Dynamics of leaf litter degradation in a small stream impacted by pesticides
Dynamique de la dégradation de litière de feuille dans un cours d'eau impacté par des phytosanitaires
Résumé
Small streams of rural watersheds are strongly dependant on leaves falling (from riparian area) as a main particulate organic matter (POM) source. This input allows the development of a trophic network that degrades and assimilates OM, assuming then the functioning and the ecological quality of the stream. In vineyard area, intensive pesticides uses could impact aquatic organisms at several trophic level (heterotrophic microorganisms, algae, invertebrates,) by modifying or reducing diversity and density of biota. The functional consequences are little known, especially those dealing with OM degradation. The present study aims to precise the degradation dynamics of leaves litter along an increasing gradient of pesticides, in a 1st order stream draining a wine-growing watershed during winter. Small bags containing Alnus sp leaves collected at leaves falling, after drying and weighting were installed on three pre selected study sites in the stream. Each 15 days, during 2 months, 5 bags per site are sampled for analysis. Parameters were the decrease of leaves dry weight, diversity and density of invertebrates colonizing the bags, bacterial density (DAPI counts), fungi biomass (ergosterol dosage), heterotrophic respiration activity (CO2 dosed on gas chromatograph). The clean upstream site (reference) had a high POM degradation rate (>50% within 1 month), associated to a high diversity of macroinvertebrates, dominated by schredders like Gammarus. The two contaminated sites showed a progressive and marked change in the invertebrates community (disappearance of Gammarus, dominance of Oligochaetes). Leaves degradation was strongly reduced at the downstream sites, characterized by high pesticides concentration: the weight loss of leaf litter was 57%, 22% and 25% respectively from the reference site (clean) to the impacted ones. Fungi and bacterial biomass remained steady while the global heterotrophic respiration decreased very slowly. Finally, the capacity of this small lotic ecosystem to process organic matter into the trophic network and so to maintain a healthy ecological state is strongly reduced by chemical gradient from upstream to downstream.