Effects of modern pesticides on bacterial activity and denitrification in lake sediment

Microcosm studies were performed to observe the effects of modern pesticides on denitrification and bacterial activity in lake sediment. Ecotoxicologically relevant test concentrations were retrieved from a Dutch study that presents Negligible Concentrations (NC) and Maximum Permissible Concentrations (MPC) for modern pesticides, i.e. concentrations critical for the environmental compartments water, soil, and sediment. Four functionally different pesticides, isoproturon (herbicide), deltamethrin and pirimicarb (insecticides) and captan (fungicide) were tested. None of the four tested pesticides had a consistent negative effect on bacterial activity or denitrification. Bacterial activity was, however, affected positively by pirimicarb. No other treatment effects were found for bacterial activity. Apparently, denitrification was affected positively by pirimicarb, isoproturon and deltametrin. Denitrifying bacteria seem to be able to use the tested pesticides as a carbon source when carbon deficiency prevails. A general decrease in denitrification over time for all treatments, especially in the controls, supports the theory that the denitrifying bacteria suffered from carbon depletion. Therefore, the addition of a carbon source in ecotoxicological studies with bacteria is recommended I n t r o d u c t i o n Pesticides are used in a number of human activities to be able to maintain high production efficiency. There is a constant demand for a stable crop production in order to support the growing human population and the future usage of pesticides can therefore be expected to increase. Most agrochemicals are designed to affect only specific target organism groups or processes, but many of them have general toxic effects. The undesired effects that can arise as these substances are transported to lakes and streams are poorly studied and the effects on benthic microbes in aquatic systems are particularly neglected. Evaluation of possible harmful effects on non-target organisms and ecosystem processes is an important element in the regulation and approval of pesticides (Kallqvist 1 9 9 4 ) . Pesticides in the environment originate from agriculture and forestry, but also from industry and from common households. Persistent chlorinated pesticides like DDT and PCB have to a great extent been replaced by modern pesticides. The critical values that exist for these modern pesticides are frequently obtained from a human health perspective. Even if the concentrations of modern pesticides are below these critical values, it is possible that sublethal effects on ecological function exist. Another problem, reported by Hessel et al. (1997), is that existing analysing methods are not applicable to many of the modern p e s t icides at ecotoxicologically relevant concentrations. The detection limit for a compound may be higher than the concentration that effects the biota. For example, for pesticides registered for agricultural use in Sweden analysing methods only exist for 60% of the herbicides, 50% of the fungicides and 70% of the insecticides (Hessel et al. 1997). Reliable data on pesticide concentrations in the environment are therefore often hard to obtain. Pesticides are often spread in low concentrations over a long period of time. Some are relatively immobile thus deposit in soil. However, shortly after application, pesticides will leave the application site in dissolved form (Kreuger et al. 1999). Hydrophobic pesticides or pesticides with strong ion-exchange to clay minerals might also enter the surface water runoff associated to suspended particles (Wauhcope 1978). Hydrophobic pesticides are relatively water-insoluble, nonpolar compounds that have high octanol-water partition coefficients (log Ko w > 2) (Elzerman and Coates 1987). Phase-transfer processes can move the pesticides from one environmental matrix to another, with sorption and volatilisation being the most important processes (Larson et al. 1997). For example, in most cases the log Ko w increases with increasing hydrophobicity of the pesticide and amount of organic matter in the sediment. The availability of particle-associated pesticides to benthic organisms depends on different sediment characteristics, chemical and physical processes as well as biological processes such as feeding habits (Kukkonen and Landrum 1996). Bacteria are critical for the turnover of carbon, nitrogen and other nutrients in lake sediment. One process of considerable ecological importance is denitrification, which involves reduction of NO3 and NO2 to the nitrogenous gases NO, N2O and N2 and mineralisation of organic matter under anaerobic conditions. Denitrification is the only process that eliminates nitrogen from the aquatic system (Jansson et al. 1994). It is carried out by a heterogeneous group of bacteria and disturbances of this process is therefore likely to have severe effects on the sediment ecosystem. Pell et al. (1998) showed that 11% out of 54 tested pesticides had stimulatory effects on denitrification in soil, whereas 15% had inhibitory effects. In lake sediment, Svensson and Leonardsson (1992) showed that the fungicide fenpropimorph inhibited denitrification and that the negative effect increased after prolonged exposure to the pesticide. Shand et al. (1995), on the other hand, reported that some pesticides were metabolised by different soil bacteria, indicating that pesticides can be used as a carbon source by bacteria. Microbial responses have been recommended as an early warning indicator of ecosystem stress, because of the quick response to changes in environmental conditions (e.g. toxicant exposure) (Burton 1991). Bacteria are probably sensitive to contaminant exposure in freshwater sediment, but there is little information on the functional response of these organisms. For pesticides, the different functional groups determining the mode of action, i.e. their toxicity, may affect the microbiota in different ways. For example, a fungicide presumably has a higher toxicity to microbiota than a herbicide, and it is therefore relevant to study how pesticides with different functional groups affect the microbial community (Walker et al. 1996). Only a few comprehensive studies have addressed the effects of modern pesticides from