User Guide to Thresholds and Classification - Environmental ...

380User Guide for Thresholds and ClassificationsEnvironmental factorsEnvironmental parameters influencing the physiology of the organism may also affect the uptake ofsubstances. For instance, when the oxygen content of the water is lowered, fish have to pass more waterover their gills in order to meet respiratory demands (McKim and Goeden, 1982). However, there may bespecies dependency as indicated by Opperhuizen and Schrap (1987). It has, furthermore, been shown thatthe temperature may have an influence on the uptake rate constant for lipophilic substances (Sijm et al,1993), whereas other authors have not found any consistent effect of temperature changes (Black et al,1991).Factors influencing the elimination rateThe elimination rate is mainly a function of the size of the organism, the lipid content, the biotransformationprocess of the organism, and the lipophilicity of the test compound.Size of organismAs for the uptake rate the elimination rate is dependent on the size of the organism. Due to the higher gillsurface to weight ratio for small organisms (for example, fish larvae) than that of large organisms, steadystateand thus ‗toxic dose equilibrium‘ has shown to be reached sooner in early life stages than injuvenile/adult stages of fish (Petersen and Kristensen, 1998). As the time needed to reach steady-stateconditions is dependent on k2, the size of fish used in bioconcentration studies has thus an importantbearing on the time required for obtaining steady-state conditions.Lipid contentDue to partitioning relationships, organisms with a high fat content tend to accumulate higher concentrationsof lipophilic substances than lean organisms under steady-state conditions. Body burdens are therefore oftenhigher for ‗fatty‘ fish such as eel, compared to ‗lean‘ fish such as cod. In addition, lipid ‗pools‘ may act asstorage of highly lipophilic substances. Starvation or other physiological changes may change the lipidbalance and release such substances and result in delayed impacts.MetabolismIn general, metabolism or biotransformation leads to the conversion of the parent compound into morewater-soluble metabolites. As a result, the more hydrophilic metabolites may be more easily excreted fromthe body than the parent compound. When the chemical structure of a compound is altered, many propertiesof the compound are altered as well. Consequently the metabolites will behave differently within theorganism with respect to tissue distribution, bioaccumulation, persistence, and route and rate of excretion.Biotransformation may also alter the toxicity of a compound. This change in toxicity may either be beneficialor harmful to the organism. Biotransformation may prevent the concentration in the organism from becomingso high that a toxic response is expressed (detoxification). However, a metabolite may be formed which ismore toxic than the parent compound (bioactivation) as known for, for example, benzo(a)pyrene.Terrestrial organisms have a developed biotransformation system, which is generally better than that oforganisms living in the aquatic environment. The reason for this difference may be the fact thatJanuary 2012 EPA0109