Effects of Organic Herbicides on Phototrophic Microbial Communities in Freshwater Ecosystems - Reviews of Environmental Contamination and Toxicology - page 115

polar organic samplers with monospecific photosynthesis bioassays (Pesce
et al. 2011 ).

4.1.3 Exposure Characteristics and Dynamics (Chronic-Acute)

The question whether there is a relevant exposure measure for periphytic micro-
organisms that are embedded in an ExoPolySaccharide (EPS) matrix is open:
depending on the solubility of pesticides, the concentrations in the water phase
may not be the most useful for predicting biological effects. Rather, periphytic
assemblage studies should address exposure, by focusing on bioilm-adsorbed
pesticide concentrations, especially for hydrophobic compounds. Indeed, the
sorption of pesticides on periphyton can enhance toxicity by extending exposure
time (Dorigo et al. 2010a ). Pesticide sorption also drives bioaccumulation pro-
cesses by favoring pesticide transfer to higher trophic levels via periphyton con-
sumers, such as grazers.
Exposures are naturally dynamic and comparing the consequences of long-term
low-dose (i.e., chronic exposure) vs. short-term high-dose (i.e., acute exposure)
exposures is very dificult to accomplish. Recent chemical monitoring studies
have shown that, during loods, many pollutant luxes - including those of pesti-
cides - can vary over several orders of magnitude, especially in small stream
systems (Rabiet et al. 2009 ). In small stream ecosystems, environmental exposure
of aquatic communities to pesticides can rapidly increase during rainfall events.
Therefore, special attention should be given to (a) studying pulsed-exposures that
result from episodic runoff events and (b) addressing the ecotoxicological ques-
tion of how to predict the lethal and sublethal consequences of such population
exposures (Tlili et al. 2008, 2011 ). Furthermore, the effect of long-term, low-dose
pesticide exposures may produce effects (e.g., biodiversity changes, tolerance
acquisition, and functional changes) that only become apparent in organisms after
several generations.

4.2 Improving Assessment of Biological Effects

4.2.1 From Monospeciic Tests to Community Assessment

There have long been efforts to enhance the integration of ecology and ecotoxicology.
However, it is now well established that a more suitable model than single-species
testing is to assess the ecological effects of pesticides at the microbial community
level. Nevertheless, data from monospeciic bioassays will also be required. Hence,
the SSD approach has become a practical ecological risk assessment method and
decision-making processes to determine water quality criteria (Schmitt-Jansen and
Altenburger 2005b ). Although SSD approaches are useful in environmental risk
research (Schmitt-Jansen et al. 2008 ), especially if toxicity datasets are suficiently