Stressor variations

Structural variations may be also produced at the microscopical scale and are able to produce significant improvements in our understanding of stressor effects. Observation of the biofilm architecture and characterisation of the different fractions (i.e. algae, bacteria, mucopolysaccharides) may be useful to identify particular effects of toxicants to selective components of the biofilm. The use of confocal laser scanning microscopy remains promising [25]. 

Life history models may be useful for considering age- or stage-specific variability in sensitivity in the extrapolation of response however, it should not be assumed that smaller or younger stages are always the most sensitive. The few studies that have addressed the impact of time of year on responses of aquatic communities to a stressor indicate that, in freshwater communities, threshold concentrations for direct toxic effects vary within a factor of 2 among seasons — well within the normal range of variation observed in laboratory toxicity tests. However, at greater exposures, the intensity and duration of direct and indirect responses may vary considerably between different periods of the year because of the influence of climatic and seasonal factors on recovery. 

Within these clinical guidelines, the unique characteristics of each patient will dictate the course to follow. Side effects, variations of clinical course, the presence of life stressors, economic considerations, relocation, concurrent illnesses, and other factors will affect the formulation of a specific plan that is suitable for a particular patient. 

The need to evaluate spatial and temporal distribution and variation is inherent in many of these example characteristics. Such information is especially useful for determining potential exposure, that is, where there is cooccurrence of or contact between the stressor and ecological components. 

The spatial and temporal distributions of ecological components also are considered in ecosystem characterization. Characteristics of ecological components that influence their exposure to the stressor are evaluated, including habitat needs, food preferences, reproductive cycles, and seasonal activities such as migration and selective use of resources. Spatial and temporal variations in the distribution of the ecological component (e.g., sediment invertebrate distribution) may complicate evaluations of exposure. When available, species-specific information about activity patterns, abundance, and life histories can be very useful in evaluating spatial and temporal distributions. 

Parts III, IV and V deal with the distribution of trace elements in biota and reservoirs (soils and sediments). In this context, it should be noted that organisms, populations, biocenoses and finally the entire ecosystem are influenced by a number of different biotic and abiotic stressors under natural conditions. These are for example climatic changes, variations in radiation regimes or availability of food resources, predator-prey relationships, parasites, diseases, intraspecific and interspecific competition. Stress is an existential prerequisite for all biological levels of organization, as it... 

The approach proposed is based on a compilation of all factors in the form of a polynomial equation (Equation 3). U sing the time samples of the psychophysiological measure and of the stressor variables as sources of variation, the coefficients were computed by a polynomial regression ... 

Processing tasks with high action frequency may induce an additional source of error. The variation of a psychophysiological measure that is caused by a stressor variable represents a subjects reaction. Thus, a delay between the occurrence or variation of a stressor variable and... 

Environmental stress is just one of many potential causes for reduction of standing variation within a population and genetic diversity measures based on RAPD-PCR are unlikely to be able to distinguish between naturally occuring and anthropogenic stressors. In addition, a reduction of genetic diversity is not necessarily the only selective response to anthropogenic stressors. 

---