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2P Modelling ecotoxic effects (MOP/150) Why herbicides do not kill submerged aquatic plants. Cedergreen, Nina1, Streibig, Jens Carl1, 1 The Royal Veterinary and Agricultural University, Frederiksberg, Copenhagen, Denmark ABSTRACT- Herbicide spray drift has been documented to cause severe damage on non-target terrestrial plants at doses down to <1% field dose. In the aquatic environment herbicide residues are regularly detected, but the effects on the aquatic flora are not well investigated. We studied the effect of 10 different herbicides, technical compounds as well as formulated products, on the growth of the floating macrophyte, Lemna minor, and the micro algae, Pseudokirchneriella subcapitata, and for three of the herbicides also on a suit of 10 macrophyte species and an epiphyte community. The general result was that foliar absorbed herbicides were practically non-toxic to aquatic plants and algae in the micromolar concentration range. The root-absorbed herbicides had some effect on growth, though still at concentrations far above those measured in the aquatic environment. Since the general physiology of aquatic plants in terms of the target sites of herbicides is not profoundly different from that of terrestrial plants, we hypothesised that the difference in tolerance between terrestrial and aquatic plants is due to herbicide uptake. To test this, we developed a generalised simulation model of herbicide uptake by aquatic plants and compared it to models for uptake by roots and leaves by terrestrial plants. Assuming that toxicity is proportional to tissue herbicide concentration, the simulation model explained the experimental data well. Thus, the larger tolerance of aquatic plants and algae relative to terrestrial plants can be explained by their differential herbicide uptake. The primary cause of this difference is that uptake of herbicides is a diffusion process driven by the concentration gradient between the uptake solution and the plant tissue. Hence, the >20 000 fold dilution that takes place if herbicides are sprayed in a volume of 150 l ha-1 onto a water column of 30 cm is enough to reduce uptake to a degree that makes most herbicides virtually non-toxic, even if sprayed at recommended agricultural doses directly onto the water surfaces. Key words: uptake, herbicides, diffusion, toxicity |
