Biological diversity versus risk for mosquito nuisance and disease transmission in constructed wetlands

Schäfer, Martina (1); Lundström, Jan O. (1); Pfeffer, Martin (2)

(1) Dept. of Population Biology, Evolutionary Biology Centre, Uppsala University, Sweden;
(2) Institute for Medical Microbiology, Infectious and Epidemic Diseases, Munich, Germany

Recently, the number of wetlands has slowly begun to increase again after a long tradition of wetland draining and river regulation. Many wetlands have been constructed in Southern Sweden, mainly for nutrient recycling, groundwater recharge, or for supporting waterfowl, but also with the aim of maintaining or increasing biodiversity. Some wetlands are constructed near human settlements, thus raising the problem of wetlands being associated with mosquitoes. Increased diversity (including mosquito diversity) is desirable, whereas mosquito nuisance from a human point of view is not.

We therefore wanted to study adult mosquito abundance, diversity and assemblages of constructed wetlands in comparison to natural wetlands, and to evaluate the potential of constructed wetlands regarding mosquito nuisance and transmission of mosquito-borne viruses. Our study areas included six constructed and three natural wetlands, differing with regard to hydrological conditions, size, location in the environment, age and purpose of construction.

We sampled from June to August 1998 (one study area), and June to September in 1999 (8 study areas), using light traps baited with dry ice. Altogether, 36,869 individuals covering 6 genera and 30 species were included in our analysis. Mosquito species assemblages differed between wetlands, and a cluster analysis based on similarity of quantitative data resulted in three distinct clusters: 1) the three natural wetlands, 2) four constructed wetlands located in forested and mixed environments, and 3) two constructed wetlands located in open environment. The average number of mosquitoes per trap night and species richness were significantly higher in the natural wetlands than in the constructed ones.

Mosquito diversity, measured by four diversity indices, did not differ significantly among natural and constructed wetlands, and was not connected to a certain feature of a wetland. All constructed wetlands contained potential nuisance species. Aedes cinereus, one of the most common species, can also transmit Sindbis virus, but so far no virus was found in the collections. Other mosquito species with potential for mass occurrence and dispersal, like Aedes vexans and Ochlerotatus sticticus occurred in all wetlands, although in very low numbers. Other hydrological conditions, like regular flooding during summer, could nevertheless favour these species.

Thus, we conclude that constructed wetlands could provide some risk for mosquito nuisance, but generally contained fewer mosquitoes than natural wetlands. Although their mosquito species richness was lower than in natural wetlands, diversity could be high in all kinds of wetlands. Therefore it seems possible to construct wetlands with low or moderate mosquito nuisance but nevertheless high diversity.

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