Abstract
The impact of small-stature alkali bulrush (Bolboschoenus maritimus) and two hydroperiod treatments (early season raised water level or ambient water level) on mosquito production and water quality was studied in replicate 0.09 ha free water surface (FWS) treatment wetlands. Following reconfiguration of a 1-ha constructed wetland into a system with six replicate wetlands, bulrush was planted on 0.5-m centers in three 5-m wide bands in each wetland in summer, 2012. Open water and the low density of emergent vegetation effectively limited mosquito production from the bands of B. maritimus in each wetland during summer and autumn of year one. After the autumnal senescence of the bulrush culms, water levels were raised in half of the wetlands during winter and early spring to enhance sinking of dead bulrush biomass to reduce harborage for mosquitoes. Macrophyte coverage continued to increase in both hydroperiod treatments during year two, but non-bulrush species proliferated and eventually overgrew B. maritimus. Immature mosquito abundance in dipper samples from wetlands in the raised water level treatment was greater than from wetlands in the constant water level treatment. During spring of year two, adult mosquito production was associated with volunteer vegetation in the center of the test cells and averaged 6–18 mosquitoes m−2·day−1, approximately twice that of the other treatment. Hydrological regime did not significantly affect water quality performance (removal of nitrogen, phosphorus and chemical oxygen demand) in the wetlands. Alkali bulrush can persist in shallow water (depth < 0.2 m), but did not persist in deeper zones (mean depth > 0.4 m) of the wetlands and after comparatively large stature grasses and cattails colonized the wetlands. Raised planting beds interspersed with zones of deeper water are recommended to facilitate persistence of alkali bulrush and to limit proliferation of superior competitors.
Highlights
Emergent vegetation in free water surface constructed wetlands fulfills important functions that enhance nutrient removal for wastewater treatment
We examined the effects of the two hydrological regimens on the sustainability of the emergent plants, water quality performance, and reduction of mosquito populations in experimental wetlands planted with B. maritimus during a two-year study
Bolboschoenus maritimus culms expanded across the 5-m planting zones within the three months after planting, increasing from 4 culms m−2 to approximately 47 culms m−2 in November (Table 1; Figure S1)
Summary
Emergent vegetation in free water surface constructed wetlands fulfills important functions that enhance nutrient removal for wastewater treatment. Emergent vegetation can (i) create slack flow and reduce wind-driven mixing which enhance sedimentation; (ii) adsorb particulates; (iii) reduce variation in environmental conditions to promote nutrient removal; (iv) provide physical structure for the attachment of microbes and periphyton important for wastewater treatment; (v) store nutrients;. Water 2016, 8, 421 concentration in the water column, provide organic carbon) that promote denitrification [1,2,3]. Large (>2.5 m in height) emergent macrophytes planted commonly in constructed treatment wetlands can support large numbers of pestiferous and pathogen-transmitting mosquitoes, especially when the water undergoing treatment carries high concentrations of nutrients and organic matter [4,5,6]. Meyer) Palla], cattail (Typha spp.) and common reed (Phragmites australis (Cav.) Trin. ex Steud.)
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