Conditions of a biological effect in the environment
The appearance of an in situ biological effect due to the presence of phytosanitary products is a complex phenomenon, depending on various environmental factors whose resulting action define what is called the exposition of the organism to the substance(s).
It is composed of several elements.
In the first place, there is the appropriateness between the product and the biological target; the phytosanitary products are chosen and formulated so as to act specifically on certain "undesirable" or bioaggressor groups of orgamisms (vegetables invading crops, fungi responsible of cryptogamic diseases, parasite insects, etc.). Logically, they will act on the same kind of organisms in water environments (respectively algae and superior plants, hyphomycetes and various water invertebrates). However, this specificity of action gives no indication of potential cascade effects which can happen within trophic networks (for instance : the direct effect of a weedkiller on the algae community could generate an indirect effect on herbivorous invertebrates by reducing their food resources). Besides, it sometimes happens that a substance, selected for its efficiency against a groups of specific bioaggressors, also has a direct effect on another group which was not targeted at first (it is the case of certain fungicides, for example, which can have an effect on the reproduction of the animals through alterations of their hormonal system.
The concentration of a phytosanitary is insufficient in itself to estimate an effect (even of it can be enough as a first approach). The effect depends on its bioavailability, i.e. the possibility for an organism to contact the substance, for the substance to get in the organism (going through the cell membrane) and to take action. Out in nature, this depends on the distribution of the substance in the various compartments of the water environment, according to whether they are hydrophilic or hydrophobic. A chemical parameter describes this feature, it is called the sharing ratio octanol-water or Kow. Substances with high Kow (> to 3 or 4) would rather tend to pile up in the sediments (apolar substances such as organo-chlorinated insecticides), on the other hand, products with lower Kow would rather appear in a dissolved form ( polar substances such as herbicides of diuron or atrazine type or fungicides of dimetomorphic or procymidone type). In aquatic systems, the substances will therefore be more specifically dissolved in the water or on the contrary present in the sediments, absorbed on the material in suspension or organic material. The interactions between phytosanitary products and the chemical matrix of the water or of the sediments modulate their bioavailability as well : absorption in organic material (humic acid, for instance). Copper, phytosanitary mineral systematically found in rivers in wine-growing regions, can therefore be more than 99% in a non-bioavailable form.
Finally, the molecules of phytosanitary products in water environments can be degraded by various processes : photodegradation by action of ultraviolet radiation (photolyse), biodegradation by water microorganisms (essentially bacterial), etc.
The effects also depend on how long and how often the organism and the substance(s) were brought together. The presence of phytosanitary products in the water systems are often associated to phenomenons of diffuse pollution (runoff, percolation) and can also be linked to localized intakes both in space (sewers, drains, etc.) and in time (accidents, treatment time). This variability explains why reliable samplings are so difficult to get, and it leads to alter the exposure of water biocenoses. Thus, it was proved that hydrological events on wine-growing zones associated with a rise of the water level generate a larger contamination with phytosanitary products during a very brief time (a few hours on small side ponds) at the opposite of chronic exposure in tide flows, which are low but permanent. Likewise, the contamination in urban zones associated to non-agricultural use (road maintenance, for instance) is also often episodic, a feature intensified by the fact that the surfaces are impermeable. The relative contribution of these two types of exposure at an ecological state of a system is very tough to gauge. Biological effects can be brought out thanks to many descriptors which have varied biological and ecological meaning.
On an individual and people scale, and whatever the considered organism, the main indicators employed are bases on the mortality rate or on physiological parameters (growth rate, average size or weight, reproduction rate, peculiar metabolic activity). Choosing them depends on the need to put forward more or less subtle effects of substances not leading to direct mortality (sub-lethal effects): lenghthy exposition to "weak" concentrations can bring about postponed effects, which will only appear after several generations...
Taking into consideration biological communities rests on in situ approaches (it might also rest on mesocosmic approaches) and are largely based on the analysis of their taxonomic structure : identification of the diversity of the taxons, presence of specific indicative species (resistant or sensitive to pollution), changings of diversity, etc. These kind of taxonomic approaches allowed to know the state of an environment and the effect of polluting pressures and, therefore, to establish many standardized and statutory evaluation methods : Diatomic Biological Index (IBD, based on microscopic algae, the diatomees), Standardized Global Biological Index (IBGN, based on invertebrates), etc. Yet, these methods are not specific (even little sensitive) to phytosanitary products, particularly in case of a multiple contamination.
(translation: Alice Gautreau)
