytes (Snel et al. 1998). Linuron inhibits photosynthesis by disturbing electron transport in photosystem II. Table 6.2 presents the kinetics of photosynthesis inhibition when shoots of macrophytes are placed in water with 50 pg/L linuron, and subsequent recovery when placed in uncontaminated water. The EC50 values are remarkably similar between macrophytes, and half-life estimates for inhibition and recovery are less than 2 hours (Table 6.2). Except for Potamogeton... 

Researchers were also able to establish the link between declines of other predatory species such as the European sparrowhawk and the use of organo-chlorine pesticides other than DDT. For instance, the cyclodiene insecticides aldrin, dieldrin, and he-ptachlor used as seed treatments caused massive mortality of both seed-eating species and their predators. All of the insecticides had the following points in common they were highly soluble in fats and refractory to metabolism. The impacts on the predatory species typically take place in periods of food stress when fat soluble residues are released from fat stores and returned into general circulation. In a food-stressed individual, the brain remains as the most lipid rich tissue and this is where contaminants move to. Toxicity results when threshold values in brain tissue are exceeded. At sublethal levels, documented effects of cyclodiene insecticides in birds have included changes in their reproductive, social, and avoidance behaviors. 

Roddie, B.D., Redshaw, C.J. and Nixon, S. (1996) Sublethal biological effects monitoring using the common mussel (Mytilus edulis) comparison of laboratory and in situ effects of an industrial effluent discharge. In Toxic Impacts of Wastes on the Aquatic Environment, Tapp, J.F., Hunt, S.M. and Wharfe, J.R. (eds), pp. 125-137. Royal Society of Chemistry, London. 

Long-term effects of sublethal effects Often the sublethal effects of a PPP are described well on the level of individuals, whereas the likely impact on population level is less well known. In some cases, the effects of density dependence, for example, will make the effects much less pronounced on the population level than on the individual level. In other cases, the sublethal effects may have pronounced effects on population density and structure. Ecological models can be used to put the effects observed on the individual level into the more ecologically relevant population level. 

Rajeshwara Rao, M., Sivaprasad Rao, K., Srinivasulu Reddy, M., Sambasiva Rao, K.R.S., Mai Reddy, N., 1983. Toxicity of phenthoate and changes in the organic constituents of the snail Pila globosa (Swainson) under sublethal and lethal impacts. Geobios. 10, 250-253. 

As in toxicity studies of chemical pesticides, the evaluation of the impact of gene products potentially expressed in GM plants can be based on a three-tiered approach where laboratory acute toxicity tests and observations under more natural conditions are combined. Although parallels can be drawn in the methodologies used in the study of the sublethal effects of chemical pesticides and the risk assessment of GM plants, the main difference relies on the fact that the evaluation of GM plant implies specifically the study of secondary changes in plant metabolites mediating their attractiveness for honey bees. 

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