Tertiary sewage treatment by a full-scale compact vertical flow constructed wetland.

A 208 m2 compact vertical flow treatment wetland (cVFTW) was studied, treating the rotating biological contactor (RBC) effluent of a busy UK visitor attraction, with widely varying daily sewage loads up to 111 resident population equivalent. Results are presented from monthly spot-sampling and analysis for inlet and outlet concentrations of BOD, SS, NH4-N and TN, including load rates, for the first four-and-a-half years of operation. From commissioning to 13 weeks, outlet BOD, SS and NH4-N were below 12 mg l-1, until a sudden outlet NH4-N spike, to 31.9 mg l-1, the reasons for which are discussed. Thereafter, final effluent quality was <3:<3:<0.5 mg l-1 BOD/SS/NH4-N almost without exception, regardless of the determinand load rates imposed. Recirculation of controlled proportions of cVFTW effluent to the RBC inlet eliminated foul odour emanating from the RBC, and considerably increased overall nitrogen removal via denitrification. Phragmites australis development was poor, with the majority of plants experiencing chlorosis, putatively owing to insufficient bioavailable iron. By year three, despite ongoing weed removal to select for Phragmites, the wetland was increasingly colonised by other species, predominantly Epilobium hirsutum. Glyceria maxima, planted in year three, outcompeted even Epilobium, recommending itself as an alternative to Phragmites in high redox potential treatment wetlands.

Mechanisms for success after long-term nutrient enrichment in a boreal forest understory.

Global levels of reactive nitrogen are predicted to rise in the coming decades as a result of increased deposition from the burning of fossil fuels and the large-scale conversion of nitrogen into a useable form for agriculture. Many plant communities respond strongly to increases in soil nitrogen, particularly in northern ecosystems where nitrogen levels are naturally very low. An experiment in northern Canada that was initiated in 1990 has been investigating the effects of long-term nutrient enrichment (fertilizer added annually) on a boreal forest understory community. We used this experiment to investigate why some species increase in abundance under nutrient enrichment whereas others decline. We focused on four species that differed in their responses to fertilization: Mertensia paniculata and Epilobium angustifolium increased in abundance, Achillea millefolium remained relatively constant and Festuca altaica declined. We hypothesized that the two species that were successful in the new high-nutrient, light-limited environment would be taller, have higher specific leaf area, change phenology by growing earlier in the season and be more morphologically plastic than their less successful counterparts. We compared plant height, specific leaf area, growth spurt date and allocation to leaves in plants grown in control and fertilized plots. We demonstrated that each of the two species that came to dominate fertilized plots has a different combination of traits and responses that likely gave them a competitive advantage; M. paniculata has the highest specific leaf area of the four species whereas E. angustifolium is tallest and exhibits morphological plasticity when fertilized by increasing biomass allocation to leaves. These results indicate that rather than one strategy determining success when nutrients become available, a variety of traits and responses may contribute to a species' ability to persist in a nutrient-enriched boreal forest understory.

The effect of protandry on siring success in Chamerion angustifolium (Onagraceae) with different inflorescence sizes.

Protandry, a form of temporal separation of gender within hermaphroditic flowers, may reduce the magnitude of pollen lost to selfing (pollen discounting) and also serve to enhance pollen export and outcross siring success. Because pollen discounting is strongest when selfing occurs between flowers on the same plant, the advantage of protandry may be greatest in plants with large floral displays. We tested this hypothesis with enclosed, artificial populations of Chamerion angustifolium (Onagraceae) by experimentally manipulating protandry (producing uniformly adichogamous or mixed protandrous and adichogamous populations) and inflorescence size (two-, six-, or 10-flowered inflorescences) and measuring pollinator visitation, seed set, female outcrossing rate, and outcross siring success. Bees spent more time foraging on and visited more flowers of larger inflorescences than small. Female outcrossing rates did not vary among inflorescence size treatments. However, seed set per fruit decreased with increasing inflorescence size, likely as a result of increased abortion of selfed embryos, perhaps obscuring the magnitude of geitonogamous selfing. Protandrous plants had a marginally higher female outcrossing rate than adichogamous plants, but similar seed set. More importantly, protandrous plants had, on average, a twofold siring advantage relative to adichogamous plants. However, this siring advantage did not increase linearly with inflorescence size, suggesting that protandry acts to enhance siring success, but not exclusively by reducing between-flower interference.