# _Vallisneria spiralis_ Promotes P and Fe Retention via Radial Oxygen Loss in Contaminated Sediments
## 3 Results and Discussion
### Net \(O_{2}\) and DIP Fluxes Measured in the Light and the Dark  
During the 3-week preincubation period _V. spiralis_ grew in the microcosms, with stolons producing 1 or 2 new plants in each microcosm. The total average biomass of transplanted _V. spiralis_ individuals in the microcosms was quantified at the end of the experiment in 70.8 \(\pm\) 8.5 g\({}_{\rm dw}\)m\({}^{-2}\) (average \(\pm\) standard deviation, n = 11). A major fraction of the total biomass was allocated in fronds (57.9 \(\pm\) 7.8 g\({}_{\rm dw}\)m\({}^{-2}\)) whereas root biomass represented 18% of the total (12.9 \(\pm\) 3.3 g\({}_{\rm dw}\)m\({}^{-2}\)). The three-way analysis of variance on dissolved O\({}_{2}\) fluxes suggests that results from measurements performed on different days were similar whereas there were significant differences between treatments (i.e., S and S + V) and incubation conditions (i.e., light versus dark fluxes, but only in sediment with _V. spiralis_) (Table 1 and Figure 3). Bare sediments always acted as a net O\({}_{2}\) sink in the dark and light incubations, with an average O\({}_{2}\) uptake of 792 \(\pm\) 57 \(\mu\)mol m\({}^{-2}\) h\({}^{-1}\) (pooled data for the three incubation days and the two incubation conditions, n = 36). Dark O\({}_{2}\) consumption in vegetated sediments was nearly 3.5-fold higher than that in bare sediments and averaged 2940 \(\pm\) 177 \(\mu\)mol m\({}^{-2}\) h\({}^{-1}\) (pooled data for dark incubations of S + V, n = 18). Photosynthetic O\({}_{2}\) production to the water column by _V. spiralis_ balanced sediment respiration results in rates of net ecosystem metabolism (NEM) not significantly different from zero (\(-\)27 \(\pm\) 69 \(\mu\)mol m\({}^{-2}\) h\({}^{-1}\), pooled data for light incubations of S + V, n = 18) and in comparable absolute values of respiration (R) and gross primary production (GPP) rates (Figure 3). Assuming a conservative value of ROL in the light of 39 \(\pm\) 16 \(\mu\)mol O\({}_{2}\) g\({}_{\rm dw}\) root\({}^{-1}\) h\({}^{-1}\)[38] a minimum amount of O\({}_{2}\) delivered by _V. spiralis_ to the sediment in this experiment can be estimated at 510 \(\pm\) 306 \(\mu\)mol O\({}_{2}\) m\({}^{-2}\) h\({}^{-1}\). This value likely underestimates the true ROL in the contaminated sediment of the Vallazza wetland due to the much larger organic matter content in the latter as compared to the sediments analyzed by [38]. Moreover, after the 3-week preincubation period, all vegetated microcosms had 1 or 2 new plants, suggesting a net growth of _V. spiralis_ (i.e., a positive daily O\({}_{2}\) budget). As rates of net ecosystem metabolism measured in the light and reported in Figure 3 equal net primary production by _V. spiralis_ plus sediment respiration and as the true net primary production by _V. spiralis_ includes the fraction released to the water column and the fraction released to the sediment (ROL), it is possible from the measured fluxes and from the data reported in the literature to reconstruct O\({}_{2}\) dynamics in the S + V treatment (Figure 4). The ratio between the calculated true NPP by _V. spiralis_ (1299 \(\pm\) 390 \(\mu\)mol O\({}_{2}\) m\({}^{-2}\) h\({}^{-1}\)) and the molar C:P stoichiometry of the plant (nearly 100, [39]) allows the estimation of a minimum theoretical net assimilation of DIP by the macrophyt  
Figure 2: Oxygen microprofiles and diffusive uptake in sediment microcosms: (**a**) O\({}_{2}\) microprofiles carried out in microcosms containing bare sediment (S) and sediment with _V. spiralis_ (S + V); (**b**) Diffusive O\({}_{2}\) uptake calculated from the microprofiles for the two treatments.